Astronomy

Have we Observed Continuum Emission from Neutron Stars?

Have we Observed Continuum Emission from Neutron Stars?


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Have we detected continuum optical emission from any rotating neutron stars that do not have an accretion disk dominating the light? I ask because I know we have observed Doppler broadening of spectral lines from the rotation of even ordinary stars rotating at ordinary rates, but it seems to me that only the rapid rotation of a neutron star would be fast enough to make the Doppler broadening of the thermal continuum peak to be observable.


Yes - quite a few isolated neutron stars have been observed, where any magnetospheric emission or accretion-related emission is either negligible or has been otherwise separated.

As you suspect, this emission is thermal in nature. Neutron stars are roughly approximated by black bodies but, like "normal" stars, they do have atmospheres and strong magnetic fields that modify the black body spectrum considerably. If the neutron star is isolated then the only reason that you might be able to detect thermal emission from the surface is that it is very young (less than about a million years after formation) and still hot. The difficulty is in ruling out any residual, pulsed magnetospheric component or a contribution from accretion of the interstellar medium itself.

A couple of examples to look at in the optical part of the spectrum would be Kulkarni & van Kerwijk (1998) who detected optical radiation from the counterpart to RX0720.4-3125; and Walter & Matthews (1997) who claimed a detection of thermal optical emission detection from the isolated neutron star RXJ185635-3754 (and see picture below).

There are several more examples of thermal emission detected at X-ray wavelengths from young, isolated neutron stars - the so-called X-ray dim neutron stars (see review by Mereghetti 2010).

Of course if you are extending your definition of continuum emission to include non-thermal (but not accretion-related) processes, then continuum emission is detected from all isolated pulsars. Synchrotron radiation is a continuum process and has been detected right across the electromagnetic spectrum in many pulsars.

A visible band HST image of the isolated neutron star RXJ185635-3754, attributable to Walter & Matthews 1997.


Hubble sees unusual features around ‘magnificent seven’ neutron star

In this artist’s impression, a disk of warm dust encircles a neutron star, producing an infrared signature detected by the Hubble Space Telescope. Not directly seen, a disk of dust is one explanation for the observed infrared emissions. Image: NASA, ESA, and N. Tr’Ehnl (Pennsylvania State University)

An unusual infrared light emission from a nearby neutron star detected by NASA’s Hubble Space Telescope, could indicate new features never before seen. One possibility is that there is a dusty disk surrounding the neutron star another is that there is an energetic wind coming off the object and slamming into gas in interstellar space the neutron star is plowing through.

Although neutron stars are generally studied in radio and high-energy emissions, such as X-rays, this study demonstrates that new and interesting information about neutron stars can also be gained by studying them in infrared light, say researchers.

The observation, by a team of researchers at Pennsylvania State University, University Park, Pennsylvania Sabanci University, Istanbul, Turkey and the University of Arizona, Tucson, Arizona could help astronomers better understand the evolution of neutron stars — the incredibly dense remnants after a massive star explodes as a supernova. Neutron stars are also called pulsars because their very fast rotation (typically fractions of a second, in this case 11 seconds) causes time-variable emission from light-emitting regions.

A paper describing the research and two possible explanations for the unusual finding appears Sept. 17, 2018 in the Astrophysical Journal.

An alternative explanation for the infrared light detected by Hubble is a “pulsar wind” blowing particles into the interstellar medium the neutron star is moving through. Image: NASA, ESA, and N. Tr’Ehnl (Pennsylvania State University)

“This particular neutron star belongs to a group of seven nearby X-ray pulsars — nicknamed ‘the Magnificent Seven’ — that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” said Bettina Posselt, associate research professor of astronomy and astrophysics at Pennsylvania State and the lead author of the paper. “We observed an extended area of infrared emissions around this neutron star — named RX J0806.4-4123 — the total size of which translates into about 200 astronomical units (approximately 18 billion miles) at the assumed distance of the pulsar.”

This is the first neutron star in which an extended signal has been seen only in infrared light. The researchers suggest two possibilities that could explain the extended infrared signal seen by the Hubble. The first is that there is a disk of material — possibly mostly dust — surrounding the pulsar.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” said Posselt. “Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation. If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

A near infrared image of neutron star RX J0806.4-4123. Image: NASA, ESA, and B. Posselt (Pennsylvania State University)

The second possible explanation for the extended infrared emission from this neutron star is a “pulsar wind nebula.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” said Posselt. “A pulsar wind can be produced when particles are accelerated in the electrical field that is produced by the fast rotation of a neutron star with a strong magnetic field. As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then emit synchrotron radiation, causing the extended infrared signal that we see. Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting.”

