INDIAN X-RAY ASTRONOMY EXPERIMENT (IXAE)
Background and Status
The early hard X-ray and gamma-ray astronomy experiments carried
out by the Indian scientists 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 X-Ray astronomy groups were formed at TIFR and at
PRL around late 1960s. Development of an instrument with an orientable
X-Ray telescope for hard X-Ray observations was undertaken at TIFR. The first
balloon flight with this instrument was made on 28, April 1968 from Hyderabad 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. The PRL
group undertook studies of several cosmic X-ray sources with the
rocket-borne instruments launched from Thumba. Later the same PRL group carried
out studies of the energy spectra and intensity variations of Cyg X-1, Her X-1
etc., and the diffuse X-ray and low-energy gamma-ray background with balloon
observations made from Hyderabad. Japanese groups, in collaboration with the
TIFR and Osmania University, carried out simultaneous X-ray and optical
observations of Sco X-1 in a series of three balloon flights made during 1971-72
from Hyderabad. They also carried out observations of the Crab Nebula in
collaboration with TIFR in 1975 during its lunar occultation to measure the size
of the hard X-ray emitting region in it. Several notable results came from these
observations: (i) change in the X-ray intensity of Sco X-1 in the 30-50 keV band
by a factor of about 3 in one and a half hours (ii) variations in the intensity
of Cyg X-1 in the 22-100 keV interval over a time-scale of minutes and longer,
(iii) first detection of a hard X-ray flare in the black hole binary Cyg X-1,
(iv) measurement of the energy spectrum of the diffuse cosmic X-ray background
in the 20-120 keV range, (v) study of the energy spectra of Her X-1, Aql X-1
etc., in the 20-150 keV range.
Starting with the rocket flights made in
1968 by the PRL group, a number of observations of the discrete X-ray sources
and the diffuse X-ray background were made in the 1968-80 period with
rocket-borne instruments launched from India. Early results included the
spectral measurement of Sco X-1 and the transient X-ray source Cen X-2. The
energy spectrum of the supernova remnant Crab Nebula was studied by the TIFR
group with thin window proportional counters flown on a Centaur rocket from
Thumba in 1973.
The diffuse cosmic soft X-ray background was mapped in the 0.1-0.19 keV and 0.1-0.28 keV spectral bands in a sky survey experiment carried out from a rocket flight in 1979. Two 450 cm2 area proportional counters equipped with 1.5 µm thick polypropylene windows were used in this experiment. Several regions of excess soft X-ray emissions including the North Polar spur were detected. The temperature of the hot interstellar gas producing diffuse soft X-ray emission in the galaxy was derived by mapping intensity distribution of the soft X-ray background in different directions. It was concluded that the soft x-ray background is of local origin from a plasma of temperature ≈ 106K and size <330 pc.
The first Indian satellite Aryabhata launched in 1975 carried an X-ray astronomy payload consisting of a collimated proportional counter of 60 cm2 area and a collimated NaI detector covering the 2.5-155 keV range. An intensity transition in Cyg X-1 and the energy spectrum of the galactic bulge sources GX 17+2 and GX 9+9 were reported from this experiment. An X-ray sky monitor camera was placed on the Bhaskara-I satellite to detect and monitor the transient X-ray sources that suddenly appear and disappear in a manner analogous to optical novae.
X-ray astronomy studies are best carried out from satellite-borne
instruments as it enables observations of objects for long periods which is
necessary to study periodic phenomenon in them. An experiment with an X-ray
astronomy payload on a three-axis stabilised Indian satellite IRS-P3 was
launched in near Earth orbit with Polar Satellite Launch Vehicle (PSLV). The
main scientific objective is to study the periodic and non-periodic intensity
variations in different classes of X-ray sources as well as to measure their
spectral characteristics. The Indian X-ray Astronomy Experiment (IXAE) payload,
developed jointly by the TIFR and ISAC groups, has two principal components: (i)
A set of three collimated proportional counters (PPCs), with a field of view of
2o x 2o and effective area of 1200 cm2 for
pointed mode observations of specific targets for studying time variability and
energy spectra in 2-20 keV region; (ii) An X-ray pin hole camera called X-Ray
Sky Monitor (XSM) to continuously monitor the intensity of bright X-Ray sources
as well as to detect and study X-Ray
transients.
Each PPC is a multi-cell proportional counter array and has an
effective area of 400 cm2.
There are 54 cells with a size of 11 mm x 11 mm arranged in 3 layers. The bottom
layer and the end cells are joined together to form a veto output for charged
particle anti-coincidence. The remaining anode cells in the top two layer form
the detection volume and they operate in mutual anti-coincidence. A passive
collimator restricts the field of view to 2.3° x 2.3°. The operating energy
range is between 2 keV and 18 keV. The overall energy resolution is 22% at 6
keV. The gain stability of the detectors are monitored continuously by X-rays
from a collimated Cd109 radioactive
source irradiating the veto cell.
