Space Astronomy Group

Bangalore, India

Space Astronomy Group

Bangalore, India
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Girish V.,Space Astronomy Group | Singh K.P.,Tata Institute of Fundamental Research
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present the analysis of archival X-ray data obtained with the XMM-Newton and Suzaku for a new intermediate polar identified as a counterpart of an INTEGRAL discovered γ-ray source, IGR J17195-4100. We report a new period of 1053.7 ± 12.2s in X-rays. A new binary orbital period of 3.52-0.80+1.43h is strongly indicated in the power spectrum of the time series. An ephemeris of the new period proposed as the spin period of the system has also been obtained. The various peaks detected in the power spectrum suggest a probable disc-less accretion system. The soft X-rays (<3keV) dominate the variability seen in the X-ray light curves. The spin modulation shows energy dependence suggesting the possibility of a variable partial covering accretion column. The averaged spectral data obtained with XMM-Newton European Photon Imaging Camera (EPIC) cameras show a multi-temperature spectrum with a soft excess. The latter can be attributed to the varying coverage of accretion curtains. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Debnath D.,Indian Center for Space Physics | Chakrabarti S.K.,Indian Center for Space Physics | Chakrabarti S.K.,Sn Bose National Center For Basic Science | Nandi A.,Space Astronomy Group
Advances in Space Research | Year: 2013

The Galactic black hole candidate H 1743-322 exhibited two X-ray outbursts in rapid succession: one in August 2010 and the other in April 2011. We analyze archival data of this object from the PCA instrument on board RXTE (2-25 keV energy band) to study the evolution of its temporal and spectral characteristics during both the outbursts, and hence to understand the behavioral change of the accretion flow dynamics associated with the evolution of the various X-ray features. We study the evolution of QPO frequencies during the rising and the declining phases of both the outbursts. We successfully fit the variation of QPO frequency using the Propagating Oscillatory Shock (POS) model in each of the outbursts and obtain the accretion flow parameters such as the instantaneous shock locations, the shock velocity and the shock strength. Based on the degree of importance of the thermal (disk black body) and the non-thermal (power-law) components of the spectral fit and properties of the QPO (if present), the entire profiles of the 2010 and 2011 outbursts are subdivided into four different spectral states: hard, hard-intermediate, soft-intermediate and soft. We attempt to explain the nature of the outburst profile (i.e., hardness-intensity diagram) with two different types of mass accretion flow. © 2013 COSPAR. Published by Elsevier Ltd. All rights reserved.

Narendranath S.,Space Astronomy Group | Sreekumar P.,Indian Institute of Astrophysics | Alha L.,University of Helsinki | Sankarasubramanian K.,Space Astronomy Group | And 2 more authors.
Solar Physics | Year: 2014

The X-ray Solar Monitor (XSM) on the Indian lunar mission Chandrayaan-1 was flown to complement lunar elemental abundance studies by the X-ray fluorescence experiment C1XS. XSM measured the ≈ 1.8 - 20 keV solar X-ray spectrum during its nine months of operation in lunar orbit. The soft X-ray spectra can be used to estimate absolute coronal abundances using intensities of emission-line complexes and the plasma temperature derived from the continuum. The best estimates are obtained from the brightest flare observed by XSM: a C2.8-class flare. The well-known first-ionization potential (FIP) effect is observed; abundances are enhanced for the low-FIP elements Fe, Ca, and Si, while the intermediate-FIP element S shows values close to the photospheric abundance. The derived coronal abundances show a quasi-mass-dependent pattern of fractionation. © 2013 Springer Science+Business Media Dordrecht.

Nandi A.,Space Astronomy Group | Nandi A.,Indian Center for Space Physics | Debnath D.,Indian Center for Space Physics | Mandal S.,Indian Institute of Space Science and Technology | And 2 more authors.
Astronomy and Astrophysics | Year: 2012