Using NASA’s upcoming James Webb Space Telescope, astronomers will be able to further explore this newly opened discovery space in the infrared to better understand neutron star evolution.


In 1932, American physicist and radio engineer Karl Jansky detected radio waves coming from an unknown source in the center of our galaxy. Jansky was studying the origins of radio frequency interference for Bell Laboratories. He found ". a steady hiss type static of unknown origin", which eventually he concluded had an extraterrestrial origin. This was the first time that radio waves were detected from outer space. [1] The first radio sky survey was conducted by Grote Reber and was completed in 1941. In the 1970s, some stars in our galaxy were found to be radio emitters, one of the strongest being the unique binary MWC 349. [2]

The Sun Edit

As the nearest star, the Sun is the brightest radiation source in most frequencies, down to the radio spectrum at 300 MHz (1 m wavelength). When the Sun is quiet, the galactic background noise dominates at longer wavelengths. During geomagnetic storms, the Sun will dominate even at these low frequencies. [3]

Jupiter Edit

Oscillation of electrons trapped in the magnetosphere of Jupiter produce strong radio signals, particularly bright in the decimeter band.

The magnetosphere of Jupiter is responsible for intense episodes of radio emission from the planet's polar regions. Volcanic activity on Jupiter's moon Io injects gas into Jupiter's magnetosphere, producing a torus of particles about the planet. As Io moves through this torus, the interaction generates Alfvén waves that carry ionized matter into the polar regions of Jupiter. As a result, radio waves are generated through a cyclotron maser mechanism, and the energy is transmitted out along a cone-shaped surface. When Earth intersects this cone, the radio emissions from Jupiter can exceed the solar radio output. [4]

Ganymede Edit

In 2021 news outlets reported that scientists, with the Juno spacecraft that orbits Jupiter since 2016, detected an FM radio signal from the moon Ganymede at a location where the planet's magnetic field lines connect with those of its moon. According to the reports these were caused by cyclotron maser instability and were similar to both WiFi-signals and Jupiter's radio emissions. [5] [6] A study about the radio emissions was published in September 2020 [7] but did not describe them to be of FM nature or similar to WiFi signals. [ clarification needed ]

The galactic center Edit

The galactic center of the Milky Way was the first radio source to be detected. It contains a number of radio sources, including Sagittarius A, the compact region around the supermassive black hole, Sagittarius A*, as well as the black hole itself. When flaring, the accretion disk around the supermassive black hole lights up, detectable in radio waves.

Supernova remnants Edit

Supernova remnants often show diffuse radio emission. Examples include Cassiopeia A, the brightest extrasolar radio source in the sky, and the Crab Nebula.

Neutron Stars Edit

Pulsars Edit

Supernovas sometimes leave behind dense spinning neutron stars called pulsars. They emit jets of charged particles which emit synchrotron radiation in the radio spectrum. Examples include the Crab Pulsar, the first pulsar to be discovered. Pulsars and quasars (dense central cores of extremely distant galaxies) were both discovered by radio astronomers. In 2003 astronomers using the Parkes radio telescope discovered two pulsars orbiting each other, the first such system known.

Rotating Radio Transient (RRAT) Sources Edit

Rotating radio transients (RRATs) are a type of neutron stars discovered in 2006 by a team led by Maura McLaughlin from the Jodrell Bank Observatory at the University of Manchester in the UK. RRATs are believed to produce radio emissions which are very difficult to locate, because of their transient nature. [8] Early efforts have been able to detect radio emissions (sometimes called RRAT flashes) [9] for less than one second a day, and, like with other single-burst signals, one must take great care to distinguish them from terrestrial radio interference. Distributing computing and the Astropulse algorithm may thus lend itself to further detection of RRATs.

Star forming regions Edit

Short radio waves are emitted from complex molecules in dense clouds of gas where stars are giving birth.

Spiral galaxies contain clouds of neutral hydrogen and carbon monoxide which emit radio waves. The radio frequencies of these two molecules were used to map a large portion of the Milky Way galaxy. [10]

Radio galaxies Edit

Many galaxies are strong radio emitters, called radio galaxies. Some of the more notable are Centaurus A and Messier 87.

Quasars (short for "quasi-stellar radio source") were one of the first point-like radio sources to be discovered. Quasars' extreme redshift led us to conclude that they are distant active galactic nuclei, believed to be powered by black holes. Active galactic nuclei have jets of charged particles which emit synchrotron radiation. One example is 3C 273, the optically brightest quasar in the sky.

Merging galaxy clusters often show diffuse radio emission. [11]

Cosmic microwave background Edit

The cosmic microwave background is blackbody background radiation left over from the Big Bang (the rapid expansion, roughly 13.8 billion years ago, [12] that was the beginning of the universe.