Each PPC having its own front-end electronics selects the genuine
events based on the pre-determined logic conditions and measures the pulse
height spectrum in 64 linear channels. In parallel mode, independent counters
store the following data i) 2 keV-6 keV genuine events of top layer. ii) 2
keV-18 keV genuine events of top layer iii) 2 keV-18 keV genuine events of middle
layer iv) >18 keV counts (ULD counts) for all layers, and v) >2 keV counts
from the veto layer. An microprocessor based system handles these basic data and
stores them in the on-board memory. The data storage is done in different modes
which can be set by ground commands. The two available modes are 1) count and
spectral mode where the five basic counts are stored in integration time of 0.01
or 10s and 64 channel spectra for three layers separately (top two layers left
and right separately) with integration time of 1, 10, 100 or 1000 s. 2) time
tagged mode where each event is time tagged to an accuracy of 0.4 msec (for
PPC-3) or 0.8 msec (for PPC-1) and for each event 8 channel linear spectral
information and layer information are also stored. The data storage can be
stopped and started by the use of time-tagged
commands.
The X-ray sky monitor consists of a position sensitive pin hole
camera to study X-ray transients and for long term monitoring of bright X-ray
binaries. It has a field of view of
900x900degree and can locate a bright star to better than 1 square
degree. The IXAE instrument is a part of the Indian Remote Sensing satellite
IRS-P3, which also includes a remote
sensing camera and an ocean monitoring instrument. IRS-P3 was launched
using the Polar Satellite Launch Vehicle (PSLV) on 21 March 1996 from
Shriharikota Range, India. The
satellite is in a circular orbit at an altitude of 830 km and inclination of
98°. Stellar pointing for any given source is done by inertial pointing by using
a star tracker. The pointing accuracy is about 0.1°. Since the remote-sensing
cameras form the primary payload, the observing time for 3-axes stabilised
stellar pointing mode is available for 3 to 4 months in a year. The high
inclination, high altitude is found to be very background prone and the useful
observation time is limited to the latitude ranges typically from -30° S to +50°
N. Further the large extent of the South Atlantic Anomaly (SAA)
region at an altitude of 800 km limits the observations to a duration of 20
minutes each in 5 to 7 out of 14 orbits during a day
.
The experiment in stellar pointing mode has observed bright
X-ray binaries like Cyg X-1, GRS1915+105, CRAB pulsar, Vela X-1, 4U1907+09 etc.
Some of the results are mentioned blow.
GRS1915+105 is a new
X-ray transient source discovered in 1992 using the WATCH experiment onboard the
GRANAT satellite. The object shows
erratic intensity variations on times scales as short as seconds to days to
months. The source is identified with a superluminal radio source and has been
termed as a microquasar. This source exhibits jets in the radio and IR bands.
Based on its X-ray luminosity and radio characteristics it has been suggested to
be a black hole.
This object is observed for a week in July 1996. The TIFR/ISAC
scientists have discovered quasi periodic oscillations in the frequency range of
0.62 to 0.82 Hz. This frequency did not vary over the duration of observation
period. In addition to this strong intensity variations over time scales of 0.1
to 10 s have also been observed. These variations are very similar to the
variations observed in black hole binaries and supports the theory that this
object consists of a black hole.
The intensity variations have also been analysed in terms of
shots. The background subtracted counts are compared in a bin with respect to a
running average. A shot is termed to occur if the counts in a particular bin
exceed the average and the shot lasts till the counts do not again reach the
running average level. The excess counts can also be expressed in terms of equivalent width.
The number distribution of shots computed from the observations of GRS 1915+105
and the duration of shots are shown to follow an exponential
distribution.
In addition it is found that the hard X-rays are delayed with
respect to the soft X-rays by about 0.2-0.4 s. This indicates that the soft
X-ray emitting regions are not the same as the hard X-ray emitting regions. Also
it supports the theory that hard X-rays may be the result of inverse Compton
scattering of soft X-ray photons in a plasma region which has a size of a
fraction of a light second.
Cyg X-1 is a well known X-ray binary source and the first
observational candidate in support of the existence of black holes. Cyg X-1 is a
high mass X-ray binary with a supergiant companion orbiting around a black hole.
Intensity variations from this
system has been reported on varying timescales from millisecons to days. Cyg X-1
is a source which generally exhibits a hard spectrum and spends most of its time
in hard state but at times also exhibits a soft state. The hard X-ray spectrum
is described by a power law with photon index of -1.5 at all times but the soft X-ray
spectrum varies considerably in the two
states.
This source is observed in the soft state and also in the hard
state prior to the transition. In the hard state (May 1996) the power density
spectrum (PDS) at frequencies (0.3 to 10 Hz) is steep as compared to that at
(0.01 to 0.3 Hz) with a break frequency of 0.3 Hz. This is very much different
as compared to the PDS obtained using ASM on RXTE at lower frequencies of 0.01
to 0.3 Hz. In the soft state the power law index at frequencies above 0.03 Hz is
less steep.
The short term intensity variations can be analysed in terms of
shots. The number distribution of the shot equivalent width can be described in
terms of an exponential function with e-folding constant of 0.35s in the hard
state and e-folding constant of 0.2 s in the soft state. Shots are termed to
occur due to the fuelling of accreting matter from the inner disc on the black
hole by means of instability. The inner portions of the disc may consist of many
small reservoirs each of which gets into instability if a critical mass is
reached due to gradual mass diffusion in the reservoir. These observations
indicating a change in e-folding time in the two states of Cyg X-1 provide vital
inputs for shot modelling as a function of the two spectral states.