GAMMA-RAY ASTRONOMY RESEARCH WITH SROSS GRB EXPERIMENT 

Extension of high energy astronomy to gamma rays with energies ranging from 100 keV to hundreds of MeV beginning with Explorer-11 in 1961, opened up a new field of high energy astronomy. Interestingly unlike the detection of X-ray sources which came as a total surprise, gamma ray emission was indeed expected from interaction of high energy cosmic ray nucleons with galactic matter, annihilation of matter with anti-matter and nuclear de-excitation from freshly synthesised material in stars and supernovae. The discovery of gamma ray sources however had to wait till the development of very sensitive and large area detectors to overcome the low flux and low interaction cross-section of gamma rays with detector material. Following Explorer-11, a number of spacecrafts such as OSO, Cosmos, SAS-2, COS-B, HEAO-3 and Compton Gamma Ray Observatory (CGRO) have been flown to investigate both diffuse gamma ray emission and discrete gamma ray sources like Crab and Vela pulsars pulsating at the same frequency as their radio counterparts, 3C-273 quasar, Seyfert galaxies and Geminga, the second largest gamma ray source after Vela. The time variability of the gamma ray sources and their spectroscopic observations have greatly helped in revealing the nature and energetics of these celestial objects. Likewise, the association of gamma ray emission with X-ray and optical emissions have helped in understanding the mechanism of production of these radiations in stellar objects. 

Equally interesting has been the detection of cosmic gamma ray bursts (GRBs), first reported in 1973, which remain an unsolved puzzle in high energy astrophysics. These  transient  events  may have duration ranging  from a few milliseconds  to  several hundreds  of seconds. These short duration events are so energetic that they exceed the total luminosity of a galaxy if they are at cosmological distances as conjectured by many of the current models of these bursts. Despite deep searches to identify their counterparts at other wavelength windows of the electromagnetic spectrum, the gamma ray burst sources have only recently (February and May 1997) been identified in other wavebands like X-ray and optical. Because of this reason, the distances to the sources and the total energy involved in  the  emission process  have not yet been established.

The gamma ray burst monitoring network include the Compton Gamma Ray Observatory (CGRO) in a near earth orbit, the Granat satellite in a highly eccentric orbit around the Earth, the Ulysses spacecraft in a solar polar orbit, and the WIND spacecraft at the first Lagrangian point of the Earth-Sun system in addition to the Indian Gamma Ray Burst (GRB) experiment on board SROSS-C2 which entered earth's atmosphere only recently (2001). (SROSS-C2 mission details).  The Burst and Transient Source Experiment (BATSE) on CGRO has been detecting about one burst per day and has therefore recorded over a thousand bursts. The most astounding result from BATSE data is that the angular distribution of GRB sources is found to be isotropic, with no concentration towards the galactic plane, or in directions of any nearby galaxy or clusters of galaxies. In addition the integral brightness distribution of these sources shows a depletion of weaker bursts with respect to the expected -3/2 power law. This would indicate that the spatial distribution of the sources, in addition to being isotropic, is also confined. These findings have important consequences to the understanding of the  nature  of  GRB  sources,  their distances and energetics. They have led to extensions,  revisions and/or new hypotheses of galactic, extragalactic and cosmological models.

Gamma ray astronomy research in India

TIFR scientists in collaboration with Russian scientists have conducted several gamma-ray astronomical observations employing spark-chamber telescopes flown on balloons from Hyderabad during 1977-80. In one of the balloon flights in November 1980, they scanned the galactic anticentre region, and  detected the Seyfert galaxy 3C 120 for the first time in gamma-rays with E>5 MeV. The measured flux from 3C120 at E>5 MeV is (3.6 ±1.2) x 10^-4 photons  cm^-2s^-1. This corresponds to a luminosity of L=2.3 x 10⁴⁶ erg s^-1 for isotropic emission, which is about 100 times its X-ray luminosity.

Observations and results from SROSS-C2 GRB experiment 

The GRB payload on the SROSS-C2 satellite has functionined very satisfactorily for ~5 years, and its performance in orbit  conforms to the design specifications. More that 25 candidate events have been recorded, and many of these have been confirmed by other satellites in interplanetary space. The observed events have a variety of temporal profiles exhibiting varying degrees of complexities. A bimodal distribution  of  event  durations and variation  in hardness  ratio have been indiacated. A spectral softening in the energy range 35-300 keV for the event GRB940730 is  observed by SROSS-C2, BATSE and Ulysses.  With  SROSS-C2 remained in orbit for  about  5  years, the  GRB  experiment  worked as part of the interplanetary  network  of  GRB detectors  for triangulation  and localisation of several  events.  

The extraordinary amount of energy involved in the gamma ray bursts is generally believed to be the result of internal rearrangement in a neutron star (Star quakes) or thermo-nuclear explosion of material from the companion star spiralling into the neutron star. Some scientists attribute the origin of these mysterious gamma ray bursts coming from all parts of the sky, to the distant collision between two neutron stars of a binary system. Interaction between pairs of neutrinos and their antimatter counterparts as they crash into each other producing positrons and electrons which in turn annihilate each other resulting in gamma ray bursts could be an alternate explanation for the origin of such phenomena. While the BATSE instrument on CGRO in particular has produced a wealth of data of about 2000 celestial gamma-ray bursts, a proper understanding of these is yet to emerge. Almost numerous theories have been proposed to explain the wide variability in the temporal, spectral and energy characteristics of the observed gamma-ray bursts, even though none of these is completely satisfactory. Gamma ray burst phenomena continues to be a great enigma even after over two decades of investigation.