Extragalactic pulses Edit

D. R. Lorimer and others analyzed archival survey data and found a 30-jansky dispersed burst, less than 5 milliseconds in duration, located 3° from the Small Magellanic Cloud. They reported that the burst properties argue against a physical association with our Galaxy or the Small Magellanic Cloud. In a recent paper, they argue that current models for the free electron content in the universe imply that the burst is less than 1 gigaparsec distant. The fact that no further bursts were seen in 90 hours of additional observations implies that it was a singular event such as a supernova or coalescence (fusion) of relativistic objects. [13] It is suggested that hundreds of similar events could occur every day and, if detected, could serve as cosmological probes. Radio pulsar surveys such as [email protected] offer one of the few opportunities to monitor the radio sky for impulsive burst-like events with millisecond durations. [14] Because of the isolated nature of the observed phenomenon, the nature of the source remains speculative. Possibilities include a black hole-neutron star collision, a neutron star-neutron star collision, a black hole-black hole collision, or some phenomenon not yet considered.

In 2010 there was a new report of 16 similar pulses from the Parkes Telescope which were clearly of terrestrial origin, [15] but in 2013 four pulse sources were identified that supported the likelihood of a genuine extragalactic pulsing population. [16]

These pulses are known as fast radio bursts (FRBs). The first observed burst has become known as the Lorimer burst. Blitzars are one proposed explanation for them.

Primordial black holes Edit

According to the Big Bang Model, during the first few moments after the Big Bang, pressure and temperature were extremely great. Under these conditions, simple fluctuations in the density of matter may have resulted in local regions dense enough to create black holes. Although most regions of high density would be quickly dispersed by the expansion of the universe, a primordial black hole would be stable, persisting to the present.

One goal of Astropulse is to detect postulated mini black holes that might be evaporating due to "Hawking radiation". Such mini black holes are postulated [17] to have been created during the Big Bang, unlike currently known black holes. Martin Rees has theorized that a black hole, exploding via Hawking radiation, might produce a signal that's detectable in the radio. The Astropulse project hopes that this evaporation would produce radio waves that Astropulse can detect. The evaporation wouldn't create radio waves directly. Instead, it would create an expanding fireball of high-energy gamma rays and particles. This fireball would interact with the surrounding magnetic field, pushing it out and generating radio waves. [18]

ET Edit

Previous searches by various "search for extraterrestrial intelligence" (SETI) projects, starting with Project Ozma, have looked for extraterrestrial communications in the form of narrow-band signals, analogous to our own radio stations. The Astropulse project argues that since we know nothing about how ET might communicate, this might be a bit closed-minded. Thus, the Astropulse Survey can be viewed [ by whom? ] as complementary to the narrow-band [email protected] survey as a by-product of the search for physical phenomena. [ citation needed ]

Other undiscovered phenomena Edit

Explaining their recent discovery of a powerful bursting radio source, NRL astronomer Dr. Joseph Lazio stated: [19] "Amazingly, even though the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, very little has been done to look for radio bursts, which are often easier for astronomical objects to produce." The use of coherent dedispersion algorithms and the computing power provided by the SETI network may lead to discovery of previously undiscovered phenomena.


Hubble Detects Never-Before-Seen Features Around a Neutron Star

This animation depicts a neutron star (RX J0806.4-4123) with a disk of warm dust that produces an infrared signature as detected by NASA’s Hubble Space Telescope. The disk wasn’t directly photographed, but one way to explain the data is by hypothesizing a disk structure that could be 18 billion miles across. The disk would be made up of material falling back onto the neutron star after the supernova explosion that created the stellar remnant. Credits: NASA, ESA, and N. Tr’Ehnl (Pennsylvania State University)

An unusual infrared light emission from a nearby neutron star detected by NASA’s Hubble Space Telescope could indicate new features never before seen. One possibility is that there is a dusty disk surrounding the neutron star another is that there is an energetic wind coming off the object and slamming into gas in interstellar space the neutron star is plowing through.

Although neutron stars are generally studied in radio and high-energy emissions, such as X-rays, this study demonstrates that new and interesting information about neutron stars can also be gained by studying them in infrared light, say researchers.

The observation, by a team of researchers at Pennsylvania State University, University Park, Pennsylvania Sabanci University, Istanbul, Turkey and the University of Arizona, Tucson, Arizona, could help astronomers better understand the evolution of neutron stars — the incredibly dense remnants after a massive star explodes as a supernova. Neutron stars are also called pulsars because their very fast rotation (typically fractions of a second, in this case 11 seconds) causes time-variable emission from light-emitting regions.

