NASA’s Hubble Discovers First of Star Cluster’s Missing Black Holes

Omega Centauri Context Image

Astronomers found ’s first stellar-mass black hole, which has a visible star companion that is shown in greater detail. They used 20-plus years of data from NASA’s Hubble Space Telescope and recent data from NASA’s James Webb Space Telescope to make the discovery. Credit Image: ESA, NASA, Maximilian Häberle (MPIA), Joseph DePasquale (STScI)

Star Orbiting Black Hole Animation

The precise data collected by NASA’s Hubble and James Webb space telescopes enabled a team of astronomers to chart the visible star’s orbital path over a 20 year-plus period. Credit Animation: NASA, ESA, Matthew Whitaker (U of Utah), Joseph Olmsted (STScI)

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The massive globular star cluster Omega Centauri has puzzled astronomers for decades. It should be filled with black holes left behind by exploding stars, yet evidence for them is scarce. Now, astronomers using archival data from NASA’s Hubble Space Telescope and supportive observations from NASA’s James Webb Space Telescope have finally located their first stellar-mass black hole in this cluster. Discovering the first of this missing black hole population will help refine current theories on black hole formation within environments such as Omega Centauri. The team’s findings published Monday in The Astrophysical Journal Letters.

Omega Centauri is composed of 10 million gravitationally bound stars. Though the astronomical community previously found evidence with Hubble that an intermediate-mass black hole lurks at its center, models suggest this star cluster should also contain about 10,000 smaller, stellar-mass black holes. This notable population of black holes evaded detection in previous observational studies, which used the radial velocity method or looked for radio and X-ray emission from material falling onto black holes.

This new discovery features a different approach, known as astrometry, to measure very small movements of stars over time. By sifting through more than 20 years of Hubble archival data and pulling in recent Webb data to further refine their astrometric measurements, the team located a star orbiting an invisible object so hefty that it has to be a black hole. Dubbed oMEGACat BH-2, it is the first stellar-mass black hole detected in Omega Centauri, and it has some surprising qualities. oMEGACat BH-2 has a lower-than-expected mass and, with its visible star companion, the black hole-star duo has the longest orbital period of any black hole binary system known to date.

“With Hubble and Webb data, we were able to see the motion of the visible main sequence star that is part of this binary, which is about 18,000 light-years away in the dense environment of Omega Centauri,” said Matthew Whitaker of the University of Utah, Salt Lake City, lead author of the paper. “The precision of these measurements is incredible, down to a fraction of a pixel on Hubble and Webb’s detectors. It would not have been possible to find this black hole without these two space telescopes.”

Long time coming

Based on the precise data from Hubble and Webb, the team could chart he star’s path over 20-plus years, during its closest approach to its black hole companion when it moved the fastest across the sky. From the extensive data, the team determined that the visible star orbits oMEGACat BH-2 once every 94 years, making it the longest-period black hole binary ever known.

Its long orbital period also gives a clue to the origin of this binary system. It was probably dynamically formed, meaning the star and its black hole companion did not start out together but rather found each other in this cluster. The researchers calculated that a system like oMEGACat BH-2 will survive for less than a billion years before it is torn apart by encounters with nearby stars, a much shorter span than the age of the cluster (approximately 12 billion years old).

“It's important to understand black hole populations in globular clusters because there's uncertainty about their physics and formation,” said Seth. “More specifically, understanding the process of forming black holes and then dynamically forming binaries is vital, because it affects our ability to interpret and understand gravitational wave events. Environments like Omega Centauri are the primary places where we think binaries are merging and creating these waves.”

The team’s discovery of stellar-mass black hole oMEGACat BH-2 with the Hubble-Webb dataset is just the start of finding these evasive black hole populations in globular star clusters.

“With Hubble and Webb, we can continue to look at Omega Centauri and expand our search for similar systems within other clusters,” said Whitaker. “We’re also very excited for the launch of NASA’s Nancy Grace Roman Space Telescope because it will image the crowded galactic bulge, including the galactic center, very regularly with Hubble-like resolution and with a much wider field of view. We’re hoping we’ll be able to find black hole binary systems like this one because of the regular cadence of Roman’s observations.”

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: NASA's Hubble Space Telescope/News

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Details

Last Updated: Jul 13, 2026
Editor: Andrea Gianopoulos
Location: NASA Goddard Space Flight Center

Contact Media:

Claire Andreoli
NASA’s Goddard Space Flight Center
Greenbelt, Maryland
claire.andreoli@nasa.gov

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X-ray panorama of the “Manatee Nebula” by SRG/eROSITA

Figure 1 shows a composite X-ray image of the radio nebula W50 taken with the eROSITA telescope. The surface brightness of the X-ray emission is colour-coded in the 0.5–1 keV (red), 1–2 keV (green), and 2–4 keV (blue) energy bands. The white arrows depict the projection of the SS433 jets' precession cone extrapolated to distances of more than 100 pc. The hard and soft X-ray diffuse emission can be convincingly split into two components: softer filamentary emission (red-yellow) and harder (green-blue) emission from EXJs. Additionally, there are numerous nearby compact sources, such as active stars and accreting white dwarfs, as well as distant compact sources, mostly AGN. For distant sources, absorption by Milky Way gas suppresses emission below 1 or 2 keV, giving them a blue colour. © MPA; eROSITA/SRG

Figure 2 shows a schematic summary of the W50 nebula superimposed on a composite X-ray (red and green) and radio (VLA at 1.4 GHz, blue) image. Radio emission most likely arises at the outer, shell-like boundary of the nebula, while soft X-ray emission (0.3–0.9 keV) traces shock-heated interstellar medium (ISM) gas behind it, filling almost the entire interior of the nebula. The harder X-ray emission (0.9–2.7 keV in this case) is of a non-thermal (synchrotron) nature and may be produced by ultrarelativistic electrons that are accelerated at the shocks in the axial outflows from the system. The central part of the nebula, within 25 pc of SS433 (dashed circle), is likely to be of very low density and could be a wind-blown cavity created by an almost spherically symmetric outflow with a kinetic luminosity close to the Eddington limit. © MPA; eROSITA/SRG

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Rare or unusual astrophysical objects are used to test the limits of theoretical models because of their extreme properties. The bright X-ray source SS433 in our galaxy undoubtedly belongs to this category. Initially identified as an Hα emitter, it was later recognised as a black hole in a binary system. Since then, SS433, which emits strongly in the radio and X-ray bands, has been targeted by almost every space- and ground-based observatory, leading to a flurry of discoveries.  In contrast, the surrounding huge W50 nebula, spanning more than two degrees, is much fainter and difficult to study. The complete radio image earned W50 the nickname 'Manatee Nebula', while X-ray maps were mostly patches from different observatories or lacked spatial or energy resolution. This shortcoming has finally been overcome by the recently published SRG/eROSITA map of W50 in multiple X-ray colours, which reveals a beautiful blend of thermal and non-thermal processes within an elongated cocoon.

