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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Telescope Tag-Team Discovers Galactic Cluster’s Bizarre Secrets

Terzan 5, located in the constellation Sagittarius, is a crowded globular cluster home to hundreds of thousands of stars.Ten unusual and exotic pulsars were recently discovered by an international team of astronomers from the U.S. National Science Foundation National Radio Astronomy Observatory, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) (AEI), and the Max Planck Institute for Radio Astronomy.Credit: US NSF, AUI, NSF NRAO, S. Dagnello. Hi-Res File

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U.S. National Science Foundation Green Bank Telescope teams up with South African Radio Astronomy Observatory MeerKAT Telescope, discovers ten strange and exotic pulsars

Towards the center of our Milky Way Galaxy, in the constellation Sagittarius, astronomers have discovered  10 monstrous neutron stars. These particular stars, called pulsars, reside together in globular cluster Terzan 5, a crowded home for hundreds of thousands of different types of stars. Pulsars are millions (or even billions) of times more dense than other stars and rotate rapidly, emitting bright pulses of light from their strong magnetic fields, making them a beacon for astronomers to find. In one of the most jam-packed places in our Milky Way, many pulsars in Terzan 5 have evolved into bizarre and eccentric forms.

Astronomers already knew that 39 pulsars call Terzan 5 home. With the teamwork of the U.S. National Science Foundation Green Bank Telescope (NSF GBT) and the South African Radio Astronomy Observatory’s MeerKAT Telescope, ten more have been added to the count. “It’s very unusual to find exotic new pulsars. But what’s really exciting is the wide variety of such weirdos in a single cluster,” shared Scott Ransom, a scientist with the U.S. National Science Foundation National Radio Astronomy Observatory (NSF NRAO). The discoveries were made by an international team of astronomers from NSF NRAO, the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) (AEI), and the Max Planck Institute for Radio Astronomy.

The Meerkat Telescope was able to determine the rough location of each pulsar by tracking and timing how quickly they rotate, matched against twenty years of Terzan 5 observations taken by the NSF GBT, which revealed the bizarre and eccentric details of these stars. “Without the NSF Green Bank Telescope’s archive, we wouldn’t have been able to characterize these pulsars and understand their astrophysics,” adds Ransom. The archival NSF GBT data allowed astronomers to pinpoint the pulsars’ position on the sky, measure their specific movements, and see how their orbits changed over time.

Among the discoveries, astronomers saw two likely neutron stars pulled into each other’s orbit as a binary system. Out of 3,600 known pulsars in the Galaxy, only 20 have been identified as double neutron-star binaries. When pulsars pair off in binaries, the gravitational pull from one to the other can steal material and energy, causing one to spin even faster, becoming a millisecond pulsar. This pair could be a record breaker, with a new contender for fastest spinning pulsar in a double neutron-star system, and the longest orbit of its kind. The current record holder for fastest spinning pulsar already resides in Terzan 5. Only future observations will reveal the truth.

Astronomers also observed three new rare pulsar “spider” binary systems (in addition to five already known in the cluster) called Redbacks or Black Widows, depending on the types of companion stars that they have. A companion star falls into the orbit of a spider pulsar, where a web of plasma fills the space between the two (caused by outflows from the companion star due to the pulsar’s energy) slowly dissolving the companion over time.

The discovery of these strange pulsars allows scientists to better understand globular clusters, neutron stars, and even test Einstein’s theory of general relativity, along with expanding what is known about pulsar categories. The research team is already making plans to find even more in Terzan 5, with the support of volunteers. Citizen scientists who’d like to share in the excitement of this discovery can help at Einstein@Home. This project, led by scientists at AEI, has already discovered more than 90 new neutron stars.

The Green Bank Observatory, home of the GBT, and the National Radio Astronomy Observatory are major facilities of the U.S. National Science Foundation and are operated by Associated Universities, Inc.

This research was shared in the journal Astronomy & Astrophysics..

Read more at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute.).

Read more at the Max Planck Institute for Radio Astronomy.

Source:National Radio Astronomy Observatory (NRAO)/News

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Media contacts:

Jill Malusky,
NRAO & GBO News & Public Information Manager
jmalusky@nrao.edu

Benjamin Knispel,
Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
benjamin.knispel@aei.mpg.de

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Monthly Roundup: Discovering, Modeling, and Characterizing Pulsars

Composite X-ray, optical, and infrared image of the Crab Nebula, which houses a pulsar at its center
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech

When a massive star goes supernova, the explosion can leave behind a pulsar: the core of a dead star containing 1–2 times the mass of the Sun in a sphere just 20 kilometers across. Pulsars are almost entirely composed of neutrons and spin extremely quickly — the fastest recorded pulsar spins 716 times every second, meaning that a point on its surface moves at roughly a quarter of the speed of light. Pulsars emit beams of radio waves from their poles, and an observer on Earth sees pulses of radio emission in time with the star’s rotation. The word pulsar comes from pulsating radio source.

Observing pulsars helps us understand the evolution of massive stars, provides a way to study the physics of ultra-dense materials, and gives us a means to search for the background gravitational hum of supermassive black holes in colliding galaxies. Today, we’ll take a look at three recent research articles that explore fundamental questions in pulsar science.

The field surrounding PSR J1032−5804 shown at, from left to right, radio, infrared, and visible wavelengths, as well as a composite of all three. Credit: Wang et al. 2024

How Do We Find Pulsars?

Jocelyn Bell Burnell discovered the first pulsar by chance in 1967, when the characteristic pulses popped up in radio observations taken with a new telescope. Today, researchers design surveys tuned to the particular properties of pulsars to make them stand out from other signals in the sky. Namely, radio surveys can search for sources with steep spectra — in other words, signals that are far brighter at low frequencies than at high frequencies — or strongly polarized light.

Ziteng Wang (Curtin University) and collaborators used the Australian Square Kilometre Array Pathfinder (ASKAP), a 36-dish radio interferometer, to search for circularly polarized signals from pulsars. In addition to known stars and pulsars, the observations pinpointed a strongly circularly polarized source with no known counterpart at other wavelengths. The team followed up on this promising discovery with the 64-meter Murriyang radio telescope at Parkes Observatory and found a pulsar with a rotation period of 78.72 milliseconds. The pulsar, cataloged as PSR J1032−5804, has an estimated age of 34,600 years, making it relatively young and possibly still associated with a visible supernova remnant. The team found a compact region of emission surrounding the pulsar, but they couldn’t rule out the possibility that the material belongs to unrelated nebulae.

PSR J1032−5804 is notable because its pulses are highly scattered by interstellar gas and dust. Highly scattered pulsar signals are hard to detect because scattering broadens and weakens the signal, especially at lower frequencies where pulsars should be at their brightest. Wang’s team has shown that searching at relatively high frequencies — the team’s observations were made at 3 gigahertz — is a viable way to detect scattered pulsars.

