All we can say is, “Wow!” In celebration of the International Year of Astronomy 2009, NASA’s Great Observatories — the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory — have collaborated to produce an unprecedented image of the central region of our Milky Way galaxy. This is a never-before-seen view of the turbulent heart of our home galaxy. The image is being unveiled by NASA to commemorate the anniversary of when Galileo first turned his telescope to the heavens in 1609. NASA provided this image and the individual images taken by each of the Great Observatories to more than 150 planetariums, museums, nature centers, libraries, and schools across the country.
In this spectacular image, observations using infrared light and X-ray light see through the obscuring dust and reveal the intense activity near the galactic core. Note that the center of the galaxy is located within the bright white region to the right of and just below the middle of the image. The entire image width covers about one-half a degree, about the same angular width as the full moon.
A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A. Credit: NASA/CXC
Supernova remnant Cassiopeia A (Cas A) has always been an enigma. While the explosion that created this supernova was obviously a powerful event, the visual brightness of the outburst that occurred over 300 years ago was much less than a normal supernova, — and in fact, was overlooked in the 1600’s — and astronomers don’t know why. Another mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. But in 1999, astronomers discovered an unknown bright object at the core of Cas A. Now, new observations with the Chandra X-Ray Observatory show this object is a neutron star. But the enigmas don’t end there: this neutron star has a carbon atmosphere. This is the first time this type of atmosphere has been detected around such a small, dense object.
A Chandra X-ray Observatory image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant. The discovery of a carbon atmosphere on this neutron star resolves a ten-year old mystery surrounding this object. Credit: Chandra image: NASA/CXC/Southampton/W.Ho; illustration: NASA/CXC/M.Weiss
The object at the core is very small – only about 20 km wide, which was key to identifying it as a neutron star, said Craig Heinke from the University of Alberta. Heinke is co-author with Wynn Ho of the University of Southampton, UK on a paper which appears in the Nov. 5 edition of Nature.
“The only two kinds of stars that we know of that are this small are neutron stars and black holes,” Heinke told Universe Today. “We can rule out that this is a black hole, because no light can escape from black holes, so any X-rays we see from black holes are actually from material falling down into the black hole. Such X-rays would be highly variable, since you never see the same material twice, but we don’t see any fluctuations in the brightness of this object.”
Heinke said the Chandra X-ray Observatory is the only telescope that has sharp enough vision to observe this object inside such a bright supernova remnant.
But the most unusual aspect of this neutron star is its carbon atmosphere. Neutron stars are mostly made of neutrons, but they have a thin layer of normal matter on the surface, including a thin–10 cm–very hot atmosphere. Previously studied neutron stars all have hydrogen atmospheres, which is expected, as the intense gravity of the neutron star stratifies the atmosphere, putting the lightest element, hydrogen, on top.
But not so with this object in Cas A.
“We were able to produce models for the X-ray radiation of a neutron star with several different possible atmospheres,” Heinke said in an email interview. “Only the carbon atmosphere can explain all the data we see, so we are pretty sure this neutron star has a carbon atmosphere, the first time we’ve seen a different atmosphere on a neutron star.” An artist's impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth's atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss An artist’s impression of the neutron star in Cas A showing the tiny extent of the carbon atmosphere. The Earth’s atmosphere is shown at the same scale as the neutron star. Credit: NASA/CXC/M.Weiss
So how does Heinke and his team explain the lack of hydrogen and helium on this neutron star? Think of Cas A as being a baby.
“We think we understand that as due to the really young age of this object–we see it at the tender age of only 330 years old, compared to other neutron stars that are thousands of years old,” he said. “During the supernova explosion that created this neutron star (as the core of the star collapses down to a city-sized object, with an incredibly high density higher than atomic nuclei), the neutron star was heated to high temperatures, up to a billion degrees. It’s now cooled down to a few million degrees, but we think its high temperatures were sufficient to produce nuclear fusion on the neutron star surface, fusing the hydrogen and helium to carbon.”
Because of this discovery, researchers now have access to the complete life cycle of a supernova, and will learn more about the role exploding stars play in the makeup of the universe. For example, most minerals found on Earth are the products of supernovae.
