NuSTAR Observes a Gamma-ray Blazar

This artist's impression shows the accreting black hole at the center of a galaxy producing a bright and powerful jet. When these jets are aligned with Earth, they become blazars, sources so bright that we can see them at enormous distances far into the Universe's past. Image credit: NASA/JPL-Caltech/GSFC.  Download Image

During the past week, NuSTAR performed a Target-of-Opportunity observation of an accreting supermassive black hole powering a relativistic jet, called PKS 1725+123, for more than two days of total exposure, together with a coordinated 11-hour XMM-Newton observation. Located at a cosmic epoch in which the universe was approximately half of its current age, PKS 1725+123 was the focus of a world-wide astrophysical observing campaign across the electromagnetic spectrum during the past two weeks after the Fermi gamma-ray telescope and numerous ground-based Very High Energy gamma-ray observatories confirmed the source to be flaring in gamma-rays. PKS 1725+123 belongs to a rare class of accreting supermassive black holes, known as Very High Energy gamma-ray blazars, which are believed to be emitting powerful relativistic jets pointed towards Earth. Despite being some of the brightest objects in the gamma-ray sky and viable sources of astrophysical neutrino emission, very little is currently known about the mechanisms responsible for the extreme radiation detected from such objects. A common prediction amongst competing models of particle acceleration and radiation is that a crucial region of the spectral energy distribution for distinguishing between models lies in the high-energy X-ray band covered by NuSTAR. The corresponding spectra and variability characteristics of the outburst of PKS1725+123 provided by NuSTAR will thus be crucial in understanding the origin of some of the most energetic astrophysical flares currently known.

Authors: Peter Boorman (Caltech), Lea Marcotulli (DESY, Germany)

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

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NuSTAR Observes a Millisecond Pulsar

An artist's impression of an accreting millisecond pulsar, showing a neutron star accreting matter from a companion star, which speeds it up to a spin period of a fraction of a second. As it rotates, the orientation of its magnetic field spins in and out of the line of sight, causing us to observe pulsations. Image credit: NASA/D. Berry. Download Image

Over the past week, NuSTAR observed the bright Galactic binary SAX J1808.4-3658 as part of an approved joint XMM-Newton and NuSTAR Target-of-Opportunity (ToO) program. SAX J1808 was the first ever discovered accreting millisecond pulsar and has been dormant for around three years before it recently increased in brightness again in a dramatic fashion, when the source was observed by Einstein Probe to have an X-ray flux exceeding the XMM/NuSTAR trigger threshold by more than a factor of 1000. The team has also triggered a 10-day-long ToO observation with IXPE, as well as requesting Director's Discretionary Time (DDT) ToO observations with XRISM, to explore the polarization and high-resolution spectral properties of the source. NuSTAR observations close to the start of IXPE observations will maximize the scientific return, probing the broadband outburst spectra beyond the IXPE, Einstein Probe, and XRISM bands to search for reflection features from the accretion disk as the outburst of this source proceeeds.

Authors: Daniel Stern (NuSTAR Deputy PI)

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

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NuSTAR Observes A Flaring Star

An image of a coronal mass ejection from the Sun, taken by the Solar Dynamics Observatory in 2015. The star that NuSTAR observed, BD-08 6022, is a G-type star, making it very similar to our own Sun. Image credit: NASA/Goddard/SDO. Download Image

During the past week, NuSTAR responded to a Director’s Discretionary Time request for a rapid follow-up of a bright stellar flare reported by the Einstein Probe Wide-field X-ray Telescope (EP-WXT) on August 2, 2025. Stellar flares are explosive releases of magnetic energy on active stars that heat plasma and accelerate particles, producing bright, rapidly evolving X-ray emission. A growing number of studies have found that some flares are accompanied by brief episodes of strong X-ray absorption, when cool, dense ejecta lifted from the low corona crosses the line of sight and temporarily boosts photoelectric opacity. Such ejecta can naturally arise if a coronal mass ejection (CME) is launched: clumpy prominences and filament material in the CME front and wake may partially cover the X-ray source, suppressing low-energy photons while higher-energy X-rays continue to escape, and producing sharp spectral hardening and energy-dependent dips. NuSTAR is ideal for probing transient X-ray absorption and searching for hard-X-ray signatures of CME-driven obscuration. By tracking spectral hardness and effective column density through the flare decay, the observations will constrain the geometry and dynamics of the absorber and evaluate the CME interpretation. Combined with EP discovery data and ongoing monitoring, this NuSTAR dataset is expected to place stringent hard-X-ray constraints on this event and provide a template for rapid characterization of similar nearby stellar flares.

Authors: Yifan Hu (Imperial College London, UK)

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

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NuSTAR Observes a Nearby Supernova

An astrophotographer's optical image of the supernova host galaxy NGC 7331. SN 2025rbs is visible as a bright point close to the galaxy center. An animated GIF showing the appearance of the supernova can be found at: https://ssr.app.astrobin.com/i/pnplmb?r=C. Image credit: GalacticRAVE/M. Steinmetz. Download Image

During the past week, NuSTAR responded to a community target-of-opportunity (ToO) request to observe the young, Type Ia supernova SN 2025rbs, which is located in the galaxy NGC 7331. Type Ia supernovae are the result of a white dwarf accreting material from a companion star until it exceeds the Chandrasekhar mass and explodes. These explosions have regular enough time profiles and overall luminosity that they are regularly used to measure the distance scale of the Universe. However, their underlying physics is relatively poorly understood since there are few Type Ia supernovae that are close enough to study in detail. In their early lives, the supernovae are powered by radioactive decay of material (primarily 56Ni) that releases gamma-rays that thermalize into the supernova atmosphere so that the ejecta glows in optical light. NuSTAR provides a unique capability to study the hard X-ray (>50 keV) emission from these systems, which arises as the ejecta expands and becomes optically thin to the gamma-ray photons so that hard X-rays “leak out” of the ejecta. SN 2025rbs is the closest Type Ia supernova to the Earth since SN 2014J exploded in M82, which NuSTAR observed in January/February 2014 for nearly a month. The NuSTAR ToO observation of SN 2025rbs occurred prior to the optical peak of the emission, only six days after the supernova was classified as a Type Ia and a few days before the optical peak. An Astronomer’s Telegram (ATel) reporting early results was posted the same day the data were received at the NuSTAR Science Operations Center (SOC), thanks to the ability of the SOC to provide “quicklook” unprocessed data products to the community. These data will provide the most stringent limits on any high-energy emission from the supernova explosion.