Simulation output showing the magnetic field lines (green curves) and plasma density (background color) in a pulsar’s magnetosphere. Credit: Bransgrove et al. 2023

How Do Pulsars Make Their Pulses?

Pulsars may be most famous for their characteristic pulses of radio emission, but the origin of those pulses is still under debate. To understand what powers these radio beacons, researchers use detailed simulations that track the behavior of individual particles to understand how they behave under the exotic conditions present at the surface of a pulsar. To date, localized simulations have been able to produce radio waves from a pulsar’s poles, and global simulations have discerned the source of pulsars’ gamma-ray pulses (10% or so of pulsars produce gamma-ray pulses in addition to radio pulses), but radio pulses have not yet been seen in global simulations.

Ashley Bransgrove (Columbia University and Princeton University) and collaborators carried out high-resolution global simulations of a pulsar’s magnetosphere: the region immediately surrounding a pulsar where its strong magnetic field dominates the motion of charged particles. The simulations show how the rapid rotation of the pulsar lofts charged particles from its surface and accelerates them, filling the magnetosphere with gamma rays and a dense sea of electrons and their positively charged counterparts, positrons. Near the pulsar’s poles and farther out in its magnetosphere, gaps form where the electric current is mismatched, and pairs of electrons and positrons are generated in these gaps. When the gaps discharge — think of a spark, or lightning — they excite waves in the plasma and, subsequently, electromagnetic waves. The emitted radiation is similar in frequency and luminosity to observed pulsars, suggesting that electric discharge may generate the radio waves that pulsars are known for.

The team notes that it’s too soon to apply their simulations to observations of individual pulsars, and more work is needed to understand the role of gamma-ray emission, explore the details of electron–positron pair production, and extend the work to pulsars whose spin axes and magnetic axes are misaligned.

The location of the Boomerang within the supernova remnant surrounding the pulsar PSR J2229+6114
Credit: Pope et al. 2024

How Do Pulsars Interact with Their Surroundings?

When pulsars are young, they’re swaddled in the gas and dust of their surrounding supernova remnants. This leads young pulsars to create a pulsar wind nebula: a glowing cloud of gas energized by winds of relativistic charged particles streaming off the pulsar. A recent article authored by the Nuclear Spectroscopic Telescope Array (NuSTAR) and Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaborations presents multiwavelength observations of the Boomerang, a 10,000-year-old pulsar wind nebula well known for its complex structure.

The teams combined archival data from radio telescopes and the Chandra X-ray Observatory with newly collected data from NuSTAR, VERITAS, and the Fermi Gamma-ray Space Telescope to probe the nebula’s multiwavelength behavior. These observations revealed that the nebula appears far larger at radio wavelengths than at X-ray wavelengths, a common feature of pulsar wind nebulae due to the difference sources of emission: the nebula’s size at radio wavelengths is set by outflowing particles, while its size at X-ray wavelengths comes from the rate at which electrons lose energy as they spiral around magnetic field lines and emit X-rays. The nebula’s size even varies across X-ray wavelengths, appearing smaller at shorter wavelengths.

Judging from how the nebula’s size changes with wavelength, its overall energy output, and its X-ray emission over the past two decades, the authors provide a new estimate on its distance and magnetic field strength, finding it to be more distant and with a far weaker magnetic field than previously thought. By modeling how the nebula’s energy output may have evolved over time, the team also found that the Boomerang is, well, boomeranging! Roughly 1,000 years ago, a backwards-moving supernova shock wave crashed into the expanding nebula, crushing the nebula and temporarily reversing its expansion. Today, the nebula is re-expanding in the wake of the shock wave, showcasing how pulsars dynamically interact with their surroundings.

By Kerry Hensley

Source: American Astronomical Society/AAS-Nova

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Citation

“Discovery of a Young, Highly Scattered Pulsar PSR J1032-5804 with the Australian Square Kilometre Array Pathfinder,” Ziteng Wang et al 2024 ApJ 961 175. doi:10.3847/1538-4357/ad0fe8

“Radio Emission and Electric Gaps in Pulsar Magnetospheres,” Ashley Bransgrove et al 2023 ApJL 958 L9. doi:10.3847/2041-8213/ad0556

“A Multiwavelength Investigation of PSR J2229+6114 and Its Pulsar Wind Nebula in the Radio, X-ray, and Gamma-ray Bands,” I. Pope et al 2024 ApJ 960 75. doi:10.3847/1538-4357/ad0120

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Four Pulsars Discovered in New Survey

The supernova remnant 30 Doradus B encloses a pulsar at its center.
X-ray: NASA/CXC/Penn State Univ./L. Townsley et al.;
Optical: NASA/STScI/HST; Infrared: NASA/JPL/CalTech/SST;
Image Processing: NASA/CXC/SAO/J. Schmidt, N. Wolk, K. Arcand

A pilot survey using the world’s largest radio dish has led to the discovery of four pulsars, two of which are ultra-precise millisecond pulsars. This survey highlights the wealth of pulsars that await discovery at intermediate galactic latitudes.

An artist’s impression of a pair of pulsars.
Credit: Michael Kramer (Jodrell Bank Observatory, University of Manchester)

Small Stars with a Big Impact

When massive stars explode as supernovae, they can leave behind their extremely dense, collapsed cores in the form of neutron stars. Neutron stars spin rapidly and have strong magnetic fields, leading many of them to produce beams of radio emission along their poles. When these beams sweep across our field of view, we see brief, regular pulses of emission and call the objects pulsars.

Several thousand pulsars have been discovered in our galaxy, but there’s a need to find even more: pulsars provide a path to studying stellar evolution, the interiors of neutron stars, and even gravitational waves. Millisecond pulsars — those with the shortest rotation periods, around 10 milliseconds or less — are especially precious, as their pulses are exceptionally regular. By monitoring the arrival times of the pulses from many millisecond pulsars at once, researchers have found evidence for the gravitational wave background, which is thought to be the combined signals of millions of distant supermassive black hole binaries.

Pulse profiles of the four newly discovered pulsars.

Click to enlarge. Credit: Zhi et al. 2024

A Small FAST Survey

Where and how do we find pulsars? The word pulsar is short for pulsating radio source, and most pulsars are identified in surveys by their characteristic pulses of radio emission. Like most stars, pulsars are concentrated in the thin disk of our galaxy, but interstellar clouds of gas and dust in this region can scatter pulsar signals. Searching the area just above the galactic plane makes for easier pulsar discovery, and current evidence suggests that millisecond pulsars may be more common in these higher-latitude regions.

