In the coming years, thousands of satellites, several next-generation space telescopes and even a few space habitats are expected to be launched into orbit. Beyond Earth, multiple missions are planned to be sent to the lunar surface, to Mars, and beyond. As humanity’s presence in space increases, the volume of data that is regularly being back sent to Earth is reaching the limits of what radio communications can handle.
For this reason, NASA and other space agencies are looking for new methods for sending information back and forth across space. Already, optical communications (which rely on lasers to encode and transmit information) are being developed, but other more radical concepts are also being investigating. These include X-ray communications, which NASA is gearing up to test in space using their XCOM technology demonstrator.
It’s relatively easy for galaxies to make stars. Start out with a bunch of random blobs of gas and dust. Typically those blobs will be pretty warm. To turn them into stars, you have to cool them off. By dumping all their heat in the form of radiation, they can compress. Dump more heat, compress more. Repeat for a million years or so.
Eventually pieces of the gas cloud shrink and shrink, compressing themselves into a tight little knots. If the densities inside those knots get high enough, they trigger nuclear fusion and voila: stars are born.
A team of researchers in the UK have observed matter falling into a black hole at 30% the speed of light. This is much faster than anything previously observed. The high velocity is a result of misaligned discs of material rotating around the black hole.
It’s not exactly an organ donor, but a star in the direction of the hyper-populated core of the Milky Way donating some of its mass to a dormant neighbor. The result? The dormant neighbor sprung back to life with an X-ray burst captured by the ESA‘s INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) space observatory.
“INTEGRAL caught a unique moment in the birth of a rare binary system” – Enrico Bozzo, University of Geneva.
The neighbors have likely been paired together for billions of years, which is not in itself noteworthy: stars often live in binary pairs. But the pair spotted by INTEGRAL on August 13th 2017 is very unusual. The donor star is a red giant, and the recipient is a neutron star. So far, astronomers only know of 10 of these pairs, called ‘symbiotic X-ray binaries’.
To understand what’s happening between these neighbors, we have to look at stellar evolution.
The donor star is in its red giant phase. That’s when a star in the same mass range as our star reaches the later stage of its life. As its mass is depleted, gravity can’t hold the star together in the same way it has for the early part of its life. The star expands outwards by millions of kilometers. As it does so, it sheds stellar material from its outer layers in a solar wind that travels several hundreds of km/sec.
Its neighbor is in a different state. It’s a star that had an initial mass of about 25 to 30 times the Sun. When a star this big approaches the end of its life, it suffers a different fate. Stars this large live fast, and burn through their fuel quickly. Then, they explode as supernovae, in this case leaving a corpse behind. In the binary system captured by INTEGRAL, the corpse is a spinning neutron star with a magnetic field.
Neutron stars are dense. In fact, they’re some of the densest stellar objects we know of, packing as much mass as one-and-a-half of our Suns into an object that’s only about 10 km across.
When the red giant’s stellar wind met the neutron star, the neutron star slowed its rate of spin, and burst into life, emitting high-energy x-rays.
“INTEGRAL caught a unique moment in the birth of a rare binary system,” says Enrico Bozzo from University of Geneva and lead author of the paper that describes the discovery. “The red giant released a sufficiently dense slow wind to feed its neutron star companion, giving rise to high-energy emission from the dead stellar core for the first time.”
After INTEGRAL spotted the x-ray burst from the binary, other observations quickly followed. The ESA’s XMM Newton and NASA’s NuSTAR and Swift space telescopes got to work, along with ground-based telescopes. These observations confirmed what initial observations showed: this is a very peculiar pair of stars.
“…we believe we saw the X-rays turning on for the first time.” – Erik Kuulkers, ESA INTEGRAL Project Scientist.
The neutron star spins very slowly, taking about 2 hours to revolve, which is remarkable since other neutron stars can spin many times per second. The magnetic field of the neutron star was also much stronger than expected. But the magnetic field around a neutron star is thought to weaken over time, making this a relatively young neutron star. And a red giant is old, so this is a very odd pairing of old red giant with young neutron star.
One possible explanation is that the neutron star didn’t form from a supernova, but from a white dwarf. In that scenario, the white dwarf would’ve collapsed into a neutron star after a very long period of feeding on material from the red giant. That would explain the disparity in ages of the two stars in the system.
