MG14 - Talk detail

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Transient black hole candidates are very interesting objects to study in X-rays as they exhibit rapid evolutions in their temporal and spectral properties during outbursts, which are strongly correlated with each other. There are a large number of phenomenological and theoretical models in literature which claim to explain physics of accretion around a black hole. However, recently after the inclusion of two Component Advective Flow (TCAF) model of Chakrabarti-Titarchuk (1995) in HEASARC's spectral analysis package XSPEC as an additive table model, we found that it is quite capable to explain both the spectral and temporal properties very successfully. From our spectral fit with TCAF, one can directly extract physical flow parameters, such as two types of accretion (Keplerian disk and sub-Keplerian halo) rates and shock parameters (location and strength of the shock). From shock parameters, one can also predict frequency of primary dominating QPOs. Based on a comparison of halo to disk accretion rate ratio (ARR) along with the nature of quasi-periodic oscillations (QPOs; if present), we are able to provide a physical understanding of the classification of the entire outburst phase of the BHCs into different spectral states. In general transient BHCs, are found to exhibit four different spectral states during their outbursts, which may form hysteresis loop in a specific sequence. These spectral states are : hard, hard-intermediate, soft-intermediate, and soft.

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ASTROSAT is India's first multi-wavelength astronomy satellite, will be launched in the last quarter of 2015. It can be used to study astronomical objects in a wide range of electromagnetic energy band from UV to hard X-rays. With a very high spectral, timing as well as spacial resolutions from different scientific instruments of the satellite, one can make a detailed spectral and timing study of transient black hole candidates (BHCs) during their outbursts. Recently, we have included Chakrabarti-Titarchuk (1995) Two-Component Advective Flow (TCAF) model in HEASARC's spectral analysis package XSPEC as a local additive table model to fit black hole with the model, and we found that it is quite capable to explain both the spectral and temporal properties of BHCs very successfully. From our spectral fits with TCAF, one can directly extract physical flow parameters, such as two types of accretion (Keplerian disk and sub-Keplerian halo) rates and shock parameters (location and strength of the shock). From shock parameters, one can also predict frequency of primary dominating quasi-periodic oscillations (QPOs). Based on a comparison of halo to disk accretion rate ratio (ARR) along with the nature of QPOs (if present), we are able to provide a physical understanding of the classification of the entire outburst phase of the BHCs into different spectral states (such as, hard, hard-intermediate, soft-intermediate, and soft, etc). Multi-wavelength data of ASTROSAT will provide us a platform to understand spectral as well as temporal variability of transient BHCs in a better sense with spectral fits using TCAF.

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Transient Galactic Black hole candidate (BHC) MAXI J1836-194 was showed its first X-ray flaring activity which continued for around 2 months after its discovery with MAXI/GSC on 2011 August 30. We make a detailed spectral analysis with Chakrabarti-Titarchuk two-component advective flow (TACF) model as a local additive table model in XSPEC using 2.5-25 keV RXTE/PCA archival data. From TCAF model spectral fits, physical flow parameters such as two types (Keplerian disk and sub-Keplerian halo) of accretion rates and shock parameters (locations and compression ratios) are extracted on a daily basis. We compared TCAF model fitted results with that of the combined disk blackbody and power-law model fits. Quasi-periodic oscillations are observed sporadically on-and-off in few observations. Unlikely as of other transient BHCs of same category, no monotonic evolutions of QPO frequencies are observed during rising as well as declining phases of the outburst. Nature of the spectral and temporal evolutions of the source during its first (2011) outburst forces us to define it as an 'unusual' or `failed' outburst.

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One of the great interesting features of transient black hole candidates (BHCs) is their outburst profile, because they show rapid spectral and timing variability in a very short time scale. A systematic detailed study on the Spectral and the Timing properties on a daily basis is necessary to understand the accretion physics around these BHCs. We make a detailed study of the evolution of spectral properties of newly discovered Galactic transient BHC MAXI J1659-152 during its very first (2010) outburst after its discovery on 2010 Sept. 25 by MAXI/GSC using Chakrabarti-Titarchuk two-component advective flow (TCAF) model as an additive table model in XSPEC. Accretion flow parameters (Keplerian disk and sub-Keplerian halo rates, shock location and shock strength) are extracted from our spectral fits with TCAF. We studied the variation of these fitted parameters during the entire outburst as it passed through three spectral classes: hard, hard-intermediate, and soft-intermediate. More interestingly, no soft state is observed, which may be due to the lack of matter from thermally cooler Keplerian disk component. So in that context, we can call this outburst as a 'failed' outburst. We also predicted mass of the BHC in between 4-8 Solar mass on the basis of keeping TCAF model normalization as constant and fitted $\chi^2_{red}$ variation throughout the outburst.

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