Using the Five-hundred Aperture Spherical Telescope (FAST) — the world’s largest radio dish — Qijun Zhi (Guizhou Normal University) and collaborators searched for pulsars in a small area of the sky about 5 degrees above the galactic midplane. The survey discovered four new pulsars and recovered all seven of the known pulsars in the search area. Of the four newly discovered pulsars, two are of the coveted millisecond variety, with rotation periods of 3.9 and 4.6 milliseconds. One of these two millisecond pulsars especially warrants further study, since it is bright enough to possibly be included in pulsar timing arrays in the future.

llustration of how galactic latitude is measured.
Credit: AAS Nova/Kerry Hensley

More Pulsars to Come

The pilot survey described in this study complements the efforts of other pulsar surveys. FAST is currently at work on the Commensal Radio Astronomy FAST Survey and the Galactic Plane Pulsar Survey, both of which aim to find pulsars at galactic latitudes below 10 degrees. These surveys have led to the discovery of roughly 800 pulsars so far, about 200 of which are millisecond pulsars.

Zhi and collaborators expect that many more pulsars await discovery at intermediate galactic latitudes, 5 to 15 degrees above the midplane of the Milky Way. Considering the success of their limited pilot study, the team expects that roughly 900 millisecond pulsars could be found in that region.

Citation

“Discovery of Four Pulsars in a Pilot Survey at Intermediate Galactic Latitudes with FAST,” Q. J. Zhi et al 2024 ApJ 960 79. doi:10.3847/1538-4357/ad0eca

By Kerry Hensley

Source: American Astronomical Society/AAS-Nova

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The Crab Nebula Seen in New Light by NASA's Webb

Crab Nebula (NIRCam and MIRI Image)
Credits> Image: NASA, ESA, CSA, STScI, Tea Temim (Princeton University)

Crab Nebula (Webb and Hubble Comparison)
Credits: Image: NASA, ESA, CSA, STScI, Jeff Hester (ASU), Allison Loll (ASU), Tea Temim (Princeton University)

Release Images | Release Videos

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NASA’s James Webb Space Telescope has gazed at the Crab Nebula, a supernova remnant located 6,500 light-years away in the constellation Taurus. Since the recording of this energetic event in 1054 CE by 11th-century astronomers, the Crab Nebula has continued to draw attention and additional study as scientists seek to understand the conditions, behavior, and after-effects of supernovae through thorough study of the Crab, a relatively nearby example.

Using Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument), a team led by Tea Temim at Princeton University is searching for answers about the Crab Nebula’s origins.

“Webb’s sensitivity and spatial resolution allow us to accurately determine the composition of the ejected material, particularly the content of iron and nickel, which may reveal what type of explosion produced the Crab Nebula,” explained Temim.

At first glance, the general shape of the supernova remnant is similar to the optical wavelength image released in 2005 from NASA’s Hubble Space Telescope : In Webb’s infrared observation, a crisp, cage-like structure of fluffy gaseous filaments are shown in red-orange. However, in the central regions, emission from dust grains (yellow-white and green) is mapped out by Webb for the first time.

Additional aspects of the inner workings of the Crab Nebula become more prominent and are seen in greater detail in the infrared light captured by Webb. In particular, Webb highlights what is known as synchrotron radiation: emission produced from charged particles, like electrons, moving around magnetic field lines at relativistic speeds. The radiation appears here as milky smoke-like material throughout the majority of the Crab Nebula’s interior.

This feature is a product of the nebula’s pulsar, a rapidly rotating neutron star. The pulsar’s strong magnetic field accelerates particles to extremely high speeds and causes them to emit radiation as they wind around magnetic field lines. Though emitted across the electromagnetic spectrum, the synchrotron radiation is seen in unprecedented detail with Webb’s NIRCam instrument.

To locate the Crab Nebula’s pulsar heart, trace the wisps that follow a circular ripple-like pattern in the middle to the bright white dot in the center. Farther out from the core, follow the thin white ribbons of the radiation. The curvy wisps are closely grouped together, outlining the structure of the pulsar’s magnetic field, which sculpts and shapes the nebula.

At center left and right, the white material curves sharply inward from the filamentary dust cage’s edges and goes toward the neutron star’s location, as if the waist of the nebula is pinched. This abrupt slimming may be caused by the confinement of the supernova wind’s expansion by a belt of dense gas.

The wind produced by the pulsar heart continues to push the shell of gas and dust outward at a rapid pace. Among the remnant’s interior, yellow-white and green mottled filaments form large-scale loop-like structures, which represent areas where dust grains reside.

The search for answers about the Crab Nebula’s past continues as astronomers further analyze the Webb data and consult previous observations of the remnant taken by other telescopes . Scientists will have newer Hubble data to review within the next year or so from the telescope’s reimaging of the supernova remnant. This will mark Hubble’s first look at emission lines from the Crab Nebula in over 20 years, and will enable astronomers to more accurately compare Webb and Hubble’s findings.

Want to learn more? Through NASA’s Universe of Learning, part of NASA’s Science Activation program, explore images of the Crab Nebula from other telescopes, a 3D visualization, data sonification, and hands-on activities. These resources and more information about supernova remnants and star lifecycles can be found at NASA’s Universe of Learning.

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency. 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 Jet Propulsion Laboratory.

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About This Release

Credits:

Media Contact:

Abigail Major
Space Telescope Science Institute, Baltimore, Maryland

Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland

Science: Tea Temim (Princeton University)

Permissions: Content Use Policy

Contact Us: Direct inquiries to the News Team.

Source: NASA's James Webb Space Telescope/News

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Looking for a Dragonfly in the Sky

Composite radio and X-ray image of the Dragonfly pulsar wind nebula.
Adapted from Jin et al. 2023

Title: Hard X-ray Observation and Multiwavelength Study of the PeVatron Candidate Pulsar Wind Nebula “Dragonfly”
Authors: Jooyun Woo et al.
First Author’s Institution: Columbia Astrophysics Laboratory
Status: Published in ApJ

Figure 1: A multi-wavelength view of the Crab Nebula that shows the X-rays from the pulsar wind nebula (pinkish-white region at the center)
Credit: NASA, ESA, NRAO/AUI/NSF and G. Dubner (University of Buenos Aires)

Pulsar Wind Nebulae: Little Space Animals

Pulsar wind nebulae are cosmic particle accelerators found all over the Milky Way (and in other galaxies too!). They’re made by the winds of pulsars — rapidly rotating and highly magnetized neutron stars, which are remnants of massive stars — pushing out winds of particles into the environments around them. The most famous example of a pulsar wind nebula is the Crab Nebula, which can be seen in Figure 1 as the small, pinkish-white, tornado-esque structure located in the larger multicolored supernova remnant left over from the original star’s explosion around a thousand years ago.

The Crab Nebula isn’t the only pulsar wind nebula with a fun nickname; in fact, most of these nebulae and their associated supernova remnants are named after animals that they (very) vaguely resemble. There’s the Mouse, the Goose, and the Kookaburra, just to name a few — and of course, the topic of today’s article, the Dragonfly (see Figure 2). Besides slightly resembling animals, pulsar wind nebulae are also thought to produce the highest-energy particles we detect on Earth. A new catalog of the highest-energy gamma-rays ever seen (see this bite) either links or tentatively associates many of these energetic systems with pulsars or pulsar wind nebulae.