“This discovery helps us understand how neutron stars are born in violent supernova explosions,” said Heinke.
•A deep new image of the center of the Milky Way by the Chandra X-ray Observatory. NASA/CXC/UMass/D. Wang et al.
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Chandra has done it again in creating some of the most visually stunning images of our Universe. This time, Chandra’s X-ray eyes show a dramatic new vista of the center of the Milky Way galaxy. This mosaic from 88 different images exposes new levels of the complexity and intrigue in the Galactic center, providing a look at stellar evolution, from bright young stars to black holes, in a crowded, hostile environment dominated by a central, supermassive black hole.
Permeating the region is a diffuse haze of X-ray light from gas that has been heated to millions of degrees by winds from massive young stars – which appear to form more frequently here than elsewhere in the Galaxy – explosions of dying stars, and outflows powered by the supermassive black hole – known as Sagittarius A* (Sgr A*). Data from Chandra and other X-ray telescopes suggest that giant X-ray flares from this black hole occurred about 50 and about 300 years earlier.
See this link for an animation that provides greater detail of the galactic center.
The area around Sgr A* also contains several mysterious X-ray filaments. Some of these likely represent huge magnetic structures interacting with streams of very energetic electrons produced by rapidly spinning neutron stars or perhaps by a gigantic analog of a solar flare.
Scattered throughout the region are thousands of point-like X-ray sources. These are produced by normal stars feeding material onto the compact, dense remains of stars that have reached the end of their evolutionary trail – white dwarfs, neutron stars and black holes.
Because X-rays penetrate the gas and dust that blocks optical light coming from the center of the galaxy, Chandra is a powerful tool for studying the Galactic Center. This image combines low energy X-rays (colored red), intermediate energy X-rays (green) and high energy X-rays (blue).
The image is being released at the beginning of the “Chandra’s First Decade of Discovery” symposium being held in Boston, Mass. This four-day conference will celebrate the great science Chandra has uncovered in its first ten years of operations. To help commemorate this event, several of the astronauts who were onboard the Space Shuttle Columbia – including Commander Eileen Collins – that launched Chandra on July 23, 1999, will be in attendance.
Cepheus B from Chandra and Spitzer: X-ray (NASA/CXC/PSU/K. Getman et al.); IR (NASA/JPL-Caltech/CfA/J. Wang et al.)
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Combining data from the Chandra X-Ray Observatory and the Spitizer Space Telescope allowed astronomers to create this gorgeous new image of Cepheus B. Besides being incredible eye candy, the new image also provides fresh insight into how some stars are born. The research shows that radiation from massive stars may trigger the formation of many more stars than previously thought.
While astronomers have long understood that stars and planets form from the collapse of a cloud of gas, the question of the main causes of this process has remained open.
“Astronomers have generally believed that it’s somewhat rare for stars and planets to be triggered into formation by radiation from massive stars,” said Konstantin Getman of Penn State University, and lead author of the study. “Our new result shows this belief is likely to be wrong.” Chandra image of Cepheus B. Credit: NASA/Chandra team
The new study suggests that star formation in the region of study in this image, Cepheus B, is mainly triggered by radiation from one bright, massive star outside the molecular cloud. According to theoretical models, radiation from this star would drive a compression wave into the cloud triggering star formation in the interior, while evaporating the cloud’s outer layers. The Chandra-Spitzer analysis revealed slightly older stars outside the cloud while the youngest stars with the most protoplanetary disks congregate in the cloud interior — exactly what is predicted from the triggered star formation scenario.
“We essentially see a wave of star and planet formation that is rippling through this cloud,” said co-author Eric Feigelson, also of Penn State. “Outside the cloud, the stars probably have newly born planets while inside the cloud the planets are still gestating.”
Cepheus B is a cloud of mainly cool molecular hydrogen located about 2,400 light years from the Earth. There are hundreds of very young stars inside and around the cloud — ranging from a few millions years old outside the cloud to less than a million in the interior — making it an important testing ground for star formation.
