Posted on by Jon Voisey

Different Supernovae; Different Neutron Stars

Artist concept of a neutron star. Credit: NASA
Artist concept of a neutron star. Credit: NASA

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Astronomers have recognized various ways that stars can collapse to undergo a supernova. In one situation, an iron core collapses. The second involves a lower mass star with oxygen, neon, and magnesium in the core which suddenly captures electrons when the conditions are just right, removing them as a support mechanism and causing the star to collapse. While these two mechanisms make good physical sense, there has never been any observational support showing that both types occur. Until now that is. Astronomers led yb Christian Knigge and Malcolm Coe at the University of Southampton in the UK announced that they have detected two distinct sub populations in the neutron stars that result from these supernova.

To make the discovery, the team studied a large number of a specific sub-class of neutron stars known as Be X-ray binaries (BeXs). These objects are a pair of stars formed by a hot B spectral class stars with hydrogen emission in their spectrum in a binary orbit with a neutron star. The neutron star orbits the more massive B star in an elliptical orbit, siphoning off material as it makes close approaches. As the accreted material strikes the neutron star’s surface it glows brightly in the X-rays, becoming, for a time, an X-ray pulsar allowing astronomers to measure the spin period of the neutron star.

Such systems are common in the Small Magellanic Cloud which appears to have a burst of star forming activity about 60 million years ago, allowing for the massive B stars to be in the prime of their stellar lives. It is estimated that the Small Magellanic Cloud alone has as many BeXs as the entire Milky Way galaxy, despite being 100 times smaller. By studying these systems as well the Large Magellanic Cloud and Milky Way, the team found that there are two overlapping but distinct populations of BeX neutron stars. The first had a short period, averaging around 10 seconds. A second group had an average of around 5 minutes. The team surmises that the two populations are a result of the different supernova formation mechanisms.

The two different formation mechanisms should also lead to another difference. The explosion is expected to give the star a “kick” that can change the orbital characteristics. The electron-captured supernovae are expected to give a kick velocity of less than 50 km/sec whereas the iron core collapse supernovae should be over 200 km/sec. This would mean the iron core collapse stars should have preferentially longer and more eccentric orbits. The team attempted to discern whether this too was supported by their evidence, but only a small fraction of the stars they examined had determined eccentricities. Although there was a small difference, it is too early to determine whether or not it was due to chance.

According to Knigge, “These findings take us back to the most fundamental processes of stellar evolution and lead us to question how supernovae actually work. This opens up numerous new research areas, both on the observational and theoretical fronts.

Posted on by Nancy Atkinson

Space Telescopes Provide New Look at 2,000 Year Old Supernova

This image combines data from four different space telescopes to create a multi-wavelength view of all that remains of the oldest documented example of a supernova, called RCW 86.

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What caused a huge explosion nearly 2,000 years ago, seen by early Chinese astronomers? Scientists have long known that a “guest star” that had mysteriously appeared in the sky and stayed for about 8 months in the year 185 was the first documented supernova. But now the combined efforts of four space observatories have provided insight into this stellar explosion and why it was so huge – and why its shattered remains — the object known as RCW 86 – is now spread out to great distances.

“This supernova remnant got really big, really fast,” said Brian Williams, an astronomer at North Carolina State University in Raleigh. “It’s two to three times bigger than we would expect for a supernova that was witnessed exploding nearly 2,000 years ago. Now, we’ve been able to finally pinpoint the cause.”

By studying new infrared observations from the Spitzer Space Telescope and data from the Wide-field Infrared Survey Explorer, and previous data from NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton Observatory, astronomers were able to determine that the ancient supernova was a Type Ia supernova. And doing some “forensics” on the stellar remains, the astronomers could piece together that prior to exploding, winds from the white dwarf cleared out a huge “cavity,” a region of very low-density surrounding the system. The explosion into this cavity was able to expand much faster than it otherwise would have. The ejected material would have traveled into the cavity, unimpeded by gas and dust and spread out quickly.

This is the first time that astronomers have been able to deduce that this type of cavity was created, and scientists say the results may have significant implications for theories of white-dwarf binary systems and Type Ia supernovae.

At about 85 light-years in diameter, RCW occupies a region of the sky that is slightly larger than the full moon. It lies in the southern constellation of Circinus.

Source: JPL

Posted on by Jon Voisey

Missing Black Holes

Artists concept of a black hole.

