arXiv subjects

Anuj Nandi

Publications and source records attributed to Anuj Nandi.

79 records · Page 5Linked to original sources

arXiv2009

Indian Payloads (RT-2 Experiment) Onboard CORONAS-PHOTON Mission

RT-2 Experiment (RT - Roentgen Telescope) is a low energy gamma-ray instrument which is designed and developed as a part of Indo-Russian collaborative project of CORONAS-PHOTON Mission to study the Solar flares in wide energy band of electromagnetic spectrum ranging from UV to high-energy gamma-rays (~2000 MeV). RT-2 instruments will cover the energy range of 15 keV to 150 keV extendable up to ~1 MeV. It consists of three detectors (two Phoswich detectors, namely, RT-2/S, RT-2/G and one solid-state imaging detector RT-2/CZT) and one processing electronic device (RT-2/E). Both Phoswich detectors will have time resolved spectrum, whereas the solid-state imaging detector will have high resolved image of the solar flares in hard X-rays. We have used Co-57 (122 keV) radio-active source for onboard calibration of all three detectors. In this paper, we briefly discuss the in-flight performance of RT-2 instruments and present initial flight data from the instruments. This mission was launched into polar LEO (Low Earth Orbit) (~550 km) on 30th January 2009 from Plesetsk Cosmodrome, Russia.

astro-ph.IM↗

arXiv2009

Fresnel Zone Plate Telescopes as high resolution imaging devices

Combination of Fresnel Zone Plates (FZP) can make excellent telescopes for imaging in X-rays. We present the results of our experiments with such telescopes with an X-ray source kept at a distance of 45 feet. We compare the patterns obtained from experiments with those obtained by our Monte-Carlo simulations. In simulations, we allow the sources to be at finite distances (diverging beam) as well as at infinite distances (parallel beam) and show that the resolution is worsened when the source is nearby. We also present simulated results for the observation of the galactic center and show that the sources may be reconstructed with accuracy. We compare the performance of such a telescope with other X-ray imaging devices used in space-astronomy. The Zone Plate based instrument has been sent for the first time in a recently launched KORONAS-FOTON satellite.

astro-ph.IM↗

arXiv2008

Evolution of the quasi-periodic oscillation frequency in GRO J1655-40 -- Implications for accretion disk dynamics

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot blobs embedded in Keplerian disks. Any movement of these perturbations is expected systematically to change the QPO frequency. Our goal is to find systems where such a systematic drifts have been observed. We also try to find the real cause of such drifts and whether they shed some light on the accretion disk dynamics. Using archival data of the recent outburst of GRO J1655-40, we report the presence of such systematic drifts not only during the rising phase from the 25th of February 2005 to the 12th March 2005, when the QPO frequency monotonically increased from 82mHz to 17.78Hz but also in the decline phase from the 15th September 2005 to the 5th of October 2005, when the QPO frequency decreased from 13.14Hz to 34mHz. We fitted the frequency drifts with the propagatory oscillating shock solution. In the shock-oscillation solution, the frequency is inversely proportional to the infall time scale from the shock location. We obtained the shock location and strength through such a fit. The astonishing smoothness of the variation of the QPO frequency over a period of weeks directly supports the view that it may due to the drift of an oscillating shock rather than the movements of a blob inside a differentially rotating disk.

astro-ph↗

arXiv2005

Class Transitions and Two Component Accretion Flow in GRS 1915+105

The light curve of the galactic micro-quasar GRS 1915+105 changes in at least thirteen different ways which are called classes. We present examples of the transitions from one class to another as observed by the IXAE instrument aboard the Indian Satellite IRS-P3. We find that the transitions are associated with changes in photon counts over a time-scale of only a few hours and they take place through unknown classes. Assuming that the transitions are caused by variation of the accretion rates, this implies that a significant fraction of the matter must be nearly freely falling in order to have such dramatic changes in such a short time.

astro-ph↗

arXiv2000

Classification of Light curves of the Black Hole Candidate GRS1915+105

The black hole candidate GRS 1915+105 exhibits a rich variety of variability. Assuming that the earlier paradigm of black hole accretion which includes a shock is applicable for this system as well, one can classify these variabilities into four classes. We present light curves of these classes and discuss the basis of our classification.

astro-ph↗

arXiv2000

Fundamental States of Accretion/Jet Configuration and the Black Hole Candidate GRS1915+105

Advective disk paradigm of black hole accretion includes self-consistent formation of shocks and outflows from post-shock region. We apply this paradigm to understand rich variation of the light curve of the black hole candidate GRS1915+105. We propose that out of five possible {\it fundamental states} the black hole candidate GRS1915+105 moves around among three of them creating all possible observed light curves.

astro-ph↗

arXiv2000

On the source of QPO of the black hole candidate GRS1915+105: some new observations and their interpretation

A few classes of the light curve of the black hole candidate GRS 1915+105 have been analyzed in detail. We discover that unlike the previous findings, QPOs occasionally occur even in the so-called `On' or softer states. Such findings may require a revision of the accretion/wind scenario of the black hole candidates. We conjecture that considerable winds which is produced in `Off' states, cool down due to Comptonization, and falls back to the disk and creating an excess accretion rate to produce the so-called `On' state. After the drainage of the excess matter, the disk goes back to the `Off' state. Our findings strengthen the shock oscillation model for QPOs.

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