A paper describing the research and two possible explanations for the unusual finding appears Sept. 17, 2018, in the Astrophysical Journal.

This is an illustration of a pulsar wind nebula produced by the interaction of the outflow particles from the neutron star with gaseous material in the interstellar medium that the neutron star is plowing through. Such an infrared-only pulsar wind nebula is unusual because it implies a rather low energy of the particles accelerated by the pulsar’s intense magnetic field. This hypothesized model would explain the unusual infrared signature of the neutron star as detected by NASA’s Hubble Space Telescope. Credits: NASA, ESA, and N. Tr’Ehnl (Pennsylvania State University)

“This particular neutron star belongs to a group of seven nearby X-ray pulsars — nicknamed ‘the Magnificent Seven’ — that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” said Bettina Posselt, associate research professor of astronomy and astrophysics at Pennsylvania State and the lead author of the paper. “We observed an extended area of infrared emissions around this neutron star — named RX J0806.4-4123 — the total size of which translates into about 200 astronomical units (approximately 18 billion miles) at the assumed distance of the pulsar.”

This is the first neutron star in which an extended signal has been seen only in infrared light. The researchers suggest two possibilities that could explain the extended infrared signal seen by Hubble. The first is that there is a disk of material — possibly mostly dust — surrounding the pulsar.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” said Posselt. “Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation. If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

The second possible explanation for the extended infrared emission from this neutron star is a “pulsar wind nebula.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” said Posselt. “A pulsar wind can be produced when particles are accelerated in the electrical field that is produced by the fast rotation of a neutron star with a strong magnetic field. As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then emit synchrotron radiation, causing the extended infrared signal that we see. Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting.”

Using NASA’s upcoming James Webb Space Telescope, astronomers will be able to further explore this newly opened discovery space in the infrared to better understand neutron star evolution.


Last year, two merging neutron stars rocked astronomy. Now it looks like they left behind a black hole.

In August 2017, a huge breakthrough in astronomy was made by a whisper of an event.

130 million light-years away, a binary neutron star system — two super-dense neutron stars orbiting one another — in the galaxy NGC 4993 spiraled in together and merged. This event, dubbed GW 170817 (for the Gravitational Wave event that happened on 17 August 2017) triggered a massive catastrophe: It literally shook the fabric of spacetime, and sent out ripples in reality called gravitational waves (hence the event's name) moving away at the speed of light. By the time they reached us here on Earth they were incredibly faint, but still detectable by LIGO, the Laser Interferometry Gravitational-Wave Observatory.

More Bad Astronomy

This was the first neutron star merger event ever detected, and it was a revolution. It was observed across the electromagnetic spectrum, and with the host galaxy pinpointed a distance to the event was found, allowing for even more interesting characteristics to be determined. Astronomers will be digging into this gift from the heavens for a long time to come.

I wrote all about this merger when it was announced, and you can get all the background info there.

But there's a big question that’s been lingering since it happened: What was left behind?

Artwork depicting the moment of collision between two neutron stars. The resulting explosion is… quite large. Credit: Dana Berry, SkyWorks Digital, Inc.

At the time I assumed it was a black hole, which is what you expect from two neutron stars crashing into each other. However, that turns out not to be clear. Usually, black holes form when a massive star goes supernova. The core collapses, and if it has enough mass — about 2.8 times the mass of the Sun, which is a lot — poof. It forms a black hole.

When two neutron stars merge, if the final product's mass adds up to more than this number, it'll form a black hole as well (the critical mass comes from combing the physics of gravity and the quantum mechanics governing how the subatomic particles making up the object behave).

What's so very interesting is that by analyzing the gravitational waves from this merger, the mass of the resulting object is 2.74 times the mass of the Sun — right on the thin edge of forming a black hole. Mind you, the 2.8 solar mass number is a bit flexible it can depend on other factors, so it's possible to get a black hole with less mass.

So it wasn't clear if whatever was left over from this event was the most massive neutron star ever found, or the lowest mass black hole ever found. Either is pretty cool, but it would be nice to know which.

New research using observations from the Chandra X-Ray Observatory have possibly tipped the balance: The astronomers who did the work think the object is actually a black hole.

Artwork showing a newly-formed black hole with material swirling around it, and jets of energy and matter blasting away from its poles. Credit: NASA/CXC/M.Weiss

Their reasoning is that if a really hefty neutron star had been left over from the merger, it would be spinning very rapidly (that's called conservation of angular momentum, and is similar to an ice skater speeding up their spin as they draw their arms in), and would have a hellaciously strong magnetic field. And I do mean hellacious it would have to be a trillion times stronger than the Earth's magnetic field! As the neutron star spun, the magnetic field would sweep up particles around the star and accelerate them to nearly the speed of light, flinging them away at ridiculously high speeds (like a dog shaking to get water off, but if the dog weighed a couple of octillion tons, spun thousands of times per second, and the water shot away at just under a billion kilometers per hour).