Figure 2: Radio (colour) and X-ray (contours) image of the Dragonfly pulsar wind nebula. Doesn’t it sort of look like a dragonfly? Credit:

Jin et al. 2023

Looking for the Dragonfly with All Sorts of Different (Wavelength) Eyes!

The authors of today’s article investigate the Dragonfly with multiple different telescopes that detect light across the electromagnetic spectrum to get a full picture of what’s going on with the particles accelerated in and around the nebula. The authors model the multi-wavelength emission to try to figure out if the Dragonfly is capable of accelerating particles (electrons, protons, and other things) up to petaelectronvolt (PeV; that’s a quadrillion electronvolts!) energies that then interact to make gamma rays, which would classify it as a PeVatron (a name that aptly describes any astronomical source that can accelerate particles up to PeV energies). We detect the highest-energy charged cosmic rays up to PeV energies, but we haven’t seen too many sources that emit gamma rays at these energies due to instrumental limitations and other things like photon absorption. Since cosmic rays (usually protons) get deviated in their travels to Earth by the swirling magnetic fields of the Milky Way, we need to search for neutral particles of similar energies, like photons (i.e., gamma rays) to find PeVatrons, since they trace a straight line back from the particle to its source.

Using model fitting, the authors can create and evolve a pulsar and pulsar wind nebula to match the observed data, which gives them information like the nebular age, the expected shape of the nebula’s emission, and whether or not it can be a PeVatron, among many other interesting clues that help narrow down what’s going on with the particles and material in this system.

In particular, one interesting thing the authors notice is that the shape of the Dragonfly is long and asymmetric in soft X-ray wavelengths (and potentially in other wavelengths, but it’s hard to say due to much coarser angular resolution; see Figure 3b). Usually we’d expect to see a more spherical shape, so the explanation for this could be that the pulsar that’s powering the nebula is zooming through space at an unusually high speed or, more likely, that the nebula lives within a supernova remnant that hasn’t been seen yet. The interaction of particles from the pulsar wind nebula with the supernova remnant can cause some funky shapes to appear in the surrounding material. The authors suggest that looking at the Dragonfly with a long exposure in radio wavelengths might be able to pick up signs of a supernova remnant that are overwhelmed in other wavelengths by the bright pulsar wind nebula to confirm this scenario.

By looking at the full multi-wavelength picture (see Figure 3), the authors note that the size of the pulsar wind nebula decreases with increasing energy in X-ray wavelengths (this isn’t apparent in Figure 3d, because the instrument isn’t able to resolve small structure and blurs everything out to look bigger than it is), meaning that the the nebula becomes a less efficient particle accelerator as we move to higher energies. By modelling this behaviour, the authors find a maximum particle energy of 1.4 PeV, meaning that the Dragonfly really can be a PeVatron.

Figure 3: The observed shape of the Dragonfly in a) radio, b) soft X-ray, c) hard X-ray, and d) very-high-energy gamma rays with X-ray contours in blue. The star or X in each figure marks the pulsar location. Adapted from Woo et al. 2023

Maybe a PeVatron? We’ll Have to Wait and See!

There’s still more work to do to figure out if we can actually see gamma rays at energies beyond a PeV from the Dragonfly and to figure out how particles are being transported around the nebula to get the weird asymmetric shape that today’s authors observed. More observations using existing radio, X-ray, and other instruments as well as future ultra-high-energy gamma-ray telescopes (like SWGO and CTAO-South) can help answer these questions and help us get an even more full picture of the Dragonfly.

Original astrobite edited by Lucie Rowland

Source: American Astronomical Society/AAS-Nova

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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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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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Selections from 2022: A Pulsar in the Large Magellanic Cloud

A view of the Large Magellanic Cloud taken with the Visible and Infrared Survey Telescope for Astronomy
Credit: ESO/VMC Survey; CC BY 4.0

Editor’s Note: In these last two weeks of 2022, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume in January.

Total intensity (left) and circularly polarized intensity (right) images of part of the Large Magellanic Cloud at 888 megahertz, as seen by ASKAP. The zoomed-in images show the location of the newly discovered pulsar. Click to enlarge. Credit: Wang et al. 2022

Discovery of PSR J0523-7125 as a Circularly Polarized Variable Radio Source in the Large Magellanic Cloud

Main takeaway:

A team led by Yuanming Wang (The University of Sydney, Australia) reported the discovery of a pulsar — the dense, rapidly spinning remnant of a massive star’s core — using radio continuum data from the Australian Square Kilometre Array Pathfinder (ASKAP). The newfound pulsar is located in the Large Magellanic Cloud, a satellite galaxy of the Milky Way, and its discovery may pave the way for astronomers to find other extragalactic pulsars with unusual properties.

Why it’s interesting:

The newly discovered pulsar, PSR J0523−7125, is one of the most luminous known radio pulsars, but several aspects of its radio signal made it difficult to find: while most pulsars are identified via their brief flashes of radio emission, PSR J0523−7125’s pulses are uncharacteristically broad, and its radio emission falls off sharply at higher frequencies. Wang and collaborators observed the new pulsar as part of the Variables and Slow Transients (VAST) survey and identified it based on its high degree of circular polarization and lack of a multiwavelength counterpart.

Prospects for finding further pulsars:

This work by Wang and collaborators shows that radio surveys are a viable means of discovering pulsars with unusual pulse properties. The combination of circular polarization data with multiwavelength images is especially useful, allowing researchers to identify sources that emit circularly polarized light but are absent in optical images. The authors also posit that future searches with the Next Generation Very Large Array — a network of 263 radio dishes scheduled to begin construction in 2026 — could lead to the first discovery of a pulsar in another neighboring galaxy, Andromeda.

By Kerry Hensley  

 

Citation

Yuanming Wang et al 2022 ApJ 930 38. doi:10.3847/1538-4357/ac61dc

Source: American Astronomical Society/AAS - Nova

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Four Perspectives on Neutron Stars, Pulsars, and Magnetars By Kerry Hensley

Illustration of a neutron star emitting a jet.
Credit: ICRAR/University of Amsterdam

When a massive star explodes as a supernova, its core collapses into a city-sized sphere of neutrons called a neutron star. These extraordinarily dense stars — just one teaspoon of a neutron star would weigh billions of tons in Earth’s gravity — exhibit some of the most intriguing behavior in the universe: rapid rotation, beams of radio emission, and extremely strong magnetic fields. Today, we’ll introduce four recent research articles that explore different aspects of these stars.