Previous observations of Cepheus B had shown a rim of ionized gas around the molecular cloud and facing the massive star. However, the wave of star formation — an additional crucial feature to identifying the source of the star formation — had not previously been seen. “We can even clock how quickly this wave is traveling and it’s going about 2,000 miles per hour,” said Getman.
The star that is the catalyst for the star formation in Cepheus B, is about 20 times as massive as the Sun, or at least five times weightier than any of the other stars in Cepheus B.
The Chandra and Spitzer data also suggest that multiple episodes of star and planet formation have occurred in Cepheus B over millions of years and that most of the material in the cloud has likely already been evaporated or transformed into stars.
“It seems like this nearby cloud has already made most of its stars and its fertility will soon wane,” said Feigelson. “It’s clear that we can learn a lot about stellar nurseries by combining data from these two Great Observatories.”
A paper describing these results was published in the July 10 issue of the Astrophysical Journal.
Ten years ago, on July 23, 1999, NASA’s Chandra X-ray Observatory was deployed into orbit by the space shuttle Columbia. Far exceeding its intened 5-year life span, Chandra has demonstrated an unrivaled ability to create high-resolution X- ray images, and enabled astronomers to investigate phenomena as diverse as comets, black holes, dark matter and dark energy.
“Chandra’s discoveries are truly astonishing and have made dramatic changes to our understanding of the universe and its constituents,” said Martin Weisskopf, Chandra project scientist at NASA’s Marshall Space Flight Center in Huntsville, Ala.
The science generated by Chandra — both on its own and in conjunction with other telescopes in space and on the ground — led to a widespread, transformative impact on 21st century astrophysics. Chandra has provided the strongest evidence yet that dark matter must exist. It has independently confirmed the existence of dark energy and made spectacular images of titanic explosions produced by matter swirling toward supermassive black holes.
To commemorate the 10th anniversary of Chandra, three new versions of classic Chandra images will be released during the next three months. These images, the first of which was released today, provide new data and a more complete view of objects that Chandra observed in earlier stages of its mission. The image being released today is of the spectacular supernova remnant E0102-72.
“The Great Observatories program — of which Chandra is a major part — shows how astronomers need as many tools as possible to tackle the big questions out there,” said Ed Weiler, associate administrator of NASA’s Science Mission Directorate at NASA Headquarters in Washington. NASA’s other “Great Observatories” are the Hubble Space Telescope, Compton Gamma-Ray Observatory and Spitzer Space Telescope.
The next image will be released in August to highlight the anniversary of when Chandra opened up for the first time and gathered light on its detectors. The third image will be released during “Chandra’s First Decade of Discovery” symposium in Boston, which begins Sept. 22.
“I am extremely proud of the tremendous team of people who worked so hard to make Chandra a success,” said Harvey Tananbaum, director of the Chandra X-ray Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass. “It has taken partners at NASA, industry and academia to make Chandra the crown jewel of high-energy astrophysics.”
Tananbaum and Nobel Prize winner Riccardo Giacconi originally proposed Chandra to NASA in 1976. Unlike the Hubble Space Telescope, Chandra is in a highly elliptical orbit that takes it almost one third of the way to the moon, and was not designed to be serviced after it was deployed.
The Chandra X-ray Observatory was named after the great Indian-born American astrophysicist Subrahmanyan Chandrasekhar, who served on the faculty at the University of Chicago for almost 60 years, winning the 1983 Nobel Prize in Physics for his work on explaining the structure and evolution of stars.
The ghost of HDF 130. Credit: X-ray (NASA/CXC/IoA/A.Fabian et al.); Optical (SDSS), Radio (STFC/JBO/MERLIN)
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The Chandra X-ray Observatory has found a cosmic “ghost” lurking around a distant supermassive black hole. Astronomers think this high-energy apparition is evidence of a huge eruption produced by the black hole. But this blue blob looks eerily similar to another cosmic blob of gas found by Galaxy Zoo member Hanny Van Arkel, the famous object called Hanny’s Voorwerp. Could the two objects be similar?
