In This Issue:

Cosmic Mystery Solved

By: Phil Plait and Lynn Cominsky, SSU E/PO

For the past 35 years since their initial discovery, one of the biggest mysteries in astrophysics was the source behind short gamma-ray bursts. Unlike their longer cousins (caused by the supernova explosions of young, massive stars) short bursts last for mere milliseconds, and are clearly a different phenomenon. Their extremely brief nature made them very difficult to study. The leading theory was that short bursts were produced when old, dense neutron stars collided, or when a neutron star was “eaten” by a black hole. But the evidence for this was maddeningly elusive.

A neutron star ripped apart by a black hole was the cause of GRB 050724, as seen here in this artist's illustration.

This past summer, three short bursts were detected that have cracked the mystery wide open. The first of the three bursts (GRB050509b) was observed by Swift on May 9, and lasted only 50 milliseconds. The X-ray afterglow seen after this burst was the first ever detected from a short burst. The GRB appeared to be located near the edge of an elliptical galaxy. Since these galaxies don’t contain young stars, this in itself was an important clue to the short burst's origin and supported the neutron star merger theory.

On July 9, the HETE-2 satellite detected another short burst, which had extensive follow-up by ground-based telescopes as well as the Chandra Observatory and the Hubble Space Telescope. The more accurate position for this burst ruled out the possibility of a supernova counterpart, and provided even better evidence for an origin due to a neutron star merger.

The light curve (brightness vs. time) of GRB 050724 showing the initial short blast of gamma rays.

On July 24, 2005, Swift detected the third short gamma-ray burst, GRB 050724, which lasted only 0.25 seconds. Ground-based observations quickly determined that the burst was approximately 3 billion light years away, and associated with an elliptical galaxy, similar to that suggested for GRB050509b.

Another important clue was that, similar to the HETE-2 burst GRB 050709a, X-ray emission for GRB 050724 persisted for some time following the initial short gamma-ray burst - flares of X-rays occurred minutes and even hours later. Most theorists believe that a neutron star merger should create a black hole with only a brief, intense flare, and no lingering emission -  so this third burst, and possibly the HETE-2 burst as well, may not have been from that kind of merger.

The secondary blasts of X-radiation may indicate that GRB050724 was caused by an incredibly violent event: the disruption of a neutron star by the enormous gravity of a black hole, the only force in the Universe powerful enough to tear apart a neutron star.  The ultradense material then formed a crescent-shaped stream, which, after orbiting for a few minutes, fell into the black hole, causing the later flares.

Observations of GRB 050724 showing the burst afterglow and the host galaxy (left), the galaxy after the burst faded (middle), and the processed image showing just the afterglow (right).

Although the initial event lasted only a fraction of a second, it put out as much energy as the Sun does in over 100,000,000 years. It took observations by Swift, HETE-2, and the cooperation of an international team of follow-up observatories to understand the causes of these mind-boggling events, and more such discoveries are expected in the months and years to come.

A NASA Science Update press conference on these exciting discoveries was held on October 5 that featured Swift PI Neil Gehrels, HETE PI George Ricker, follow-up lead scientist Derek Fox and 2 other prominent scientists from the astrophysics community.  See the Swift in the News article for the links to the press releases about all three of these bursts.

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Swift's Other Burst Observations

By: Peter Brown, Penn State

Swift ultraviolet image of supernova 2005ke in the spiral galaxy NGC 1371. The supernova is circled.

In addition to chasing Gamma Ray Bursts, Swift has also spent some time chasing other kinds of exploding stars -- supernovae. The Swift observations are very important because the Ultraviolet/Optical Telescope (UVOT) can measure the brightness of these explosions in the ultraviolet, most of which is blocked by the Earth's atmosphere (thankfully) and cannot be observed by ground-based telescopes. We are also simultaneously able to put limits on the amount of x-rays coming from the explosion with the X-Ray Telescope (XRT). Observations in multiple wavelengths like this give astronomers better insight on the physics and mechanisms of the explosion as well.

One type of supernova (designated Ia) is caused when an old, dead star called a white dwarf accumulates matter from a companion star. When enough matter piles up, it can undergo nuclear fusion, the energy source of thermonuclear bombs.

Swift ultraviolet image of supernova 2005cs in the nearby spiral galaxy M51, the Whirlpool Galaxy. The supernova is red, and to the right of the galactic center.

The effect is similar for the star, too: the white dwarf can explode. We hope to find out if these types of explosions are always the same brightness. This would help measure distances in the universe, just as you can tell how far away an oncoming car is based on the brightness of the headlights. An example of this is SN2005ke (the letters indicate the order it was discovered in the year 2005).  The ultraviolet image shows that its galaxy has lots of new stars forming in its spiral arms.

Other supernovae signal the early deaths of young, massive stars. Some of these might also result in Gamma Ray Bursts, which is why Swift is especially interested in them.  One of these is SN2005cs which exploded in the photogenic Whirlpool Galaxy. The pretty images the Hubble Space Telescope took some years ago have been used to identify the kind of star that exploded. The Swift UVOT observations showed the supernova to be extremely hot and bright, fading in the ultraviolet as it cooled very quickly. These supernova observations are among the most exciting of Swift's non-GRB science.

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GRB 050904: another milestone set by Swift

By: Giancarlo Cusumano, I.A.S.F

One of the most important discoveries that Swift has made in the last months is the detection of the most distant gamma-ray burst (GRB) ever observed.  This burst, known as GRB 050904, triggered the Burst Alert Telescope (BAT) onboard Swift on September 4, 2005; follow-up observations through ground-based optical telescopes determined its redshift to be 6.3 (the redshift is an indication of the distance of a cosmic source—the higher the number, the farther away the object is). This redshift value means that the burst occurred 12.8 billion years ago, only 890 million years after the Big Bang—when the Universe was 6% of its present age. GRB 050904 is by far the most distant cosmic explosion ever observed (the previous record for a GRB was at a redshift of 4.5) and one of the farthest cosmic sources detected (the most distant known quasar is at a redshift of 6.4, and the most distant galaxy at a redshift possibly as high as 7).

Due to the expansion of the Universe in the Big Bang, objects at high redshift appear to be receding from us at very large velocities. This has two main effects: one is to lower the energy emitted by the object (due to the Doppler effect), and the other is to stretch the apparent timescale of the event because of the effects of Einstein's relativity. Due to its large distance, most of GRB 050904’s gamma-ray energy emission was therefore shifted into the energy range of Swift’s X-Ray Telescope (XRT). The XRT’s excellent sensitivity therefore allowed the prompt gamma-ray emission of the burst to be observed with an unprecedented richness of detail. The XRT started observing the burst 161 seconds after Swift’s initial detection, but due to relativistic time contraction this translates to only 22 seconds after the actual explosion in the GRB’s timeframe.

The GRB’s gamma-ray emission was exceptionally high and its evolution over time particularly complex, showing the presence of several secondary explosions at times long after the initial explosion. The burst lasted more than 2 hours and it is among the brightest and longest GRBs ever observed. It represents an exceptional event among the GRBs detected so far, and its unique characteristics could be linked to its distance and to the different star evolution and environment in the early Universe.

Thanks to its sensitivity, Swift is detecting very distant GRBs, opening a new frontier in the study of these explosions. Phenomenological and statistical differences of the very distant GRBs may offer an important tool for studies of the early Universe. A paper about the Swift discovery and observations of GRB 050904 has been accepted for publication in "Nature".
See lightcurve for this burst

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