Authors: Brian Grefenstette (NuSTAR Instrument Scientist, Caltech)

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

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An artist's impression of a white dwarf polar system, consisting of a magnetic white dwarf accreting matter from its companion—for EF Eri, this is a star that has lost so much mass that it is now too small to undergo stellar fusion. Image credit: International Gemini Observatory/NOIRLab/NSF/AURA/University of Leicester (UK)/M. A. Garlick. Download Image

During the past week, NuSTAR observed the magnetic Cataclysmic Variable (mCV) EF Eridani, after it awoke from a nearly 30-year-long dormant period. mCVs are binary star systems consisting of a white dwarf and a companion star, where material from the companion is accreted onto the white dwarf. As this material falls, it reaches supersonic speeds, creating a shock wave that heats the material to over 100 million Kelvin and produces intense X-ray emission, detectable by NuSTAR. mCVs are of particular astrophysics interest since they are potential progenitors to Type Ia supernovae, a critical component of the cosmological distance ladder, and because they contribute significantly to the X-ray source population in the Galactic Center. This NuSTAR observation is coordinated with XRISM. NuSTAR’s broadband spectral sensitivity, combined with XRISM's precision spectroscopy, will provide scientists with unique insights into the accretion flow onto EF Eridani, revealing details of the heating, dynamics, and radiative processes that govern mCV systems.

Authors: Gabriel Bridges (PhD Student, Columbia University)

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

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NuSTAR Observes an Ultraluminous X-ray Pulsar

In this artist's impression of an ultraluminous X-ray pulsar, rivers of hot gas are funneled down the poles of the neutron star's magnetic field, where they shine at extreme luminosities. As the system rotates, around once per second, NuSTAR observes X-ray pulses. Image credit: NASA/JPL-Caltech/R. Hurt (IPAC) Download Image

During the past week, NuSTAR observed the ultraluminous X-ray pulsar (ULXP) NGC5907 ULX1 in coordination with ESA's soft X-ray observatory XMM-Newton. Ultraluminous X-ray sources are now widely understood to be the best nearby examples of super-Eddington accretion, an extreme accretion regime that may be required to rapidly grow supermassive black holes in the early Universe. This was spectacularly confirmed with the discovery by NuSTAR that some of these sources are unambiguously powered by highly super-Eddington neutron stars and not black holes as previously thought (see https://www.nustar.caltech.edu/news/nustar141008). Investigations are underway to determine exactly how these neutron stars are able to reach such high luminosities. NGC5907 ULX1 is the most extreme of the ULXPs currently known, able to reach an apparent luminosity of ~1e41 erg/s, an astonishing ~500 times its "Eddington limit" (the theoretical maximum luminosity any source can achieve in a simplified situation of spherical symmetry and weak magnetic fields). However, NGC5907 ULX1 had spent almost two years in a low-luminosity state, but has recently recovered again to the extreme luminosity it has exhibited previously. Scientists had been waiting for this change to occur and so new NuSTAR and XMM-Newton target-of-opportunity coordinated observations were triggered and scheduled this month. Preliminary inspection of these data suggest that they provide a new measurement of the spin period of the neutron star powering NGC5907 ULX1. This is the first spin measurement obtained since 2021 and will be vital in refining models of how the neutron star spins up while at high luminosity, and spins down while at low luminosity. In turn, this will improve understanding of both the extreme magnetic field strength and the connection to the super-Eddington accretion flow in this remarkable system.

Authors: Dominic Walton (Senior Lecturer in Astrophysics, University of Hertfordshire, UK)

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

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NuSTAR Observes A Flaring Star

An artist's impression of an M-dwarf star undergoing violent outbursts, stripping the atmosphere from an orbiting planet.
Image credit: NASA/ESA/D. Player. Download Image

AU Mic is one of the closest (∼9.7 pc), brightest, and most prolific flaring M-dwarf stars known—stars smaller and cooler than our Sun, but that are capable of launching massive stellar flares. AU Mic averages 6–10 low-energy X-ray flares per day and ∼2.3 high-energy X-ray-detectable events per day. NuSTAR received a Director's Discrentionary Time request to observe this star in order to address how a host star’s high-energy X-ray output—specifically stellar flares—drives exoplanet atmospheric escape and thus habitability. This is significant because not only is this a novel science topic for NuSTAR, but it was submitted by a summer undergraduate research fellow undertaking his project in the high-energy astrophysics group at Caltech, making him the first undergrad to be the primary investigator (PI) of a NuSTAR observation. The PI hopes to use the NuSTAR data to directly measure the broadband high-energy X-ray fluence of these flares—a critical metric for quantifying the total energy deposition into a planet’s upper atmosphere. This measurement is essential because the Neupert effect implies that only ∼35% of low-energy X-ray flares yield a measurable high-energy X-ray peak, and the ratio of high-energy to low-energy emission varies from flare to flare, so the fluence cannot be inferred reliably from low-energy X-ray data or past statistics alone.

Authors: Murray Brightman (NuSTAR Operations Scientist, Caltech)

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

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NuSTAR Investigates Stars At The End Of Their Lives

An artist's impression of an exploding star, showing the explosion colliding with gas in the space around it.
Image credit: NASA - Download Image

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Last week, NuSTAR performed a two-day long observation of the symbiotic binary star system, RT Cru. Previous observations of the X-ray emission from the accreting white dwarf star in this system indicate that is likely to be very massive, close to the Chandrasekhar gravitational collapse limit, and so is a candidate progenitor for a Type Ia supernova. The NuSTAR observation is scheduled to be simultaneous with an observation by the NASA/JAXA X-ray observatory XRISM which, with its high spectral sensitivity microcalorimeter instrument Resolve, has the potential to allow a direct measurement of the gravitational redshift on the white dwarf's surface and hence an accurate measurement of the mass of the star. To achieve this, the wide energy sensitivity of NuSTAR will be used to support this measurement by providing an observation that accurately characterizes the broad-band X-ray spectrum of RT Cru. The NuSTAR observation will also supply an independent mass estimate of the white dwarf through modeling of the X-ray continuum. This summer NuSTAR will be performing a series of observations coordinated with the XRISM observing schedule, and more observations are planned over the next year that combine the unique capabilities of NuSTAR and XRISM.