Astronomers say the “ghost” found by Chandra is the remains of a diffuse X-ray source, lingering after other radiation from the black hole’s outburst died away. The object, HDF 130 is over 10 billion light years away and existed at a time 3 billion years after the Big Bang, when galaxies and black holes were forming at a high rate. Hanny's Voorwerp. Credit: Galaxy Zoo
Hanny’s Voorwerp has been a mystery ever since it was found in 2007 as part of the Galaxy Zoo project. Recent research on the object reveals that the Voorwerp is also likely to be a remnant from a black hole outburst. In the original Sloan Digital Sky Survey images of Hanny’s Voorwerp, the object showed up as blue, however further spectral analysis showed it is actually green. The Voorwerp was studied by the Swift gamma-ray satellite, which also can pick up ultraviolet and X-ray emissions, but the satellite didn’t come up with anything conclusive. However, the Westerbork Synthesis Radio Telescope (WSRT) took a look at Hanny’s Voorwerp and determined that indeed, black hole jets were allowing beams of intense optical and ultraviolet emissions from the black hole to heat and illuminate a small part of a large gas cloud that partially surrounds the nearby galaxy, IC 2497.
But Galaxy Zoo astronomers suspect X-rays might play a role in the Voorwerp, too. It was recently imaged by the Suzaku X-ray telescopes to see if is visible in that part of the spectrum, as well as to probe the current activity of the supermassive black hole. The results of that observation are still being analyzed. Yale astronomer Kevin Schawinski recently wrote in the Galaxy Zoo Blog that detecting hard X-ray photons would provide evidence of an active supermassive black hole in IC 2497, which would be illuminating the Voorwerp. “If on the other hand we don’t pick up anything, then we can be sure that the black hole has stopped feeding, i.e. it has genuinely shut down,” Schawinski wrote.
So are the two objects, the “ghost” of HDF 130 and Hanny’s Voorwerp similar? Yes – and no – said Chandra scientist Dr. Peter Edmonds.
“There are indeed some basic similarities between these two objects, in that both were generated by eruptions from a supermassive black hole, either in the form of bright radiation or jets, Edmonds told Universe Today.”Also, in both cases the eruption from the black hole seems to have died down.”
The details of the two objects, however, are very different, Edmonds said. “Hanny’s Voorwerp involves a light echo while the X-ray ghost was thought to form by an interaction between the comic background radiation and particles in a jet. They’re obviously seen at very different wavelengths. Also, the ghost is found in the early Universe at much greater distances than Hanny’s Voorwerp and is physically much larger.”
Additionally, the Chandra team suspects a very powerful and large eruption was responsible for the formation of the ghost, much more powerful than the one for Hanny’s Voorwerp.
Andy Fabian of the Cambridge University in the United Kingdom, lead author on the paper on the ghost of HDF 130, thinks the object’s X-ray glow is evidence of an outburst equivalent to about a billion supernovas, which blasted particles at almost the speed of light. When the eruption was ongoing, it produced prodigious amounts of radio and X-radiation, but after several million years, the radio signal faded from view as the electrons radiated away their energy.
This is the first X-ray ghost ever seen after the demise of radio-bright jets. Astronomers have observed extensive X-ray emission with a similar origin, but only from galaxies with radio emission on large scales, signifying continued eruptions. In HDF 130, only a point source is detected in radio images, coinciding with the massive elliptical galaxy seen in its optical image.
This radio source indicates that HDF 130’s supermassive black hole may be growing.
With Hanny’s Voorwerp, however, astronomers are still searching for any sign of activity from the black hole.
Another argument that the two objects are different is their shape. The linear shape of the HDF 130’s X-ray source is consistent with the shape of radio jets and not with that of a galaxy cluster, which is expected to be circular. The energy distribution of the X-rays is also consistent with the interpretation of an X-ray ghost. WSRT observations of Hanny's Voorwerp. Credit: ASTRON
Hanny’s Voorwerp has all the hallmarks of an interacting system. “The gas probably arises from a tidal interaction between IC 2497 and another galaxy, which occurred several hundred million years ago,” said Dr. Tom Oosterloo, part of the team that studied the Voorwerp with WSRT.
