A Mystery:
Gamma-Ray Bursts

The sky glows at gamma ray wavelengths as well as visible ones. If our eyes could see gamma rays (and the atmosphere didn't absorb them), the sky would look like the adjacent image, which shows the intensity of high energy gamma rays plotted in galactic coordinates as measured by the Compton Gamma Ray Observatory (Ref). The diffuse glow running horizontally through the image is from gamma ray sources in the plane of our galaxy. The bright spots to the right of center in the galactic plane are pulsars. Brighter spots above and below the plane of the galaxy are gamma rays coming from distant quasars. Many of the fainter sources are of unknown origin at this point.

In addition to the constant flux of gamma rays illustrated in the adjacent image, there are also observed sudden pulses of gamma rays lasting typically seconds in which enormous amounts of energy appear to be poured out in gamma rays by still unknown sources. These Gamma Ray Bursts pose one of the greatest mysteries of modern astronomy. First discovered by military satellites looking for the telltale signature of clandestine nuclear explosions, they are now studied extensively for their scientific merit. Here is a history of their discovery.

Credit: NASA, Compton Gamma Ray Observatory, BATSE

Distribution of Gamma Ray Bursts

On average, about one every day the gamma-ray sky lights up with a spectacular explosion. The above map was created by Robert Nemiroff and represents the entire sky in galactic coordinates. In this map, spot size is proportional to peak flux during the gamma ray burst (which can be as short as a few seconds) and spot color indicates the average energy, with the more blue dots being highest in energy. The map shows the positions of over 800 gamma ray bursts detected by the BATSE instrument on NASA's Compton Gamma Ray Observatory. (BATSE stands for "Burst and Transient Source Experiment") The BATSE Group to date has about 2000 gamma ray bursts recorded. The following image shows the position of 2000 bursts detected by BATSE, with no indication of strength or average energy.

Before the systematic BATSE observations, a common hypothesis was that gamma ray bursts occurred in the disk of our galaxy. The above sky maps contradict this hypothesis because it indicates that the distribution of bursts is isotropic (no preferred direction). This tells us that either the gamma ray bursts come from events at great distances (cosmological distances), or that they come from events in the more spherical halo of our galaxy.

The Distance to Gamma Ray Bursts

More recent observations make it highly likely that gamma ray bursts are not coming from our galaxy, but are at very large distances. This makes them extremely interesting, because for them to be seen at very large distances they must correspond to events in which as much as 100 times the energy of a supernova is being liberated in a short period in the form of gamma rays. Furthermore, the mechanism producing the gamma rays must be such as to allow the gamma rays to escape without too much interaction with surrounding matter, because that interaction would convert the gamma rays to light of longer wavelength.

Identification of Optical Counterparts

Recently, substantial progress has been made in understanding gamma ray bursts. For the first time, it has been possible to correlate some gamma ray bursts with other sources in the visible, RF, and X-ray portions of the spectrum. The breakthrough was the ability of a small Dutch-Italian satellite called BeppoSAX to pinpoint the position of gamma-ray bursts with 2 arcsecond resolution in a matter of hours. This permitted other satellite and ground-based instruments to look quickly at the burst sight, and this in turn has allowed transient fading sources at other wavelengths to be correlated with the burst. The adjacent image shows an X-ray transient observed by BeppoSAX at the location of a previously seen gamma-ray burst (Ref).

Such observations have allowed a distance to be estimated to the gamma ray bursts because spectral lines and their Doppler shift have been observed in the transients after the burst. Assuming these transients to be near the location of the gamma ray burst and the Doppler shifts to be Hubble redshifts, these observations have almost conclusively shown that gamma ray bursts are at cosmological distances and not in the halo of the local galaxy.

What Are They?

Such enormous amounts of energy suggest a gravitational source. Two popular ideas are the merger of two black holes or the merger of two neutron stars. However, although such events might yield the required energy, it is not clear that they can be made consistent with all the observations. Thus, the source of gamma ray bursts remains one of the most important mysteries in modern cosmology.

SOURCE: http://antwrp.gsfc.nasa.gov/apod/ap950827.html

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