“These objects are puzzling,” says Enrico. “It might be that either the neutron star magnetic field does not decay substantially with time after all, or the neutron star actually formed later in the history of the binary system. That would mean it collapsed from a white dwarf into a neutron star as a result of feeding off the red giant over a long time, rather than becoming a neutron star as a result of a more traditional supernova explosion of a short-lived massive star.”
The next question is how long will this process go on? Is it short-lived, or the beginning of a long-term relationship?
“We haven’t seen this object before in the past 15 years of our observations with INTEGRAL, so we believe we saw the X-rays turning on for the first time,” says Erik Kuulkers, ESA’s INTEGRAL project scientist. “We’ll continue to watch how it behaves in case it is just a long ‘burp’ of winds, but so far we haven’t seen any significant changes.”
The INTEGRAL space observatory was launched in 2002 to study some of the most energetic phenomena in the universe. It focuses on things like black holes, neutron stars, active galactic nuclei and supernovae. INTEGRAL is a European Space Agency mission in cooperation with the United States and Russia. Its projected end date is December, 2018.
In addition to being the largest and most massive planet in our Solar system, Jupiter is also one of its more mysterious bodies. This is certainly apparent when it comes to Jupiter’s powerful auroras, which are similar in some ways to those on Earth. In recent years, astronomers have sought to study patterns in Jupiter’s atmosphere and magnetosphere to explain how aurora activity on this planet works..
For instance, an international team led by researchers from University College London recently combined data from the Juno probe with X-ray observations to discern something interesting about Jupiter’s northern and southern auroras. According to their study, which was published in the current issue of the scientific journal Nature – Jupiter’s intense, Jupiter’s X-ray auroras have been found to pulsate independently of each other.
As already noted, Jupiter’s auroras are somewhat similar to Earth’s, in that they are also the result of charged particles from the Sun (aka. “solar wind”) interacting with Jupiter’s magnetic field. Because of the way Jupiter and Earth’s magnetic fields are structured, these particles are channeled to the northern and southern polar regions, where they become ionized in the atmosphere. This results in a beautiful light display that can be seen from space.
In the past, auroras have been spotted around Jupiter’s poles by NASA’s Chandra X-ray Observatory and by the Hubble Space Telescope. Investigating this phenomena and the mechanisms behind it has also been one of the goals of the Juno mission, which is currently in an ideal position to study Jupiter’s poles. With every orbit the probe makes, it passes from one of Jupiter’s poles to the other – a maneuver known as a perijove.
For the sake of their study, Dr. Dunn and his team were forced to consult data from the ESA’s XMM-Newton and NASA’s Chandra X-ray observatories. This is due to the fact that while it has already acquired magnificent images and data on Jupiter’s atmosphere, the Juno probe does not have an X-ray instrument aboard. Once they examined the X-ray data, Dr. Dunn and his team noticed a difference between Jupiter’s northern and southern auroras.
Whereas the X-ray emissions at the north pole were erratic, increasing and decreasing in brightness, the ones at the south pole consistently pulsed once every 11 minutes. Basically, the auroras happened independently of each other, which is different from how auroras on Earth behave – i.e. mirroring each other in terms of their activity. As Dr. Dunn explained in a recent UCL press release:
“We didn’t expect to see Jupiter’s X-ray hot spots pulsing independently as we thought their activity would be coordinated through the planet’s magnetic field. We need to study this further to develop ideas for how Jupiter produces its X-ray aurora and NASA’s Juno mission is really important for this.”
The X-ray observations were conducted between May and June of 2016 and March of 2017. Using these, the team produced maps of Jupiter’s X-ray emissions and identified hot spots at each pole. The hot spots cover an area that is larger than the surface area of Earth. By studying them, Dr. Dunn and his colleagues were able to identify patterns of behavior which indicated that they behaved differently from each other.
Naturally, the team was left wondering what could account for this. One possibility they suggest is that Jupiter’s magnetic field lines vibrate, producing waves that carry charged particles towards the poles. The speed and direction of these particles could be subject to change over time, causing them to eventually collide with Jupiter’s atmosphere and generate X-ray pulses.