Simulated light curves during an X-ray burst, showing the effects of incorporating different physics. A model without neutrino cooling (labeled “No DU” in reference to the neutrino cooling pathway called direct Urca), peaks at a lower luminosity than models incorporating neutrino cooling. Credit: Adapted from Dohi et al. 2022

Bursting, Cooling, and Bursting Again

Sometimes, neutron stars reveal themselves by interacting with other stars. When a neutron star gathers gas from a stellar companion, the gas can ignite on the star’s scorching surface, resulting in a sudden burst of X-rays. After this sudden influx of heat, how does the neutron star cool, and how is the cooling reflected in the star’s light curve? While this may seem like a simple question, the answer hinges on our understanding of the conditions within the neutron star’s interior as well as the characteristics of the gas being accreted.

In a recent publication, a team led by Akira Dohi (土肥明; Kyushu University, Japan) explored the issue of neutron star cooling with general relativistic stellar evolution models. Specifically, the team investigated the effects of cooling by emitting neutrinos — chargeless, nearly massless particles that scarcely interact with matter — which is expected to speed up the cooling rate. The authors found that neutrino cooling increases the time between outbursts but makes them brighter at their peak, though additional physics to be included in future modeling might suppress this effect.

Simulated pulses showing a change in the phase of the pulse due to the shifting motion of the sparks.
Credit: Adapted from Basu et al. 2022

Simulating Pulsar Sparks

Rahul Basu (University of Zielona Góra, Poland) and collaborators reported on simulations of conditions very close to the surface of a neutron star that emits beams of radio emission. Neutron stars that emit beamed radio waves are called pulsars for the way the beams sweep across our field of view, generating what we see as pulses of emission. Near a pulsar’s surface, extremely high temperatures and strong magnetic and electric fields combine forces to summon a sea of charged particles that are then accelerated to relativistic speeds.

Basu and collaborators focused on a phenomenon called sparking, in which charged particles jump the gap between the pulsar’s surface at its poles and its plasma-rich magnetosphere. The team’s modeling demonstrated that a pulsar’s poles are tightly filled with constant sparks, and the arrangement of these sparks slowly shifts over time. By modeling the emission associated with the simulated sparks, the team showed that the shifting motion of the sparks appears to be responsible for the observed periodic variations in the phases and amplitudes of some pulsars’ pulses.

Example of a pulse observed with the Giant Metrewave Radio Telescope.
Credit: Adapted from Sharma et al. 2022

Pulsars Probing Gravitational Waves

By studying large groups of pulsars, astronomers hope to learn about something seemingly unrelated: gravitational waves. Pulsars provide a method to detect gravitational waves by way of these stars’ impeccable timekeeping abilities — because a pulsar’s radio beat is so reliable, the slight distortion of space caused by a passing gravitational wave should impact the arrival times of a pulsar’s pulses.

However, there’s a complication to this technique: spatial and temporal changes in the interstellar medium plasma can also affect when a pulsar’s radio pulses arrive at Earth. In order to compensate for the effect of the interstellar medium, we need to be able to make precise observations of pulsars across a range of radio frequencies. In a recent research article, Shyam Sharma (Tata Institute of Fundamental Research, India) and collaborators tested a pulsar-timing measurement technique using the Giant Metrewave Radio Telescope, which is highly sensitive to low-frequency radio waves. Sharma and coauthors showed that observing using a wide frequency band yields results comparable to typical narrowband observations, indicating that this technique could be used to disentangle the effects of the interstellar medium and more accurately time the pulses of arrays of pulsars, opening a new window onto gravitational waves.

Temperature maps of the top of a magnetar’s crust (top) and the magnetar’s surface (bottom) after a hotspot is injected.
Credit: De Grandis et al. 2022

Magnetic Outbursts

As if neutron stars could get any wilder: some neutron stars, dubbed magnetars, have extremely strong magnetic fields and exhibit frequent X-ray flares. While the cause of these X-ray outbursts is still unknown, some researchers have suggested that they arise from a sudden upwelling of magnetic energy beneath the magnetar’s crust, creating a hot spot that cools gradually over days or months.

To understand how the injection of heat into a magnetar’s crust might create the spectral features seen during X-ray outbursts, Davide De Grandis (University of Padova, Italy) and coauthors employed a three-dimensional magnetothermal model of hotspot formation and cooling. This model allowed the team to study the effects of asymmetrical hot spots under a magnetar’s crust for the first time. The team was able to confirm that these hot spots can be responsible for outbursts, though we’ll have to wait for future research to fully explore the evolution of the spectral features generated during these events.

By Kerry Hensley

Citation

“Impacts of the Direct Urca and Superfluidity inside a Neutron Star on Type I X-Ray Bursts and X-Ray Superbursts,” A. Dohi et al 2022 ApJ 937 124. doi:10.3847/1538-4357/ac8dfe

“Two-dimensional Configuration and Temporal Evolution of Spark Discharges in Pulsars,” Rahul Basu et al 2022 ApJ 936 35. doi:10.3847/1538-4357/ac8479

“Wide-band Timing of GMRT-discovered Millisecond Pulsars,” Shyam S. Sharma et al 2022 ApJ 936 86. doi:10.3847/1538-4357/ac86d8

“Three-dimensional Magnetothermal Simulations of Magnetar Outbursts,” Davide De Grandis et al 2022 ApJ 936 99. doi:10.3847/1538-4357/ac8797

Source: American Astronomical Society - AAS Nova

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"Black Widow" Star Devours Its Rapidly Circling Companion 'Black Widow' Pulsar Animation

An illustrated view of a black widow pulsar and its stellar companion. The pulsar's gamma-ray emissions (magenta) strongly heat the facing side of the star (orange). The pulsar is gradually evaporating its partner. Credit: NASA's Goddard Space Flight Center/Cruz deWilde

Black Widow' Pulsar Animation

This animation from NASA's Goddard Space Flight Center, which depicts a different black widow system, illustrates how a pulsar evaporates its companion over time. Credit: NASA's Goddard Space Flight Center/Cruz deWilde This animation from NASA's Goddard Space Flight Center, which depicts a different black widow system, illustrates how a pulsar evaporates its companion over time. Credit: NASA's Goddard Space Flight Center/Cruz deWilde

Finding represents the shortest-period black widow binary found to date

In black widow star systems, a rapidly spinning dead star, called a pulsar, blasts its orbiting companion with radiation, slowly evaporating it. Like their namesake spiders, the pulsars take advantage of their lower-mass companions before destroying them, by harnessing material and energy from the doomed partner stars.

A new study published in the journal Nature reports a new candidate black widow star system, named ZTF J1406+1222, in which the stars orbit around each other every 62 minutes—the shortest orbital period observed to date for this type of binary star system. The previous black widow record holder, PSR J1653-0158, contains stars orbiting around each other every 75 minutes.

The new candidate system was found using the Zwicky Transient Facility (ZTF) instrument at Caltech's Palomar Observatory near San Diego. ZTF is funded by the National Science Foundation (NSF) and an international consortium of partners.

"This 62-minute orbit is remarkable because we don't understand how the stars could get into such a tight orbit," says Kevin Burdge (PhD '21), a postdoctoral scholar at MIT who performed the research while at Caltech. "The process of the pulsar ablating its companion should actually drive them apart. This is pushing the boundaries of what we thought possible."