This blast of particles is called a pulsar wind, and as the particles slam into the material around them they'd generate a ferocious glow of X-rays. However, while the Chandra observations do indicate the presence of X-rays, they are far too weak to be from a pulsar wind. That's strong evidence no neutron star was left over.

Also, much of the X-ray emission was delayed by days and weeks after the event, and that's likely coming from pre-existing material many billions of kilometers or more away. The shock wave generated from the merger would have slammed into that material, heated the matter up, and as a result it would glow in X-rays on its own. The delay was caused by the time it took the shock wave to reach this material, and for it to react.

The flip side of this is that if the merger created a black hole, you wouldn't see strong X-rays because black holes don't work like neutron stars. They don't generate a pulsar wind, so the X-ray brightness would be far fainter. And since the X-rays are pretty faint, a black hole is the likely result.

However, this isn't yet confirmed. It's possible that, if a neutron stars resulted and if a pulsar wind is being made, it’s just taking longer to flow out and strike the material surrounding the event than first thought. If that’s the case, then over the next year or two the spot of the event will brighten in X-rays, and that should be detectable by Chandra. If that doesn't happen, then it's safe to assume a lightweight black hole was born on that day in August 2017.

And that's pretty awesome. But I also have to say, if the brightening occurs, that would be very interesting as well. This would be a big challenge to how we think neutron stars behave, so theoreticians would have to go back to their equations and figure this out.

Either way, it's a win for astronomy. We learned something really amazing. And that's on top of all the phenomenal things we've already learned from this singular event.

… but it won't be singular for long. LIGO is still watching the skies, waiting for the now-familiar tremble of spacetime resulting from these cosmic collisions. Neutron star binary mergers are rare in any given galaxy, but there are a lot of galaxies out there. We'll be feeling the next one soon enough. What will we learn from that one?


Burst of radio waves in Milky Way probably came from neutron star

For more than a decade, astronomers have puzzled over the origins of mysterious and fleeting bursts of radio waves that arrive from faraway galaxies.

Now, scientists have discovered the first such blast in the Milky Way and traced it back to its probable source: a small, spinning remnant from a collapsed star about 30,000 light years from Earth.

The surprise detection has handed researchers their strongest evidence yet that some if not all fast radio bursts, or FRBs, are unleashed by compact, highly magnetised neutron stars called magnetars – exotic objects born in the embers of supernovae.

“This is the most luminous radio burst ever detected in our galaxy,” said Daniele Michilli, an astrophysicist at McGill University in Montreal who works on the Canadian Hydrogen Intensity Mapping Experiment, or Chime telescope.

The first FRB sighting came in 2007 when Duncan Lorimer and his student David Narkevic worked through archived observations from the Parkes radio dish in Australia. The intense burst of radio waves lasted less than five milliseconds, and what had produced it was a mystery. Scientists have recorded dozens more since, all from beyond our own galaxy.

The latest discovery came on 28 April when the Chime telescope detected a millisecond-long FRB coming from a region of the sky where a magnetar called SGR1935+2154 lurks. A second, less sophisticated telescope – made from metal poles and cake tins – known as the Survey for Transient Astronomical Radio Emission 2, or Stare2, swiftly confirmed the sighting, along with an outburst of x-rays from the same source.

Chris Bochenek, an astrophysicist at Caltech who helped to build Stare2, said he and his colleagues had given the project a 10% chance of detecting an FRB in the Milky Way. “When I looked at the data for first time, I froze and was basically paralysed with excitement,” he said. “The fact that we detected such a burst in the Milky Way at all is surprising.”

Analysis of the signal, named FRB 200428, found that the magnetar emitted as much energy in radiowaves in one millisecond as the sun does in half a minute. Details of the discovery are published in three independent studies in Nature.

While the discovery does not mean that all fast radio bursts come from magnetars, it pinpoints the objects as one source that astronomers will now observe more closely.

A major question that remains is how magnetars unleash such intense blasts of radiation. One idea is that magnetars are distorted by “starquakes” that tear open their surfaces and release vast blasts of energy. Another is that powerful flares from magnetars collide with particles in space, producing intense shockwaves and magnetic fields that whip electrons around, releasing bursts of radio waves in the process.

Duncan Lorimer, an astrophysicist at West Virginia University, who was not involved in the latest work, called the finding “an incredibly important development” that showed magnetars were “really credible sources of FRBs”.

