| dc.description.abstract | We study the spectro-temporal properties of the black hole X-ray binaries (BHXBs) GRS 1915+105
and IGR J17091-3624 with data from RXTE, Chandra and XMM-Newton. From the analysis
result, we propose models of accretion modes in terms of known accretion classes that might be
responsible for the different correlated spectral states with temporal classes, namely: Keplerian
disc flow (Shakura & Sunyaev, 1973a), slim disc flow (Abramowicz et al., 1988), advection
dominated accretion flow or ADAF (Narayan & Yi, 1994) and general advective accretion flow
or GAAF (Rajesh & Mukhopadhyay, 2010). We further posit that accretion rate must play an
important role in transition between these states. While GRS 1915+105 exhibits all four modes,
in IGR J17091-3624, only two of the modes – ADAF and slim disc – are evident, although
the effect of Poisson noise cannot be ruled out. Thus, we infer that while both sources show a
lot of similarities in their lightcurves, the underlying nonlinear dynamical properties of their
accretion flows may be different. With long, continuous optical/UV and X-ray data from AGNs,
134 Summary and future prospects
this model can be tested, as any consistent result will throw more light on how the mechanisms
at work in BHXBs and AGNs are related.
The energy spectra of the Seyfert 1 AGN Zw 229.015 reveals the presence of strong soft
X-ray emission below 1keV in excess of the primary X-ray power-law. In an attempt to probe
the plausible origin of the soft excess, we apply four different models, namely: multicolour disc
blackbody (Mitsuda et al., 1984; Makishima et al., 1986), smeared wind absorption (Gierli´nski
& Done, 2004), thermal Comptonisation (Titarchuk, 1994) and relativistically blurred reflection
(Ross & Fabian, 2005) models. We find that both thermal Comptonisation and relativistically
blurred reflection models provide acceptable explanation to the origin of the soft X-ray excess.
Further, based on soft/hard X-ray time lag, we constrain the size of its corona to be ∼ 20Rg
– a value which is consistent with earlier works. Through cross-correlation analysis of the
UV/X-ray lightcurves of the Seyfert 1 AGN Mrk 493, we measure a lag of 5ks in which the
UV emission lead the X-rays in their variability. To explain this lag, we calculate different
variability timescales associated with accretion discs. From our estimations, the measured
lag is consistent with the time required for UV photons produced in the disc to travel to the
hot corona and the time required for repeated inverse Compton scattering in the corona. This
implies that thermal Comptonisation of disc UV emission in the corona is responsible for the
observed X-rays. Although this scenario has been predicted theoretically, we have been able to
confirm it observationally for the first time.
Therefore, overall: (1) we have shown through correlated spectral and nonlinear time series
analysis that accretion flows around compact object – black holes – could reveal deterministic
or stochastic behaviour over time with implications for modes of accretion. (2) Through
arguments from cross-correlation analysis and energy spectral properties, we put constraints on
corona geometry and show its emission mechanism | en_US |