Also over the course of last week, NuSTAR performed a series of observations of the supernova SN2025mvn in coordination with NASA's Neil Gehrels Swift Observatory. SN2025mvn is a recent stellar explosion in the nearby galaxy NGC 5033, at a distance of 40 million light years, which was discovered at a very early stage of evolution of the supernova. SN2025mvn showed spectroscopic signatures of strong shock interaction in its optical spectra and so researchers triggered Target-of-Opportunity observations with NuSTAR and Swift. These spectroscopic features are caused by the interaction of the explosion's shock with a dense medium created by the mass lost through winds from the star in the decades before its explosive demise; a phase of stellar evolution that is poorly explored. NuSTAR observations of SN2025mvn, acquired within just a few days after the explosion, allow an exploration of this pristine part of the parameter space of supernova events. Specifically, NuSTAR observations of SN2025mvn will allow constraints to be placed on the physical parameters of the radiating electrons, which directly depend on a combination of the stellar explosion's parameters and its environment, as well as the fundamental physics of strong shocks. This is a unique set of observations, enabled by the recently-developed fast repointing capabilities of NuSTAR, the unique hard X-ray frequency coverage of NuSTAR, and the coordination with the Swift observatory.

Authors: Karl Forster (NuSTAR Science Operations Manager), Raffaella Margutti (Associate Professor, UC Berkeley)

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

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Distant Flares and Nearby Remnants

An X-ray image of the Tycho supernova remnant built up from years of observations by the Chandra space telescope, showing the clumpy shape of the explosion's debris. New NuSTAR data will locate energetic sites of cosmic ray acceleration within the remnant. Image credit: NASA/CXC/RIKEN & GSFC/T. Sato et al; DSS.   Download Image

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NuSTAR recently observed the distant gamma-ray blazar 4FGL J1428.9+5406 in response to a flare detected by NASA’s Fermi-LAT gamma-ray telescope. Blazars are a subclass of active galaxies—that is, galaxies containing a central supermassive black hole that is actively consuming matter—capable of launching relativistic jets aligned with our line of sight. These objects are highly variable and can produce bright flares lasting from a few days to several weeks. In the early Universe, blazars are typically faint gamma-ray sources and are only detectable during such flare events. These rare flares offer valuable insights into the physics of black hole jets at redshifts greater than 3—that is, within the first 2 billion years after the Big Bang. This NuSTAR observation of 4FGL J1428.9+5406 was triggered by a team monitoring about 80 high-redshift blazars with Fermi-LAT. Their program aims to collect near-simultaneous data across multiple wavelengths, from radio to X-ray, which are essential for probing the origin of the flare and constraining the power of the jet. Understanding how such powerful jets are launched and sustained in the early Universe will inform models of black hole growth and feedback during this epoch of high activity.

Last week, NuSTAR observed the Tycho supernova remnant, the remains of a stellar explosion that was famously visible to the naked eye 453 years ago. Young supernova remnants like Tycho are known to accelerate cosmic rays, such as electrons, to ultra-relativistic energies exceeding 1 TeV. This extreme phenomenon can be detected in the high energy X-ray band through synchrotron radiation emitted by these energetic electrons. NuSTAR first observed the Tycho remnant in 2014 for a total of 750 ks—more than eight days of exposure time—showcasing its exceptional high energy X-ray imaging and spectral capabilities by pinpointing the most energetic acceleration sites down to arcminute scales within this 9-arcminute-wide remnant, and precisely measuring their synchrotron spectra to high X-ray energies of 40 keV. A new observation begun last week totaling 500 ks, or nearly six days, will reveal how Tycho's electron acceleration has evolved over the past decade, providing a unique opportunity to tightly constrain the spectrum of accelerated electrons, deepen our understanding of cosmic-ray acceleration mechanisms, and estimate Tycho's contribution to the most energetic Galactic cosmic rays. For a related study by the same team of researchers on a similar source, see this recent article about Cassiopeia A.

Authors: Andrea Gokus (McDonnell Center Postdoctoral Fellow, Washington University), Jooyun Woo (Postdoctoral scholar, Columbia University), Hannah Earnshaw (NuSTAR Project Scientist, Caltech).

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

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The Explosion's Aftermath: Cosmic Rays from the Remnant of a Supernova

The Cassiopeia A supernova remnant as seen in X-rays during its original observation, with low-energy X-rays detected by Chandra in red, yellow, and green, and high-energy X-rays detected by NuSTAR in blue. Credit: NASA/JPL-Caltech/CXC/SAO. Download Image

A gigantic explosion may be the end of a massive star's life, but it is by no means the end of its story. Take Cassiopeia A (Cas A for short), the remnants of the most recent known core-collapse supernova in our Galaxy—a stellar explosion about 11,000 light-years away that would have been visible to Earth around 350 years ago. Since then, debris from the explosion has been blasting into the universe as fast-moving material plows into its slow-moving surroundings and creates powerful shockwaves.

These shockwaves heat the gas to millions of degrees, causing it to glow brightly in optical, ultraviolet, and even X-ray light. These shocks can also accelerate particles like electrons to nearly the speed of light, becoming what are known as cosmic rays. These high-energy cosmic rays also emit X-rays, which carry information about the heating and cooling processes happening within the remnant.

Low-energy X-ray observations taken by NASA’s Chandra X-ray Observatory over the past two decades have shown that the Cas A supernova remnant is expanding and slowly cooling down. However, since low-energy X-rays are produced by both hot gas and high-energy cosmic rays, it is difficult to determine which of these light sources contribute the most to these changes.

That's where NASA’s NuSTAR satellite comes in. With its ability to detect the high-energy X-rays that are only produced by the high-energy cosmic rays, NuSTAR can produce maps of the most energetic regions of the supernova remnant and watch how these regions evolve over time.

In a recent paper led by Dr Jooyun Woo, then a graduate student at Columbia University, astronomers used new NuSTAR observations of Cas A and compared them with observations taken ten years ago. If the electrons had been accelerated all at once in the initial shock wave, then we would have expected them to have cooled down and become dimmer. In comparing the two images, Woo and her co-authors found that the X-ray brightness of these shock regions did not decrease as much as expected. This tells us that cosmic ray heating is still taking place, keeping the supernova remnant bright in the latest NuSTAR image. Studying such changes in brightness over time allows astronomers to compare different models of electron acceleration, enabling the remnants of the relatively nearby and recent Cas A supernova to act as a laboratory in which we can test physical theories in environments that we can't reproduce in labs on Earth.

Even with its slower-than-expected rate of dimming, one day Cas A will fade away and become too faint for a telescope like NuSTAR to detect, possibly within a century. It is incredible to think of how many advances in astronomy have taken place over the last 350 years to allow us to see the high-energy emission from this explosion before it vanishes!

