Gamma Ray Bursts Archives

February 18, 2010December 24, 2015 by Jean Tate

What is a Supernova?

[/caption]
What is a supernova? Well, “nova” means “new star”, and “super” means “really big”, like supermarket, so a supernova is a really bright new star. That’s where the word comes from, but today it has a rather more precise meaning, namely a once-off variable star which has a peak brightness similar to, or greater than, that of a typical galaxy.

Supernovae aren’t new stars in the sense that they were not stars before they became supernovae; the progenitor – what the star was before it went supernova – of a supernova is just a star (or a pair of stars), albeit an unusual one.

From what we see – the rise of the intensity of light (and electromagnetic radiation in general) to a peak, its decline; the lines which show up in the spectra (and the ones which don’t), etc – we can classify supernovae into several different types. There are two main types, called Type I and Type II. The difference between them is that Type I supernovae have no lines of hydrogen in their spectra, while Type II ones do.

Centuries of work by astronomers and physicists have given us just two kinds of progenitors: white dwarfs and massive (>8 sols) stars; and just two key physical mechanisms: nuclear detonation and core collapse.

Core collapse supernovae happen when a massive star tries to fuse iron in its core … bad move, because fusing iron requires energy (rather than liberates it), and the core suddenly collapses due to its gravity. A lot of interesting physics happens when such a core collapses, but it either results in a neutron star or a black hole, and a vast amount of energy is produced (most of it in the form of neutrinos!). These supernovae can be of any type, except a sub-type of Type I (called Ia). They also produce the long gamma-ray bursts (GRB).

Detonation is when a white dwarf star undergoes almost simultaneous fusion of carbon or oxygen throughout its entire body (it can do this because a white dwarf has the same temperature throughout, unlike an ordinary star, because its electrons are degenerate). There are at least two ways such a detonation can be triggered: steady accumulation of hydrogen transferred from a close binary companion, or a collision or merger with a neutron star or another white dwarf. These supernovae are all Type Ia.

One other kind of supernova: when two neutron stars merge, or a ~solar mass black hole and a neutron star merge – as a result of loss of orbital energy due to gravitational wave radiation – an intense burst of gamma-rays results, along with a fireball and an afterglow (as the fireball cools). We see such an event as a short GRB, but if we were unlikely enough to be close to such a stellar death, we’d certainly see it as a spectacular supernova!

Would you like to read more about what a supernova is? Check out these webpages: Hubblesite’s News Releases on Supernova, Supernova Cosmology Project (Lawrence Berkeley Lab), and Supernovae, Supernova Remnants (etc) (Talk Origins).

Everyone has a fascination for things which go bang!, and so you won’t be at all surprised to learn that Universe Today has many articles on supernovae, what a supernova is, etc. Here is selection for your enjoyment and education: Merging White Dwarfs Set Off Supernovae, GRB Central Engines Observed in Nearby Supernovae?, and Another Antimatter Supernova Discovered.

Astronomy Cast too has several episodes on what a supernova is; for example We’re All Made of Supernovae, and Gamma-Ray Bursts.

Reference:
NASA

February 4, 2010December 24, 2015 by Jean Tate

GRB Central Engines Observed in Nearby Supernovae?

SN 2009bb (Image Credit: NASA, Swift, Stefan Immler)

[/caption]
Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?

The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.

“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”

Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.

“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”

The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.

“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”

The 28 January, 2010 issue of Nature contains two papers reporting these discoveries: A relativistic type Ibc supernova without a detected γ-ray burst (arXiv:0908.2817 is the preprint), and A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr (arXiv:1001.5060 is the preprint).

Sources: Newborn Black Holes May Add Power to Many Exploding Stars, Newborn Black Holes Boost Explosive Power of Supernovae

January 28, 2010December 24, 2015 by Jean Tate

Hypernova

Artist impression of the twin jets from a GRB. Credit: Dana Berry/SkyWorks Digital

[/caption]
Nova, “new star”; supernova, a “super” nova; hypernova, a super-duper, or super super, nova!