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As astronomers began working out how stars die, they expected that the mass of remnants, whether white dwarfs, neutron stars, or black holes, should be essentially continuous. In other words, there should be a smooth distribution of remnant masses from a fraction of a solar mass, up to nearly 100 times the mass of the sun. Yet observations have shown a distinct lack of objects at the borderline of neutron stars and black holes weighing 2-5 solar masses. So where have they all gone and what might this imply about the explosions that create such objects?

The gap was first noted in 1998 and was originally attributed to a lack of observations of black holes at the time. But in the past 13 years, the gap has held up.

In an attempt to explain this, a new study has been conducted by a team of astronomers led by Krzystof Belczynski at Warsaw University. Following the recent observations, the team assumed the paucity was not caused by a lack of observations or selection effect, but rather, there simply weren’t many objects in this mass range.

Instead, the team looked at the engines of supernovae that would create such objects. Stars less than ~20 solar masses are expected to explode into supernovae, leaving behind neutron stars, while ones greater than 40 solar masses should collapse directly into black holes with little to no fanfare. Stars between these ranges were expected to fill this gap of 2-5 solar mass remnants.

The new study proposes that the gap is created by a fickle switch in the supernova explosion process. In general, supernovae occur when the cores are filled with iron which can no longer create energy through fusion. When this happens, the pressure supporting the star’s mass disappears and the outer layers collapse onto the immensely dense core. This creates a shockwave which is reflected by the core and rushes outwards, slamming into more collapsing material and creates a stalemate, where the outwards pressure balances the infalling material. For the supernova to proceed, that outwards shockwave needs an extra boost.

While astronomers disagree on exactly what might cause this revitalization, some suggest that it is generated as the core, superheated to hundreds of billions of degrees, emits neutrinos. Under normal densities, these particles travel right past most matter, but in the superdense regions inside the supernova, many are captured, reheating the material and driving the shockwave back out to create the event we observe as a supernova.

Regardless of what causes it, the team suggests that this point is critical for the final mass of the object. If it explodes, much of the mass of the progenitor will be lost, pushing it towards a neutron star. If it fails to push outwards, the material collapses and enters the event horizon, piling on mass and driving the final mass upwards. It’s an all or nothing moment.

And moment is a good description of how fast this occurs. At most, astronomers suggest that this interplay between the outwards shock and the inwards collapse takes a single second. Other models place the timescale at a tenth of a second. The new study notes that the more quickly the decision takes place, the more pronounced the gap is in the resulting objects. As such, the fact that the gap exists may be taken as evidence for this being a split second decision.

Posted on by Jon Voisey

A Magnified Supernova

Galaxy Cluster Abell 1689

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Supernovae are among astronomers most important tools for exploring the history of the universe. Their frequency allows us to examine how active star formation was, how heavy elements have developed, and the distance to galaxies across vast distances. Yet even these titanic explosions are only so bright, and there’s an effective limit on how far we can detect them with the current generation of telescopes. However, this limit can be extended with a little help from gravity.

One of the consequences of Einstein’s theory of general relativity is that massive objects can distort space, allowing them to act as a lens. While first postulated in 1924, and proposed for galaxies by Fritz Zwicky in 1937, the effect wasn’t observed until 1979 when a distant quasar, an energetic core of a distant galaxy, was split in two by the gravitational disturbances of an intervening cluster of galaxies.

While lensing can distort images, it also provides the possibility that it may magnify a distant object, increasing the amount of light we receive. This would allow astronomers to probe even more distant regions with supernovae as their tool. But in doing so, astronomers must look for these events in a different manner than most supernova searches. These searches are generally limited to the visible portion of the spectrum, the portion we see with our eyes, but due to the expansion of the universe, the light from these objects is stretched into the near-infrared portion of the spectrum where few surveys to search for supernovae exist.

But one team, led by Rahman Amanullah at Stockholm University in Sweden, has conducted a survey using the Very Large Telescope array in Chile, to search for supernovae lensed by the massive galaxy cluster Abell 1689. This cluster is well known as a source of gravitationally lensed objects, making visible some galaxies that formed shortly after the Big Bang.

In 2009, the team discovered one supernova that was magnified by this cluster that originated 5-6 billion lightyears away. In a new paper, the team reveals details about an even more distant supernova, nearly 10 billion lightyears distant. This event was magnified by a factor of 4 from the effects of the foreground cluster. From the distribution of energy in different portions of the spectrum, the team concludes that the supernova was an implosion of a massive star leading to a core-collapse type of supernova. The distance of this event puts it among the most distant supernovae yet observed. Others at this distance have required extensive time using the Hubble telescope or other large telescopes.