Context. The Galactic transient black hole candidate (BHC) GX 339-4 exhibited several outbursts at regular intervals of about two to three years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for three long years, it again became X-ray active in January, 2010, continuing to be so over the next ~14 months. Aims. We study the timing and spectral properties of the BHC during its recent outburst and understand the behavioral change in the accretion flow dynamics associated with the evolution of the various X-ray features. Methods. The detailed analysis of the temporal and spectral properties of the source during this outburst are carried out using archival data of the RXTE PCA instrument. We analyze a total of 236 observational intervals consisting of 419 days of data observed by RXTE, from 2010 January 12 to 2011 March 6. Results. Our study provides a comprehensive understanding of the mass accretion processes and properties of the accretion disk of the BHC. The PCA spectra of 2.5-25 keV are mainly fitted with a combination of two components, namely, a disk black body and a power-law. The entire outburst as observed by RXTE, is divided into four spectral states, namely, hard, hard-intermediate, soft-intermediate, and soft. Quasi-periodic oscillations (QPOs) were found in three out of the four states, namely hard, hard-intermediate, and soft-intermediate states. The QPO frequencies increase monotonically from 0.102 Hz to 5.692 Hz in the rising phase of the outburst, while during the declining phase QPO frequencies decrease monotonically from 6.420 to 1.149 Hz. The evolution pattern, i.e. the hardness-intensity diagram, of the present outburst can be reproduced by two different components of the flow of accreting material. Conclusions. The recent outburst of GX 339-4 gives us an opportunity to understand the evolution of the two-component accretion rates starting from the onset to the end of the outburst phase. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity. © 2012 ESO.

Das S.,Indian Institute of Technology Guwahati | Chattopadhyay I.,ARIES | Nandi A.,Space Astronomy Group | Molteni D.,University of Palermo
Monthly Notices of the Royal Astronomical Society | Year: 2014

We investigate the behaviour of low angular momentum viscous accretion flows around black holes using smooth particle hydrodynamics method. Earlier, it has been observed that in a significant part of the energy and angular momentum parameter space, rotating transonic accretion flow undergoes shock transition before entering in to the black hole and a part of the post-shock matter is ejected as bipolar outflows, which are supposed to be the precursor of relativistic jets. In this work, we simulate accretion flows having injection parameters from the inviscid shock parameter space, and study the response of viscosity on them. With the increase of viscosity, shock becomes time dependent and starts to oscillate when the viscosity parameter crosses its critical value. As a result, the in fallingmatter inside the post-shock region exhibits quasi-periodic variations and causes periodic ejection of matter from the inner disc as outflows. In addition, the same hot and dense post-shock matter emits high energy radiation and the emanating photon flux also modulates quasi-periodically. Assuming a 10 M⊙ black hole, the corresponding power density spectrum peaks at the fundamental frequency of few Hz followed by multiple harmonics. This feature is very common in several outbursting black hole candidates. We discuss the implications of such periodic variations. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Rajaguru S.P.,Indian Institute of Astrophysics | Wachter R.,Stanford University | Sankarasubramanian K.,Space Astronomy Group | Couvidat S.,Stanford University
Astrophysical Journal Letters | Year: 2010

Using a high-cadence imaging spectropolarimetric observation of a sunspot and its surroundings in magnetically sensitive (Fe I 6173 Å) and insensitive (Fe I 7090 Å) upper photospheric absorption lines, we map the instantaneous wave phases and helioseismic travel times as a function of observation height and inclination of magnetic field to the vertical. We confirm the magnetic inclination-angle-dependent transmission of incident acoustic waves into upward propagating waves and derive (1) proof that helioseismic travel times receive direction-dependent contributions from such waves and hence cause errors in conventional flow inferences, (2) evidences for acoustic wave sources beneath the umbral photosphere, and (3) significant differences in travel times measured from the chosen magnetically sensitive and insensitive spectral lines. © 2010 The American Astronomical Society. All rights reserved.

Radhika D.,Space Astronomy Group | Radhika D.,University of Calicut | Nandi A.,Space Astronomy Group
Advances in Space Research | Year: 2014