At the core of the W50 nebula lies a compact source (most likely a stellar-mass black hole) that accretes matter from a companion star at an astonishingly high rate — thousands of times greater than the amount the black hole can digest. This limiting rate (known as the Eddington accretion rate) arises due to the pressure exerted by the radiation produced by the infalling gas. This configuration has an immediate impact on the observational appearance of the compact source and its large-scale environment. The key prediction of the accretion theory is that most of the gas supplied to the black hole will be expelled from the system, depositing a large amount of energy into the ambient medium in the process (see Highlight September 2024).

The W50 nebula is well known in radio astronomy for its croissant-like shape. Mapping this large nebula in X-rays used to be problematic due to the limited field of view of space telescopes. Additionally, strong and inhomogeneous absorption by gas and dust occurs in the direction of W50, which is located just two degrees away from the Galactic Plane. These problems can be resolved by using a telescope with a large field of view and high sensitivity to diffuse emission — the very characteristics of the eROSITA telescope on board the SRG observatory.

The full-size X-ray map of the W50 nebula is shown in Fig. 1. The central bright spot is the black hole that powers the entire nebula. It appears extended because it is much brighter than the nebula emission, causing the central part of the image to become saturated.

The 'X-ray colours' in this figure serve the same role as red, green, and blue colours in visible light. Specifically, red corresponds to X-ray photons with a longer wavelength, while green and blue correspond to progressively shorter wavelengths. Remarkably, this simple approach immediately reveals the nature of the X-ray emission: red and yellow colours dominate where thermal plasma with a temperature of 2–10 million degrees is present. Conversely, in the bluer regions, non-thermal emission from relativistic particles dominates.

The nebula is clearly asymmetric, most likely due to a gradient in the ambient gas density surrounding it. The most remarkable feature is the so-called 'Extended X-ray Jets' (EXJs), which have sharp inner edges located around 25 parsecs from the central black hole SS433. Their spectra do not have the emission lines characteristic of thermal plasma. Rather, they must be due to the emission of relativistic particles accelerated by shocks powered by SS433’s outflows. These structures have recently been detected at TeV energies; each TeV photon carries a billion times more energy than a soft X-ray photon at keV energies.

These new X-ray data support the idea that the energy flow from SS433 evolves through three distinct stages:

1) an invisible 'dark' flow of energy between the black hole and the EXJs, presumably carried by a cold wind from the binary system;

2) a 'non-thermal' flow of energy over some 30 pc in the form of EXJs; and

3) a thermal flow (i.e., shock-heated interstellar medium (ISM)) that envelops the EXJs.

The thermal part of the W50 X-ray emission can be reasonably well described by a shock-heated plasma that has not yet reached temperature and ionisation equilibrium. Such emission is typical of middle-aged or old supernova remnants (SNRs). The outer radio boundary of the nebula also resembles SNR shocks (see Fig. 2).

In contrast, the 'extended X-ray jets' are the most remarkable features of this system on tens-of-pc scales. Their sharp inner edges plausibly correspond to extreme shocks that accelerate particles and power the X-ray (synchrotron) and TeV emission, which is 9–10 orders of magnitude more energetic. The W50/SS433 system clearly illustrates the important role that hyper-Eddington accretors might play in the energetics of the interstellar medium in galaxies at different redshifts, as well as in the production of ultra-high-energy particles.

Source: Max Planck Institute for Astrophysics/Research Highlights

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Authors:

Rashid Sunyaev
Emeritus Director
Tel: 2244
Email: rsunyaev@mpa-garching.mpg.de

Eugene Churazov
Scientific Staff
Tel: 2219
Email: echurazov@mpa-garching.mpg.de

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Original publication

Sunyaev R., Khabibullin I., Churazov E., Gilfanov M., Medvedev P., Sazonov S.
X-ray panorama of the SS433/W50 complex by SRG/eROSITA
A&A, in press

DOI

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NuSTAR Observes a Nearby Obscured Black Hole

An artist's impression of the GRS 1915+105 black hole system, showing its companion star and massive accretion disk.  Image credit: NASA/GSFC. Download Image

Over last week, NuSTAR observed the stellar-mass black hole GRS 1915+105, which resides in the Milky Way Galaxy. A fraction of massive black holes in active galaxies are largely hidden from view by intervening gas. In most cases, this is likely due to obscuration by a distant reservoir of cold gas and dust; however, a fraction of cases may be hidden by gas that is much closer to the black hole. In surveys, such black holes can only be directly detected via hard X-ray emission above 15 keV and indirectly detected using neutral iron emission lines at an energy of around 6.5 keV. GRS 1915+105 is a stellar-mass black hole in the Milky Way that was originally famous for being bright, but is now highly obscured, like some massive black holes in the centers of distant galaxies. NuSTAR accepted a Director's Discretionary Time request to observe the stellar-mass black hole GRS 1915+105 simultaneously with JAXA/NASA/ESA’s XRISM mission. The pairing of the two missions delivers exactly the hard X-ray sensitivity and sharp line response needed to study how and why black holes become obscured. Starting in January 2026, scientists will be able to submit joint observing proposals with XRISM observing time made available in the NuSTAR General Observer (GO) program, and a reciprocal agreement for NuSTAR time available in XRISM GO Cycle 3. In the future, surveys of obscured black holes with NuSTAR and XRISM will advance our understanding of black hole fueling, and how much accretion power is hidden from view in the local Universe.

Author: Jon Miller (Professor of Astronomy, University of Michigan), Daniel Stern (NuSTAR Deputy PI, Caltech)

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/Weekly Highlights

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Sun-Like Stars Found Orbiting Hidden Companions

Astronomers have discovered 21 stars like our Sun in orbit around neutron stars—heavy, compact remains of massive stars that previously exploded. The hidden neutron stars were discovered through their gravitational effects alone. Though the neutron stars are heavier than Sun-like stars, the two objects mutually orbit one another around a common center of mass. As the neutron stars orbit around, they tug on the Sun-like stars, causing them to wobble. The European Space Agency's Gaia mission detected this wobble by observing the orbits of the Sun-like stars (yellow dots) over a period of three years. The Sun-like stars are green in this animation, and the neutron stars (and their orbits) are purple. Credit: Caltech/Kareem El-Badry

This illustration depicts a binary star system consisting of a dense neutron star and a normal Sun-like star (upper left). Using data from the European Space Agency's Gaia mission, astronomers found several systems like this one, in which the two bodies are widely separated. Because the bodies in these systems are far apart, with separations on average 300 times the size of a Sun-like star, the neutron star is dormant—it is not actively stealing mass from its companion and is thus very faint. To find these hidden neutron stars, the scientists used Gaia observations to look for a wobble in the Sun-like stars caused by a tugging action of the orbiting neutron stars. These are the first neutron stars discovered purely due to their gravitational effects. As depicted in this illustration, the intense gravity of the compact neutron star—which is about 100,000 times smaller than the Sun-like star yet heavier—warps our view of the sky around it, producing a distorted mirrored view of the nearby star. Credit: Caltech/R. Hurt (IPAC)

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Most stars in our universe come in pairs. While our own Sun is a loner, many stars like our Sun orbit similar stars, while a host of other exotic pairings between stars and cosmic orbs pepper the universe. Black holes, for example, are often found orbiting each other. One pairing that has proved to be quite rare is that between a Sun-like star and a type of dead star called a neutron star.