Posted on by Jon Voisey

Homeless Supernovae

NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF
NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF

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In a post earlier this month, we looked at a team of astronomers searching for stars that were on ejected from their birthplaces in clusters. These stars could receive the needed kick from a gravitational swing by the core of the cluster to achieve a velocity of a few tens of km/sec. But a similar mechanism can function in the cores of galaxies giving stars a speed of roughly 1,000 km/sec, enough to leave their parent galaxies. a new study asks whether we have ever witnessed any of these stellar cast offs explode as supernovae.

The team, led by Peter-Christian Zinn at Ruhr University in Bochum, Germany, searched through roughly 6,000 supernovae listed in the Sternbarg Astronomical Institute Supernova Catalog, for which no host galaxy was apparent, yet weren’t too distant from any known galaxy. The latter criteria was added because, even at the high velocities, stars still couldn’t get too far before they reached the end of their fuses. The team imposed a rough inner cut off of around 10 kiloparsecs (roughly 1/3 of the width of the disk of the Milky Way). They expected stars should be at least this distance from the cores of the parent galaxy.

The initial list contained five candidate stars, dating back as far as 1969. The first step the team used to determine if the supernova was truly in a galaxy or not, was to take long exposure images of the immediate area, to draw out potential low surface brightness hosts. The team also used archival data in the far ultraviolet as well as the x-ray spectrum to determine whether or not the nearby galaxies from which the supernovae could potentially be ejected had an extended disk, invisible in the visible portion of the spectrum that would have allowed the progenitor star to form in the outskirts of the galaxy. These wavelengths are tracers of ongoing star formation which are sites in which high mass stars that would lead to core-collapse supernovae, would likely be found.

The oldest candidate, SN 1969L, was located near the flocculent spiral NGC 1058. While the deep exposures did not show a host galaxy, the x-ray and UV images both showed some extended structure of the parent galaxy at the distance of the supernova. This led to the conclusion that this supernova, while far removed from its host galaxy, was still gravitationally tied to it.

With the second candidate, SN 1970L, the team again failed to find any faint host galaxy. However, the supernova was situated between two galaxies, NGC 2968 and a faint elliptical, NGC 2970. A 1994 study had revealed a faint bridge of matter connecting the two, implying that they had had an interaction in the past. This interaction would likely have pulled off gas and stars, of which SN 1970L could have been one.

SN 1997C was the third candidate and also lacked a discernible host galaxy, even with long exposures. This one also did not have an indication of an extended disk of which the supernova could have been part. Given the characteristics of the supernova, the team estimated that it had an original mass of 15 times that of the Sun. Given its projected distance and the lifetime of such stars, the team noted that this would correspond to a velocity of some 3,000 km/sec, which is several times the speed of the highest confirmed hypervelocity star. As such, the team expected that this star would have to be ejected in a similar manner to SN 1970L, using an interaction between galaxies. Given that the host galaxy is known to be one in a small cluster and the disk shows some signs of perturbation, they suggested this was likely.

The fourth candidate, SN 2005nc, the team selected because there was no nearby galaxy they could assign as a possible parent. They suggested this was due to an extremely distant host galaxy, too faint to resolve with previous studies. The basis for this assertion was that the supernova came with a gamma ray burst that indicated an origin some 5-6 billion light years distant. Due to the associated GRB, the Hubble telescope swung in to take a look. These archival pictures failed to reveal any objects that could readily be identified as host galaxies leaving the team to presume the host was simply too far away to resolve.

The last candidate was SN 2006bx located near the galaxy UGC 5434. This supernova did not appear to be in a faint background galaxy and did not have hints of being formed in an extended disk. The estimated velocity from the projected distance was ~850 km/sec which placed it in the realm of plausible speeds for stars ejected by gravitational assists from the supermassive black hole at the center of galaxies.

Posted on by Jon Voisey

Pan-STARRS Discovers two Super Supernovae

Artist illustration of a supernova. Image credit: ESO

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Supernovae are the brightest phenomenon in the current universe. As massive stars die as supernovae, they briefly outshine the rest of the stars in their galaxy and are visible, at least once the light gets there, from across the universe. Until recently, astronomers thought they pretty much had supernovae figured out; they could either form from the direct collapse of a massive core or the tipping over the Chandrasekhar limit as a white dwarf accreted neighbor. These methods seemed to work well until astronomers began to discover “ultra-luminous” supernovae beginning with SN 2005ap. The usual suspects could not produce such bright explosions and astronomers began looking for new methods as well as new ultra-luminous supernovae to help understand these outliers. Recently, the automated sky survey Pan-STARRS netted two more.