There are more differences between the two objects, primarily that ghosts like the one from HDF 130 may be prevalent in the universe, while the Voorwerp might just be a one-time occurance. “The stream of gas ends three hundred thousand light years westwards of IC2497, and all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident,” said Oosterloo.
Chandra astronomer Caitlin Casey, also of Cambridge said, “This result hints that the X-ray sky should be littered with such ghosts, especially if black hole eruptions are as common as we think they are in the early Universe.”
So now that astronomers know where and now to look for X-ray objects like the one by HDF 130, we’re likely to hear about more cosmic X-ray ghosts in the future. But Hanny’s Voorwerp appears to be unique.
The plane of the Milky Way, recorded with the Chandra satellite in three colours: Photons with energies between 0.5 and 1keV appear red, those between 1 and 3keV green, and those between 3 and 7keV blue. Discrete sources are indicated by circles. Image: Mikhail Revnivtsev
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25 years ago, astronomers discovered diffuse X-ray emissions coming from the plane of the Milky Way, but were puzzled by the source of those emissions. The mystery has now been solved by an international team of astronomers using the Chandra X-ray Observatory. These diffuse emissions do not originate from one single source but from white dwarf stars and stars with active outer gas layers.
Energetic X-ray emissions usually originate from very hot gases in a temperature range between 10 and 100 million degrees Celsius. And this so called “Galactic Ridge X-ray Emission” (GRXE) can also be found in very hot, optically thin plasma.
However, a gas with these thermal properties would immediately dissipate. Cosmic particles colliding with the interstellar medium could also be ruled out as an explanation for the GRXE.
Recently observations from two different satellites, the RXTE and Integral satellites, have shown that the X-ray emissions of the Milky Way exhibit the same distribution pattern as the stars. Since then, it has been assumed that a large portion of the GRXE originates from individual stars. These findings motivated the international team to carry out more precise measurements with the Chandra X-ray telescope.
The test area chosen was a small celestial region near the center of the Milky Way, and was about one and a half time the size of a full moon. Chandra identified 473 point sources of X-rays in a sector of the search field covering only 2.6 arcminutes. In a further step, the group used measurements from the Spitzer space telescope to prove that the results of the sector observed could be applied to the whole galaxy.
Most of the 473 X-ray sources are likely white dwarfs, which accrete matter from their surroundings. The sources could also be stars that have high activity in their outermost gas layer, the corona. White dwarfs are the remnants of extinct, low-mass suns. These cooling dead stars frequently orbit a partner, and in such a binary star system the white dwarf extracts matter from its larger partner until it becomes a Type Ia supernova.
The resolution of the diffuse X-ray emissions in our galaxy into discrete sources has far-reaching consequences for our understanding of a number of astrophysical phenomena. Astronomers can use the GRXE as a calibration for the spatial distribution of star populations within the Milky Way, for example. The results are also relevant for research into other galaxies, to determine if diffuse X-ray radiation from these objects also originates from white dwarfs and active stars.
The work was done by Mikhail Revnivtsev from the Excellence Cluster Universe at the TU Munich and his colleagues at the Max Planck Institute for Astrophysics in Garching, the Space Research Institute in Moscow and the Harvard-Smithsonian Center for Astrophysics in Cambridge, and was published in the April 30, 2009 edition of Nature.
The image shows in red the X-ray emission produced by high-energy particles accelerated at the shock front where Centaurus A's expanding radio lobe (shown in blue) collides with the surrounding galaxy. (In the top-left corner X-ray emission from close to the central black hole, and from the X-ray jet extending in the opposite direction can also be seen.) Credit: NASA
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The Chandra X-ray observatory has taken a closer look at the galaxy Centaurus A, and new images have revealed in detail the effects of a shock wave blasting through the galaxy. Powerful jets of plasma emanating from a supermassive black hole at the galactic core are creating the shock wave, and the new observation, have enabled astronomers to revise dramatically their picture of how jets affect the galaxies in which they live.
