V. K. Agrawal

Publications (5)6.02 Total impact

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  Available from: A. R. Rao

  Article: Energy Dependent time lags in the Seyfert 1 galaxy NGC 4593

  K. Sriram, V. K. Agrawal, A. R. Rao
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  ABSTRACT: We investigate the energy-time lag dependence of the source NGC 4593 using XMM-{\it Newton}/EPIC-pn data. We found that the time lag dependency is linear in nature with respect to the logarithm of different energy bands. We also investigate the frequency dependent time lags and identify that at some frequency range (5 $\times$ 10$^{-5}$ Hz -- 2 $\times$ 10$^{-4}$ Hz) the X-ray emission is highly coherent, mildly frequency dependent and very strongly energy dependent. These observations can be explained in the frame work of the thermal Comptonization process and they indicate a truncated accretion disk very close to the black hole. We discuss the plausible spectral state to explain the phenomenon and conclude that the observed properties bear a close resemblance to the intermediate state or the steep power-law state, found in galactic black hole sources. Comment: accepted for the publication in The Astrophysical journal, 5 pages
  06/2009;
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  Available from: A. R. Rao

  Article: A truncated accretion disk in the galactic black hole source H1743-322

  K. Sriram, V. K. Agrawal, A. R. Rao
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  ABSTRACT: To investigate the geometry of the accretion disk in the source H1743-322, we have carried out a detailed X-ray temporal and spectral study using RXTE pointed observations. We have selected all data pertaining to the Steep Power Law (SPL) state during the 2003 outburst of this source. We find anti-correlated hard X-ray lags in three of the observations and the changes in the spectral and timing parameters (like the QPO frequency) confirm the idea of a truncated accretion disk in this source. Compiling data from similar observations from other sources, we find a correlation between the fractional change in the QPO frequency and the observed delay. We suggest that these observations indicate a definite size scale in the inner accretion disk (the radius of the truncated disk) and we explain the observed correlation using various disk parameters like Compton cooling time scale, viscous time scale etc..
  05/2009;
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  Available from: arxiv.org

  Article: Anticorrelated Hard X-Ray Time Lags in Galactic Black Hole Sources

  K. Sriram, V. K. Agrawal, Jayant K. Pendharkar, and A. R. Rao
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  ABSTRACT: We investigate the accretion disk geometry in Galactic black hole sources by measuring the time delay between soft and hard X-ray emissions. Similar to the recent discoveries of anticorrelated hard X-ray time lags in Cygnus X-3 and GRS 1915+105, we find that the hard X-rays are anticorrelated with soft X-rays with a significant lag in another source, XTE J1550-564. We also find the existence of pivoting in the model-independent X-ray spectrum during these observations. We investigate time-resolved X-ray spectral parameters and find that the variation in these parameters is consistent with the idea of a truncated accretion disk. The QPO frequency, which is a measure of the size of truncated accretion disks, also changes, indicating that the geometric size of the hard X-ray emitting region changes along with the spectral pivoting and soft X-ray flux. A similar kind of delay is also noticed in 4U 1630-47.
  The Astrophysical Journal 12/2008; 661(2):1055. · 6.02 Impact Factor
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  Available from: A. R. Rao

  Article: Anti-correlated hard X-ray time lag in GRS 1915+105: evidence for a truncated accretion disc

  Manojendu Choudhury, A. R. Rao, Surajit Dasgupta, J. Pendharkar, K. Sriram, V. K. Agrawal
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  ABSTRACT: Multi-wavelength observations of Galactic black hole candidate sources indicate a close connection between the accretion disk emission and the jet emission. The recent discovery of an anti-correlated time lag between the soft and hard X-rays in Cygnus X-3 (Choudhury & Rao 2004) constrains the geometric picture of the disk-jet connection into a truncated accretion disk, the truncation radius being quite close to the black hole. Here we report the detection of similar anti-correlated time lag in the superluminal jet source GRS 1915+105. We show the existence of the pivoting in the X-ray spectrum during the delayed anti-correlation and we also find that the QPO parameters change along with the spectral pivoting. We explore theoretical models to understand this phenomenon. Comment: Accepted in ApJ: (to appear on 1 Oct, 2005 issue)
  06/2005;
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  Available from: A. R. Rao

  Article: Detection of absorption features in the X-ray spectrum of the narrow-line Quasar PG 1404+226 : Possible evidence for accretion disk winds

  Surajit Dasgupta, A. R. Rao, G. C. Dewangan, V. K. Agrawal
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  ABSTRACT: We present the results of an analysis of data from XMM-Newton and CHANDRA observations of the high luminosity narrow-line quasar PG 1404+226. We confirm a strong soft X-ray excess in the X-ray spectrum and we find rapid variability (a factor of two in about 5000 s). When the X-ray spectrum is fit with a two component model which includes a power-law and a blackbody component, we find that low energy absorption lines are required to fit the data. If we interpret these lines as due to highly ionized species of heavy elements in an outflowing accretion disk wind, an outflow velocity of about 26000 km/s could be derived. One interesting feature of the present observation is the possible detection of variability in the absorption features: the absorption lines are visible only when the source is bright. From the upper limits of the equivalent widths (EW) of the absorption lines during the low flux states and also from the model independent pulse height ratios, we argue that the strength of absorption is lower during the low flux states. This constraints the physical size of the absorbing medium within 100 Schwartzschild radius (Rg) of the putative supermassive black hole. We also find a marginal evidence for a correlation between the strength of the absorption line and the X-ray luminosity. Comment: 4 pages, 4 figures, Astrophysical Journal Letter (in press)
  09/2004;