Since 2010, the Panoramic Survey Telescope & Rapid Response System (Pan-STARR) has been conducting observations atop Mount Haleakala and is controlled by the University of Hawaii. Its primary mission is to search for objects that may pose a threat to Earth. To do this, it repeatedly scans the northern sky, looking at 10 patches per night and cycling through various color filters. While it has been very successful in this area, the observations can also be used to study objects that change on short timescales such as supernovae.

The first of the two new supernovae, PS1-10ky was already in the process of exploding as Pan-STARRS came into operation, thus, the brightness curve was incomplete since it was discovered near peak brightness and no data exists to catch it as it brightened. However, for the second, PS1-10awh, the team caught while in the process of brightening and have a complete light curve for the object. Combining the two, the team, led by Laura Chomiuk at the Harvard-Smithsonian Center for Astrophysics, was able to get a full picture of just how these titanic supernovae behave. And what’s more, since they were observed with multiple filters, the team was able to understand just how the energy was distributed. Additionally, the team was able to use other instruments, including Gemini, to get spectroscopic information.

The two new supernovae are very similar in many regards to the other ultra-luminous supernovae discovered previously, including SN 2010gx and SCP 06F6. All of these objects have been exceptionally bright with little absorption in their spectra. What little they did have was due to partially ionized carbon, silicon, and magnesium. The average peak brightness was -22.5 magnitudes where as typical core collapse supernovae peak around -19.5. The presence of these lines allowed astronomers to measure the expansion velocity for the new objects as 40,000 km/sec and place a distance to these objects as around 7 billion light years (previous ultra-luminous supernovae like these have been between 2 and 5 billion light years).

But what could power these leviathans? The team considered three scenarios. The first was radioactive decay. The violence of supernovae explosions injects atomic nuclei with additional protons and neutrons creating unstable isotopes which rapidly decay giving off visible light. This process is generally implicated in the fading out of supernovae as this decay process withers out slowly. However, based on the observations, the team concluded that it should not be possible to create sufficient amounts of the radioactive elements necessary to account for the observed brightness.

Another possibility was a rapidly rotating magnetar underwent a rapid change in its rotation. This sudden change would throw off large large chunks of material from the surface which could, in extreme cases, match the observed expansion velocity of these objects.

Lastly, the team considers a more typical supernova expanding into a relatively dense medium. In this case, the shockwave produced by the supernova would interact with the cloud around the star and the kinetic energy would heat the gas, causing it to glow. This too could reproduce many of the observed features of the supernova, but had the requirement that the star shed large amounts of material just before exploding. Some evidence is given for this as being a common occurrence in massive Luminous Blue Variable stars observed in the nearby universe. The team notes that this hypothesis may be tested by searching for radio emission as the shockwave interacted with the gas.

Posted on by VirtualAstro

Supernova Discovered in M51 The Whirlpool Galaxy

M51 Hubble Remix

A new supernova (exploding star) has been discovered in the famous Whirlpool Galaxy, M51.

M51, The Whirlpool galaxy is a galaxy found in the constellation of Canes Venatici, very near the star Alkaid in the handle of the saucepan asterism of the big dipper. Easily found with binoculars or a small telescope.

The discovery was made on June 2nd by French astronomers and the supernova is reported to be around magnitude 14. More information (In French) can be found here or translated version here.

Image by BBC Sky at Night Presenter Pete Lawrence

The supernova will be quite tricky to spot visually and you may need a good sized dobsonian or similar telescope to spot it, but it will be a easy target for those interested in astro imaging.

The whirlpool galaxy was the first galaxy discovered with a spiral structure and is one of the most recognisable and famous objects in the sky.

Posted on by Jon Voisey

Finding the Failed Supernovae

Recipe for a pair instability supernova. It is hypothesised that in extremely massive stars, gamma rays radiating from the core become so energetic that they can undergo pair production after interaction with a nucleus. Essentially, the gamma ray creates a paired particle and antiparticle (commonly an electron and a positron). The loss of radiation pressure as gamma rays convert to particles results in gravitational collapse of the star's core - and kaboom! Credit: chandra.harvard.edu

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When high mass stars end their lives, they explode in monumental supernovae. But, when the most massive of these monsters die, theory has predicted that they may not even reveal as much as a whimper as their massive cores implode. Instead, the implosion occurs so quickly, that the rebound and all photons created during it, are immediately swallowed into the newly formed black hole. Estimates have suggested that as much as 20% of stars that are massive enough to form supernovae collapse directly into a black hole without an explosion. These “failed supernovae” would simply disappear from the sky leaving such predictions seemingly impossible to verify. But a new paper explores the potential for neutrinos, subatomic particles that rarely interact with normal matter, could escape during the collapse, and be detected, heralding the death of a giant.

