Science with Astrosat LAXPC
The three LAXPC detectors, with their unprecedented sensitivity in the hard X-ray band, combined with the Soft X-ray Telescope (SXT) with its low energy capability will make ASTROSAT an unique observatory in its time frame. The estimated sensitivity of the LAXPCs is such that with an exposure of 1 day one will be able to obtain spectra with good statistical significance for an X-ray source with a flux of 0.1 milli Crab.
A Crab nebula-like X-ray source will register 12,000 counts per sec in the LAXPCs. One will be able to successfully search quasi-periodic oscillations from the X-ray sources in the kHz range if the source intensity rises above 50 milli Crab level. The sources to be observed with these detectors will range from the nearby solar-mass Galactic X-ray binaries to the largest structures in the universe, the clusters of galaxies. Some of the important astrophysical areas to be probed by the LAXPCs on the ASTROSAT are the following:
1. Broad band spectrum of accreting binary X-ray pulsars will be characterized to energies extending to 80 keV - a study that has so far been limited to only a handful of bright sources. Measurement of magnetic field strength of the X-ray pulsars will be performed through detection of cyclotron absorption lines.Due to larger area and better energy resolution, the sensitivity of the LAXPC array for this measurement is unmatched in comparison to any other existing experiment. The simulation shown in the figure below demonstrates the superior capability of the LAXPC to detect cyclotron lines in a source such as 4U0115+63.
Simulated X-ray spectrum of the X-ray Pulsar 4U 0115+63 with LAXPC and CZTI on Astrosat. Cyclotron resonance Absorption features are expected to be detected unambiguously with better sensitivity compared to the earlier measurements.
LAXPC 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.
2.Characterisation of the broad band X-ray spectra of Galactic and extragalactic black hole sources will be carried out. LAXPCs will be able to measure the hard X-ray spectrum accurately for a very large number of galactic and extragalactic black hole sources.
3.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 ASTROSAT Scanning Sky Monitor. These objects are the missing link between the millisecond radio pulsars and the low mass X-ray binaries and the discovery of many more of this type of objects will give a complete understanding of the evolution of neutron stars.
4.Possible glitches and non-linear pulse period evolution of Anomalous X-ray Pulsars (AXPs) and Soft Gamma Repeaters (SGRs) are important studies related to the test of the Magnetar hypothesis. After RXTE, the LAXPCs will be the only instrument capable of carrying out continuous temporal study of about a dozen of objects suspected to be magnetars.
5.Detection of the kilo-Hertz quasi-periodic oscillations (kHz QPOs) in X-ray radiation in the low mass X-ray binary systems is yet to be made in the hard X-ray band. For most sources, the RXTE-PCA measurements extend only up to about 15 keV. The LAXPCs will be able to probe the kHz QPOs and kHz oscillations up to about 50 keV.
6.Non-thermal hard X-ray emission component reported in some clusters of galaxies is an unresolved mystery. Observations of several clusters of galaxies with the LAXPCs will enable the study of Compton scattering of the cosmic microwave radiation in them. These measurements will also allow the estimation of the magnetic field strength and the energy density of relativistic electrons in galaxy clusters.
7.Temporal and spectral studies of X-ray emission from micro-quasars will be performed to understand the accretion process close to the central black hole. High frequency QPOs from the micro-quasars can be detected with high sensitivity by LAXPCs, providing a tool for measuring the mass and spin of the black holes.
8.Quasi-periodic and aperiodic variations of Active Galactic Nuclei (AGNs) have not yet been studied in detail. Very long exposures of some selected AGN will help detect and verify quasi-periodic intensity variations as well as calculate the power density spectrum of AGNs. With these measurements, one will be able to compare the temporal properties of the AGNs with their galactic analogues, the Black Hole Candidates.