We re-investigated the 'spectro-temporal' behavior of the source XTE J1859+226 in X-rays during its outburst phase in 1999, by analysing the RXTE PCA/HEXTE data in 2-150 keV spectral band. Detailed analysis shows that the source evolves through different spectral states during its entire outburst as indicated by the variation in the spectral and temporal characteristics. Although the evolution pattern of the outburst followed the typical q-shaped profile, we observed an absence of 'canonical' soft state and a weak presence of 'secondary' emission during the decay phase of the outburst. The broad-band spectra, modeled with high energy cutoff, shows that the fold-energy increases monotonically in the hard and hard-intermediate states followed by a random variation in the soft-intermediate state. We attempted to estimate the mass of the source based on the evolution of Quasi-Periodic Oscillation (QPO) frequencies during rising phase modeled with the propagating oscillatory shock solution, and from the correlation of photon index and QPO frequency. It is also observed that during multiple ejections (observed as radio flares) the QPO frequencies are not present in the power spectra and there is an absence of lag in the soft to hard photons. The disk flux increases along with a decrease in the high energy flux, implying the soft nature of the spectrum. These results are the 'possible' indication that the inner part of the disk (i.e., Comptonized corona), which could be responsible for the generation of QPO and for the non-thermal Comptonized component of the spectrum, is disrupted and the matter gets evacuated in the form of jet. We attempted to explain the complex behavior of 'spectro-temporal' properties of the source during the entire outburst and the nature of the disk-jet connection before, during and after the ejection events in the context of two different types of accreting flow material, in presence of magnetic field. © 2014 COSPAR. Published by Elsevier Ltd. All rights reserved.

Radhika D.,University of Calicut | Nandi A.,Space Astronomy Group | Agrawal V.K.,Space Astronomy Group | Mandal S.,Indian Institute of Space Science and Technology
Monthly Notices of the Royal Astronomical Society | Year: 2016

We study the spectral and temporal characteristics of the source GS 2023+338 (V404 Cyg) during the initial phase of its 2015 June outburst, over the energy range of 0.5-150 keV. This is the first detailed study of the characteristics of this source based on SWIFT observations, being reported. Based on our analysis, we understand that the source existed in the hard, intermediate and soft spectral states. We find that the evolution of the spectral parameters, the hardness-intensity diagram and the rms-intensity diagram are not similar to those observed for most of the outbursting black hole sources. We also observe presence of weak peaked components in the power density spectra during the intermediate state of the source. Dramatic changes in the spectral and temporal properties are also exhibited before the ejection of a radio jet suggesting it to be associated with the coronal mass ejection. It seems that may be due to evacuation of the inner part of the Keplerian disc for a short duration, the disc component is not observed after the huge radio flare. The absorption features observed in the low-energy spectra suggest the presence of wind emission and the evolution of the characteristics of the variable Fe line emission during both hard and intermediate states, indicate its origin to be probably related to the wind/outflow. © 2016 The Authors.

Garai B.,Indian Institute of Science | Radhakrishna V.,Space Astronomy Group | Rajanna K.,Indian Institute of Science
Optical Materials Express | Year: 2013

CsI can be used as a photocathode material in UV photon detectors. The detection efficiency of the detector strongly depends on the photoemission property of the photocathode. CsI is very hygroscopic in nature. This limits the photoelectron yield from the photocathode when exposed to humid air even for a short duration during photocathode mounting or transfer. We report here on the improvement of photoemission properties of both thick (300 nm) and thin (30 nm) UV-sensitive CsI film exposed to humid air by the process of vacuum treatment. © 2013 Optical Society of America.

Agrawal V.K.,Space Astronomy Group | Nandi A.,Space Astronomy Group
Monthly Notices of the Royal Astronomical Society | Year: 2014

We report the discovery of a quasi-periodic oscillation (QPO) at 642 mHz in an XMM-Newton observation of the ultraluminous X-ray source (ULX) IC 342 X-1. The QPO has a centroid at νQPO = 642 ± 20 mHz, a coherence factor Q = 11.6 and an amplitude (rms) of 4.1 per cent with significance of 3.6s. The energy dependence study shows that the QPO is stronger in the energy range 0.3-5.0 keV. A subsequent observation (6 d later) does not show any signature of the QPO in the power-density spectrum. The broad-band energy spectra (0.3-40.0 keV) obtained by quasi-simultaneous observations of XMM-Newton and NuSTAR can be described well by an absorbed diskbb plus cutoffpl model. The best-fitting spectral parameters are powerlaw index (Γ) ~ 1.1, cut-off energy (Ec) ~ 7.9 keV and disc temperature (kTin) ~ 0.33 keV where the QPO is detected. The unabsorbed bolometric luminosity is ~5.34 × 1039 erg s-1. Comparing this with the well-known X-ray binary GRS 1915+105, our results are consistent with the mass of the compact object in IC 342 X-1 being in the range ~20-65M⊙.We discuss the possible implications of our results. © 2014 The Authors.

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