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

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Investigating Neutron Star Evolution

An artist's impression of a neutron-star X-ray binary accreting from its companion star and powering a jet.
Image credit: NASA/JPL-Caltech/R. Hurt (SSC)
Download Image

Over the past week, NuSTAR has conducted an intensive observing campaign on the neutron star X-ray binary GX 340+0, in coordination with the Australian Telescope Compact Array (ATCA, radio) and the X-ray Telescope aboard NASA’s Gehrels-Swift observatory. This well-known system belongs to the so-called "Z-source" family – a group of bright, accreting neutron stars that persistently trace a distinctive "Z-shaped" pattern in diagrams of their X-ray "hardness" (or color) plotted against their X-ray intensity. As these systems move back and forth along this Z-shaped track, typically over a timescale of about a week, their X-ray spectral and timing properties undergo a clear evolution. However, this evolution is significantly less pronounced compared to most other X-ray binaries, whether they contain neutron stars or black holes as the central accreting object. Interestingly, despite the relatively subtle changes in their X-ray properties, the radio emission from Z-sources has been observed to vary dramatically depending on their position along the Z-track. This suggests that the physical properties of their radio jets evolve far more than the underlying accretion flow. This behavior is unexpected, as such dramatic jet evolution is typically accompanied by equally dramatic changes in X-ray spectral-timing properties – especially in black hole systems. This discrepancy raises the possibility of different jet-launching mechanisms, distinct physical conditions, and/or unique correlations between jets and accretion flows in neutron star versus black hole X-ray binaries. To investigate this intriguing scenario, six simultaneous NuSTAR and ATCA observations were coordinated with a near-daily cadence over the course of a week. This observing strategy will, for the first time, track the co-evolution of the jet (radio) and accretion flow (X-rays) in this system, providing unique and unprecedented insights into the factors that govern jet evolution in Z-sources.

Authors: Alessio Marino (Postdoctoral fellow, ICE-CSIC, Spain)

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

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NuSTAR Observes Merging Clusters

An optical image of the Abell 399 (right) and Abell 401 (left) galaxy clusters, with microwave data from the Planck satellite overlaid in orange showing the location of hot gas in the clusters and linking them together. NuSTAR will help to measure the temperature of this gas. Image credit: ESA/Planck Collaboration/STScI. Download Image

NuSTAR has recently spent more than two days observing one of the most significant galaxy cluster interactions in the local Universe – the early stage merger of the massive systems Abell 401 and Abell 399. This merger is one of the most energetic events in the Universe, and has profound effects on the Intra-Cluster Medium (ICM); the hot, diffuse, gas that fills the volume between cluster member galaxies, which emits strongly in soft X-rays, and is responsible for a significant percentage of the thermal pressure support that balances a cluster’s gravitational collapse. The ICM of each cluster is undergoing shocks caused by the interaction, becoming even hotter (increasing its thermal pressure), and producing hard X-ray photons that NuSTAR is sensitive to. Without NuSTAR observations, temperature measurements will be biased toward lower temperatures. This shock heating will preferentially occur at the interface between the galaxy clusters, and the high-energy spatial resolution of NuSTAR images is indispensable to being able to make multiple temperature measurements in different spatial regions around the clusters. These NuSTAR observations are also being used to searching for another, more elusive, source of hard X-ray emission from galaxy clusters – Cosmic Microwave Background photons (the leftover microwave radiation from the Big Bang) that have been inverse-compton scattered to X-ray energies by populations of cosmic rays in the ICM. It is extremely hard to identify this emission, but in combination with radio observations, they can both provide information about the cluster magnetic field strength, and the contributions of the magnetic field and the cosmic ray population to the pressure support of a galaxy cluster. In an era where it is now possible to constrain the other known source of ICM pressure (turbulence, using observations from the recently launched JAXA/NASA/ESA mission XRISM), NuSTAR will help build a full picture of the ongoing astrophysical processes in galaxy clusters.

Authors: David Turner (Research Associate, Michigan State University Astronomy Group), Karl Forster (NuSTAR Science Operations Manager)

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

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Expecting the Unexpected

An artist's impression of the black hole and its surrounding accretion disk in the system IGR J17091-3624, mid-pulse. In the top right, a plot shows the "heartbeat"—the regular increase and decrease in brightness measured as the system pulses. Image credit: NASA/Goddard Space Flight Center/CI Lab. Download Image

During the past week, a Target of Opportunity large observing program with NuSTAR was triggered to monitor the outburst of the black hole binary IGR J17091-3624. This X-ray transient black hole binary candidate was first detected in 2003 and has since exhibited fascinating behavior, including radio jets, X-ray quasi-periodic oscillations, and state transitions characteristic of microquasars. Most peculiar are its X-ray flarings, which resemble a “heartbeat” pattern over time—a phenomenon observed in only one other source, GRS 1915+105. This large observing program, consisting of a total of 500 ks of NuSTAR exposure time coordinated NASA’s NICER mission, was designed to track the full evolution of a transient black hole outburst, capturing rapid changes in X-ray spectra and variability as the system transitions through different accretion states. However, after the initial observations, the outburst appeared to fade rather than fully develop, leading to a pause in the program. This unexpected behavior exemplifies one of the key challenges—and excitements—of time-domain astrophysics: black hole outbursts are not always predictable, and what appears to be the beginning of a dramatic event can sometimes fizzle out. We continue to monitor the source with other observatories to determine whether the NuSTAR program should be resumed. The three observations taken so far remain highly valuable, revealing clear signatures of X-ray reflection off an accretion disk, with spectra showing atomic fluorescence lines distorted by the black hole’s gravity. As new data is acquired, we continue to refine our understanding of the factors that drive complete versus failed outbursts in black hole systems.

Last Wednesday was the due date for NuSTAR General Observer (GO) cycle-11, an annual call soliciting proposals for basic research relevant to the NuSTAR mission. This is the primary opportunity for the scientific community to request observing time with NuSTAR, and includes the possibility of proposing for multi-mission investigations by coordinating with NASA’s NICER and Swift and ESA’s XMM-Newton observatories. This year, the project has received a record number of proposals, 15% higher than previous years. The proposal oversubscription rate of available NuSTAR observing time is the highest for many years, and the interest in proposing for Target of Opportunity investigations continues an increasing trend seen in the last five years. Target of Opportunity investigations are the most oversubscribed and competitive aspect of the NuSTAR GO program, mirroring the community's focus on time-domain astrophysics. These proposals will be peer-reviewed by independent panels of experts in April, with recommendations for selection submitted by the end of that month, in time for cycle-11 observations to begin on June 1st this year.

