R. Misra — Page 5 | Searcharxiv

arXiv1997

Broadening of the Iron emission line in MCG-6-30-15 by Comptonization

We show that the Iron K emission line from MCG-6-30-15 could be broadened due to Comptonization by a surrounding highly ionized cloud with radius $\sim 10^{14}$ cms. We calculate the temperature of the cloud to be $\sim0.21\kev$, provided a reasonable estimate of the UV flux is made. The X-ray/$γ$-ray emission observed from the source is compatible with this model. Such a cloud should be highly ionized and strong absorption edges are not expected from the source (Fabian et al 1995). For a $\onlyten{6}\msol$ black hole the size of the could corresponds to about 300 Schwarzschild radius. The intrinsic line could then be emitted far from the black hole and gravitational red-shift and Doppler effects would be negligible. If the black hole mass is much larger than $\onlyten{6}\msol$, gravitational/Doppler red-shifts would also contribute significantly to the broadening. We argue that the broad red wing observed in the source does not by itself imply emission from regions close ($R < 5 r_s$) to the black hole. However, Comptonization cannot produce a double peak. The presence of such a feature is a clear sign of inner disk emission influenced by gravitational and Doppler effects, perhaps broadened by the Comptonization. We note that simultaneous broad band (2-100 keV) study of this source can also reveal (or rule out) the presence of such a Comptonizing cloud.

astro-ph↗

arXiv1997

A fit to the simultaneous broadband spectrum of Cygnus X-1 using the transition disk model

We have used the transition disk model to fit the simultaneous broad band ($2-500$ keV) spectrum of Cygnus X-1 from OSSE and Ginga observations. In this model, the spectrum is produced by saturated Comptonization within the inner region of the accretion disk, where the temperature varies rapidly with radius. In an earlier attempt, we demonstrated the viability of this model by fitting the data from EXOSAT, XMPC balloon and OSSE observations, though these were not made simultaneously. Since the source is known to be variable, however, the results of this fit were not conclusive. In addition, since only once set of observations was used, the good agreement with the data could have been a chance occurrence. Here, we improve considerably upon our earlier analysis by considering four sets of simultaneous observations of Cygnus X-1, using an empirical model to obtain the disk temperature profile. The vertical structure is then obtained using this profile and we show that the analysis is self- consistent. We demonstrate conclusively that the transition disk spectrum is a better fit to the observations than that predicted by the soft photon Comptonization model. Since the temperature profile is obtained by fitting the data, the unknown viscosity mechanism need not be specified. The disk structure can then be used to infer the viscosity parameter $α$, which appears to vary with radius and luminosity. This behavior can be understood if $α$ depends intrinsically on the local parameters such as density, height and temperature. However, due to uncertainties in the radiative transfer, quantitative statements regarding the variation of $α$ cannot yet be made.

astro-ph↗