This word appeared in the astronomical literature at least as early as 1982, and refers to a kind of core-collapse supernova far brighter (>100 times) than usual; its meaning has changed somewhat, and today generally refers to the core collapse of particularly massive stars (>100 sols), whether or not they are spectacularly brighter than other core-collapse supernovae (though they are that too).

Most times you’ll come across hypernovae in material on gamma ray bursts (GRBs), many of which seem to involve emission of electromagnetic radiation with total energy many times that from ordinary supernovae (whether core collapse or Type Ia). Long-duration GRBs have jets, presumably from the poles of the temporary accretion disk which forms around the new black hole at the heart of the collapsed core of the progenitor (short-duration GRBs, which also produce jets, are thought to be the merger of two neutron stars, or a neutron star and a stellar-mass black hole), but even when viewed side-on (i.e. not looking into one of the jets), these GRBs are intrinsically much brighter than other core collapse supernovae.

If a supernova were to occur a few hundred light-years from us, we’d certainly notice it, and there might be some impact on our atmosphere; if there was a hypernova the same distance away, we’d suffer (not only from the increased incidence of cancer due to the far greater intensity of cosmic rays, but also from changes in weather and climate, and damage to ecosystems); if the jet were aimed directly at us, we’d be toast (while those on the other side of the world would survive the few seconds-long blast, they’d die from the consequences).

Fortunately, it seems there are no stars likely to go hypernova on us … at least not within a few tens of thousands of light-years. Whew!

Have I whet your appetite for more? Check these sites out! Brighter than an Exploding Star, It’s a Hypernova! (NASA’s Imagine the Universe), Face on Beauty (Phil Plait), and Hypernova (Swinburne University).

Like everyone else, Universe Today writers love a good story about explosions … so there are quite a few on hypernovae! Some examples: Gamma Ray Bursts and Hypernovae Linked, ESO Watches Burst Afterglow for Five Weeks, and Carbon/Oxygen Stars Could Explode as Gamma Ray Bursts.

No surprise that Astronomy Cast episode Gamma-Ray Bursts features hypermovae! Back in 2007, after attending the American Astronomical Society meeting, Pamela learning something new about hypernovae; what? Well, check out the episode, What We Learned from the American Astronomical Society and find out for yourself!

References:
NASA
ESO

January 27, 2010December 24, 2015 by Nancy Atkinson

Supernova or GRB? Radio Observations Allow Astronomers to Find Unusual Object

Core-collapse supernova explosion expelling nearly-spherical debris shell. CREDIT: Bill Saxton, NRAO/AUI/NSF

[/caption]

For the first time, astronomers have found a supernova explosion with properties similar to a gamma-ray burst, but without seeing any gamma rays from it. Radio observations with the Very Large Array (VLA) showed material expelled from supernova explosion SN2009bb at speeds approaching the speed of light. The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. But astronomers don’t think this blast is one-of-a-kind, and say that more radio observations will point the way toward locating many more examples of these mysterious explosions.

“We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites,” said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.

Usually supernova explosions blasts the star’s material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.

Engine-driven" supernova explosion with accretion disk and high-velocity jets. CREDIT: Bill Saxton, NRAO/AUI/NSF — Engine-driven

When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star’s material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a “core-collapse supernova” as the cause of one kind of gamma-ray burst.

The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.

“This is the only way we know that a supernova explosion could accelerate material to such speeds,” Soderberg said.

Until now, no such “engine-driven” supernova had been found any way other than by detecting gamma rays emitted by it.

“Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough. With the new capabilities of the Expanded VLA coming soon, we believe we’ll find more in the future through radio observations than with gamma-ray satellites,” Soderberg said.

Why didn’t anyone see gamma rays from this explosion? “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” Soderberg said. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.

“Another possibility,” Soderberg adds, “is that the gamma rays were ‘smothered’ as they tried to escape the star. This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.”

One important question the scientists hope to answer is just what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” Soderberg said. “We’d like to find out what that property is.”

One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen. However, Soderberg points out, that does not seem to be the case for this supernova.

This research will be published in January 28 issue of the journal Nature.

