Science with the ASTROSAT Large Area Hard X-ray Detectors :

Large Area Proportional Counters (LAXPCs) and

Cadmium-Zink-Telluride (CZT) Array

One of the Principal Scientific Objectives of the ASTROSAT is broad band X-ray spectroscopic studies of binary X-ray sources, AGNs and other Galactic and extragalactic X-ray sources. It is proposed that the thermal and non-thermal components in the X-ray spectra of these sources will be decoupled to understand the energy generation and dissipation mechanisms. For this purpose, a soft X-ray imaging telescope with an X-ray CCD at its image plane and a large area proportional counter for the hard X-ray band are proposed to be used. Lack of efficient hard X-ray focusing devices, difficulties in building large area good energy resolution hard X-ray detectors and the existence of multiple spectral components in Cosmic sources have made the hard X-ray continuum spectroscopy one of the most difficult and challenging branches in observational astronomy. The ASTROSAT satellite aims to fill this niche area.

Three Large Area X-ray Proportional Counters (LAXPCs), each with an effective area of at least 2,000 cm² and a total minimum effective area of 6,000 cm² alongwith a CZT array with an area of about 1,000 cm² are proposed to constitute the hard X-ray detector system on board the ASTROSAT. A large depth of 15 cm of the LAXPC detectors, and high gas pressure of 2.5-3.0 atm along with the large geometric area will make it a very sensitive instrument in a wide energy band of 2-100 keV. In the 20-100 keV energy band, the proposed LAXPC detector array will have an effective area that is about 6-10 times larger than that of the present largest instruments in this energy band, the PCA and HEXTE detectors onboard the American satellite Rossi X-ray Timing Explorer (RXTE) and the PDS detector onboard the Dutch-Italian satellite Beppo-SAX. A comparison of the LAXPC and CZT array effective areas with the other presently operating instruments is shown in the adjoining figure. The combination of the LAXPC array, with its unprecedented sensitivity in the hard X-ray band and the Soft X-ray Telescope (SXT) with its low energy sensitivity and very good spectral capability will make ASTROSAT an unique observatory in its time frame. The CZT array with its superior energy resolution (about 3% at 6 keV and 1.5% at 60 keV) will carry out the spectral measurements in the 10-100 keV band with an accuracy which will be unmatched by any existing or planned mission in the 2005 time frame. The CZT detector array will (a) extend the energy band-width to 200 keV for bright X-ray sources, (b) measure the contribution from the sky X-ray background from 4 keV to 200 keV, simultaneously with the source intensity measurements, (c) enable a simultaneous spectral fit to the X-ray spectra so as to get a measure of systematic errors in the data, and (d) allow us to measure the contribution from confusing neighboring sources.

The minimum effective area of the LAXPC array will be 6,000 cm² upto 50 keV

and will have an equivalent area of 2,000 cm² even at 100 keV. The sensitivity of the LAXPC detectors has been estimated for a nominal orbit of altitude 600 km. The computation shows that with an exposure of 1 day one will be able to obtain spectra with good statistical significance for a 0.1 milli Crab intensity X-ray source. A Crab nebula like X-ray source will give 12,000 counts per sec in the LAXPCs. One will be able to successfully search quasi-periodic oscillations form the X-ray sources in the kHz range if the source intensity rises above 50 milli Crab level.

Owing to its very large effective area (6,000 cm²) , high detection efficiency over a broad energy band (~100% below 60 keV and ~50% in 60-100 keV with the LAXPCs and 100% at 100 keV with CZT array), high time resolution (10 micro-second), and good energy resolution (8% and 1.5% at 60 keV with the LAXPC and CZT array respectively), we will be able to carry out internationally competitive and frontline research work in a variety of astrophysical subjects which are presently of great interest in astrophysics. The cosmic X-ray sources , which will be observed with these detectors, range from the nearby solar-mass Galactic X-ray binaries to the largest structures in the universe, the clusters of galaxies. The important astrophysical questions to be probed by the LAXPCs and the CZT array and in many cases in conjunction with the low energy counterpart, the Soft X-ray Telescope (SXT) of the ASTROSAT, are related to the physics of :

(a) Neutron stars in the form of low magnetic field ordinary pulsars and high magnetic field X-ray pulsars,

(b) Black holes of a wide mass range from the solar mass Galactic variety, medium mass predicted in some nearby galaxies, to the supermassive ones in the center of the Active Galactic Nuclei (AGN),

(c) Accretion onto the stellar mass and the supermassive compact objects.