Authors: Javier Garcia (Senior Scientist, GSFC), Karl Forster (NuSTAR Science Operations Lead, Caltech)

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

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Teaming Up To Observe the Perseus Cluster

An X-ray image of the Perseus cluster taken by Hitomi, the precursor mission to XRISM. Image credit: NASA/CXC/SAO/E.Bulbul, et al. Download Image

During the past week, NuSTAR observed the Perseus Cluster, the brightest galaxy cluster in the sky in the X-rays, in coordination with the JAXA-NASA-ESA mission XRISM. Perseus is a calibration source for the wide-field Xtend imager on XRISM, but it also provides extremely valuable science for the primary XRISM instrument, Resolve. Resolve is the first high-spectral-resolution X-ray imaging spectrometer to fly an extended mission, replacing the similar instrument lost on the Hitomi mission. The XRISM science team has studied earlier observations of Perseus to try to resolve Doppler motions of the super-heated intra-cluster medium (ICM) gas, groundbreaking studies that will help us understand how these enormous galaxy clusters formed and evolved. However, the supermassive black hole at the center of the Perseus cluster also emits copious X-rays, which need to be disentangled from the ICM X-ray signal to properly isolate and measure the cluster gas motions. Simultaneous observations by NuSTAR are in a higher energy band than XRISM where the data is dominated by X-rays from the black hole. This will allow the XRISM calibration team to account for the contribution from the black hole in the XRISM observations, and precisely measure not only the gas motion but also the abundances of key elements and the temperature structure of the ICM. Since this is a calibration target observed twice a year by XRISM, a very deep total exposure will be obtained, and the vital collaboration with NuSTAR will enable a transformative view into the astrophysics of galaxy clusters.

Authors: Eric D. Miller (XRISM In-Flight Calibration Lead, MIT Kavli Institute for Astrophysics and Space Research)

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

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Extreme Variability at the Edge of the Universe

An artist’s illustration of a close-up view of a black hole and its jet, like the one in CFHQS J1429+5447. Image credit: NASA/CXC/M. Weiss (CXC). Download Image

Black holes are the most powerful and scary monsters in our universe, lurking at the centers of galaxies. Some, such as the black hole at the center of our own Milky Way Galaxy, have already finished their cosmic meals, with just occasional nibbles observed today. Others, however, are seen ravenously devouring delicious matter from their surroundings. At such times, black holes are noisy eaters, dominating all the activity in their host galaxy centers. As matter spirals in towards the bottomless maw, it collides, heats up, and becomes very bright from X-ray to infrared energies. The accretion disk around a supermassive black hole can easily outshine the billions of stars in a galaxy, and that incredible brightness can make them some of the most distant objects we can observe in both space and time. Black holes can also be messy eaters, spewing out material in cosmic jets that can reach thousands and even millions of light years from the black hole—material that can then go on to influence the universe around it.

Of the many mysteries that keep astronomers up all night observing and pondering these enigmatic beasts, one of the most perplexing is how black holes grow to such enormous sizes. We see supermassive black holes with masses hundreds of millions of times that of the Sun, observed when the universe was only a few hundred million years old. It’s like finding 7-foot basketball players or 300-pound football players with appetites to match in a Kindergarten classroom: just how were they able to grow so big so quickly?

Recent observations by NASA’s NuSTAR and Chandra X-ray observatories might offer some clues. In a paper recently published by the Astrophysical Journal, scientists led by Lea Marcotulli at Yale University and Thomas Connor at the Center for Astrophysics | Harvard & Smithsonian report on observations of the most X-ray luminous accreting black hole, or quasar, ever discovered in the first billion years of the universe. This quasar, called CFHQS J1429+5447, was initially found 15 years ago using data from a ground-based telescope that surveyed wide patches of the sky. Far more recently it was observed by Chandra, which was able to pick up X-rays from this incredibly distant source. Only four months afterwards, NuSTAR also observed it, finding that the quasar had doubled in X-ray brightness in that time.

Such a dramatic variation in such a short time for something this massive is evidence towards this quasar being a particularly messy eater, expelling a powerful jet of material at close to the speed of light. This jet is pointed straight at Earth—a chance alignment that boosts the amount of light making its way to us, allowing telescopes in Earth's orbit like NuSTAR and Chandra to see it at such a great distance.

"These results have significant implications for supermassive black holes and jet evolution theories," said Marcotulli. "The presence of a jet may be a necessity to grow such extreme black holes so early in the Universe."

Because the light observed from this quasar was emitted when the Universe was still very young, this lets us see into an era soon after the Big Bang called the Epoch of Reionization. This time period was when light began to be able to pass through the Universe unimpeded, which is what allows us to see stars and galaxies and distant quasars today. Exactly what kind of objects helped to clear the way for light to travel through space is a mystery that astronomers are still seeking to unravel, but the discovery of a cosmic jet like this one suggests that the Universe's biggest, messiest eaters might have been involved.

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

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How Many Black Holes Are Hiding? NASA Study Homes in on Answer

A supermassive black hole surrounded by a torus of gas and dust is depicted in four different wavelengths of light in this artist’s concept. Visible light (top right) and low-energy X-rays (bottom left) are blocked by the torus; infrared (top left) is scattered and re-emitted; and some high energy X-rays (bottom right) can penetrate the torus. Credit: NASA/JPL-Caltech. Download Image

Multiple NASA telescopes recently helped scientists search the sky for supermassive black holes — those up to billions of times heavier than the Sun. The new survey is unique because it was as likely to find massive black holes that are hidden behind thick clouds of gas and dust as those that are not.

Astronomers now think that every large galaxy in the universe has a supermassive black hole at its center. But testing this hypothesis is difficult because researchers can’t hope to count the billions or even trillions of supermassive black holes thought to exist in the universe, and instead have to extrapolate from smaller samples to learn about the larger population. So accurately measuring the ratio of hidden supermassive black holes in a given sample will help scientists better estimate the total number of supermassive black holes in the universe.

The new study published in the Astrophysical Journal found that about 35% of supermassive black holes are heavily obscured, meaning the surrounding clouds of gas and dust are so thick they even block low-energy X-ray light. Comparable searches have previously found less than 15%. Scientists think the true split should be closer to 50/50 based on models of how galaxies grow. But if observations continue to show a lower percentage, scientists will need to adjust some key ideas they have about supermassive black holes and the role they play in shaping galaxies.

