AstroSat Completes One Year in Orbit
The Indian multi-wavelength space astronomy observatory AstroSat has completed one year in orbit on September 28, 2016. The spacecraft has orbited the Earth more than 5400 times and has executed 343 individual pointings to 141 different cosmic sources. During this scientifically rewarding one-year, several celestial sources have been observed and studied in detail. The results obtained are as expected and scientific findings are being published. The satellite is functioning normal and all the state-of-the-art world-class payloads are being operated as planned. A science meet was organized at IUCAA on September 29, 2016 to commemorate the one year completion of AstroSat.
AstroSat observes the Universe in optical, near and far Ultraviolet and X-ray regions of the electromagnetic spectrum. The interesting feature is the simultaneous multi-wavelength capability on the same satellite platform. These studies are being extended by co-ordinated observations using other spacecrafts and ground based observatories.
The first six months were dedicated to performance verification and onboard calibration of payloads indicated, which has shown that all the payloads conformed to the designed parameters. The first set of scientific observations have been completed from April to September 2016 as guaranteed time for the payload teams.
Some of the salient science observations are:
- Large Area X-ray Proportional Counter (LAXPC) has observed rapid variability of high energy (particularly >20keV) X-ray emission from a black hole system, GRS 1915+105 for the very first time.
Figure shows the power density spectrum for the source GRS 1915+105. As seen in the figure, Quasi periodic oscillations (QPOs) at 2.55 Hz are observed in the energy band 20 – 80 keV for the first time. This QPO changes in frequency as the flux of the source changes.
LAXPC, also measured the arrival time difference between the high and low energy X-rays (which is of the order of tens of milli-seconds). This provides direct clues to the geometry and dynamic behavior of the gas swirling around a spinning black hole (Ref: arXiV:1608.07023 [astro-ph.HE]). A publication based on the above results is accepted in Astrophysical Journal.
The Cadmium Zinc Telluride Imager (CZTI) normally operates in the energy range 15 – 100 keV. At higher energies, the Field of View of this imager becomes wide and therefore it can detect Gamma Ray Bursts (GRBs). These are transient gamma ray events that can occur at any time, in any part of the sky, and are considered to arise at cosmological distances beyond our Galaxy. CZTI has detected over 40 GRBs. CZTI has demonstrated capability to detect polarization in GRBs.
Figure above shows the identified events corrected for geometry contributing to the GRB 1510016A (in black) as a function of azimuthal angle. The red solid is the fit to the modulation curve. The fitted modulation factor is 0.32 with the detection significance of 1.5 sigma.
The scientific details will be published shortly in the Astrophysical Journal (Ref : arXiV:1608.07388 [astro-ph.HE]. Polarisation properties and their relation to the spectral evolution have potential to clearly distinguish between various models of GRB prompt emission mechanism.
Polarisation estimates are being undertaken for Crab and several other X-ray sources.
- Ultraviolet Imaging Telescope (UVIT) observed the field stars in the open cluster NGC 188. One source which was earlier thought to be a sub-dwarf is found to be a binary with sources having temperatures around 12500K and 5750K.
- Scanning Sky Monitor (SSM) observed the X-ray pulsar 4U0115+63 in its outburst phase and the pulsations of 3.6 seconds were detected.
- Soft X-ray Telescope (SXT) detected the continuum and lines from bright Supernova Remnant such as Tycho. The advantage of having a good spectral resolution along with large field of view is expected to be extremely important to study the clusters of galaxies.
Regular Satellite Tracking and data processing is being done at ISRO. The data is distributed to the scientific community by the Indian Space Science Data Centre (ISSDC).
An announcement of opportunity (AO) was made in June 2016 through ISRO website, for Indian scientists/ researchers working at institutes/Universities/colleges to submit proposals. The proposal entry and review is done online through AstroSat Proposal Processing System (APPS) software developed for this purpose. A total of 59 Indian proposals have been accepted and observations have started.
ISRO, in collaboration with IUCAA, has established an AstroSat support Cell (ASC) at IUCAA, Pune, considering the imperative long term scientific support for AstroSat and to train the young scientists to explore possibilities for proposal submission and data analysis. This cell was formally dedicated by Chairman, ISRO to the scientific community on the occasion of one year completion of AstroSat.
Details on the science meet organized at IUCAA to commemorate the one year completion and the posters released during the event is available at ISRO Website.