As Dr Licia Ray, a physicist from Lancaster University and a co-author on the paper, explained:
“The behavior of Jupiter’s X-ray hot spots raises important questions about what processes produce these auroras. We know that a combination of solar wind ions and ions of Oxygen and Sulfur, originally from volcanic explosions from Jupiter’s moon, Io, are involved. However, their relative importance in producing the X-ray emissions is unclear.”
And as Graziella Branduardi-Raymont- a professor from UCL’s Space & Climate Physics department and another co-author on the study – indicated, this research owes its existence to multiple missions. However, it was the perfectly-timed nature of the Juno mission, which has been in operation around Jupiter since July 5th, 2016, that made this study possible.
“What I find particularly captivating in these observations, especially at the time when Juno is making measurements in situ, is the fact that we are able to see both of Jupiter’s poles at once, a rare opportunity that last occurred ten years ago,” he said. “Comparing the behaviours at the two poles allows us to learn much more of the complex magnetic interactions going on in the planet’s environment.”
Looking ahead, Dr. Dunn and his team hope to combine X-ray data from XMM-Newton and Chandra with data collected by Juno in order to gain a better understanding of how X-ray auroras are produced. The team also hopes to keep tracking the activity of Jupiter’s poles for the next two years using X-ray data in conjunction with Juno. In the end, they hope to see if these auroras are commonplace or an unusual event.
“If we can start to connect the X-ray signatures with the physical processes that produce them, then we can use those signatures to understand other bodies across the Universe such as brown dwarfs, exoplanets or maybe even neutron stars,” said Dr. Dunn. “It is a very powerful and important step towards understanding X-rays throughout the Universe and one that we only have while Juno is conducting measurements simultaneously with Chandra and XMM-Newton.”
In the coming decade, the ESA’s proposed JUpiter ICy moons Explorer (JUICE) probe is also expected to provide valuable information on Jupiter’s atmosphere and magnetosphere. Once it arrives in the Jovian system in 2029, it too will observe the planet’s auroras, mainly so that it can study the effect these have on the Galilean Moons (Io, Europa, Ganymede and Callisto).
Astronomers have long understood that there is a link between a star’s magnetic activity and the amount of X-rays it emits. When stars are young, they are magnetically active, due to the fact that they undergo rapid rotation. But over time, the stars lose rotational energy and their magnetic fields weaken. Concurrently, their associated X-ray emissions also begin to drop.
Interestingly, this relationship between a star’s magnetic activity and X-ray emissions could be a means for finding potentially-habitable star systems. Hence why an international team led by researchers from Queen’s University Belfast conducted a study where they cataloged the X-ray activity of 24 Sun-like stars. In so doing, they were able to determine just how hospitable these star systems could be to life.
To understand how stellar magnetic activity (and hence, X-ray activity) changes over time, astronomers require accurate age assessments for many different stars. This has been difficult in the past, but thanks to mission like NASA’s Kepler Space Observatory and the ESA’s Convection, Rotation and planetary Transits (CoRoT) mission, new and precise age estimates have become available in recent years.
Using these age estimates, Booth and her colleagues relied on data from the Chandra X-ray observatory and the XMM-Newton obervatory to examine 24 nearby stars. These stars were all similar in mass to our Sun (a main sequence G-type yellow dwarf star) and at least 1 billion years of age. From this, they determined that there was a clear link between the star’s age and their X-ray emissions. As they state in their study:
“We find 14 stars with detectable X-ray luminosities and use these to calibrate the age-activity relationship. We find a relationship between stellar X-ray luminosity, normalized by stellar surface area, and age that is steeper than the relationships found for younger stars…”
In short, of the 24 stars in their sample, the team found that 14 had X-ray emissions that were discernible. From these, they were able to calculate the star’s ages and determine that there was a relationship between their longevity and luminosity. Ultimately, this demonstrated that stars like our Sun are likely to emit less high-energy radiation as they exceed 1 billion years in age.
And while the reason for this is not entirely clear, astronomers are currently exploring various possible causes. One possibility is that for older stars, the reduction in spin rate happens more quickly than it does for younger stars. Another possibility is that the X-ray brightness declines more quickly for older, more slowly-rotating stars than it does for younger, faster ones.