Burdge says that upcoming observations from NASA's Chandra X-ray Observatory should help confirm the result. "Our data indicate we are looking at a black widow binary, but it could be something entirely new."

The first black widow star was discovered in the 1980s, and, since then, a few dozen have been found along with a similar spidery "species" of stars called redbacks, which also consist of pulsars and doomed partners. Scientists say that the rapidly spinning pulsar stars, in both redback and black widow systems, pick up energy and speed by siphoning material away from their companions.

"The lower-mass stars donate material to their partner stars but suffer the consequences," explains Burdge.

The new candidate black widow binary stars were identified by scanning millions of stars for those that rapidly blink on and off in the night sky. ZTF captures images of the entire night sky every two nights thanks to its camera's large field of view, which covers an area of sky equivalent to a grid of 247 full moons. The sky survey enables researchers to search for objects that change in brightness or location on rapid timescales. "ZTF has revealed a zoo of exotic stellar species," says co-author Tom Prince, the Ira S. Bowen Professor of Physics, Emeritus, at Caltech and a co-investigator of ZTF. "Rather than being boring, static objects, a large fraction of stars exhibit dips, pulsations, or periodic brightenings that are a key to understanding their nature."

In this case, Burdge developed an algorithm to search for ZTF objects that dramatically change in brightness on timescales of less than 80 minutes.

That led them to ZTF J1406+1222, which varies in brightness by a factor of 13 every 62 minutes. The periodic dimming and brightening of the system is caused by the companion to the pulsar, a cool failed star called a brown dwarf, rotating around on its axis. The brown dwarf, which is being blasted by the pulsar, retains heat on one side while its other side remains cooler. Telescopes cannot distinguish between the pulsar and the brown dwarf, but when the brown dwarf's hot side rotates into our view from Earth, the overall brightness of the system goes up; conversely, when the cool side swings around, the brightness goes down.

Interestingly, this black widow system also has a third companion orbiting much farther out, at a distance of roughly 600 astronomical units, or AUs (an AU is the distance between Earth and the sun). While the primary pulsar and brown dwarf orbit each other every 62 minutes, the third partner star orbits the tight pair every 10,000 years.

The scientists also point out that this is the first black widow system discovered using optical light. "The population of black widows we've been finding so far with other wavelengths of light, such as X-rays, gamma rays, and radio waves, is probably biased because we haven't been catching all of them," says Burdge. "Now we have a new lens through which we can identify these systems."

Observations from the W. M. Keck Observatory atop Maunakea in Hawaiʻi; the Grand Canary Telescope in the Canary Islands, Spain; the European Space Agency's Gaia space observatory; and the Sloan Digital Sky Survey were also used to confirm the findings. The study titled "A 62-minute orbital period black widow binary in a wide hierarchical triple," was funded primarily by the NSF, NASA, and the MIT Pappalardo Fellowships program. Other Caltech authors include: Jim Fuller, Illaria Caiazzo, Matthew Graham, Amruta Jaodand, Shri Kulkarni, Sterl Phinney (BS '80), Jan van Roestel, Andrew Drake, Richard Dekany (BS '89), Dmitry Duev, Ashish Mahabal, and Reed Riddle.

Written by Whitney Clavin

Contact:

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

Source: California Institute of Technology Caltech/News

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Globular cluster billowing in the Galactic wind

Globular cluster 47 Tuc (upper right) and the Small Magellanic Cloud in the same field-of-view. The inset is a close-up of the cluster showing the detected magnetic field in a colour scale. The lines indicate the effect of the Galactic wind on the magnetic field. © ESO/VISTA VMC (background image); F. Abbate et al., Nature Astronomy (inset)

Investigation of pulsars in 47 Tuc provides constraints on the magnetic field in the halo of the Milky Way

March 02, 2020. The Galactic magnetic field plays an important role in the evolution of our Galaxy, but its small-scale behaviour is still poorly known. It is also unknown whether it permeates the halo of the Galaxy or not. By using observations of pulsars in the halo globular cluster 47 Tuc, an international research team led by Federico Abbate from the Max Planck Institute for Radio Astronomy in Bonn, Germany who started this work at University of Milano Bicocca and INAF-Astronomical Observatory of Cagliari, could probe the Galactic magnetic field at scales of a few light years for the first time. They discovered an unexpected strong magnetic field in the direction of the cluster. This magnetic field points perpendicularly to the Galactic disk and could be explained by an interaction with the Galactic wind. This is a magnetized outflow that extends from the Galactic disk into the surrounding halo and its existence has never been proven before.

47 Tucanae, or 47 Tuc as it is usually called, is a spectacular globular cluster visible with the naked eye in the constellation “Tucana” in the southern sky close to the Small Magellanic Cloud. The first pulsar in this cluster was discovered in 1990 with the Parkes 64-m radio telescope in Australia, and soon more were found with the same telescope. Currently there are 25 pulsars known in 47 Tuc. For this reason, this very well-studied globular cluster became one of the most important for pulsar astronomers as well.

Pulsars are periodic sources that allow astronomers to measure the so-called dispersion measure which is a delay of the arrival time of the single pulses at different frequencies. This delay is proportional to the density of free electrons along the path from the pulsar to the Earth. “In 2001, we noticed that the pulsars in the far side of the cluster had a higher dispersion measure than those in the near side, which implied the presence of gas in the cluster”, says Paulo Freire from the Max Planck Institute for Radio Astronomy (MPIfR) who led a number of research projects on 47 Tuc.

What makes 47 Tuc even more interesting is that the cluster is at a distance of about 15,000 light years, located in a relatively undisturbed area in the Galactic halo. The halo surrounds the Galactic disk and hosts very few stars and very small quantities of gas. “The pulsars in this cluster can give us a unique and unprecedented insight into the large-scale geometry of the magnetic field in the Galactic halo.” says Federico Abbate, lead author of the paper and now working at MPIfR, who performed the analysis during his PhD at the University of Milano-Bicocca and at INAF - Cagliari Astronomical Observatory.

Understanding the geometry and strength of Galactic magnetic fields is essential to draw a complete picture of our Galaxy. The magnetic fields can affect star formation, regulate the propagation of high-energy particles and help establish the presence of a Galactic scale outflow of gas from the disk to the surrounding halo. Despite their importance, the large-scale geometry of the magnetic fields in the Galactic halo is not fully known.

Magnetic fields are not observable directly, but scientists make use of the effects they have on the low-density plasma that permeates the Galactic disk. In this plasma, the electrons are separated from the atomic nuclei and they behave like small magnets. The electrons are attracted by the magnetic field and are forced to orbit the magnetic field lines, emitting radiation known as synchrotron radiation. Other than emitting their own radiation, the free electrons also leave a peculiar signature on the polarized radiation that travels through the plasma. The electromagnetic field of the polarized radiation oscillates always in the same direction and the electrons in a magnetized medium will rotate this direction by different amounts at different frequencies. This effect is called Faraday rotation and is measurable only at radio frequencies.