Now, astronomers led by Caltech's Kareem El-Badry have uncovered what appear to be 21 neutron stars orbiting in binary systems with stars like our Sun. Neutron stars are dense burned-out cores of massive stars that exploded. On their own, they are extremely faint and usually cannot be detected directly. They are heavier than Sun-like stars, but the two objects mutually orbit each other around a common center of mass. As the neutron stars orbit, they tug on the Sun-like stars, causing their companions to shift back and forth in the sky. Using the European Space Agency's Gaia mission, the astronomers were able to catch these telltale wobbles to reveal a new population of dark neutron stars.

"Gaia is continuously scanning the sky and measuring the wobbles of more than a billion stars, so the odds are good for finding even very rare objects," says El-Badry, an assistant professor of astronomy at Caltech and an adjunct scientist at the Max Planck Institute for Astronomy in Germany.

Neutron Star and Sun-like Star in Orbit Around Each Other

This animation depicts a binary star system in which a massive compact neutron star is orbiting a larger Sun-like star. The intense gravity of this high-density neutron star produces significant warping effects that distort the view of the sky around it, not unlike what occurs around more compact black holes. Animation credit: Caltech/R. Hurt (IPAC)

The new study, which includes a team of co-authors from around the world, was published in The Open Journal for Astrophysics. Data from several ground-based telescopes, including the W. M. Keck Observatory on Maunakea, Hawai‘i; La Silla Observatory in Chile; and the Whipple Observatory in Arizona, were used to follow up the Gaia observations and learn more about the masses and orbits of the hidden neutron stars.

While neutron stars have previously been detected in orbit around stars like our Sun, those systems have all been more compact. With little distance separating the two bodies, a neutron star (which is heavier than a Sun-like star) can steal mass away from its partner. This mass transfer process makes the neutron star shine brightly at X-ray or radio wavelengths. In contrast, the neutron stars in the new study are much farther from their partners—on the order of one to three times the distance between Earth and the Sun.

That means the newfound stellar corpses are too far from their partners to be stealing material from them. They are instead quiescent and dark. "These are the first neutron stars discovered purely due to their gravitational effects," El-Badry says.

The discovery comes as somewhat of a surprise because it is not clear how an exploded star winds up next to a star like our Sun.

"We still do not have a complete model for how these binaries form," explains El-Badry. "In principle, the progenitor to the neutron star should have become huge and interacted with the solar-type star during its late-stage evolution." The huge star would have knocked the little star around, likely temporarily engulfing it. Later, the neutron star progenitor would have exploded in a supernova, which, according to models, should have unbound the binary systems, sending the neutron stars and Sun-like stars careening in opposite directions.

"The discovery of these new systems shows that at least some binaries survive these cataclysmic processes even though models cannot yet fully explain how," he says.

Gaia was able to find the unlikely companions due to their wide orbits and long periods (the Sun-like stars orbit around the neutron stars with periods of six months to three years). "If the bodies are too close, the wobble will be too small to detect," El-Badry says. "With Gaia, we are more sensitive to the wider orbits." Gaia is also most sensitive to binaries that are relatively nearby. Most of the newly discovered systems are located within 3,000 light-years of Earth—a relatively small distance compared, for example, to the 100,000 light-year-diameter of the Milky Way Galaxy.

The new observations also suggest just how rare the pairings are. "We estimate that about one in a million solar-type stars is orbiting a neutron star in a wide orbit," he said

El-Badry also has an interest in finding unseen dormant black holes in orbit with Sun-like stars. Using Gaia data, he has found two of these quiet black holes hidden in our galaxy. One, called Gaia BH1, is the closest known black hole to Earth at 1,600 light-years away.

"We don't know for sure how these black hole binaries formed either," El-Badry says. "There are clearly gaps in our models for the evolution of binary stars. Finding more of these dark companions and comparing their population statistics to predictions of different models will help us piece together how they form."

The paper titled "A population of neutron star candidates in wide orbits from Gaia astrometry" was funded by the National Science Foundation, the European Research Council, and the Gordon and Betty Moore Foundation. Other Caltech authors include graduate student Natsuko Yamaguchi and Professor of Astronomy Andrew Howard. Additional authors include Hans-Walter Rix and René Andrae of the Max-Planck Institute for Astronomy, David Latham and Allyson Bieryla of the Center for Astrophysics/Harvard & Smithsonian, Sahar Shahaf of the Weizmann Institute for Science, Tsevi Mazeh of Tel Aviv University; Lars Buchhave of the Technical University of Denmark, Howard Isaacson of UC Berkeley and University of Southern Queensland; Alessandro Savino of UC Berkeley, and Ilya Ilyin of Leibniz Institute for Astrophysics Potsdam.

Written by Whitney Clavin

Contact:

Whitney Clavin
(626) 395‑1944
wclavin@caltech.edu

Source: Caltech/News

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Ghost (Particle) Hunting in Gamma-ray Bursts

Illustration of a jet of particles emitted when a massive star explodes.
These jets could be one cause of gamma-ray bursts.
Credit: NASA/Swift/Cruz deWilde

Title: Search for 10–1,000 GeV Neutrinos from Gamma-ray Bursts with IceCube
Authors: IceCube Collaboration
Status: Published in ApJ

Particle Accelerators, but in Space!

Gamma-ray bursts are some of the most powerful explosions in the universe, releasing a “fireball” of particle-filled plasma in a powerful jet that accelerates these particles through the universe. Gamma-ray bursts are understood to originate either from compact object (neutron star or stellar-mass black hole) mergers (these are called short gamma-ray bursts and last less than two seconds!) or from core-collapse supernovae (these are called long gamma-ray bursts but are just defined as anything longer than two seconds). Gamma-ray bursts are the most powerful particle accelerators in the universe and are really useful for looking for new particles and new particle interactions!

Today’s authors look at gamma-ray bursts as a possible source of ghost particles, i.e., neutrinos. Neutrinos rarely interact with other matter, which makes them really hard to detect — like a ghost! The IceCube Neutrino Observatory sees neutrinos all over the sky but can’t pinpoint where they’re coming from. Since gamma-ray bursts could produce neutrinos in their outbursts, the authors search through all of IceCube’s data to see if there are any bursts of high-energy neutrinos that came in at the same time as a gamma-ray bursts.

Figure 1: A: histogram of the initial gamma-ray burst emission (called prompt emission) duration for all 2,268 gamma-ray bursts used in this study. The time windows investigated in this article are shown as red arrows. [Adapted from IceCube Collaboration et al. 2024

How Many Gamma-ray Bursts Does It Take to Find a Neutrino?