“Back in the day, we thought about magnetars, but I think I was more prone to a one-off source like a neutron star merger,” he said.


Have we Observed Continuum Emission from Neutron Stars? - Astronomy

Fast radio bursts (FRBs) at cosmological distances have recently been discovered, whose duration is about milliseconds. We argue that the observed short duration is difficult to explain by giant flares of soft gamma-ray repeaters, though their event rate and energetics are consistent with FRBs. Here, we discuss binary neutron star (NS-NS) mergers as a possible origin of FRBs. The FRB rate is within the plausible range of the NS-NS merger rate and its cosmological evolution, while a large fraction of the NS-NS mergers must produce observable FRBs. A likely radiation mechanism is coherent radio emission, like radio pulsars, by magnetic braking when magnetic fields of neutron stars are synchronized to binary rotation at the time of coalescence. Magnetic fields of the standard strength (∼ 10 12-13 G) can explain the observed FRB fluxes, if the conversion efficiency from magnetic braking energy loss to radio emission is similar to that of isolated radio pulsars. Corresponding gamma-ray emission is difficult to detect by current or past gamma-ray burst satellites. Since FRBs tell us the exact time of mergers, a correlated search would significantly improve the effective sensitivity of gravitational wave detectors.


Hubble Detects Unusual Infrared Emission from Nearby Neutron Star

An unusual infrared emission from the neutron star RX J0806.4-4123 detected by the NASA/ESA Hubble Space Telescope could indicate new features never before seen: one possibility is that there is a disk of material surrounding the neutron star another is that there is an energetic wind coming off RX J0806.4-4123 and slamming into gas in interstellar space the neutron star is plowing through.

This is an illustration of a pulsar wind nebula produced by the interaction of the outflow particles from the neutron star with gaseous material in the interstellar medium that the neutron star is plowing through. Such an infrared-only pulsar wind nebula is unusual because it implies a rather low energy of the particles accelerated by the pulsar’s intense magnetic field. This hypothesized model would explain the unusual infrared signature of RX J0806.4-4123. Image credit: NASA / ESA / Nahks Tr’Ehnl, Pennsylvania State University.

“RX J0806.4-4123 belongs to a group of seven nearby X-ray pulsars (the Magnificent Seven) that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” said Dr. Bettina Posselt, a researcher in the Department of Astronomy & Astrophysics at Pennsylvania State University.

“We observed an extended area of infrared emissions around RX J0806.4-4123, the total size of which translates into about 200 AU (astronomical units) at the assumed distance of the pulsar.”

This is the first neutron star in which an extended emission has been seen only in the infrared.

Dr. Posselt and colleagues suggest two possibilities that could explain this extended infrared emission.

The first is that there is a disk of material (possibly mostly dust) surrounding RX J0806.4-4123. The second is a so-called ‘pulsar wind nebula.’

This animation depicts RX J0806.4-4123 with a disk of warm dust that produces an infrared signature as detected by the NASA/ESA Hubble Space Telescope. The disk wasn’t directly photographed, but one way to explain the data is by hypothesizing a disk structure that could be 18 billion miles across. The disk would be made up of material falling back onto the neutron star after the supernova explosion that created the stellar remnant. Image credit: NASA / ESA / Nahks Tr’Ehnl, Pennsylvania State University.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” Dr. Posselt said.

“Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation.”

“If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” she added.

“A pulsar wind can be produced when particles are accelerated in the electric field that is produced by the fast rotation of a neutron star with a strong magnetic field. As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then radiate synchrotron emission, causing the extended infrared emission that we see.”

“Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting.”

The findings appear in the Astrophysical Journal.

B. Posselt et al. 2018. Discovery of Extended Infrared Emission around the Neutron Star RXJ0806.4-4123. ApJ 865, 1 doi: 10.3847/1538-4357/aad6df


Odd infrared emission makes this neutron star special

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An unusual infrared emission that the Hubble Space Telescope detected from a nearby neutron star could indicate that the pulsar has features never seen before, researchers report.

The observation could help astronomers better understand the evolution of neutron stars—the incredibly dense remnants of massive stars after a supernova. A paper describing the research and two possible explanations for the unusual finding appears in the Astrophysical Journal.

“This particular neutron star belongs to a group of seven nearby X-ray pulsars—nicknamed ‘the Magnificent Seven’…”

“This particular neutron star belongs to a group of seven nearby X-ray pulsars—nicknamed ‘the Magnificent Seven’—that are hotter than they ought to be considering their ages and available energy reservoir provided by the loss of rotation energy,” says lead author Bettina Posselt, associate research professor of astronomy and astrophysics at Penn State.

