Weekly Space Hangout – May. 3, 2013

Another busy episode of the Weekly Space Hangout, with more than a dozen space stories covered by a collection of space journalists. This week’s panel included Alan Boyle, Dr. Nicole Gugliucci, Amy Shira Teitel, David Dickinson, Dr. Matthew Francis, and Jason Major. Hosted by Fraser Cain. We discussed:

We record the Weekly Space Hangout every Friday at 12 pm Pacific / 3 pm Eastern. You can watch us live on Google+, Cosmoquest or listen after as part of the Astronomy Cast podcast feed (audio only).

Weekly Space Hangout – January 25, 2013

Back by popular demand… the Weekly Space Hangout has returned. This is a weekly broadcast on Google+, where I’m joined by a wide and varied team of space and astronomy journalists to discuss the big breaking stories this week.

This week, we talked about:

We record the Weekly Space Hangout every Friday on Google+ at 12:00 pm PST / 3:00 pm EST / 2000 GMT. You’ll want to circle Cosmoquest on Google+ to find out when we’re recording next. The audio for the Weekly Space Hangout is also released to the Astronomy Cast podcast feed.

X-ray Burst May Be the First Sign of a Supernova

GRB 080913, a distant supernova detected by Swift. This image merges the view through Swift’s UltraViolet and Optical Telescope, which shows bright stars, and its X-ray Telescope. Credit: NASA/Swift/Stefan Immler

The first moments of a massive star going supernova may be heralded by a blast of x-rays, detectable by space telescopes like Swift, which could then tell astronomers where to look for the full show in gamma rays and optical wavelengths. These findings come from the University of Leicester in the UK where a research team was surprised by the excess of thermal x-rays detected along with gamma ray bursts associated with supernovae.

“The most massive stars can be tens to a hundred times larger than the Sun,” said Dr. Rhaana Starling of the University of Leicester  Department of Physics and Astronomy. “When one of these giants runs out of hydrogen gas it collapses catastrophically and explodes as a supernova, blowing off its outer layers which enrich the Universe.

“But this is no ordinary supernova; in the explosion narrowly confined streams of material are forced out of the poles of the star at almost the speed of light. These so-called relativistic jets give rise to brief flashes of energetic gamma-radiation called gamma-ray bursts, which are picked up by monitoring instruments in space, that in turn alert astronomers.”

Powerful gamma ray bursts — GRBs — emitted from supernovae can be detected by both ground-based observatories and NASA’s Swift telescope. Within seconds of detecting a burst (hence its name) Swift relays its location to ground stations, allowing both ground-based and space-based telescopes around the world the opportunity to observe the burst’s afterglow.

But the actual moment of the star’s collapse, when its collapsing core reacts with its surface, isn’t observed — it happens too quickly, too suddenly. If these “shock breakouts” are the source of the excess thermal x-rays (a.k.a. black body emission) that have been recently identified in Swift data, some of the galaxy’s most energetic supernovae could be pinpointed and witnessed at a much earlier moment in time — literally within the first seconds of their birth.

“This phenomenon is only seen during the first thousand seconds of an event, and it is challenging to distinguish it from X-ray emission solely from the gamma-ray burst jet,” Dr. Starling said. “That is why astronomers have not routinely observed this before, and only a small subset of the 700+ bursts we detect with Swift show it.”

Read more: Finding the Failed Supernovae

More observations will be needed to determine if the thermal emissions are truly from the initial collapse of stars and not from the GRB jets themselves. Even if the x-rays are determined to be from the jets it will provide valuable insight to the structure of GRBs… “but the strong association with supernovae is tantalizing,” according to Dr. Starling.

Read more on the University of Leicester press release here, and see the team’s paper in the Nov. 28 online issue of the Monthly Notices of the Royal Astronomical Society here (Full PDF on arXiv.org here.)

Inset image: An artist’s rendering of the Swift spacecraft with a gamma-ray burst going off in the background. Credit: Spectrum Astro. Find out more about the Swift telescope’s instruments here.

 

Solving the Mystery of Dark Gamma Ray Bursts

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Unraveling the mystery of Gamma Ray Bursts (GRBs) is a story filled with international intrigue, fantastic claims, serious back-tracking, and incremental improvements in our understanding of the true nature and implications of the most energetic, destructive forces in the Universe. New results from a team of scientists studying so-called “dark gamma-ray bursts” have firmly snapped a new piece into the GRB puzzle. This research is presented in a paper to appear in the journal Astronomy & Astrophysics on December 16, 2010.