A team led by Dr. Judith Croston from the University of Hertfordshire and Dr. Ralph Kraft, of the Harvard-Smithsonian Center for Astrophysics used very deep X-ray observations from Chandra to get a new view of the jets in Centaurus A. The jets inflate large bubbles filled with energetic particles, driving a shock wave through the stars and gas of the surrounding galaxy. By analyzing in detail the X-ray emission produced where the supersonically expanding bubble collides with the surrounding galaxy, the team were able to show for the first time that particles are being accelerated to very high energies at the shock front, causing them to produce intense X-ray and gamma-ray radiation. Very high-energy gamma-ray radiation was recently detected from Centaurus A for the first time by another team of researchers using the High Energy Stereoscopic System (HESS) telescope in Namibia.
“Although we expect that galaxies with these shock waves are common in the Universe, Centaurus A is the only one close enough to study in such detail,” said Croston. “By understanding the impact that the jet has on the galaxy, its gas and stars, we can hope to understand how important the shock waves are for the life cycles of other, more distant galaxies.”
Centaurus A (NGC 5128) is one of our closest galactic neighbors, and is located in the southern constellation of Centaurus. The supermassive black hole is the source of strong radio and X-ray emissions. Visible in the image below, (click here for a zoomable image from Chandra) a combined image from Chandra and the Atacama Pathfinder Experiment (APEX) telescope in Chile, is a dust ring encircling the giant galaxy, and the fast-moving radio jets ejected from the galaxy center. Centaurus A. Credit: ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al. (Submillimetre); NASA/CXC/CfA/R.Kraft et al. (X-ray)
The powerful jets are found in only a small fraction of galaxies but are most common in the largest galaxies, which are thought to have the biggest black holes. The jets are believed to be produced near to a central supermassive black hole, and travel close to the speed of light for distances of up to hundreds of thousands of light years. Recent progress in understanding how galaxies evolve suggests that these jet-driven bubbles, called radio lobes, may play an important part in the life cycle of the largest galaxies in the Universe.
Energetic particles from radio galaxies may also reach us directly as cosmic rays hitting the Earth’s atmosphere. Centaurus A is thought to produce many of the highest energy cosmic rays that arrive at the Earth. The team believes that their results are important for understanding how such high-energy particles are produced in galaxies as well as for understanding how massive galaxies evolve.
The results of this research will be published in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society and were presented at the European Week of Astronomy and Space Science in the UK.
This X-ray nebula pictured above measures 150 light-years across. At its center is a very young and powerful pulsar known as PSR B1509-58, or B1509 for short.
How big is the pulsar?
B1509 is only 12 miles (19 km) across!
The small, dense pulsar is a rapidly spinning neutron star which is spewing energy out into the space around it to create complex and intriguing structures, including one that resembles a large cosmic hand. In this image, the lowest energy X-rays that Chandra detects are red, the medium range is green, and the most energetic ones are colored blue. Astronomers think B1509 is about 1,700 years old, and located about 17,000 light years away.
Neutron stars are created when massive stars run out of fuel and collapse. B1509 is spinning completely around almost seven times a second and is releasing energy into its environment at a prodigious rate — presumably because it has an intense magnetic field at its surface, estimated to be 15 trillion times stronger than the Earth’s magnetic field.
The combination of rapid rotation and ultra-strong magnetic field makes B1509 one of the most powerful electromagnetic generators in the Galaxy, pushing an energetic wind of electrons and ions away from the neutron star. As the electrons move through the magnetized nebula, they radiate away their energy and create the elaborate nebula seen by Chandra.
In the innermost regions, a faint circle surrounds the pulsar, and marks the spot where the wind is rapidly decelerated by the slowly expanding nebula. In this way, B1509 shares some striking similarities to the famous Crab Nebula. However B1509’s nebula is 15 times wider than the Crab’s diameter of 10 light years.
Finger-like structures extend to the north, apparently energizing knots of material in a neighboring gas cloud known as RCW 89. The transfer of energy from the wind to these knots makes them glow brightly in X-rays (orange and red features to the upper right). The temperature in this region appears to vary in a circular pattern around this ring of emission, suggesting that the pulsar may be precessing like a spinning top and sweeping an energizing beam around the gas in RCW 89.