“We observed an extended area of infrared emissions around this neutron star—named RX J0806.4-4123—the total size of which translates into about 200 astronomical units (or 2.5 times the orbit of Pluto around the Sun) at the assumed distance of the pulsar,” Posselt says.

This is the first neutron star in which researchers have seen an extended emission only in the infrared. The researchers suggest two possibilities that could explain the extended infrared emission the Hubble Space Telescope saw. The first is that there is a disk of material—possibly mostly dust—surrounding the pulsar.

“One theory is that there could be what is known as a ‘fallback disk’ of material that coalesced around the neutron star after the supernova,” says Posselt. “Such a disk would be composed of matter from the progenitor massive star. Its subsequent interaction with the neutron star could have heated the pulsar and slowed its rotation. If confirmed as a supernova fallback disk, this result could change our general understanding of neutron star evolution.”

Illustrated GIF showing a neutron star with a circum-pulsar disk. If seen at the proper angle the scattered emission from the inner part of the disk could produce the extended infrared emission astronomers observed around the neutron star RX J0806.4-4123. (Credit: Nahks Tr’Ehnl, Penn State)

The second possible explanation for the extended infrared emission from this neutron star is a “pulsar wind nebula.”

“A pulsar wind nebula would require that the neutron star exhibits a pulsar wind,” says Posselt. “A pulsar wind can be produced when particles are accelerated in the electric field that is produced by the fast rotation of a neutron star with a strong magnetic field.

“As the neutron star travels through the interstellar medium at greater than the speed of sound, a shock can form where the interstellar medium and the pulsar wind interact. The shocked particles would then radiate synchrotron emission, causing the extended infrared emission that we see. Typically, pulsar wind nebulae are seen in X-rays and an infrared-only pulsar wind nebula would be very unusual and exciting,” Posselt explains.

Illustrated GIF showing a neutron star with a pulsar wind nebula produced by the interaction of the pulsar wind and the oncoming interstellar medium. A pulsar wind nebula could explain the extended infrared emission astronomers observed. Such an infrared-only pulsar wind nebula is unusual because it implies a rather low energy of the accelerated particles.(Credit: Nahks Tr’Ehnl, Penn State)

Although researchers generally study neutron stars in radio and high-energy emissions, such as X-rays, this study demonstrates that they can gain new and interesting information about neutron stars by studying them in the infrared. Using the new NASA James Webb Space Telescope, due to launch in 2021, astronomers will be able to further explore this newly opened discovery space in the infrared to better understand neutron star evolution.


An Update on Fast Radio Bursts: New Discovery in Our Own Galaxy

Have we recently spotted the first equivalent of a fast radio burst (FRB) — a mysterious and brief extragalactic flash of radio emission — in our own galaxy? Some astronomers think so, and argue that the new discovery solidifies the connection between these exotic radio bursts and powerfully magnetized neutron stars.

Origin of a Burst

Artist’s impression of telescopes observing an extragalactic fast radio burst. [CSIRO/Andrew Howells]

There’s plenty of evidence pointing to magnetars as the source of FRBs, from polarization measurements that suggest FRB sources are strongly magnetized, to localizations of several FRBs to star-forming regions typical of magnetar environments. And some magnetars, known as soft gamma-ray repeaters (SGRs), emit repeated high-energy flares and bursts across their lifetime — another sign of volatility that could tie into the FRB picture.

But there’s a major challenge to the magnetar model for FRBs: we’ve never observed radio emission remotely similar to an FRB coming from a magnetar in our own galaxy.

A Missing Link Found?

In a new study led by Sandro Mereghetti (INAF, Italy), scientists have reported the detection of a series of bright X-ray bursts from the magnetar SGR 1935+2154 using the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) spacecraft.

The X-ray light curve from INTEGRAL (black line) is shown here with the positions of the associated radio pulses (orange line) marked for comparison. The inset box shows the brightest part of the burst. Click to enlarge. [Mereghetti et al. 2020]

The radio burst exhibits similar structure to the associated X-ray burst from the magnetar, occurred at roughly the same time, and is within a factor of 10 of the energy of some extragalactic FRBs. These clues strongly suggest that SGR 1935+2154’s outburst may be the missing link that connects magnetars with FRBs.

Pinpointing Production

Can we use these new observations of SGR 1935+2154 to narrow down models of FRB production?

The existence of a second, rare population of magnetars with stronger magnetic fields and higher activity levels could explain a number of properties of the FRBs we’ve observed. As an example, the observed rates of repeating and non-repeating FRBs can be reproduced with the authors’ two-population model. [Adapted from Margalit et al. 2020]

Margalit and collaborators also use the radio and X-ray observations from SGR 1935+2154 to evaluate specific magnetar emission mechanisms, providing constraints on models of how these energetic flashes are produced.