Hidden Treasure

Although black holes are inherently dark — not even light can escape their gravity — they can also be some of the brightest objects in the universe: When gas gets pulled into orbit around a supermassive black hole, like water circling a drain, the extreme gravity creates such intense friction and heat that the gas reaches hundreds of thousands of degrees and radiates so brightly it can outshine all the stars in the surrounding galaxy.

The clouds of gas and dust that surround and replenish the bright central disk may roughly take the shape of a torus, or doughnut. If the doughnut hole is pointed toward Earth, the bright central disk within it is visible; if the doughnut is edge-on, the disk is obscured. Most telescopes can rather easily identify face-on supermassive black holes, but not edge-on ones.

But there’s an exception to this that the authors of the new paper took advantage of: The doughnut absorbs light from the central source and reemits lower-energy light in the infrared range, or wavelengths slightly longer than what human eyes can detect. Essentially, the doughnuts glow in infrared.

These wavelengths of light were detected by NASA’s Infrared Array Survey, or IRAS, mission, which operated for 10 months in 1983 and was managed by NASA’s Jet Propulsion Laboratory in Southern California. A survey telescope that imaged the entire sky, IRAS was able to see the infrared emissions from the clouds surrounding supermassive black holes. Most importantly, it could spot edge-on and face-on black holes equally well.

IRAS caught hundreds of initial targets. Some of them turned out to not be heavily obscured black holes, but galaxies with high rates of star formation that emit a similar infrared glow. So, the team used ground-based, visible-light telescopes to identify the latter and separate them.

To confirm edge-on, heavily obscured black holes, the researchers relied on NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array), an X-ray observatory also managed by JPL. X-rays are radiated by some of the hottest material around the black hole. Lower-energy X-rays will be absorbed by the surrounding clouds of gas and dust, while the higher-energy X-rays observed by NuSTAR can penetrate and scatter off the clouds. This can take hours of observation, so scientists working with NuSTAR first need a telescope like IRAS to tell them where to look.

“It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago,” said study lead Peter Boorman, an astrophysicist at Caltech in Pasadena, California. “I think it shows the legacy value of telescope archives and the benefit of using multiple instruments and wavelengths of light together.”

Numerical Advantage

Determining the number of hidden black holes compared to non-hidden ones can help scientists understand how these black holes get so big. If they grow by consuming material, then a significant number of black holes should be surrounded by thick clouds and potentially obscured. Boorman and his coauthors say this first unbiased look at the population supports this hypothesis.

In addition, black holes influence the galaxies they live in, mostly by impacting how galaxies grow. This happens because black holes surrounded by massive clouds of gas and dust can consume vast — but not infinite — amounts of material. If too much falls toward a black hole at once, the black hole starts coughing up the excess and firing it back out into the galaxy. That can disperse gas clouds within the galaxy where stars are forming, slowing the rate of star formation in the galaxy.

“If we didn’t have black holes, galaxies would be much larger,” said Poshak Gandhi, a professor of astrophysics at Southampton University in the United Kingdom and a coauthor on the new study. “So if we didn’t have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky. That’s just one example of how black holes can influence a galaxy’s evolution.”

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

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More About NuSTAR

NuSTAR launched on June 13, 2012. A Small Explorer mission led by Caltech in Pasadena, California, and managed by JPL for NASA’s Science Mission Directorate in Washington, it was developed in partnership with the Danish Technical University (DTU) and the Italian Space Agency (ASI). The telescope optics were built by Columbia University, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and DTU. The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. The NuSTAR mission’s operations center is at the University of California, Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror data archive. Caltech manages JPL for NASA.

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News Media Contact:

Calla Cofield

Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov

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Fireworks at Closest Approach: Repeated X-ray Flares from a Young Binary System

Two young protostars in a cosmic tango that brings them within 10 stellar radii every two weeks. This artist’s illustration of the DQ Tau system shows the intense fireworks that occur every fortnight as these two swiftly moving stars are ever, ever getting back together. Image credit: NASA/JPL-Caltech/R. Hurt (IPAC).

DQ Tau is a unique binary system. Approximately 650 light-years away in the Taurus constellation, DQ Tau consists of two young stars still in the process of forming. The protostars have not yet ignited hydrogen burning in their cores (the fusion process that heats mature stars). Instead, they are glowing as they evolve from their natal form (diffuse clouds of gas) and are heated by this gravitational collapse. Each is half the mass of the Sun and currently twice its radius, dancing in a highly elongated orbit which has them plunging in towards each other every 15.8 days. At closest approach in this violent cosmic tango, the separation between the two stars is exceptionally small, only 8-10 stellar radii. Characteristic of this early phase, the protostars in DQ Tau harbor strong magnetic fields on their surfaces. Also, like most protostars, the DQ Tau system is surrounded by a disk in which planets are also forming. Understanding planet formation, including how the intense flares characteristic of protostars affect disk heating and chemistry, are areas of active research.

DQ Tau provides an exceptional laboratory for such studies. Like clockwork, the DQ Tau system brightens at closest approach. While large X-ray flares in young stars are generally rare and unpredictable (as on our own star, the Sun), the presence of the predictable X-ray super-flares and outbursts in DQ Tau enables synchronized studies of these cosmic fireworks. Like tourists at Yellowstone National Park timing their visit to Old Faithful Geyser, astronomers can plan ahead, coordinating telescopes to jointly investigate these intense flares and understand how they affect the protoplanetary disk. X-ray flares come as the protostar magnetospheres collide, while the lower energy optical and ultraviolet flares also come from accretion of material onto the young stars. Infrared and radio studies probe the changing temperature and chemistry of the protoplanetary disk.

In a recent paper published in the Astrophysical Journal, scientists led by Konstantin Getman at Pennsylvania State University report on new observations of a single orbit of DQ Tau in July and August 2022 using the NuSTAR, Swift, and Chandra X-ray telescopes. NuSTAR accesses higher energy X-rays, while Swift and Chandra access lower energy X-rays. The observations indicate that most of the X-ray emission is from interactions of the magnetospheres of these young stars at closest approach. In a process similar to what is seen on our own Sun, magnetic field collisions and reconnections produce strong high-energy X-ray emission. This heats the surrounding region to high temperature, detectable as thermal emission in the lower energy X-rays. Notably, however, flares on our Sun occur among coronal magnetic loops much smaller than the star, with sizes of 1000 to 10,000 km. In contrast, the DQ Tau super-flares occur on spatial scales a thousand times larger, corresponding to approximately 10 million km or tens of stellar radii. The current study is part of a broader campaign using additional ground-based telescopes to investigate the influence of DQ Tau’s stellar radiation on the chemistry within its surrounding disk.