Artist concept of ancient pulsar J0108. Image credit: X-ray: NASA/CXC/Penn State/G.Pavlov et al. Optical: ESO/VLT/UCL/R.Mignani et al. Illustration: CXC/M. Weiss
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It may be old, but it ain’t dead. The Chandra X-Ray Observatory has found the oldest isolated pulsar ever detected. While this pulsar is ancient, this exotic object is still kicking and is surprisingly active. According to radio observations, the pulsar, PSR J0108-1431 (J0108 for short) is about 200 million years old. Among isolated pulsars — ones that have not been spun-up in a binary system — it is over 10 times older than the previous record holder. A team of astronomers led by George Pavlov of Penn State University observed J0108 in X-rays with Chandra, and found that it glows much brighter in X-rays than was expected for a pulsar of such advanced years.
At a distance of 770 light years, it is also one of the nearest pulsars we know of.
Pulsars are created when stars that are much more massive than the Sun collapse in supernova explosions, leaving behind a small, incredibly weighty core, known as a neutron star. At birth, these neutron stars, which contain the densest material known in the Universe, are spinning rapidly, up to a hundred revolutions per second. As the rotating beams of their radiation are seen as pulses by distant observers, similar to a lighthouse beam, astronomers call them “pulsars”.
Astronomers observe a gradual slowing of the rotation of the pulsars as they radiate energy away. Radio observations of J0108 show it to be one of the oldest and faintest pulsars known, spinning only slightly faster than one revolution per second. J0108 in a combination of optical and X-ray. Image credit: X-ray: NASA/CXC/Penn State/G.Pavlov et al. Optical: ESO/VLT/UCL/R.Mignani et al.
Some of the energy that J0108 is losing as it spins more slowly is converted into X-ray radiation. The efficiency of this process for J0108 is found to be higher than for any other known pulsar.
“This pulsar is pumping out high-energy radiation much more efficiently than its younger cousins,” said Pavlov. “So, although it’s clearly fading as it ages, it is still more than holding its own with the younger generations.”
It’s likely that two forms of X-ray emission are produced in J0108: emission from particles spiraling around magnetic fields, and emission from heated areas around the neutron star’s magnetic poles. Measuring the temperature and size of these heated regions can provide valuable insight into the extraordinary properties of the neutron star surface and the process by which charged particles are accelerated by the pulsar.
The younger, bright pulsars commonly detected by radio and X-ray telescopes are not representative of the full population of objects, so observing objects like J0108 helps astronomers see a more complete range of behavior. At its advanced age, J0108 is close to the so- called “pulsar death line,” where its pulsed radiation is expected to switch off and it will become much harder, if not impossible, to observe.
“We can now explore the properties of this pulsar in a regime where no other pulsar has been detected outside the radio range,” said co- author Oleg Kargaltsev of the University of Florida. “To understand the properties of ‘dying pulsars,’ it is important to study their radiation in X-rays. Our finding that a very old pulsar can be such an efficient X-ray emitter gives us hope to discover new nearby pulsars of this class via their X-ray emission.”
The Chandra observations were reported by Pavlov and colleagues in the January 20, 2009, issue of The Astrophysical Journal. However, the extreme nature of J0108 was not fully apparent until a new distance to it was reported on February 6 in the PhD thesis of Adam Deller from Swinburne University in Australia. The new distance is both larger and more accurate than the distance used in the Chandra paper, showing that J0108 was brighter in X-rays than previously thought.
“Suddenly this pulsar became the record holder for its ability to make X-rays,” said Pavlov, “and our result became even more interesting without us doing much extra work.” The position of the pulsar seen by Chandra in X-rays in early 2007 is slightly different from the radio position observed in early 2001. This implies that the pulsar is moving at a velocity of about 440,000 miles per hour, close to a typical value for pulsars.
Currently the pulsar is moving south from the plane of the Milky Way galaxy, but because it is moving more slowly than the escape velocity of the Galaxy, it will eventually curve back towards the plane of the Galaxy in the opposite direction.