Source: NRAO

November 26, 2009December 24, 2015 by Nancy Atkinson

Fermi Finds Gamma-Ray Microquasar

Fermi’s Large Area Telescope has detected bursts of gamma-rays in the binary system Cygnus X-3, which astronomers say are coming from a microquasar. While microquasars have strong emissions across is a broad range of wavelengths, this is the first time this type of object has been detected in gamma rays. “Cygnus X-3 is a genuine microquasar and it’s the first for which we can prove high-energy gamma-ray emission,” said Stéphane Corbel at Paris Diderot University in France.

n Cygnus X-3, an accretion disk surrounding a black hole or neutron star orbits close to a hot, massive star. Gamma rays (purple, in this illustration) likely arise when fast-moving electrons above and below the disk collide with the star's ultraviolet light. Fermi sees more of this emission when the disk is on the far side of its orbit. Credit: NASA's Goddard Space Flight Center

Microquasars are stellar mass object that displays in miniature some of the properties of quasars: a normal star begins shedding its matter onto either a neutron star or a black hole. This phenomenon produces large amounts of radiation and “jets” of material moving at relativistic speeds—more than 10% the speed of light—away from the star. These “relativistic jets” are a great mystery that astronomers are still trying to understand, but this new gamma-ray microquasar could provide new ways to study them.

At the center of Cygnus X-3 lies a massive Wolf-Rayet star. With a surface temperature of 100,255.372 Kelvin (180,000 degrees F,) or about 17 times hotter than the sun, the star is so hot that its mass bleeds into space in the form of a powerful outflow called a stellar wind. “In just 100,000 years, this fast, dense wind removes as much mass from the Wolf-Rayet star as our sun contains,” said Robin Corbet at the University of Maryland, Baltimore County.

The researchers matched the gamma-rays to the known orbital period of the Cygnus X-3 microquasar in order to confirm that the strong pulses of radiation were, in fact, originating from the object. They also matched the gamma-rays with radio emission from the relativistic jets of Cygnus X-3.

Brighter colors indicate greater numbers of gamma rays detected in this Fermi LAT view of a region centered on the position of Cygnus X-3 (circled). The brightest sources are pulsars. Credit: NASA/DOE/Fermi LAT Collaboration

Every 4.8 hours, a compact companion embedded in a disk of hot gas wheels around the star. “This object is most likely a black hole, but we can’t yet rule out a neutron star,” Corbet said.

Between Oct. 11 and Dec. 20, 2008, and again between June 8 and Aug. 2, 2009, Cygnus X-3 was unusually active. The team found that outbursts in the system’s gamma-ray emission preceded flaring in the radio jet by roughly five days, strongly suggesting a relationship between the two.

These new findings should provide more information about the formation of such mysterious and fast-moving relativistic jets. This research appears in the 26 November issue of Science Express.

Read the team’s abstract

Sources: Science, Goddard Spaceflight Center

October 28, 2009April 26, 2016 by Nancy Atkinson

More Observations of GRB 090423, the Most Distant Known Object in the Universe

This image shows the afterglow of GRB 090423 (red source in the centre) and was created from images taken in the z, Y and J filters at Gemini-South and VLT (credit: A. J. Levan).

[/caption]
This image shows the afterglow of GRB 090423 (red source in the centre) and was created from images taken in the z, Y and J filters at Gemini-South and VLT (credit: A. J. Levan).

On April 23, 2009 the Swift satellite detected a gamma ray burst and as we reported back in April, scientists soon realized that it was more than 13 billion light-years from Earth. GRB 090423 occurred 630 million years after the Big Bang, when the Universe was only four percent of its current age of 13.7 billion years. Now, continued observations of the GRB by astronomers around the world have yielded more information about this dramatic and ancient event: the GRB didn’t come from a monster star, but it produced a fairly sizable explosion.

Several of the world’s largest telescopes turned to the region of the sky within the next minutes and hours after Swift’s announcement of the GRB’s detection, and were able to locate the faint, fading afterglow of the GRB. Detailed analysis revealed that the afterglow was seen only in infrared light and not in the normal optical. This was the clue that the burst came from very great distance.

The Very Large Array radio telescope first looked for the object the day after the discovery, detected the first radio waves from the blast a week later, then recorded changes in the object until it faded from view more than two months later.

