2004 ASTRONOMY & ASTROPHYSICS - Indian Academy of Sciences

CHAPTER 6
accessible. This advance would be equivalent to
going from UHURU to EINSTEIN in the soft X-rays.
The development of a hard X-ray imaging
telescope, based on multi-layered mirrors in a
configuration similar to the soft X-ray telescope
proposed for ASTROSAT, is now technically
feasible and scientifically compelling. Therefore,
a programme to develop hard X-ray mirrors and
hard X-ray detectors could be initiated as one of
the most promising areas of future
development in X-ray astronomy.
A simultaneous development of solid state
detectors like CdZnTe and CdTe would
provide the best match for hard X-ray mirrors. It
would be desirable to undertake such a detector
development in collaboration with institutions
such as Indian Institute of Science, Bangalore;
Centre for Advanced Technology, Inder; Bhabha
Atomic Research Centre, Mumbai, etc.
The detector and optics development should
address two goals – efficiency and reasonable
spectral capability in hard X-rays, and high
spectral resolution in soft X-rays. Although a lot
of spectral work in soft X-rays will be done by
Chandra and XMM Telescopes, there will still be
ample scope for further work if one has good
spectral resolution.
2. Sky Monitors
Development of an efficient and light soft X-ray
detector technology would also be of help in
taking up another area where we could make
some impact, namely, monitoring the X-ray sky.
Having an X-ray sky monitor up is always
useful, and the current programme of X-ray
missions leave large gaps of time between the
availability of such instruments. We could think
of a sustained programme of building and
launching relatively light, low-power, X-ray sky
monitors as piggyback payloads or small
spacecrafts in themselves, in addition to those
which might be integrated into future X-ray
astronomy missions. One should aim to reach
at least an order of magnitude improvement in
sensitivity over RXTE ASM in the near term. If
we establish such a programme, we would
most likely be able to collaborate with, and
have observing possibilities on, major X-ray
missions planned by other countries. Continuous
availability of a sensitive monitor would help
address, among other issues, the nature of the
multitude of X-ray transients in the Galaxy.
3. X-ray Polarimetry
X-ray polarimtery has the potential to provide a
direct picture of the state of matter in extreme
magnetic and gravitational fields. In binary
pulsars, for example, it would allow one to
probe the geometry of the magnetic field.
Similarly, a polarization map would provide
clinching evidence for the presence of a central
rotating Kerr black hole at the centres of active
galaxies. Polarimetry will enable one to
separate the thermal emission from synchrotron
emission, etc. Lack of sensitivity has, however,
hampered any progress in the direction of
utilization of X-ray polarimetry as an important
diagnostic. Recent developments in X-ray
polarimetry have led to a factor of 100
improvement in sensitivity.
Efforts are underway at TIFR to design gasbased
proportional counters that are sensitive to
the electric field orientation of the incoming
X-ray photon. Polarization detectors, if
successfully demonstrated in laboratory
conditions, would form an exciting payload on a
small satellite to study the high degree of
polarization expected from pulsars, pulsar
produced nebulae and γ-ray burst sources.
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