Presently, only one supernova has been detected by its neutrinos. This was supernova 1987a, a relatively close supernova which occurred in the Large Magellanic Cloud, a satellite galaxy to our own. When this star exploded, the neutrinos escaped the surface of the star and reached detectors on Earth three hours before the shockwave reached the surface, producing a visible brightening. Yet despite the enormity of the eruption, only 24 neutrinos (or more precisely, electron anti-neutrinos), were detected between three detectors.

The further away an event is, the more its neutrinos will be spread out, which in turn, decreases the flux at the detector. With current detectors, the expectation is that they are large enough to detect supernovae events around a rate of 1-3 per century all originating from within the Milky Way and our satellites. But as with most astronomy, the detection radius can be increased with larger detectors. The current generation uses detectors with masses on the order of kilotons of detecting fluid, but proposed detectors would increase this to megatons, pushing the sphere of detectability to as much as 6.5 million light years, which would include our nearest large neighbor, the Andromeda galaxy. With such enhanced capabilities, detectors would be expected to find neutrino bursts on the order of once per decade.

Assuming the calculations are correct and that 20% of supernova implode directly, this means that such gargantuan detectors could detect 1-2 failed supernovae per century. Fortunately, this is slightly enhanced due to the extra mass of the star, which would make the total energy of the event higher, and while this wouldn’t escape as light, would correspond to an increased neutrino output. Thus, the detection sphere could be pushed out to potentially 13 million lightyears, which would incorporate several galaxies with high rates of star formation and consequently, supernoave.

While this puts the potential for detections of failed supernovae on the radar, a bigger problem remains. Say neutrino detectors record a sudden burst of neutrinos. With typical supernovae, this detection would be quickly followed with the optical detection of a supernova, but with a failed supernova, the followup would be absent. The neutrino burst is the beginning and end of the story, which could not initially positively define such an event as different from other supernovae, such as those that form neutron stars.

To tease out the subtle differences, the team modeled the supernovae to examine the energies and durations involved. When comparing failed supernovae to ones forming neutron stars, they predicted that the failed supernovae neutrino bursts would have shorter durations (~1 second) than ones forming neutron stars (~10 seconds). Additionally, the energy imparted in the collision that makes up the detection would be higher for failed supernovae (up to 56 MeV vs 33 MeV). This difference could potentially discriminate between the two types.

Posted on by Nancy Atkinson

‘Ring’ in the Holidays with New Hubble Bubble Image

SNR 0509 is the visible remnant of a powerful stellar explosion in the Large Magellanic Cloud. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgement: J. Hughes (Rutgers University)

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From a Hubble/ESA press release:

A festive, delicate ring –photographed by the Hubble Space Telescope — appears to float serenely in the depths of space, but this apparent calm hides an inner turmoil. The gaseous envelope formed as the expanding blast wave and ejected material from a supernova tore through the nearby interstellar medium. Called SNR B0509-67.5 (or SNR 0509 for short), the bubble is the visible remnant of a powerful stellar explosion in the Large Magellanic Cloud (LMC), a small galaxy about 160,000 light-years from Earth.

Ripples seen in the shell’s surface may be caused either by subtle variations in the density of the ambient interstellar gas, or possibly be driven from the interior by fragments from the initial explosion. The bubble-shaped shroud of gas is 23 light-years across and is expanding at more than 18 million km/h.

Astronomers have concluded that the explosion was an example of an especially energetic and bright variety of supernova. Known as Type Ia, such supernova events are thought to result when a white dwarf star in a binary system robs its partner of material, taking on more mass than it is able to handle, so that it eventually explodes.

Hubble’s Advanced Camera for Surveys observed the supernova remnant on 28 October 2006 with a filter that isolates light from the glowing hydrogen seen in the expanding shell. These observations were then combined with visible-light images of the surrounding star field that were imaged with Hubble’s Wide Field Camera 3 on 4 November 2010.

With an age of about 400 years, the supernova might have been visible to southern hemisphere observers around the year 1600, although there are no known records of a “new star” in the direction of the LMC near that time. A much more recent supernova in the LMC, SN 1987A, did catch the eye of Earth viewers and continues to be studied with ground- and space-based telescopes, including Hubble.