Today’s authors search for coincident neutrinos in the time periods surrounding the 2,298 bursts that happened during the lifetime of IceCube-DeepCore (IceCube’s highest-energy neutrino detector). They do this by looking at each time window individually and by combining many time windows to add faint signals that might not be seen in individual windows, but together might show an association between neutrinos and gamma-ray bursts.

In the first search, the authors define search windows before and after each burst to look for neutrinos (see Figure 1). Since neutrinos don’t interact with matter very often, they can easily stream out of dusty environments from which photons struggle to escape, meaning that the neutrinos could actually be expected to arrive at Earth before gamma-ray (and other photon) emission. The authors search the entire sky for neutrinos in these windows and assess the probability that there is an excess of neutrinos coming from the source location compared to the neutrino background that we see all over the sky.

The second search looks at groups of gamma-ray bursts that are associated in location and time with neutrino events. The authors look at the combined probability of burst/neutrino association of all the events in this group. This makes it possible to correlate gamma-ray bursts with neutrinos even if the events don’t individually stand out. Using this method, the authors didn’t find any groups of neutrinos that are any more statistically significant than individual neutrinos that fall within gamma-ray burst time windows.

Trials Factors and Tribulations

The winning burst of the first search (i.e., the most significant neutrino–gamma-ray burst correlation) is GRB bn 140807500. (Since there are a lot of gamma-ray bursts recorded by burst-hunting instruments like the Fermi Gamma-ray Burst Monitor (Fermi-GBM) and the Swift Burst Alert Telescope (Swift-BAT), it’s too much of a hassle to give the bursts individual names. Instead, the bursts get “telephone numbers” corresponding to the date they were detected.) The corresponding neutrino falls within 100 seconds of GRB bn 140807500 and has a p-value of 4.6 x 10^-5, which is the probability that the correlation between the burst and the neutrino is just a lucky coincidence and not from actual correlation (i.e., small p-values mean a more likely detection of neutrinos from gamma-ray bursts!).

This probability seems really small, and at first glance it seems like the neutrino and the gamma-ray burst are most likely connected here! Unfortunately, this doesn’t take into account trials factors (also called the look-elsewhere effect), which quantify the statistical statement that if you look at enough gamma-ray bursts and neutrinos, there will be some events that line up with each other in space and time, just by chance. To account for this, the authors must correct for 2,268 trials, one for each burst. After correcting for trials, this leaves us with a much larger p-value of 0.097, meaning there’s a one in ten chance that the gamma-ray burst and the neutrino aren’t really connected. Generally particle (and astroparticle) physicists require a p-value of 3×10^-7 (about 1 in 3.5 million, corresponding to the [in]famous 5-sigma threshold!) to feel confident in saying that these events are actually correlated.

Figure 2: Estimated neutrino flux (number of neutrinos detected at a given energy per area) for 2,264 gamma-ray bursts combined (blue) compared with the BOAT gamma-ray burst (orange). (Four of the bursts used in this study were excluded from this analysis.) Credit: IceCube Collaboration et al. 2024

Don’t Forget About the BOAT

At the same time as this article was being prepared, the brightest of all time (BOAT) gamma-ray burst was detected. The authors didn’t include the BOAT directly in their dataset, but they made some predictions as to how it would measure up to the other gamma-ray bursts that were considered. The BOAT was so bright that the authors calculated the expected neutrino signal to be 6–8 times the combined expected signal of all 2,264 gamma-ray bursts used in this part of the study (see Figure 2)! This is because the BOAT is so much more energetic in gamma rays than other gamma-ray bursts, implying a large flux of high-energy neutrinos, which are localized to more precise regions in the sky than the lower-energy neutrinos expected to accompany other bursts. This means that we can more confidently associate any observed neutrinos with the location of the burst.

There’s still work to be done to see if there are any neutrino events that seem to come from the BOAT gamma-ray burst, but this leaves the idea open that neutrinos could come from gamma-ray bursts (or at least, that we can more confidently say that they don’t)! The universe sometimes throws surprises like the BOAT at us, allowing astronomers to study the high-energy universe a lot more easily. Luckily, we’ve entered into the era of time-domain astronomy where instruments like Fermi-GBM and Swift-BAT allow us to catch more bursts and explosions than ever before, giving us an increasingly large sample of gamma-ray bursts to study!

Original astrobite edited by Cole Meldorf.

Source: American Astronomical Society/AAS-Nova

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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

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About the author, Samantha Wong:

I’m a graduate student at McGill University, where I study high-energy astrophysics. This includes studying all sorts of extreme environments in the universe like active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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Astronomers Measure Heaviest Black Hole Pair Ever Found

PR Image noirlab2405a
Artist’s Impression of Heaviest Supermassive Binary Black Hole

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Data from Gemini North provide possible explanation for supermassive binary black hole’s halted merger

Using archival data from the Gemini North telescope, a team of astronomers have measured the heaviest pair of supermassive black holes ever found. The merging of two supermassive black holes is a phenomenon that has long been predicted, though never observed. This massive pair gives clues as to why such an event seems so unlikely in the Universe.

Nearly every massive galaxy hosts a supermassive black hole at its center. When two galaxies merge, their black holes can form a binary pair, meaning they are in a bound orbit with one another. It’s hypothesized that these binaries are fated to eventually merge, but this has never been observed [1]. The question of whether such an event is possible has been a topic of discussion amongst astronomers for decades. In a recently published paper in The Astrophysical Journal, a team of astronomers have presented new insight into this question.

The team used data from the Gemini North telescope in Hawai‘i, one half of the International Gemini Observatory operated by NSF’s NOIRLab, which is funded by the U.S. National Science Foundation, to analyze a supermassive black hole binary located within the elliptical galaxy B2 0402+379. This is the only supermassive black hole binary ever resolved in enough detail to see both objects separately [2], and it holds the record for having the smallest separation ever directly measured — a mere 24 light-years [3]. While this close separation foretells a powerful merger, further study revealed that the pair has been stalled at this distance for over three billion years, begging the question; what’s the hold-up?

To better understand the dynamics of this system and its halted merger the team looked to archival data from Gemini North’s Gemini Multi-Object Spectrograph (GMOS), which allowed them to determine the speed of the stars within the vicinity of the black holes. “The excellent sensitivity of GMOS allowed us to map the stars’ increasing velocities as one looks closer to the galaxy’s center,” said Roger Romani, Stanford University physics professor and co-author of the paper. “With that, we were able to infer the total mass of the black holes residing there.”

The team estimates the binary’s mass to be a whopping 28 billion times that of the Sun, qualifying the pair as the heaviest binary black hole ever measured. Not only does this measurement give valuable context to the formation of the binary system and the history of its host galaxy, but it supports the long-standing theory that the mass of a supermassive binary black hole plays a key role in stalling a potential merger [4].

“The data archive serving the International Gemini Observatory holds a gold mine of untapped scientific discovery," says Martin Still, NSF program director for the International Gemini Observatory. "Mass measurements for this extreme supermassive binary black hole are an awe-inspiring example of the potential impact from new research that explores that rich archive.”