Regardless of the cause, the relationship between a star’s age and its X-ray emissions could provide astronomers and exoplanet hunters with another tool for gauging the possible habitability of a system. Wherever a G-type or K-type star is to be found, knowing the age of the star could help place constraints on the potential habitability of any planets that orbit it.
When massive stars reach the end of their life cycle, they explode in a massive supernova and cast off most of their material. What’s left is a “milliscond pulsar”, a super dense, highly-magnetized neutron star that spins rapidly and emit beams of electromagnetic radiation. Eventually, these stars lose their rotational energy and begin to slow down, but they can speed up again with the help of a companion.
According to a recent study, an international team of scientists witnessed this rare event when observing an ultra-slow pulsar located in the neighboring Andromeda Galaxy (XB091D). The results of their study indicated that this pulsar has been speeding up for the past one million years, which is likely the result of a captured a companion that has since been restoring its rapid rotational velocity.
Typically, when a pulsars pairs with an ordinary star, the result is a binary system consisting of a pulsar and a white dwarf. This occurs after the pulsar pulls off the outer layers of a star, turning it into a white dwarf. The material from these outer layer then forms an accretion disk around the pulsar, which creates a “hot spot” that radiates brightly in the X-ray specturum and where temperatures can reach into the millions of degrees.
As they state in their paper, the detection of this pulsar was made possible thanks to data collected by the XMM-Newton space observatory from 2000-2013. In this time, XMM-Newton has gathered information on approximately 50 billion X-ray photons, which has been combined by astronomers at Lomosov MSU into an open online database.
This database has allowed astronomers to take a closer look at many previously-discovered objects. This includes XB091D, a pulsar with a period of seconds (aka. a “second pulsar”) located in one of the oldest globular star clusters in the Andromeda galaxy. However, finding the X-ray photos that would allow them to characterize XB091D was no easy task. As Ivan Zolotukhin explained in a MSU press release:
“The detectors on XMM-Newton detect only one photon from this pulsar every five seconds. Therefore, the search for pulsars among the extensive XMM-Newton data can be compared to the search for a needle in a haystack. In fact, for this discovery we had to create completely new mathematical tools that allowed us to search and extract the periodic signal. Theoretically, there are many applications for this method, including those outside astronomy.”
Based on a total of 38 XMM-Newton observations, the team concluded that this pulsar (which was the only known pulsar of its kind beyond our galaxy at the time), is in the earliest stages of “rejuvenation”. In short, their observations indicated that the pulsar began accelerating less than 1 million years ago. This conclusion was based on the fact that XB091D is the slowest rotating globular cluster pulsar discovered to date.
The neutron star completes one revolution in 1.2 seconds, which is more than 10 times slower than the previous record holder. From the data they observed, they were also able to characterize the environment around XB091D. For example, they found that the pulsar and its binary pair are located in an extremely dense globular cluster (B091D) in the Andromeda Galaxy – about 2.5 million light years away.
This cluster is estimated to be 12 billion years old and contains millions of old, faint stars. It’s companion, meanwhile, is a 0.8 solar mass star, and the binary system itself has a rotation period of 30.5 hours. And in about 50,000 years, they estimate, the pulsar will accelerate sufficiently to once again have a rotational period measured in the milliseconds – i.e. a millisecond pulsar.
Interestingly, XB910D’s location within this vast region of super-high density stars is what allowed it to capture a companion about 1 million years ago and commence the process “rejuvenation” in the first place. As Zolotukhin explained:
“In our galaxy, no such slow X-ray pulsars are observed in 150 known globular clusters, because their cores are not big and dense enough to form close binary stars at a sufficiently high rate. This indicates that the B091D cluster core, with an extremely dense composition of stars in the XB091D, is much larger than that of the usual cluster. So we are dealing with a large and rather rare object—with a dense remnant of a small galaxy that the Andromeda galaxy once devoured. The density of the stars here, in a region that is about 2.5 light years across, is about 10 million times higher than in the vicinity of the Sun.”
Thanks to this study, and the mathematical tools the team developed to find it, astronomers will likely be able to revisit many previously-discovered objects in the coming years. Within these massive data sets, there could be many examples of rare astronomical events, just waiting to be witnessed and properly characterized.