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

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NuSTAR Reveals Black Hole Shredding a Star

A star is being consumed by a nearby supermassive black hole, in a rare event that astronomers call a tidal disruption event (TDE). What makes this event, AT2022cmc, even more rare is that as the black hole ripped the star apart, jets of material moving at almost the speed of light were launched. AT2022cmc, depicted here in an artist’s impression, was the first jetted-TDE discovered in over a decade and the first since the launch of NuSTAR. The sensitive, broadband X-ray observation by NuSTAR provided critical data for understanding the event. Image credit: ESO/M.Kornmesser

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NuSTAR unveiled crucial details for understanding one of the most energetic types of event in the universe. If a star comes too close to a supermassive black hole, it will be torn apart by the black hole’s tidal force. Fundamentally, the side of the star closer to the black hole feels a stronger gravitational pull than the far side of the star, much like people on Earth feel stronger gravity than an astronaut in the International Space Station. However, you’d have to imagine an astronaut so tall that their feet were on Earth while their head was in orbit. And the difference in gravitational field would also be much larger. When a star comes extremely close to the black hole, it becomes stretched and is pulled apart. The resulting stream of material loops around the black hole. Some stellar material is captured into orbit, creating an accretion disk around the black hole. This disk becomes quite bright. In a very rare subclass of events, a relativistic jet is also produced, creating another source of light. As the disk material is consumed by the black hole over the course of months to years, the event gradually fades.

Tidal disruption events, or TDEs for short, were first predicted in the 1970s, and first observed in the 1990s. Currently, approximately 100 TDEs are known, only four of which are of the rare jetted-TDE variety.

Just after midnight on February 11, 2022, the Zwicky Transient Facility at Palomar Observatory in Southern California detected a new transient source. Data obtained over the next two nights led to this new source, AT2022cmc, being flagged as unusual, rising and falling faster than a typical supernova. This inspired follow-up observations using telescopes around the planet and in space. Those data provided the distance and energetics of the system, ultimately classifying it as a jetted-TDE. This subclass of TDE is very bright at X-ray energies, and only four have been detected to date. The most recent jetted-TDE occurred more than a decade ago, prior to the launch of NuSTAR. NuSTAR is the first focusing high-energy, or hard X-ray telescope in orbit, providing two orders of magnitude improvement in sensitivity compared to previous instruments. NuSTAR significantly extends the range of X-ray energies that can be studied in detail for astrophysical phenomena. AT2022cmc provided the first opportunity to study this type of rare, X-ray bright, transient event and motivated three NuSTAR observations in the month after its discovery.

While the lower energy emission from jetted-TDEs is relatively well understood (e.g., at radio, optical, and UV energies), the location and mechanism producing the bright X-ray emission in jetted-TDEs is a topic of active debate. The most popular scenario is that the X-rays come from less energetic photons in the radio and optical bands being scattered by energetic relativistic electrons in the surrounding plasma up to X-ray energies. This scenario predicts that we should see the high-energy X-ray spectrum as a smooth extrapolation of the lower-energy X-ray spectrum. However, when NuSTAR observed AT2022cmc, it detected a pronounced break in the X-ray spectrum within the NuSTAR band. This break gives an important clue to the X-ray emission mechanisms of jetted-TDEs.

In a recent paper published in the Astrophysical Journal, Dr. Yuhan Yao of the University of California, Berkeley and her team report that the NuSTAR data and spectral break are consistent with a phenomenon known as synchrotron radiation, created as relativistic charged particles (i.e., electrons) move through a strong magnetic field. This is naturally expected from astrophysical jets, though the leading jetted-TDE models had predicted it to be subdominant to the up-scattered emission in jetted-TDEs. Modeling the X-ray data, Dr. Yao and her team were able to constrain the properties of the jet and determine what part of the jet is dominating its X-ray emission. They find that the jet is likely to be starved of protons, and instead is dominated by electrons moving in a highly magnetized jet.

AT2022cmc represents a significant leap in our understanding of relativistic jets in astrophysical phenomena. “The NuSTAR data challenge existing models and suggest that magnetic reconnection plays a key role in accelerating particles within these jets,” noted Dr. Yao. This result not only sheds light on the inner workings of TDEs but also has broader implications for understanding relativistic jets in other high-energy astrophysical sources, such as gamma-ray bursts. Dr. Yao explained, “overall, this work contributes to the ongoing quest to decipher the composition and acceleration mechanisms of relativistic jets in the Universe.”

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

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Unveiling the Secrets of our Galaxy’s Supermassive Black Hole

By monitoring the high-energy X-ray emission from the center of our Galaxy, NuSTAR probes the current and past activity of Sagittarius A*, the supermassive at the center of the Milky Way. These images capture a flare observed in July 2012 over a 2-day period. In the middle panel, the black hole was consuming and heating matter to temperatures up to 180 million degrees Fahrenheit (100 million degrees Celsius). Image Credit: NASA/JPL-Caltech.

Black holes are notoriously difficult to study, in part because not even light can escape their immense gravity. Researchers typically infer their properties by observing the gravitational influence of a black hole on nearby stars, gas, energetic plasmas, and other related phenomena. Astronomers also learn both about the environment around the black hole and its past activity by observing echoes reflected off nearby structures (with the caveat that astronomers might have a different understanding of "nearby" than other people). Using X-ray data from NuSTAR, researchers at Michigan State University (MSU) have made groundbreaking discoveries about Sagittarius A*, the supermassive black hole at the center of our Milky Way galaxy. These findings were presented at the 244th meeting of the American Astronomical Society, held in Madison, Wisconsin in June.

Galactic fireworks

Grace Sanger-Johnson, a postbaccalaureate researcher at MSU, analyzed 10 years of NuSTAR data looking for X-ray flares from Sagittarius A*. In doing so, she discovered nine flares that had previously gone unnoticed. These flares are dramatic bursts of high-energy light that provide a unique opportunity to study the environment around the black hole. This region is typically invisible due to the dense gas and dust that blocks most light, but not the penetrating, high-energy X-rays observed by NuSTAR. Sagittarius A* is the closest supermassive black hole to Earth. Data from Sagittarius A*, and its flares, are one of the only ways currently known to study the physics and physical environment of a black hole. Sanger-Johnson meticulously sifted through a decade's worth of X-ray data collected between 2015 and 2024 by NuSTAR. Each of the nine newly-discovered flares provides unique insight into the black hole's environment and activities.