Understanding how this binary formed can help predict if and when it will merge — and a handful of clues point to the pair forming via multiple galaxy mergers. The first is that B2 0402+379 is a ‘fossil cluster,’ meaning it is the result of an entire galaxy cluster’s worth of stars and gas merging into one single massive galaxy. Additionally, the presence of two supermassive black holes, coupled with their large combined mass, suggests they resulted from the amalgamation of multiple smaller black holes from multiple galaxies.

Following a galactic merger, supermassive black holes don’t collide head-on. Instead they begin slingshotting past each other as they settle into a bound orbit. With each pass they make, energy is transferred from the black holes to the surrounding stars. As they lose energy, the pair is dragged down closer and closer until they are just light-years apart, where gravitational radiation takes over and they merge. This process has been directly observed in pairs of stellar-mass black holes — the first ever recorded instance being in 2015 via the detection of gravitational waves — but never in a binary of the supermassive variety.

With new knowledge of the system’s extremely large mass, the team concluded that an exceptionally large number of stars would have been needed to slow the binary’s orbit enough to bring them this close. In the process, the black holes seem to have flung out nearly all the matter in their vicinity, leaving the core of the galaxy starved of stars and gas. With no more material available to further slow the pair’s orbit, their merger has stalled in its final stages.

“Normally it seems that galaxies with lighter black hole pairs have enough stars and mass to drive the two together quickly,” said Romani. “Since this pair is so heavy it required lots of stars and gas to get the job done. But the binary has scoured the central galaxy of such matter, leaving it stalled and accessible for our study.”

Whether the pair will overcome their stagnation and eventually merge on timescales of millions of years, or continue in orbital limbo forever, is yet to be determined. If they do merge, the resulting gravitational waves would be a hundred million times more powerful than those produced by stellar-mass black hole mergers. It’s possible the pair could conquer that final distance via another galaxy merger, which would inject the system with additional material, or potentially a third black hole, to slow the pair’s orbit enough to merge. However, given B2 0402+379’s status as a fossil cluster, another galactic merger is unlikely.

“We’re looking forward to follow-up investigations of B2 0402+379’s core where we’ll look at how much gas is present,” says Tirth Surti, Stanford undergraduate and the lead author on the paper. “This should give us more insight into whether the supermassive black holes can eventually merge or if they will stay stranded as a binary.”

Source: NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab)/News

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Notes

[1] While there is evidence of supermassive black holes coming within a few light-years of each other, it seems none have been able to overcome that final distance. The question of whether such an event is possible is known as the final-parsec problem and has been a topic of discussion amongst astronomers for decades.

[2] Previous observations have been made of galaxies containing two supermassive black holes, but in these cases they are thousands of light-years apart — too far to be in a bound orbit with one another like the binary found in B2 0402+379.

[3] Other black hole-powered sources exist with possible smaller separations, though these have been inferred using indirect observations and therefore can best be classified as candidate binaries.

[4] This theory was first put forth in 1980 by Begelman et al. and has long been argued to occur based on decades of observations of the centers of galaxies.

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More information

This research was presented in a paper accepted in The Astrophysical Journal. DOI: 10.3847/1538-4357/ad14fa

The team is composed of: Tirth Surti (Kavli Institute for Particle Astrophysics and Cosmology, Stanford University), Roger W. Romani (Kavli Institute for Particle Astrophysics and Cosmology, Stanford University), Julia Scharwächter (Gemini Observatory/NSF’s NOIRLab), Alison Peck (University of Maryland) and Greg B. Taylor (University of New Mexico, Albuquerque).

NSF’s NOIRLab (National Optical-Infrared Astronomy Research Laboratory), the US center for ground-based optical-infrared astronomy, operates the International Gemini Observatory (a facility of NSF, NRC–Canada, ANID–Chile, MCTIC–Brazil, MINCyT–Argentina, and KASI–Republic of Korea), Kitt Peak National Observatory (KPNO), Cerro Tololo Inter-American Observatory (CTIO), the Community Science and Data Center (CSDC), and Vera C. Rubin Observatory (operated in cooperation with the Department of Energy’s SLAC National Accelerator Laboratory). It is managed by the Association of Universities for Research in Astronomy (AURA ) under a cooperative agreement with NSF and is headquartered in Tucson, Arizona. The astronomical community is honored to have the opportunity to conduct astronomical research on Iolkam Du’ag (Kitt Peak) in Arizona, on Maunakea in Hawai‘i, and on Cerro Tololo and Cerro Pachón in Chile. We recognize and acknowledge the very significant cultural role and reverence that these sites have to the Tohono O’odham Nation, to the Native Hawaiian community, and to the local communities in Chile, respectively.

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Links

• Read the paper: The Central Kinematics and Black Hole Mass of 4C+37.11
• Images of the Gemini North telescope
• Videos of the Gemini North telescope
• Other Gemini North news
• Other discoveries made with GMOS-N
• Check out other NOIRLab Science Releases

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Contacts:

Roger Romani
Stanford University
Email: rwr@astro.stanford.edu

Josie Fenske
Jr. Public Information Officer
NSF’s NOIRLab
Email: josie.fenske@noirlab.edu

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'Listen' to the Light Echoes From a Black Hole Quick Look: 'Listen' to the Light Echoes From a Black Hole

V404 Cygni

Credit X-ray: Chandra: NASA/CXC/U.Wisc-Madison/S. Heinz et al.; Swift: NASA/Swift/Univ. of Leicester/A. Beardmore; Optical: DSS; Sonification: NASA/CXC/SAO/K.Arcand, SYSTEM Sounds (M. Russo, A. Santaguida)

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A Quick Look at V404 Cygni - More Animations

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One of the surprising features of black holes is that although light (such as radio, visible, and X-rays) cannot escape from them, surrounding material can produce intense bursts of electromagnetic radiation. As they travel outward, these blasts of light can bounce off clouds of gas and dust in space, similar to how light beams from a car’s headlight will scatter off fog.

A new sonification turns these “light echoes” from the black hole called V404 Cygni into sound. Located about 7,800 light-years from Earth, V404 Cygni is a system that contains a black hole, with a mass between five and 10 times the Sun’s, that is pulling material from a companion star in orbit around it. The material is funneled into a disk that encircles the stellar-mass black hole.

This material periodically generates bursts of radiation, including X-rays. As the X-rays travel outward they encounter clouds of gas and dust in between V404 Cygni and Earth and are scattered at various angles. NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory have imaged the X-ray light echoes around V404 Cygni. Because astronomers know exactly how fast light travels and have determined an accurate distance to this system, they can calculate when these eruptions occurred. This data, plus other information, helps astronomers learn more about the dust clouds, including their composition and distances.