For over sixty years, astronomers have been exploring the Universe for x-ray sources. Known to be associated with stars, clouds of super heated gas, interstellar mediums, and destructive events, the detection of cosmic x-rays is challenging work. In recent decades, astronomers have been benefited immensely from by the deployment of orbital telescopes like the Chandra X-ray Observatory.
Since it was launched on July 23rd, 1999, Chandra has been NASA’s flagship mission for X-ray astronomy. And this past week (on Thurs. March 30th, 2017), the Observatory accomplished something very impressive. Using its suite of advanced instruments, the observatory captured a mysterious flash coming from deep space. Not only was this the deepest X-ray source ever observed, it also revealed what could be an entirely new phenomenon.
Located in the region of the sky known as the Chandra Deep Field-South (CDF-S), this X-ray emission source appeared to have come from a small galaxy located approximately 10.7 billion light-years from Earth. It also had some remarkable properties, producing more energy in the space of a few minutes that all the stars in the galaxy combined.
Originally detected in 2014 by a team of researchers from Penn State University and the Pontifical Catholic University of Chile in Santiago, Chile, this source was not even detected in the X-ray band at first. However, it quickly caught the team’s attention as it erupted and became 1000 brighter in the space of a few hours. At this point, the researchers began gathering data using Chandra’s Advanced CCD Imaging Spectronomer.
A day after the flare-up, the X-ray source had faded to the point that Chandra was no longer able to detect it. As Niel Brandt – the Verne M. Willaman Professor of Astronomy and Astrophysics at Penn State and part of the team that first observed it – described the discovery in a Penn State press release:
“This flaring source was a wonderful surprise bonus that we accidentally discovered in our efforts to explore the poorly understood realm of the ultra-faint X-ray universe. We definitely ‘lucked out’ with this find and now have an exciting new transient phenomenon to explore in future years.”
Thousands of hours of legacy data from the Hubble and Spitzer Space Telescopes was then consulted in order to determine the location of the CDF-S X-ray source. And though scientists were able to determine that the image of the X-ray source placed it beyond any that had been observed before, they are not entirely clear as to what could have caused it.
On the one hand, it could be the result of some sort of destructive event, or something scientists have never before seen. The reason for this has to do with the fact that X-ray bursts also come with a gamma-ray burst (GRB), which appears to be missing here. Essentially, GRBs are jetted explosions that are triggered by the collapse of a massive star or by the merger of two neutron stars (or a neutron star with a black hole).
Because of this, three possible explanations have been suggested. In the first, the CDF-S X-ray source is indeed the result of a collapsing star or merger, but the resulting jets are not pointed towards Earth. In the second, the same scenario is responsible for the x-ray source, but the GRB lies beyond the small galaxy. The third possible explanation is that the event was caused by a medium-sized black hole shredding a white dwarf star.
Unfortunately, none of these explanations seem to fit the data. However, these research team also noted that these possibilities are not that well understood, since none have been witnessed in the Universe. As Franz Bauer – an astronomer from the Pontifical Catholic University of Chile – said: “Ever since discovering this source, we’ve been struggling to understand its origin. It’s like we have a jigsaw puzzle but we don’t have all of the pieces.”
Not only has Chandra not observed any other X-ray sources like this one during the 17 years it has surveyed the CDF-S region, but no similar events have been observed by the space telescope anywhere in the Universe during its nearly two decades of operation. On top of that, this event was brighter, more short-lived, and occurred in a smaller, younger host galaxy than other unexplained X-ray sources.
From all of this, the only takeaway appears to be that the event was likely the result of a cataclysmic event, like a neutron star or a white dwarf being torn apart. But the fact that none of the more plausible explanations seem to account for it’s peculiar characteristics would seem to suggest that astronomers may have witnessed an entirely new kind of cataclysmic event.
And of course, future surveys conducted using Chandra and next-generation X-ray telescopes will also be on the lookout for these kind of short-lived, high-energy X-ray bursts. It’s always good when the Universe throws us a curve ball. Not only does it show us that we have more to learn, but it also teaches us that we must never grow complacent in our theories.
Be sure to check out this animation of the CDF-S X-ray source too, courtesy of the Chandra X-ray Observatory:
Does a distant black hole provide a new definition of pain and suffering?
The black hole, named XJ1500+0154, appears to be the real-life equivalent of the Pit of Carkoon, the nesting place of the all-powerful Sarlacc in Star Wars, which slowly digested its victims.