Observations of polarized radio emission work well to constrain the magnetic field in the Galactic disk where the plasma is dense enough. In the Galactic halo, however, the plasma density is too low to directly observe the effects. For this reason, the geometry and strength of the magnetic field in the halo is unknown and models predict that it could either be parallel or perpendicular to the disk. The presence of a magnetized outflow from the disk to the halo has been suggested following observations in other galaxies. It can also explain the diffuse X-ray emission in the Galaxy.

Recent observations of the pulsars in 47 Tuc, also performed with the Parkes radio telescope in Australia, were able to measure their polarized radio emission and their Faraday rotation. These reveal the presence of a magnetic field in the globular cluster that is surprisingly strong - so strong, in fact, that it cannot be maintained by the globular cluster itself but requires an external source located in the Galactic halo. The direction of the magnetic field is compatible with that of the Galactic wind, perpendicular to the Galactic disk. The interaction of the Galactic wind and the cluster forms a shock that amplifies the magnetic field to the values observed.

This work reveals a new technique to study the magnetic field in the Galactic halo. This cluster is a perfect target for observations with the innovative MeerKAT radio telescope in South Africa. “In the near future, the MeerKAT telescope will greatly improve the polarization measurements and possibly not only confirm the presence of the Galactic wind but also constrain its properties,” says Andrea Possenti from the INAF – Cagliari Astronomical Observatory who is involved in the globular cluster pulsars efforts with MeerKAT together with the MPIfR. Moreover, this powerful telescope in particular with its further development towards the Square Kilometre Array (SKA) has the capabilities to observe other globular clusters in the halo and corroborate the results.

The results are published in this week’s issue of „Nature Astronomy“.

Source: Max Planck Institute for Radio Astronomy/News

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The research team consists of Federico Abbate, Andrea Possenti, Caterina Tiburzi, Ewan Barr, Willem van Straten, Alessandro Ridolfi and Paulo Freire. The first author, Federico Abbate, is now at the MPIfR. Co-authors Ewan Barr and Paulo Freire are both affiliated with the MPIfR.

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Original Paper

Constraints on the magnetic field in the Galactic halo from globular cluster pulsars 

F. Abbate et al., Nature Astronomy, 02 March 2020. DOI: 10.1038/s41550-020-1030-6.

The URL will become valid after the embargo expires on Monday, March 02, 19:00 CET (13:00 US EST).

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Links

Fundamental Physics in Radio Astronomy
Research Department "Fundamental Physics in Radio Astronomy" at MPIfR, Bonn, Germany

Parkes
CSIRO Parkes Observatory

Millisecond Pulsars in 47 Tuc 
Information on millisecond pulsars in globular cluster 47 Tuc (Website Paulo Freire)

MeerKAT
South African MeerKAT radio telescope

SKA Observatory 
Square Kilometre Array Observatory

Pulsar Dispersion Measure 
Website "Pulsar Dispersion Measure" at Swinburne University, Australia

Cosmic Magnetism
Website "Cosmic Magnetism" at Square Kilometre Array (SKA)

Galactic Magnetic Fields
Scholarpedia article "Galactic Magnetic Fields" by Rainer Beck/MPIfR

Pulsars in 47 Tuc (Movie)

Ensemble of pulsars in 47 Tuc: movie simulation with pulsar sounds (Jodrell Bank; Andrew Lyne & Michael Kramer; 40 MB)

Pulsars in 47 Tuc (Audio file)

Sounds of an ensemble of millisecond pulsars in 47 Tuc. Audio file (Jodrell Bank; Andrew Lyne & Michael Kramer)

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Most Massive Neutron Star Ever Detected, Almost too Massive to Exist

Artist impression of the pulse from a massive neutron star being delayed by the passage of a white dwarf star between the neutron star and Earth. Credit: BSaxton, NRAO/AUI/NSF

Astronomers using the GBT have discovered the most massive neutron star to date, a rapidly spinning pulsar approximately 4,600 light-years from Earth. This record-breaking object is teetering on the edge of existence, approaching the theoretical maximum mass possible for a neutron star.

Neutron stars – the compressed remains of massive stars gone supernova – are the densest “normal” objects in the known universe. (Black holes are technically denser, but far from normal.) Just a single sugar-cube worth of neutron-star material would weigh 100 million tons here on Earth, or about the same as the entire human population. Though astronomers and physicists have studied and marveled at these objects for decades, many mysteries remain about the nature of their interiors: Do crushed neutrons become “superfluid” and flow freely? Do they breakdown into a soup of subatomic quarks or other exotic particles? What is the tipping point when gravity wins out over matter and forms a black hole?

A team of astronomers using the National Science Foundation’s (NSF) Green Bank Telescope (GBT) has brought us closer to finding the answers.

The researchers, members of the NANOGrav Physics Frontiers Center, discovered that a rapidly rotating millisecond pulsar, called J0740+6620, is the most massive neutron star ever measured, packing 2.17 times the mass of our Sun into a sphere only 30 kilometers across. This measurement approaches the limits of how massive and compact a single object can become without crushing itself down into a black hole. Recent work involving gravitational waves observed from colliding neutron stars by LIGO suggests that 2.17 solar masses might be very near that limit.

“Neutron stars are as mysterious as they are fascinating,” said Thankful Cromartie, a graduate student at the University of Virginia and Grote Reber pre-doctoral fellow at the National Radio Astronomy Observatory in Charlottesville, Virginia. “These city-sized objects are essentially ginormous atomic nuclei. They are so massive that their interiors take on weird properties. Finding the maximum mass that physics and nature will allow can teach us a great deal about this otherwise inaccessible realm in astrophysics.”

Pulsars get their name because of the twin beams of radio waves they emit from their magnetic poles. These beams sweep across space in a lighthouse-like fashion. Some rotate hundreds of times each second. Since pulsars spin with such phenomenal speed and regularity, astronomers can use them as the cosmic equivalent of atomic clocks. Such precise timekeeping helps astronomers study the nature of spacetime, measure the masses of stellar objects, and improve their understanding of general relativity.

In the case of this binary system, which is nearly edge-on in relation to Earth, this cosmic precision provided a pathway for astronomers to calculate the mass of the two stars.

Artist impression and animation of the Shapiro Delay. As the neutron star sends a steady pulse towards the Earth, the passage of its companion white dwarf star warps the space surrounding it, creating the subtle delay in the pulse signal. Animation: BSaxton, NRAO/AUI/NSF

As the ticking pulsar passes behind its white dwarf companion, there is a subtle (on the order of 10 millionths of a second) delay in the arrival time of the signals. This phenomenon is known as “Shapiro Delay.” In essence, gravity from the white dwarf star slightly warps the space surrounding it, in accordance with Einstein’s general theory of relativity. This warping means the pulses from the rotating neutron star have to travel just a little bit farther as they wend their way around the distortions of spacetime caused by the white dwarf.