SGR 1935+2154 may have more X-ray and radio activity in store for us in the future, so you can bet we’ll be keeping an eye on it. With any luck, upcoming observations will help us to further address the mystery of FRBs — right here in our own galaxy.

Citation

“INTEGRAL Discovery of a Burst with Associated Radio Emission from the Magnetar SGR 1935+2154,” S. Mereghetti et al 2020 ApJL 898 L29. doi:10.3847/2041-8213/aba2cf

“Implications of a Fast Radio Burst from a Galactic Magnetar,” Ben Margalit et al 2020 ApJL 899 L27. doi:10.3847/2041-8213/abac57


Excess x-rays from neutron stars could lead to discovery of new particle

The XMM-Newton space telescope, shown above in an artistic rendering, detected unusually high levels of X-ray emission from nearby neutron stars. A team of scientists, including a University of Minnesota researcher, found that the excess of X-rays may be the first evidence of axions, hypothetical particles that could help physicists unravel several mysteries of the universe. Image courtesy of D. Ducros and the European Space Agency.

A team of scientists, including a University of Minnesota researcher, have found that mysterious x-rays detected from nearby neutron stars may be the first evidence of axions, hypothetical particles that many physicists believe make up dark matter. If their theory is confirmed, the researchers’ findings could help physicists unravel several mysteries of the universe.

Their paper is published in Physical Review Letters, a peer-reviewed academic journal published by the American Physical Society.

There are many kinds of particles that make up matter in the universe. The most common are protons, neutrons, and electrons. These particles collide with each other in certain settings, such as inside a star’s core or in particle accelerators built by scientists on Earth. Axions have long been elusive to physicists because they are “weakly interacting,” which means they rarely collide with other particles and instead often pass through them.

“Finding axions has been one of the major efforts in high-energy particle physics, both in theory and in experiments,” said Raymond Co, an author on the paper and postdoctoral researcher in the University of Minnesota’s School of Physics and Astronomy. “We think axions could exist, but we haven’t discovered them yet. You can think of axions as ghost particles. They can be anywhere in the universe, but they don’t interact strongly with us so we don’t have any observations of them yet.”

Theoretically, axions can be created by other particles colliding or exist naturally as dark matter, which physicists believe makes up a large percentage of the universe that we cannot directly see. The discovery of axions would answer many questions about dark matter and other particle physics mysteries. Axions are also predicted by string theory, or the idea that all the forces and particles in the universe are tied together as part of the same framework.

In 2019, Co’s former colleagues at the University of Michigan observed a mysterious, inexplicable increase in x-rays emitted from several neutron stars, which are extremely dense stars made up mostly of neutrons. In their recent paper, Co worked with his former colleagues to propose that these extra x-rays are caused by axions being produced in the neutron stars’ cores.

The researchers used a previously proposed theory about axions to explain this phenomenon. The theory states that axions are produced in the core of a neutron star as byproducts of colliding neutrons and protons. The particles then shoot out into the star’s strong magnetic field, where they are converted into photons—particles of light—which make up the x-rays detected by telescopes on Earth. Since axions carry much more energy than the photons these neutron stars typically emit, the photons produced from the axions would yield more energy as well, explaining the unexpected increase in x-rays.

“We’re not claiming that we’ve made the discovery of the axion yet, but we’re saying that the extra x-ray photons can be explained by axions,” Co said. “It is an exciting discovery of the excess in the x-ray photons, and it’s an exciting possibility that’s already consistent with our interpretation of axions.”

The current telescope data is not sufficient enough to prove that the x-rays come from axions, but the researchers hope that more data from other telescopes may provide further insight in the future. Other scientists in the particle physics community will also be able to branch off this research in their searches for axions.

Other members of the research team include Malte Buschmann, a postdoctoral researcher at Princeton University Christopher Dessert, a graduate student at the University of Michigan and Benjamin Safdi, a researcher at Lawrence Berkeley National Lab.

This research was funded by grants from the Department of Energy’s Office of Science and supported by Advanced Research Computing at the University of Michigan the National Science Foundation the Mainz Institute for Theoretical Physics (MITP) of the Cluster of Excellence PRISMA+ the Munich Institute for Astro- and Particle Physics (MIAPP) of the DFG Excellence Cluster Origins and the CERN Theory department.

Read the research paper entitled, “Axion Emission Can Explain a New Hard X-ray Excess from Nearby Isolated Neutron Stars” on the Physical Review Letters website.