Illustration showing how the rings seen by Chandra were produced
Credit: Univ. of Wisconsin-Madison/S.Heinz

The sonification of V404 Cygni translates the X-ray data from both Chandra and Swift into sound. During the sonification, the cursor moves outward from the center of the image in a circle. As it passes through the light echoes detected in X-rays (seen as concentric rings in blue by Chandra and red by Swift in the image), there are tick-like sounds and changes in volume to denote the detection of X-rays and the variations in brightness. To differentiate between the data from the two telescopes, Chandra data is represented by higher-frequency tones while the Swift data is lower. In addition to the X-rays, the image includes optical data from the Digitized Sky Survey that shows background stars. Each star in optical light triggers a musical note. The volume and pitch of the note are determined by the brightness of the star.

V404 Cygnus Sonification (Wavelengths) (104 mb)

More sonifications of astronomical data, as well as additional information on the process, can be found at the "A Universe of Sound" website:https://chandra.si.edu/sound/

These sonifications were led by the Chandra X-ray Center (CXC) and included as part of NASA's Universe of Learning (UoL) program. The collaboration was driven by visualization scientist Kimberly Arcand (CXC), astrophysicist Matt Russo, and musician Andrew Santaguida (both of the SYSTEM Sounds project). NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science from Cambridge Massachusetts and flight operations from Burlington, Massachusetts. NASA's Universe of Learning materials are based upon work supported by NASA under cooperative agreement award number NNX16AC65A to the Space Telescope Science Institute, working in partnership with Caltech/IPAC, Center for Astrophysics | Harvard & Smithsonian, and the Jet Propulsion Laboratory.

Quick Look: 'Listen' to the Light Echoes From a Black Hole

Source: NASA’s Chandra X-ray Observatory

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Fast Facts for V404 Cygni:

About the Sound:

• This is an inside-out scan of the light echo rings formed by dust scattering and the background stars.
• Dust scattering rings
• The sound is generated by a series of tick-like sounds. The volume and density of ticks is controlled by the ring brightness.
• Listening to the pattern of rings in this way traces the density of dust clouds that the light has scattered off of on its way towards Earth.
• The sound generated by the Swift X-ray data is represented as lower frequencies.
• The Chandra X-ray data represents higher frequency light and its corresponding sound is limited to higher frequencies.
• Background stars (DSS Optical data)
• Each visible light star triggers a musical note. The volume and pitch of the note are determined by the brightness of the star. Brighter stars are louder and higher pitched.

Scale: Image is about 35 arcmin (80 light-years) across.
Category: Black Holes
Coordinates (J2000): RA 20h 24m 03s | Dec +33° 52´ 02"
Constellation: Cygnus
Observation Date: 2 observations: July 13th and 29th, 2015
Observation Time: 18 hours 51 minutes
Obs. ID: 17701, 17704
Instrument: ACIS
References: Heinz, S., et al., ApJ, 2016, 825, 15; arXiv:1605.01648
Color Code: X-ray: Chandra: blue & teal, Swift: red, green, blue; Optical: red, green blue
Distance Estimate: About 7,800 light-years

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MAXI J1820+070: Black Hole Outburst Caught on Video

MAXI J1820+070
Credit: X-ray: NASA/CXC/Université de Paris/M. Espinasse et al.; Optical/IR:PanSTARRS

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A Tour of a Star Survives Close Call with a Black Hole - More Animations

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Astronomers have caught a black hole hurling hot material into space at close to the speed of light. This flare-up was captured in a new movie from NASA's Chandra X-ray Observatory.

The black hole and its companion star make up a system called MAXI J1820+070, located in our Galaxy about 10,000 light years from Earth. The black hole in MAXI J1820+070 has a mass about eight times that of the Sun, identifying it as a so-called stellar-mass black hole, formed by the destruction of a massive star. (This is in contrast to supermassive black holes that contain millions or billions of times the Sun's mass.)

The companion star orbiting the black hole has about half the mass of the Sun. The black hole's strong gravity pulls material away from the companion star into an X-ray emitting disk surrounding the black hole.

While some of the hot gas in the disk will cross the "event horizon" (the point of no return) and fall into the black hole, some of it is instead blasted away from the black hole in a pair of short beams of material, or jets. These jets are pointed in opposite directions, launched from outside the event horizon along magnetic field lines. The new footage of this black hole's behavior is based on four observations obtained with Chandra in November 2018 and February, May, and June of 2019, and reported in a paper led by Mathilde Espinasse of the Université de Paris.

The main panel of the graphic is a large optical and infrared image of the Milky Way galaxy from the PanSTARRS optical telescope in Hawaii, with the location of MAXI J1820+070 above the plane of the galaxy marked by a cross. The inset shows a movie that cycles through the four Chandra observations, where "day 0" corresponds to the first observation on November 13th, 2018, about four months after the jet's launch. MAXI J1820+070 is the bright X-ray source in the middle of the image and sources of X-rays can be seen moving away from the black hole in jets to the north and south. MAXI J1820+070 is a point source of X-rays, although it appears to be larger than a point source because it is much brighter than the jet sources. The southern jet is too faint to be detected in the May and June 2019 observations.

Just how fast are the jets of material moving away from the black hole? From Earth's perspective, it looks as if the northern jet is moving at 60% the speed of light, while the southern one is traveling at an impossible-sounding 160% of light speed!

This is an example of superluminal motion, a phenomenon that occurs when something travels towards us near the speed of light, along a direction close to our line of sight. This means the object travels almost as quickly towards us as the light it generates, giving the illusion that the jet's motion is more rapid than the speed of light. In the case of MAXI J1820+070, the southern jet is pointing towards us and the northern jet is pointing away from us, so the southern jet appears to be moving faster than the northern one. The actual velocity of the particles in both jets is greater than 80% of the speed of light.

Illustration of a Black Hole Accreting Matter from a Companion Star and Producing Jets

Credit: NASA/CXC/M.Weiss

Only two other examples of such high-speed expulsions have been seen in X-rays from stellar-mass black holes.

MAXI J1820+070 has also been observed at radio wavelengths by a team led by Joe Bright from the University of Oxford, who previously reported the detection of superluminal motion of compact sources based on radio data alone that extended from the launch of the jets on July 7, 2018 to the end of 2018.

Because the Chandra observations approximately doubled the length of time the jets were followed, a combined analysis of the radio data and the new Chandra data by Espinasse and her team gave more information about the jets. This included evidence that the jets are decelerating as they travel away from the black hole.

Most of the energy in the jets is not converted into radiation, but is instead released when particles in the jets interact with surrounding material. These interactions might be the cause of the jets' deceleration. When the jets collide with surrounding material in interstellar space, shock waves — akin to the sonic booms caused by supersonic aircraft — occur. This process generates particle energies that are higher than that of the Large Hadron Collider.

The researchers estimate that about 400 million billion pounds of material was blown away from the black hole in these two jets launched in July 2018. This amount of mass is comparable to what could be accumulated on the disk around the black hole in the space of a few hours, and is equivalent to about a thousand Halley's Comets or about 500 million times the mass of the Empire State Building.