Images of the afterglow of GRB 090423 taken (left to right) with the Y, J, H and K filters. The absence of any flux in the Y filter is a strong indication that the GRB is very high redshift (Credit: A. J. Levan & N. R. Tanvir)

Astronomers have thought that the very first stars in the Universe might be very different — brighter, hotter, and more massive — from those that formed later.

“This explosion provides an unprecedented look at an era when the Universe was very young and also was undergoing drastic changes. The primal cosmic darkness was being pierced by the light of the first stars and the first galaxies were beginning to form. The star that exploded in this event was a member of one of these earliest generations of stars,” said Dale Frail of the National Radio Astronomy Observatory.

Universe Today spoke with Edo Berger with the Gemini Telescope shortly after the GRB was detected, and he said the burst itself was not all that unusual. But even that can convey a lot of information. “That might mean that even these early generations of stars are very similar to stars in the local universe, that when they die they seem to produce similar types of gamma ray bursts, but it might be a little early to speculate.”

“This happened a little more than 13 billion years ago,” said Berger. “We’ve essentially been able to find gamma ray bursts throughout the Universe. The nearest ones are only about 100 million light years away, and this most distant one is 13 billion light years away, so it seems that they populate the entire universe. This most distant one demonstrates for the first time that massive stars exist at those very high red shifts. This is something people have suspected for a long time, but there was no direct observational proof. So that is one of the cool results from this observation.”

The scientists concluded the explosion was more energetic than most GRBs, but was certainly not the most energetic ever detected. The blast was nearly spherical that expanded into a tenuous and relatively uniform gaseous medium surrounding the star.

Antennas of the Very Large Array CREDIT: NRAO/AUI/NSF

“It’s important to study these explosions with many kinds of telescopes. Our research team combined data from the VLA with data from X-ray and infrared telescopes to piece together some of the physical conditions of the blast,” said Derek Fox of Pennsylvania State University. “The result is a unique look into the very early Universe that we couldn’t have gotten any other way,” he added.

Sources: NRAO, University of Leicester

September 18, 2009December 24, 2015 by Nancy Atkinson

Blaming Black Holes for Gamma Ray Bursts

Artist's rendering of a black hole. Image Credit: NASA

[/caption]
Black holes get a bad rap. Most people are afraid of them, and some think black holes might even destroy Earth. Now, scientists from the University of Leeds are blaming black holes for causing the most energetic and deadly outbursts in the universe: gamma ray bursts.

The conventional model for GRBs is that a narrow beam of intense radiation is released during a supernova event, as a rapidly rotating, high-mass star collapses to form a black hole. This involves plasma being heated by neutrinos in a disk of matter that forms around the black hole. A subclass of GRBs (the “short” bursts) appear to originate from a different process, possibly the merger of binary neutron stars.

But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.

Their theory is based on recent observations by the Swift satellite which indicates that the central jet engine operates for up to 10,000 seconds – much longer than the neutrino model can explain.

The scientists believe that this is evidence for an electromagnetic origin of the jets, i.e. that the jets come directly from a rotating black hole, and that it is the magnetic stresses caused by the rotation that focus and accelerate the jet’s flow.

For the mechanism to operate the collapsing star has to be rotating extremely rapidly. This increases the duration of the star’s collapse as the gravity is opposed by strong centrifugal forces.

One particularly peculiar way of creating the right conditions involves not a collapsing star but a star invaded by its black hole companion in a binary system. The black hole acts like a parasite, diving into the normal star, spinning it with gravitational forces on its way to the star’s centre, and finally eating it from the inside.

“The neutrino model cannot explain very long gamma ray bursts and the Swift observations, as the rate at which the black hole swallows the star becomes rather low quite quickly, rendering the neutrino mechanism inefficient, but the magnetic mechanism can,” says Professor Komissarov from the School of Mathematics at the University of Leeds.

“Our knowledge of the amount of the matter that collects around the black hole and the rotation speed of the star allow us to calculate how long these long flashes will be – and the results correlate very well with observations from satellites,” he adds.

Source: EurekAlert