BALLOON X-RAY ASTRONOMY EXPERIMENTS FROM INDIA

Background Information

1.   Introduction:

Indian space era dawned when the first two-stage sounding rocket was launched from Thumba in 1963. However even before this epoch making event, noteworthy contributions were made by the Indian scientists in the following areas of space science research: 

*         Cosmic rays and high energy astronomy using both ground based as well as balloon borne experiments/studies such as neutron/meson monitors, Geiger Muller particle detectors/counters etc. 

*         Ionospheric research using ground based radio propagation techniques such as ionosonde, VLF/HF/VHF radio probing , a chain of magnetometer stations etc. 

*         Upper atmospheric research using ground based optical techniques such as Dobson’s spectrophotometers for measurement of total ozone content, airglow photometers etc. 

*         Indian astronomers have been carrying out major investigations using a number of     ground based optical and radio telescopes with varying sophistication.  

With the advent of Indian space programme, main emphasis was laid on indigenous, self-reliant and state-of-the-art development of technology for immediate practical applications in the fields of improved communication, broadcasting, weather prediction, assessment of natural and man-made resources.  In addition to implementing such down-to-earth utility projects, ISRO has been providing necessary thrust for conducting basic space science research activities in the country being pursued by a number of research institutions and universities. ISRO’s efforts in encouraging and growing space science research activities in national institutions/universities have been mainly directed to provide national facilities for conducting space borne experiments using balloons, sounding rockets and satellites.  

There is a national balloon launching facility at Hyderabad jointly supported by TIFR and ISRO. This facility has been extensively used for carrying out research in high energy (i.e., x- and gamma ray) astronomy, IR astronomy, middle atmospheric trace constituents including CFCs & aerosols, ionisation, electric conductivity and electric fields.

2.      High energy astronomy programmes- achievements and prospects 

High energy astronomy or the X-ray and gamma ray astronomy field, unlike optical astronomy, requires space platforms for observations. Earth’s atmosphere acts as a shield preventing the solar as well as celestial X-rays and gamma rays to reach the surface. Hence the whole new field of high energy astronomy opened up with the advent of space research and in particular satellite technology. However early work was carried out with balloon and rocket-borne instruments. Satellite-borne studies in this area became possible only during the seventies. 

3.   X-ray Astronomy-Global scene 

The new branch of X-ray astronomy was born in 1962 when the first extra-solar X-ray source ScoX-1 was accidentally discovered in a sounding rocket flight experiment to detect X-rays from the moon. It was found that Sco X-1 had an X-ray luminosity of over a million times that of the luminosity of the sun and over a thousand times the total luminosity of the sun in all wavelengths. This clearly indicated that X-ray stars may in fact be a different class of objects as compared to sun like stars. A large number of X-ray sources were discovered soon after by a succession of rocket-borne observations. Precise determination of the position of Sco X-1 followed by its optical identification in 1966 and determination of the position and size of the X-ray source in Crab Nebula, which was identified as a supernova remnant, during a lunar occultation in 1964, gave a tremendous impetus to the development of X-ray astronomy. By 1970 when the first X-ray astronomy satellite UHURU was launched the number of known X-ray sources was already close to 60. The launch of the UHURU satellite was a major landmark in the history of  astronomy as it brought about a profound change in our concepts both relating to theory and observations because of the scope of the field by discovery of X-ray emission from a variety  of galactic and extragalactic objects. The discovery of pulsating X-ray sources Cen X-3 and Her X-1 and revelation of their binary nature from UHURU observations demonstrated that accretion of matter onto a compact object in a binary system plays a dominant role in the production of X-rays in these stars. It was soon established that a majority of bright X-ray sources in our galaxy are binary stars powered by the release of gravitational energy by matter accreting onto a magnetised neutron star. Based on these considerations it was realised that X-ray emission occurs close to objects of high density  (~10¹⁴gm /cc) having regions with temperatures over a million K and probably having extreme magnetic fields (10¹²Gauss). Some of the emission processes include thermal and non-thermal Brhemstrahlung, synchrotron emission and inverse Compton radiation. X-ray astronomy studies have also provided the first credible observational evidence for the existence of black holes in nature. The 5.6-day period X-ray binary Cyg X-1, in which the X-ray source is most likely a black hole with mass greater than 5 times the solar mass, is the most well-known example of a likely black hole. These earlier studies were all confined to 2-10 keV energy region. In mid-sixties, from balloon-borne observations of Cyg X-1 and Crab Nebula, it was established that these sources also produce hard (>20 keV) X-rays. Both of them were found to be rather bright sources in 20-60 keV band. In the following decade hard X-ray studies of many more objects were carried out with balloon-borne X-ray telescopes culminating in the first hard X-ray survey of the sky with the HEAO-A4 experiment. 

