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
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.
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.
Compton Gamma Ray Observatory,
Distribution of Gamma Ray Bursts
On average, about one every day the
gamma-ray sky lights up with a
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
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
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
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.