The discovery of GRBs was an unexpected result of the American space program and the military keeping tabs on the Russians to verify compliance with a cold war nuclear test ban treaty. In order to be sure the Russians weren’t detonating nuclear weapons on the far side of the Moon, the 1960’s era Vela spacecraft were equipped with gamma ray detectors. The Moon might shield the obvious signature of x-rays from the far side, but gamma rays would penetrate right through the Moon and would be detectible by the Vela satellites.

By 1965, it became apparent that events which triggered the detectors but were clearly not signatures of nuclear detonations, so they were carefully, and secretly, filed away for future study. In 1972, astronomers were able to deduce the directions to the events with sufficient accuracy to rule out the Sun and Earth as sources. They came to the conclusion that these gamma-ray events were “of cosmic origin”. In 1973, this discovery was announced in the Astrophysical Journal.

This created quite stir in the astronomical community and dozens of papers on GRBs and their causes began appearing in the literature. Initially, most hypothesized the origin of these events came from within our own galaxy. Progress was painfully slow until the 1991 launch of the Compton Gamma Ray Observatory. This satellite provided crucial data indicating that the distribution of GRBs is not biased towards any particular direction in space, such as toward the galactic plane or the center of the Milky Way Galaxy. GRBs came from everywhere all around us. They are “cosmic” in origin. This was a big step in the right direction, but created more questions.

For decades, astronomers searched for a counterpart, any astronomical object coincident with a recently observed burst. But the lack of precision in the location of GRBs by the instruments of the day frustrated attempts to pin down the sources of these cosmic explosions. In 1997, BeppoSAX detected a GRB in x-rays shortly after an event and the optical after glow was detected 20 hours later by the William Herschel Telescope. Deep imaging was able to identify a faint, distant galaxy as the host of the GRB. Within a year the argument over the distances to GRBs was over. GRBs occur in extremely distant galaxies. Their association with supernovae and the deaths of very massive stars also gave clues to the nature of the systems that produce GRBs.

It wasn’t too long before the race to identify optical afterglows of GRBs heated up and new satellites helped pinpoint the locations of these after glows and their host galaxies. The Swift satellite, launched in 2004, is equipped with a very sensitive gamma ray detector as well as X-ray and optical telescopes, which can be rapidly slewed to observe afterglow emissions automatically following a burst, as well as send notification to a network of telescopes on the ground for quick follow up observations.

Today, astronomers recognize two classifications of GRBs, long duration events and short duration events. Short gamma-ray bursts are likely due to merging neutron stars and not associated with supernovae. Long-duration gamma-ray bursts (GRBs) are critical in understanding the physics of GRB explosions, the impact of GRBs on their surroundings, as well as the implications of GRBs on early star formation and the history and fate of the Universe.

While X-ray afterglows are usually detected for each GRB, some still refused to give up their optical afterglow. Originally, those GRBs with X-ray but without optical afterglows were coined “dark GRBs”. The definition of “dark gamma-ray burst” has been refined, by adding a time and brightness limit, and by calculating the total output of energy of the GRB.

This lack of an optical signature could have several origins. The afterglow could have an intrinsically low luminosity. In other words, there may just be bright GRBs and faint ones. Or the optical energy could be strongly absorbed by intervening material, either locally around the GRB or along the line-of-sight through the host galaxy. Another possibility is that the light could be at such a high redshift that blanketing and absorption by the intergalactic medium would prohibit detection in the R band frequently used to make these detections.

In the new study, astronomers combined Swift data with new observations made using GROND, a dedicated GRB follow-up instrument attached to the 2.2-metre MPG/ESO telescope at La Silla in Chile. GROND is an exceptional tool for the study of GRB afterglows. It can observe a burst within minutes of an alert coming from Swift, and it has the ability to observe through seven filters simultaneously, covering the visible and near-infrared parts of the spectrum.

By combining GROND data taken through these seven filters with Swift observations, astronomers were able to accurately determine the amount of light emitted by the afterglow at widely differing wavelengths, all the way from high energy X-rays to the near-infrared. They then used this data to directly measure the amount of obscuring dust between the GRB and observers on Earth. Thankfully, the team has found that dark GRBs don’t require exotic explanations.

What they found is that a significant proportion of bursts are dimmed to about 60–80 percent of their original intensity by obscuring dust. This effect is exaggerated for the very distant bursts, letting the observer see only 30–50 percent of the light. By proving this to be so, these astronomers have conclusively solved the puzzle of the missing optical afterglows. Dark gamma-ray bursts are simply those that have had their visible light completely stripped away before it reaches us.