The UHURU satellite was followed by a succession of other X-ray observatories, which include Ariel-5, HEAO-1, Einstein Observatory, EXOSAT, Ginga and ROSAT. The imaging X-ray telescope on the Einstein X-ray observatory, launched in 1978, brought about a revolutionary change in X-ray astronomy owing to its orders of magnitude higher sensitivity and imaging capability. For the first time it became possible to do structural and spectroscopic studies of a variety of extended and compact objects. The more recent ROSAT mission employing scanning X-ray optics for the first time has discovered over 60,000 X-ray sources thus enabling a statistical study of the X-ray stars. In July,1999 NASA has recently launched its Chandra X-ray observatory with four pairs of collimating mirrors and science experiments to record as accurately as possible the number, position and energy of the incoming X-rays in order to make an X-ray image and study other properties of the source, such as its temperature. The incoming X-rays are focused by the mirrors to a tiny spot (about half as wide as a human hair) on the focal plane, about 30 feet away. The four science instruments have complementary capabilities to record and analyse X-ray images of celestial objects and probe their physical conditions with unprecedented accuracy. The first  Chandra imagery showed the remnant of an exploded star in the Large Magellanic Cloud. It showed a highly structured remnant, or shell, of 10-million-degree gas that is 80 light years across. The remnant is thought to be about 3,000 years old. The Large Magellanic Cloud, a companion galaxy to the Milky Way, is 180,000 light years from Earth.

Some of the other currently working satellites include the Rossi X-ray Timing Explorer (RXTE), the Beppo SAX experiment and the Japanese Advanced Satellite for Cosmology and Astrophysics(ASCA) in addition to the Indian X-ray Astronomy Experiment (IXAE) on board IRS-P3. The RXTE has enabled the long term temporal studies of different objects with resolution down to microseconds proving the capability of a timing mission.  The Beppo SAX experiment has both narrow and wide field instruments with a larger energy coverage of 0.1 to 300 keV.  The ASCA with its fine spectral resolution capability all the way upto 10 keV has heralded the era of X-ay spectroscopy. 

4.      Gamma ray astronomy(Global scene) 

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 yet another 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 spacecraft’s 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.  

Currently, the gamma ray burst monitoring network includes 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.  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 our 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.  

5.      Indian Scenario: Hard X-ray (>20 keV) Astronomy Studies with Balloons 

There are some unique advantages of conducting hard X-ray and gamma-ray studies from India. The flux of primary cosmic rays over Hyderabad is only about one-fifth or less of that at Palestine and other launch stations in the USA from where most balloon flights are conducted. The flux of secondary particles and X-ray and gamma-rays of atmospheric origin produced by the interaction of the cosmic rays is also correspondingly low. This low background, in the presence of which one has to detect the feeble signal from cosmic sources is a major advantage in conducting hard X-ray observations from India. This was an important consideration in the early hard X-ray and gamma-ray astronomy experiments which used a simple collimated sodium iodide (NaI) detector with no active shield of NaI/Caesium Iodide (CsI), like the ones used in the later improved detector systems, to discriminate against atmospheric background. The second advantage is that many bright sources like Cyg X-1, Crab-Nebula, Sco X-1 and Galactic Centre sources are observable from Hyderabad due to their favourable declination. With these considerations, an X-Ray astronomy group was formed at TIFR in 1967 and development of an instrument with an orientable X-Ray telescope for hard X-Ray observations was undertaken. The first balloon flight with the new instrument was made on 28, April 1968 in which observations of Sco X-1 were successfully carried out. In a succession of balloon flights made with this instrument between 1968 and 1974 a number of binary X-ray sources including Sco X-1, Cyg X-1, Her X-1 etc. and the diffuse cosmic X-ray background were studied. Many new and astrophysically important results were obtained from these observations.