(d) 'Magnetars', objects with the highest known magnetic field strength in the universe,

(e) Nonthermal processes in the largest structures in the universe, the clusters of galaxies,

(f) Investigation of the hard X-ray background radiation and identification of its possible constituents.

Some of the most interesting scientific topics to be probed in the initial stage of the operation of the ASTROSAT hard X-ray instruments are briefly mentioned here.

• Measurement of magnetic field strength of the X-ray pulsars will be performed through detection of cyclotron absorption lines. The sensitivity of the LAXPCs and the CZT array for this measurement is unmatched compared to any other existing or upcoming experiment. We will characterise the broad band spectrum of the accreting binary X-ray pulsars extending upto 100 keV, this has so far been limited only to a handful of bright sources. Our observations of pulse period variations with luminosity in the transient X-ray pulsars will also improve the understanding of the accretion disk-magnetosphere interactions in these sources.

• Characterisation of the broad band X-ray spectrum of Galactic and extragalactic black hole sources will be carried out. In most of these sources, the spectrum in the 20-80 keV band is not available due to lack of sensitive measurements. LAXPCs, alongwith the CZT array will be able to measure the hard X-ray spectrum accurately for a very large number of galactic and extragalactic black hole sources.

• Discovery of new `Accreting Millisecond X-ray Pulsars' will be possible by LAXPC follow-up observations of the transient sources that will be discovered by the Scanning Sky Monitor (SSM) of the ASTROSAT. These objects are the missing link between the millisecond radio pulsars and the low mass X-ray binaries and the discovery of a few of this type of objects will give a complete understanding of the evolution of neutron stars. Only one of this type of object is known so far.

• Do the Anomalous X-ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs) have `magnetars' (neutron stars with magnetic field strength of ~10¹⁵ G) ? Detection of glitches and nonlinear pulse period evolution of AXPs and SGRs is a very important subject to study and is a test of the magnetar hypothesis. After RXTE, LAXPCs will be the only instrument capable to carry out continuous temporal study of about a dozen of objects suspected to be magnetars.

• Nature of the Kilo Hertz quasi-periodic oscillations (kHz QPOs) of the X-ray radiation in the low mass X-ray binary systems is yet to be measured in the hard X-ray band. For most sources, the RXTE-PCA measurements extend only upto about 15 keV. With the LAXPCs, we hope to be able to probe the kHz QPOs and kHz oscillations upto about 50 keV.

• Among the other type of transient sources, temporal and spectral measurements of X-ray emission from new micro-quasars will be performed. Detection of high frequency QPOs from BHCs will give us a way of measuring the mass and spin of the black holes.

• With with an energy resolution of 3% at 6 keV and effective area of 500 cm² of the CZT detector array, we will be able to measure the strength of the Iron K-alpha emission line in X-ray sources with very good accuracy. This will allow us to properly quantify the thermal components in the X-ray spectra. We will also be able to study the variability of emission line strength and its relation with the continuum variability of the AGNs.

• Quasi-periodic and aperiodic variations of AGNs has not yet been studied in detail. Very long exposures of some selected AGNs will help us to detect and verify quasi-periodic intensity variations and also to calculated the power density spectrum of AGNs. With these measurements, we will be able to compare the temporal properties of the AGNs with their galactic analogues, the BHCs.

• We aim to make accurate measurement of the Cosmic X-ray Background (CXB) spectrum in the hard X-ray band. The quasars are believed to contribute most of the soft X-ray background (0.1-2.0 keV band), while normal galaxies and some obscured AGNs are believed to constitute the medium energy X-ray background (2-10 keV band). Accurate spectral measurements of different types of AGNs in the hard X-ray band may lead to identification of the constituents of CXB in this energy band.

• Nonthermal emission component in the cluster of galaxies is an unresolved mystery. Observation of several clusters of galaxies with the LAXPCs and the CZT array, which have very high hard X-ray sensitivity will allow us to study the Compton scattering of the cosmic microwave radiation in the clusters. We will be able to measure the magnetic field strength in the galaxy clusters and also the energy density of the electrons.