"We are sitting in the front row to observe these unique cosmic fireworks at the center of our own Milky Way galaxy," said Prof. Shuo Zhang, Sanger-Johnson's advisor. "Flares light up the darkness and help us observe things we wouldn't normally be able to." Analyzing the properties of these X-ray flares will help astronomers infer the physical conditions of the extreme environment around the supermassive black hole.

`Echoes' of a black hole

While Sanger-Johnson focused on the brilliant flares from Sagittarius A*, Jack Uteg, an undergraduate researcher, examined the black hole's activity using a technique akin to listening to echoes. Uteg analyzed more than 20 years of data targeting a giant molecular cloud known as "the Bridge" near Sagittarius A*. This monitoring began before Uteg was born.

Unlike the central black hole, clouds of gas and dust in interstellar space cannot generate their own X-rays. So, when X-ray telescopes detected photons from the Bridge, the emission was inferred to be delayed reflection of past X-ray outbursts by Sagittarius A*. The X-ray luminosity of the Bridge first began to increase around 2008 and then increased over the next 12 years until it hit peak brightness in 2020. This "echo" light from the black hole traveled for hundreds of years from Sgr A* to the molecular cloud, and then traveled another roughly 26,000 years before reaching Earth.

By analyzing these X-ray echoes, Uteg was able to reconstruct a timeline of our black hole's past activity, offering insights that would not be possible through direct observations alone. Uteg's analysis used data from NuSTAR as well as from the European Space Agency's XMM-Newton satellite, which studies the universe in lower energy X-rays.

"One of the main reasons we care about this cloud getting brighter is that it lets us constrain how bright Sagittarius A* was in the past," Uteg said. Based on these data, the team determined that, about 200 years ago, the black hole at the center of the Milky Way was about 10,000 times brighter in X-rays compared to how we see it today. As the United States was celebrating its first July 4th, Sagittarius A* was feasting on nearby material, heating it up, and producing copious amounts of X-rays. Today, the black hole just nibbles. "The black hole at the center of our Galaxy was producing fireworks just 200 years ago, but today it is merely a sparkler," said Dr. Brian Grefenstette, a NuSTAR staff scientist at the California Institute of Technology.

This is the first long-term X-ray variability study of a molecular cloud surrounding Sagittarius A* to detect its peak X-ray luminosity. "This allows us to tell the past activity of Sgr A*, and we will continue this astro-archaeological study to further unravel the mysteries of the Milky Way's center," Zhang said.

While the exact mechanisms triggering X-ray flares and the life cycles of black holes remain mysteries and areas of active study, NuSTAR is helping spark further investigations and improve our understanding of these enigmatic objects.

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

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Clumpy Doughnuts around Supermassive Black Holes

By monitoring the high-energy X-ray emission from accreting supermassive black holes, NuSTAR probes the structure of the surrounding dusty, doughnut-shaped structure and finds evidence that the “torus” is clumpy. This artist’s conception of a clumpy region around a supermassive black hole includes the inner disk of orbiting material and ejected jets. Overlay credit: Bill Saxton, NRAO/AUI/NSF. Background image credit: NASA, ESA, CSA, and M. Zamani (ESA).

NuSTAR is providing new information about the structures around supermassive black holes. All large galaxies have a supermassive black hole at their center, with a mass millions to billions of times that of the Sun. With an effective radius measured in light-hours, the black hole is tiny compared to the galaxy, whose size is measured in tens of light-millennia. However, when the black hole is rapidly accreting, a hot disk of orbiting material forms and can outshine the hundreds of billions of stars in the host galaxy. This hot, energetic central region produces strong X-ray and ultraviolet light and is embedded in a larger, cooler structure, believed to have a toroidal or doughnut-like shape. However, the structure of this dusty region, including its size and shape, is still broadly unknown. In the last decades, evidence has pointed toward the gas and dust being clumpy rather than smoothly diffuse. This is commonly understood as a collection of clouds orbiting the central supermassive black hole rather than a persistent haze of smog hiding the black hole from view.

Historically, the dusty region was modeled as a uniform torus or a sphere with conical regions carved out above and below the black hole. Recent models, however, consider more complicated structures, including clouds, outflows, and warped disks. To further complicate matters, models generally do not include any physical scale, which means the obscuration could be taking place anywhere between the central black hole and the very edge of the host galaxy.

NuSTAR high-energy X-ray observations, above 10 keV, provide a powerful tool to distinguish changes in intrinsic brightness from changes in obscuration. Using an analogy connected to everyday life, a dimmer lamp could be due to a thicker lampshade or a weaker lightbulb. NuSTAR detects penetrating, high-energy X-rays, which are largely insensitive to any lampshade: dimming seen in the NuSTAR range must be intrinsic to the source (i.e., a dimmer lightbulb) rather than from variable absorption (i.e., a thicker lampshade). Using NuSTAR we can get the best physical picture of what’s happening when we see a source get brighter or dimmer. NuSTAR also detects reflected X-ray emission, which depends on the properties and geometry of the whole torus (i.e., the extent and thickness or opacity of the cloud cover). This provides valuable insight into the global structure, particularly when compared to the line-of-sight properties determined from variable absorption.

In a paper recently published in Astronomy & Astrophysics, Dr. Nuria Torres-Alba of Clemson University and her team measure the scale of the obscurers around supermassive black holes based on NuSTAR monitoring of a sample of nearby sources. Obscuration changes on timescales shorter than a few days would require small structures, likely clouds close to the black hole, while obscuration changes on timescales of years would likely originate from larger, more distant structures.

Dr. Torres-Alba and her team analyzed 53 individual NuSTAR observations of a sample of 12 nearby galaxies known to host a heavily obscured central accreting supermassive black hole. Roughly half of the galaxies (5/12) show clear evidence of obscuration variability on timescales of years, which begs the question: if the torus is clumpy, why do we see variability in less than half of the sources? One clue might come from the observation that sources that show variable obscuration tend to have thicker columns of obscuring material and broader cloud distributions than their counterparts.

Currently, the models of the clumpy torus used by astronomer are not calibrated to consider any of these observational facts. Previous studies tended to be anecdotal, investigating a single object or even a single event, or studies focused on samples of less heavily obscured sources for which less variability due to the dusty torus would be expected. This recent paper is the first comprehensive NuSTAR study of a sample of heavily obscured systems. It provides a new benchmark against which clumpy torus models can be tested, constraining key model parameters such as the number and density of clouds, their sizes, and their orbits.

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

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