Astronomers can use the amount of that delay to calculate the mass of the white dwarf. Once the mass of one of the co-orbiting bodies is known, it is a relatively straightforward process to accurately determine the mass of the other.

Cromartie is the principal author on a paper accepted for publication in Nature Astronomy. The GBT observations were research related to her doctoral thesis, which proposed observing this system at two special points in their mutual orbits to accurately calculate the mass of the neutron star.

“The orientation of this binary star system created a fantastic cosmic laboratory,” said Scott Ransom, an astronomer at NRAO and coauthor on the paper. “Neutron stars have this tipping point where their interior densities get so extreme that the force of gravity overwhelms even the ability of neutrons to resist further collapse. Each “most massive” neutron star we find brings us closer to identifying that tipping point and helping us to understand the physics of matter at these mindboggling densities.”

These observation were also part of a larger observing campaign known as NANOGrav, short for the North American Nanohertz Observatory for Gravitational Waves, which is a Physics Frontiers Center funded by the NSF.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The Green Bank Observatory is supported by the National Science Foundation, and is operated under cooperative agreement by Associated Universities, Inc. Any opinions, findings and conclusions or recommendations expressed in this material do not necessarily reflect the views of the National Science Foundation.

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‘Pulsar in a Box’ Reveals Surprising Picture of a Neutron Star’s Surroundings

Electrons (blue) and positrons (red) from a computer-simulated pulsar. These particles become accerlated to extreme energies in a pulsar's powerful magnetic and electric fields; lighter tracks show particles with higher energies. Each particle seen here actually represents trillions of electrons or positrons. Better knowledge of the particle environment around neutron stars will help astronomers understand how they behave like cosmic lighthouses, producing precisely timed radio and gamma-ray pulses. Credit: NASA's Goddard Space Flight Center

An international team of scientists studying what amounts to a computer-simulated “pulsar in a box” are gaining a more detailed understanding of the complex, high-energy environment around spinning neutron stars, also called pulsars. The model traces the paths of charged particles in magnetic and electric fields near the neutron star, revealing behaviors that may help explain how pulsars emit gamma-ray and radio pulses with ultraprecise timing. 

“Efforts to understand how pulsars do what they do began as soon as they were discovered in 1967, and we’re still working on it,” said Gabriele Brambilla, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and the University of Milan who led a study of the recent simulation. “Even with the computational power available today, tracking the physics of particles in the extreme environment of a pulsar is a considerable challenge

.”

A pulsar is the crushed core of a massive star that ran out of fuel, collapsed under its own weight and exploded as a supernova. Gravity forces more mass than the Sun’s into a ball no wider than Manhattan Island in New York City while also revving up its rotation and strengthening its magnetic field. Pulsars can spin thousands of times a second and wield the strongest magnetic fields known.

Explore a new “pulsar in a box” computer simulation that tracks the fate of electrons (blue) and their antimatter kin, positrons (red), as they interact with powerful magnetic and electric fields around a neutron star. Lighter tracks indicate higher particle energies. Each particle seen in this visualization actually represents trillions of electrons or positrons. Better knowledge of the particle environment around neutron stars will help astronomers understand how they produce precisely timed radio and gamma-ray pulses. Credits: NASA’s Goddard Space Flight Center.  Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

These characteristics also make pulsars powerful dynamos, with superstrong electric fields that can rip particles out of the surface and accelerate them into space.

NASA’s Fermi Gamma-ray Space Telescope has detected gamma rays from 216 pulsars. Observations show that the high-energy emission occurs farther away from the neutron star than the radio pulses. But exactly where and how these signals are produced remains poorly known.

Various physical processes ensure that most of the particles around a pulsar are either electrons or their antimatter counterparts, positrons.

“Just a few hundred yards above a pulsar’s magnetic pole, electrons pulled from the surface may have energies comparable to those reached by the most powerful particle accelerators on Earth,” said Goddard’s Alice Harding. “In 2009, Fermi discovered powerful gamma-ray flares from the Crab Nebula pulsar that indicate the presence of electrons with energies a thousand times greater.”  

Speedy electrons emit gamma rays, the highest-energy form of light, through a process called curvature radiation. A gamma-ray photon can, in turn, interact with the pulsar’s magnetic field in a way that transforms it into a pair of particles, an electron and a positron.

To trace the behavior and energies of these particles, Brambilla, Harding and their colleagues used a comparatively new type of pulsar model called a “particle in cell” (PIC) simulation. Goddard’s Constantinos Kalapotharakos led the development of the project’s computer code. In the last five years, the PIC method has been applied to similar astrophysical settings by teams at Princeton University in New Jersey and Columbia University in New York.   

“The PIC technique lets us explore the pulsar from first principles. We start with a spinning, magnetized pulsar, inject electrons and positrons at the surface, and track how they interact with the fields and where they go,” Kalapotharakos said. “The process is computationally intensive because the particle motions affect the electric and magnetic fields and the fields affect the particles, and everything is moving near the speed of light.”

The simulation shows that most of the electrons tend to race outward from the magnetic poles. The positrons, on the other hand, mostly flow out at lower latitudes, forming a relatively thin structure called the current sheet. In fact, the highest-energy positrons here — less than 0.1 percent of the total — are capable of producing gamma rays similar to those Fermi detects, confirming the results of earlier studies.

Some of these particles likely become boosted to tremendous energies at points within the current sheet where the magnetic field undergoes reconnection, a process that converts stored magnetic energy into heat and particle acceleration.

One population of medium-energy electrons showed truly odd behavior, scattering every which way — even back toward the pulsar.

The particles move with the magnetic field, which sweeps back and extends outward as the pulsar spins. Their rotational speed rises with increasing distance, but this can only go on so long because matter can’t travel at the speed of light.

The distance where the plasma’s rotational velocity would reach light speed is a feature astronomers call the light cylinder, and it marks a region of abrupt change. As the electrons approach it, they suddenly slow down and many scatter wildly. Others can slip past the light cylinder and out into space.

The simulation ran on the Discover supercomputer at NASA’s Center for Climate Simulation at Goddard and the Pleiades supercomputer at NASA’s Ames Research Center in Silicon Valley, California. The model actually tracks “macroparticles,” each of which represents many trillions of electrons or positrons. A paper describing the findings was published May 9 in The Astrophysical Journal. 

“So far, we lack a comprehensive theory to explain all the observations we have from neutron stars. That tells us we don’t yet completely understand the origin, acceleration and other properties of the plasma environment around the pulsar,” Brambilla said. “As PIC simulations grow in complexity, we can expect a clearer picture.”

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the U.S. Department of Energy and with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden and the United States.

For more about NASA’s Fermi mission, visit:  https://www.nasa.gov/fermi

By Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Editor: Rob Garner

 Source: NASA/Pulsar

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