Browsing by Author "Dewangan, Gulab Chand"
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Item Active Galactic Nuclei(2013-07-16) Dewangan, Gulab Chand; Misra, RanjeevItem Associated spectral and temporal state transition of the bright ULX NGC 1313 X-1(2010-05-21) Dewangan, Gulab Chand; et al.Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to different accretion disc geometries. We present analysis of two XMM-Newton observations of the Ultra-Luminous X-ray source (ULX) NGC 1313 X-1, which reveal that the system was in two different spectral states. While spectral variations have been observed in this source before, this data provides clear evidence that the spectral states also have distinct temporal properties. With a count rate of ~ 1.5 counts/s and a fractional variability amplitude of ~ 15%, the ULX was in a high flux and strongly variable state in March 2006. In October 2006, the count rate of the ULX had reduced by a factor of ~ 2 and the spectral shape was distinctly different with the presence of a soft component. No strong variability was detected during this low flux state with an upper limit on the amplitude < 3%. Moreover, the spectral properties of the two states implies that the accretion disc geometry was different for them. The low flux state is consistent with a model where a standard accretion disc is truncated at a ra- dius of ~ 17 Schwarzschild radius around a ~ 200M⊙ black hole. The inner hot region Comptonizes photons from the outer disc to give the primary spectral component. The spectrum of the high flux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disc and Comptonizes the disc photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a lower temperature (~ 2 keV) of the hot Comptonizing plasma and higher optical depth (~ 15) than what is observed for the Galactic ones.Item The effect of UV/Soft X-ray excess emission on the warm absorber properties of active galactic nuclei-A case study of IRAS 13349+2438(IUCAA, 2015-02) Laha, Sibasish; Dewangan, Gulab Chand; Chakravorty, SusmitaItem On the origin of the featureless soft X-ray excess emission from the Seyfert 1 galaxy ESO 198–G24(IUCAA, 2015-02) Laha, Sibasish; Dewangan, Gulab Chand; Kembhavi, A.K.Item Partial covering absorption and variability in AGN(2011-01-29) Dewangan, Gulab ChandItem Soft time lags in the X-ray emission of mrk 1040(2011-01-30) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab Chand; et al.Temporal analysis of X-ray binaries and Active Galactic Nuclei have shown that hard X-rays react to variation of soft ones after a time delay. The opposite trend, or soft lag, has only been seen in a few rare Quasi-periodic Oscillations in X-ray binaries and recently for the AGN, 1H 0707-495, on short timescales of ∼ 103 secs. Here, we report analysis of a XMM-Newton observation of Mrk 1040, which reveals that on the dominant variability timescale of ∼ 104 secs, the source seems to exhibit soft lags. If the lags are frequency independent, they could be due to reverberation effects of a relativistically blurred reflection component responding to a varying continuum. Alternatively, they could be due to Comptonization delays in the case when high energy photons impinge back on the soft photon source. Both models can be verified and their parameters tightly constrained, because they will need to predict the photon spectrum, the r.m.s variability and time lag as a function of energy. A successful application of either model will provide unprecedented information on the radiative process, geometry and more importantly the size of the system, which in turn may provide stringent test of strong general relativistic effects.Item Spectral variability of IRAS 18325-5926 and Contraints on the Geometry of the Scattering Medium(IUCAA, 2015-02) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab ChandItem Warm absorbers in X-rays (WAX), a comprehensive high resolution grating spectral study of a sample of Seyfert galaxies:I. A global view and frequency of occurrence of warm absorbers(IUCAA, 2015-02) Laha, Sibasish; Guainazzi, Matteo; Dewangan, Gulab ChandItem X-ray emission from active galactic nuclei with intermediate mass black holes(2008-12) Dewangan, Gulab ChandWe present a systematic X-ray study of eight active galactic nuclei (AGNs) with intermediate mass black holes (MBH ∼ 8 − 95 × 104 M⊙) based on 12 XMM-Newton observations. The sample includes the two prototype AGNs in this class – NGC 4395 and POX 52 and six other AGNs discovered with the Sloan Digitized Sky Survey. These AGNs show some of the strongest X-ray variability with the normalized excess variances being the largest and the power density break time scales being the shortest observed among radio-quiet AGNs. The excess variance – luminosity correlation appears to depend on both the BH mass and the Eddington luminosity ratio. The break time scale – black hole mass relations for AGN with IMBHs are consistent with that observed for massive AGNs. We find that the FWHM of the Hβ/Hα line is uncorrelated with the BH mass, but shows strong anticorrelation with the Eddington luminosity ratio. Four AGNs show clear evidence for soft X-ray excess emission (kTin ∼ 150 −200 eV). X-ray spectra of three other AGNs are consistent with the presence of the soft excess emission. NGC 4395 with lowest L/LEdd lacks the soft excess emission. Evidently small black mass is not the primary driver of strong soft X-ray excess emission from AGNs. The X-ray spectral properties and optical-to-X-ray spectral energy distributions of these AGNs are similar to those of Seyfert 1 galaxies. The observed X-ray/UV properties of AGNs with IMBHs are consistent with these AGNs being low mass extension of more massive AGNs; those with high Eddington luminosity ratio looking more like narrow-line Seyfert 1s while those with low L/LEdd looking more like broad-line Seyfert 1s.Item X-Ray warm absorption and emission in the polar scattered seyfert 1 galaxy MRK 704(2011-04-10) Laha, Sibasish; Dewangan, Gulab Chand; Kembhavi, A.K.