Over ten years ago, this giant black hole ripped apart a star and has since continued a very long lunch, feasting on the stars’ remains. Astronomers have been carefully monitoring this slow ‘digestion,’ because it is so unusual for what are called tidal disruption events (TDEs), where tidal forces from black holes tear stars apart.
“We have witnessed a star’s spectacular and prolonged demise,” said Dacheng Lin from the University of New Hampshire in Durham, New Hampshire, who led the observations of this event. “Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one.”
This decade-long feast has gone on ten times longer than any other observed TDE.
XJ1500+0154 is located in a small galaxy about 1.8 billion light years from Earth, and three telescopes have been monitoring this X-ray event: the Chandra X-ray Observatory, the Swift satellite, and the XMM-Newton.
TDEs are different from another, more common black-hole related source of X-rays in the galaxy, active galactic nuclei (AGN). Like the digestion of the Sarlacc, AGNs really can last for thousands of years. These are supermassive black holes at the center of galaxies that pull in surrounding gas and “emit copious amounts of radiation, including X-rays,” explained Lin in a blog post on the Chandra website. “Radiation from AGNs do not vary a lot because the gas surrounding them extends over a large scale and can last for tens of thousands of years.”
In contrast, TDEs are relatively short-lived, lasting only a few months. During a TDE, some of the stellar debris is flung outward at high speeds, while the rest falls toward the black hole. As it travels inwards to be consumed by the black hole, the material heats up to millions of degrees, generating a distinct X-ray flare.
XJ1500+0154 has provided an extraordinarily long, bright phase, spanning over ten years. Lin and his team said one explanation could be the most massive star ever to be completely torn apart during a TDE.
“To have the event last so long at such high luminosity requires full disruption of a relatively massive star, about twice the mass of the sun,” Lin wrote; however, “disruption of such massive stars by the SMBH is very unlikely because stars this massive are rare in most galaxies, unless the galaxy is young and actively forming stars, as in our case.
So, another more likely explanation is that this is the first TDE observed where a smaller star was completely torn apart.
Lin also said this event has broad implications for black hole physics.
“To fully explain the super-long duration of our event requires the application of recent theoretical progress on the study of TDEs,” he wrote. “In the last two years, several groups independently found that it can take a long time after the disruption of the star for the stellar debris to settle onto the accretion disk and into the SMBH. Therefore, the event can evolve much more slowly than previously thought.”
Additionally, the X-ray data also indicate that radiation from material surrounding this black hole has consistently surpassed what is called the Eddington limit, which is defined as a balance between the outward pressure of radiation from the hot gas and the inward pull of the gravity of the black hole.
Seeing evidence of such rapid growth may help astronomers understand how supermassive black holes were able to reach masses about a billion times higher than the sun when the universe was only about a billion years old.
“This event shows that black holes really can grow at extraordinarily high rates,” said co-author Stefanie Komossa of QianNan Normal University for Nationalities in Duyun City, China. “This may help understand how precocious black holes came to be.”
Lin and his team will continue to monitor this event, and they expect the X-ray brightness to fade over the next few years, meaning the supply of ‘food’ for this long lunch will soon be consumed.
Since they were first discovered in the late 1960s, pulsars have continued to fascinate astronomers. Even though thousands of these pulsing, spinning stars have been observed in the past five decades, there is much about them that continues to elude us. For instance, while some emit both radio and gamma ray pulses, others are restricted to either radio or gamma ray radiation.
However, thanks to a pair of studies from two international teams of astronomers, we may be getting closer to understanding why this is. Relying on data collected by the Chandra X-ray Observatory of two pulsars (Geminga and B0355+54), the teams was able to show how their emissions and the underlying structure of their nebulae (which resemble jellyfish) could be related.
Located 800 and 3400 light years from Earth (respectively), the Geminga and B0355+54 pulsars are quite similar. In addition to having similar rotational periods (5 times per second), they are also about the same age (~500 million years). However, Geminga emits only gamma-ray pulses while B0355+54 is one of the brightest known radio pulsars, but emits no observable gamma rays.