Studies of MAXI J1820+070 and similar systems promise to teach us more about the jets produced by stellar-mass black holes and how they release their energy once their jets interact with their surroundings.

Radio observations conducted with the Karl G. Jansky Very Large Array and the MeerKAT array were also used to study MAXI J1820+070's jets.

A paper describing these results is published in the latest edition of The Astrophysical Journal Letters and is available online. The authors of the paper are Mathilde Espinasse and Stéphane Corbel (Université de Paris, Paris, France), Philip Kaaret (University of Iowa, Iowa City, Iowa), Evangelia Tremou (Université de Paris , Paris, France), Giulia Migliori (Institute of Radio Astronomy of Bologna, Bologna, Italy), Richard M. Plotkin (University of Nevada, Reno, Nevada), Joe Bright (University of Oxford, Oxford, UK), John Tomsick (University of California, Berkeley, California), Anastasios Tzioumis (Australia Telescope National Facility, CSIRO, Epping, Australia), Rob Fender (University of Oxford, Oxford, UK), Jerome A. Orosz (San Diego State University, San Diego, California), Elena Gallo (University of Michigan, Ann Arbor, Michigan), Jeroen Homan (Eureka Scientific, Oakland, California), Peter G. Jonker (Radboud University, Nijmegen, the Netherlands), James C. A. Miller-Jones (Curtin University, Perth, Australia), David M. Russell (New York University Abu Dhabi, Abu Dhabi, UAE), and Sara Motta (University of Oxford, Oxford, UK).

NASA's Marshall Space Flight Center manages the Chandra program. The Smithsonian Astrophysical Observatory's Chandra X-ray Center controls science and flight operations from Cambridge and Burlington, Massachusetts.

A Quick Look at a Black Hole Outburst Caught on Video

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Fast Facts for MAXI J1820+070:

Scale: Optical/infrared image is about 44.5 degrees (8,000 light years) across. X-ray inset image is about 30 arcsec (1.5 light years) across.
Category: Black Holes
Coordinates (J2000): RA 18h 20m 21.9s | Dec 07° 11´ 7.2"
Constellation: Ophiuchus
Observation Date: 5 pointings from November 13, 2018 to June 11, 2019
Observation Time: 39 hours 50 minutes (1 day 15 hours 50 minutes)
Obs. ID: 20207, 20208, 22080, 21203, 21205
Instrument: ACIS

References: Espinasse, M., et al. 2020, ApJ Letters. arXiv:2004.06416
Color Code: X-ray: purple; Optical/IR: red, green, blue

Distance Estimate: About 10,000 light years

Source:  NASA’s Chandra X-ray Observatory

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NASA’s NICER Mission Maps ‘Light Echoes’ of New Black Hole

In this illustration of a newly discovered black hole named MAXI J1820+070, a black hole pulls material off a neighboring star and into an accretion disk. Above the disk is a region of subatomic particles called the corona. Credit: Aurore Simonnet and NASA’s Goddard Space Flight Center. Hi-res image

Scientists have charted the environment surrounding a stellar-mass black hole that is 10 times the mass of the Sun using NASA’s Neutron star Interior Composition Explorer (NICER) payload aboard the International Space Station. NICER detected X-ray light from the recently discovered black hole, called MAXI J1820+070 (J1820 for short), as it consumed material from a companion star. Waves of X-rays formed “light echoes” that reflected off the swirling gas near the black hole and revealed changes in the environment’s size and shape.

“NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before,” said Erin Kara, an astrophysicist at the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who presented the findings at the 233rd American Astronomical Society meeting in Seattle. “Previously, these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and undergo changes slowly. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.”

A paper describing the findings, led by Kara, appeared in the Jan. 10 issue of Nature and is available online.

Watch how X-ray echoes, mapped by NASA’s Neutron star Interior Composition Explorer (NICER) revealed changes to the corona of black hole MAXI J1820+070. Credits: NASA’s Goddard Space Flight Center. Download this video and related multimedia in HD formats from NASA Goddard's Scientific Visualization Studio

J1820 is located about 10,000 light-years away toward the constellation Leo. The companion star in the system was identified in a survey by ESA’s (European Space Agency) Gaia mission, which allowed researchers to estimate its distance. Astronomers were unaware of the black hole’s presence 

until March 11, 2018, when an outburst was spotted by the Japan Aerospace Exploration Agency’s Monitor of All-sky X-ray Image (MAXI), also aboard the space station. J1820 went from a totally unknown black hole to one of the brightest sources in the X-ray sky over a few days. NICER moved quickly to capture this dramatic transition and continues to follow the fading tail of the eruption.

“NICER was designed to be sensitive enough to study faint, incredibly dense objects called neutron stars,” said Zaven Arzoumanian, the NICER science lead at Goddard and a co-author of the paper. “We’re pleased at how useful it’s also proven in studying these very X-ray-bright stellar-mass black holes.”

A black hole can siphon gas from a nearby companion star into a ring of material called an accretion disk. Gravitational and magnetic forces heat the disk to millions of degrees, making it hot enough to produce X-rays at the inner parts of the disk, near the black hole. Outbursts occur when an instability in the disk causes a flood of gas to move inward, toward the black hole, like an avalanche. The causes of disk instabilities are poorly understood.

Above the disk is the corona, a region of subatomic particles around 1 billion degrees Celsius (1.8 billion degrees Fahrenheit) that glows in higher-energy X-rays. Many mysteries remain about the origin and evolution of the corona. Some theories suggest the structure could represent an early form of the high-speed particle jets these types of systems often emit.

Astrophysicists want to better understand how the inner edge of the accretion disk and the corona above it change in size and shape as a black hole accretes material from its companion star. If they can understand how and why these changes occur in stellar-mass black holes over a period of weeks, scientists could shed light on how supermassive black holes evolve over millions of years and how they affect the galaxies in which they reside.

One method used to chart those changes is called X-ray reverberation mapping, which uses X-ray reflections in much the same way sonar uses sound waves to map undersea terrain. Some X-rays from the corona travel straight toward us, while others light up the disk and reflect back at different energies and angles.

X-ray reverberation mapping of supermassive black holes has shown that the inner edge of the accretion disk is very close to the event horizon, the point of no return. The corona is also compact, lying closer to the black hole rather than over much of the accretion disk. Previous observations of X-ray echoes from stellar black holes, however, suggested the inner edge of the accretion disk could be quite distant, up to hundreds of times the size of the event horizon. The stellar-mass J1820, however, behaved more like its supermassive cousins.  

As they examined NICER’s observations of J1820, Kara’s team saw a decrease in the delay, or lag time, between the initial flare of X-rays coming directly from the corona and the flare’s echo off the disk, indicating that the X-rays traveled shorter and shorter distances before they were reflected. From 10,000 light-years away, they estimated that the corona contracted vertically from roughly 100 to 10 miles — that’s like seeing something the size of a blueberry shrink to something the size of a poppy seed at the distance of Pluto.