What’s more, their PWNs are structured quite differently. Based on composite images created using Chandra X-ray data and Spitzer infrared data, one resembles a jellyfish whose tendrils are relaxed while the other looks like a jellyfish that is closed and flexed. As Bettina Posselt – a senior research associate in the Department of Astronomy and Astrophysics at Penn State, and the lead author on the Geminga study – told Universe Today via email:
“The Chandra data resulted in two very different X-ray images of the pulsar wind nebulae around the pulsars Geminga and PSR B0355+54. While Geminga has a distinct three-tail structure, the image of PSR B0355+54 shows one broad tail with several substructures.”
In all likelihood, Geminga’s and B0355+54 tails are narrow jets emanating from the pulsar’s spin poles. These jets lie perpendicular to the donut-shaped disk (aka. a torus) that surrounds the pulsars equatorial regions. As Noel Klingler, a graduate student at the George Washington University and the author of the B0355+54 paper, told Universe Today via email:
“The interstellar medium (ISM) isn’t a perfect vacuum, so as both of these pulsars plow through space at hundreds of kilometers per second, the trace amount of gas in the ISM exerts pressure, thus pushing back/bending the pulsar wind nebulae behind the pulsars, as is shown in the images obtained by the Chandra X-ray Observatory.”
Their apparent structures appear to be due to their disposition relative to Earth. In Geminga’s case, the view of the torus is edge-on while the jets point out to the sides. In B0355+54’s case, the torus is seen face-on while the jets points both towards and away from Earth. From our vantage point, these jets look like they are on top of each other, which is what makes it look like it has a double tail. As Posselt describes it:
“Both structures can be explained with the same general model of pulsar wind nebulae. The reasons for the different images are (a) our viewing perspective, and (b) how fast and where to the pulsar is moving. In general, the observable structures of such pulsar wind nebulae can be described with an equatorial torus and polar jets. Torus and Jets can be affected (e.g., bent jets) by the “head wind” from the interstellar medium the pulsar is moving in. Depending on our viewing angle of the torus, jets and the movement of the pulsar, different pictures are detected by the Chandra X-ray observatory. Geminga is seen “from the side” (or edge-on with respect to the torus) with the jets roughly located in the plane of the sky while for B0355+54 we look almost directly to one of the poles.”
This orientation could also help explain why the two pulsars appear to emit different types of electromagnetic radiation. Basically, the magnetic poles – which are close to their spin poles – are where a pulsar’s radio emissions are believed to come from. Meanwhile, gamma rays are believed to be emitted along a pulsar’s spin equator, where the torus is located.
“The images reveal that we see Geminga from edge-on (i.e., looking at its equator) because we see X-rays from particles launched into the two jets (which are initially aligned with the radio beams), which are pointed into the sky, and not at Earth,” said Klingler. “This explains why we only see Gamma-ray pulses from Geminga. The images also indicate that we are looking at B0355+54 from a top-down perspective (i.e., above one of the poles, looking into the jets). So as the pulsar rotates, the center of the radio beam sweeps across Earth, and we detect the pulses; but the gamma-rays are launched straight out from the pulsar’s equator, so we don’t see them from B0355.”
“The geometrical constraints on each pulsar (where are the poles and the equator) from the pulsar wind nebulae help to explain findings regarding the radio and gamma-ray pulses of these two neutron stars,” said Posselt. “For example, Geminga appears radio-quiet (no strong radio pulses) because we don’t have a direct view to the poles and pulsed radio emission is thought to be generated in a region close to the poles. But Geminga shows strong gamma-ray pulsations, because these are not produced at the poles, but closer to the equatorial region.”
These observations were part of a larger campaign to study six pulsars that have been seen to emit gamma-rays. This campaign is being led by Roger Romani of Stanford University, with the collaboration of astronomers and researchers from GWU (Oleg Kargaltsev), Penn State University (George Pavlov), and Harvard University (Patrick Slane).
Not only are these studies shedding new light on the properties of pulsar wind nebulae, they also provide observational evidence to help astronomers create better theoretical models of pulsars. In addition, studies like these – which examine the geometry of pulsar magnetospheres – could allow astronomers to better estimate the total number of exploded stars in our galaxy.
By knowing the range of angles at which pulsars are detectable, they should be able to better estimate the amount that are not visible from Earth. Yet another way in which astronomers are working to find the celestial objects that could be lurking in humanity’s blind spots!