The NICER instrument installed on the International Space Station, as captured by a high-definition external camera on Oct. 22, 2018. Credits: NASA

“NICER’s observations of J1820 have taught us something new about stellar-mass black holes and about how we might use them as analogs for studying supermassive black holes and their effects on galaxy formation,” said co-author Philip Uttley, an astrophysicist at the University of Amsterdam. “We’ve seen four similar events in NICER’s first year, and it’s remarkable. It feels like we’re on the edge of a huge breakthrough in X-ray astronomy.”

NICER is an Astrophysics Mission of Opportunity within NASA's Explorer program, which provides frequent flight opportunities for world-class scientific investigations from space utilizing innovative, streamlined and efficient management approaches within the heliophysics and astrophysics science areas. NASA's Space Technology Mission Directorate supports the SEXTANT component of the mission, demonstrating pulsar-based spacecraft navigation.

“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” said co-author Jack Steiner, an astrophysicist at the Massachusetts Institute of Technology’s Kavli Institute for Astrophysics and Space Research in Cambridge. “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”

To confirm the decreased lag time was due to a change in the corona and not the disk, the researchers used a signal called the iron K line created when X-rays from the corona collide with iron atoms in the disk, causing them to fluoresce. Time runs slower in stronger gravitational fields and at higher velocities, as stated in Einstein’s theory of relativity. When the iron atoms closest to the black hole are bombarded by light from the core of the corona, the X-ray wavelengths they emit get stretched because time is moving slower for them than for the observer (in this case, NICER).

Kara’s team discovered that J1820’s stretched iron K line remained constant, which means the inner edge of the disk remained close to the black hole — similar to a supermassive black hole. If the decreased lag time was caused by the inner edge of the disk moving even further inward, then the iron K line would have stretched even more.

These observations give scientists new insights into how material funnels in to the black hole and how energy is released in this process.

By Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

 Source: NASA/NICER

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ULX in M51: Beaming with the Light of Millions of Suns

ULX in M51 

Credit  X-ray: NASA/CXC/Caltech/M. Brightman et al.; Optical: NASA/STScI

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In the 1980s, scientists started discovering a new class of extremely bright sources of X-rays in galaxies. These sources were a surprise, as they were clearly located away from the supermassive black holes found in the center of galaxies. At first, researchers thought that many of these ultraluminous X-ray sources, or ULXs, were black holes containing masses between about a hundred and a hundred thousand times that of the sun. Later work has shown some of them may be stellar-mass black holes, containing up to a few tens of times the mass of the sun.

In 2014, observations with NASA's NuSTAR (Nuclear Spectroscopic Telescope Array) and Chandra X-ray Observatory showed that a few ULXs, which glow with X-ray light equal in luminosity to the total output at all wavelengths of millions of suns, are even less massive objects called neutron stars. These are the burnt-out cores of massive stars that exploded. Neutron stars typically contain only about 1.5 times the mass of the sun. Three such ULXs were identified as neutron stars in the last few years. Scientists discovered regular variations, or "pulsations," in the X-ray emission from ULXs, behavior that is exhibited by neutron stars but not black holes.

Now, researchers using data from NASA's Chandra X-ray Observatory have identified a fourth ULX as being a neutron star, and found new clues about how these objects can shine so brightly. The newly characterized ULX is located in the Whirlpool galaxy, also known as M51. This composite image of the Whirlpool contains X-rays from Chandra (purple) and optical data from the Hubble Space Telescope (red, green, and blue). The ULX is marked with a circle.

Neutron stars are extremely dense objects — a teaspoon would weigh more than a billion tons, as much as a mountain. The intense gravity of the neutron stars pulls surrounding material away from companion stars, and as this material falls toward the neutron star, it heats up and glows with X-rays. As more and more matter falls onto the neutron star, there comes a time when the pressure from the resulting X-ray light becomes so intense that it pushes the matter away. Astronomers call this point — when the objects typically cannot accumulate matter any faster and give off any more X-rays — the Eddington limit. The new result shows this ULX is surpassing the Eddington limit for a neutron star.

The scientists analyzed archival X-ray data taken by Chandra and discovered an unusual dip in the ULX's X-ray spectrum, which is the intensity of X-rays measured at different wavelengths. After ruling out other possibilities, they concluded that the dip was likely from a process called cyclotron resonance scattering, which occurs when charged particles — either positively charged protons or negatively charged electrons — circle around in a magnetic field. The size of the dip in the X-ray spectrum, called a cyclotron line, implies magnetic field strengths that are at least 10,000 times greater than those associated with matter spiraling into a stellar-mass black hole, but are within the range observed for neutron stars. This provides strong evidence that this ULX is a neutron star rather than a black hole, and is the first such identification that did not involve the detection of X-ray pulsations.

An accurate determination of the magnetic field strength depends on whether the cause of the cyclotron line, either protons or electrons, is known. If the line is from protons, then the magnetic fields around the neutron star are extremely strong, comparable to the strongest magnetic fields produced by neutron stars, and may in fact be helping to break the Eddington limit. Such strong magnetic fields could reduce the pressure from a ULX's X-rays — the pressure that normally pushes away matter — allowing the neutron star to consume more matter than expected.

If the cyclotron line is from circling electrons, by contrast, then the magnetic field strength around the neutron star would be about 10,000 times less strong, and thus not powerful enough for the flow onto this neutron star to break the Eddington limit.

The researchers currently don't have a spectrum of the new ULX with enough detail to determine the cyclotron line's origin. To further address this mystery, the researchers are planning to acquire more X-ray data on the ULX in M51 and look for cyclotron lines in other ULXs.

A paper describing this research, led by Murray Brightman of the California Institute of Technology, appears in the latest issue of Nature Astronomy. The other authors include F. Fürst of the European Space Astronomy Centre; M.J. Middleton of University of Southampton, United Kingdom; D.J. Walton and A.C. Fabian of University of Cambridge, United Kingdom; D. Stern of NASA's Jet Propulsion Laboratory; M. Heida of Caltech; D. Barret of France's Centre national de la recherche scientifique and University of Toulouse; and M. Bachetti of Italy's Istituto Nazionale di Astrofisica.

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.

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Fast Facts for ULX in M51:

Scale: Image is 6 x 6 arcmin across. (About 52,000 x 52,000 light years.)
Category: Neutron Stars/X-ray Binaries, Normal Galaxies & Starburst Galaxies
Coordinates (J2000): RA 13h 29m 55.7s | Dec +47° 13´ 53"
Constellation: Canes Venatici
Observation Date: 11 pointings between Mar 2000 and Oct 2012
Observation Time: 232 hours 10 min (9 days 16 hours 10 min )
Obs. ID: 353, 354, 1622, 3932, 13812-13816, 15496, 15553
Instrument: ACIS
References: "Magnetic field strength of a neutron-star-powered ultraluminous X-ray source", M. Brightman et al., 2018, Nature Astronomy, in press.
Color Code: X-ray (Purple); Optical (Red, Green, Blue)
Distance Estimate: About 30 million light years

Source: NASA’s Chandra X-ray Observatory