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 $\alpha$, which appears to vary with radius and luminosity. This behavior can be understood if $\alpha$ 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 $\alpha$ cannot yet be made.Comment: 8 figures. uses aasms4.sty, accepted by ApJ (Mar 98

Broad band X-ray spectrum of Cygnus X-1

We present the hard X-ray (20-100 keV) observations of Cygnus X-1 obtained using a large area balloon-borne Xenon filled Multi-anode Proportional Counter (XMPC) telescope. The observations were carried out during the gamma_2 state of the source and we obtain a power law photon index of 1.62 +- 0.07. To constrain the spectral shape of the source, we have analyzed the archival EXOSAT ME argon and GSPC data in the low energies (2-20 keV band) as well as the archival OSSE data in the high energies (50-500 keV). The data in different energy bands are not obtained in simultaneous observations, but they pertain to the gamma_2 state of the source. We have attempted a combined fit to the wide band data using appropriate mutual detector calibrations. A combined fit to the EXOSAT and XMPC data (2-100 keV) shows that the observed spectrum requires a low energy absorption corresponding to the Galactic interstellar absorption, a low energy excess modeled as blackbody, a narrow emission line due to iron K_alpha and a continuum. The continuum can be either modeled as a power law with a reflection bump or a Comptonisation model with an additional bump which can be modeled as the partial covering with a heavy absorber. To resolve between these two models, we have attempted combined fit to the 2-500 keV data obtained from EXOSAT, XMPC and OSSE. We find that two component Comptonisation model adequately represents the data. We explore the possible emission region that is responsible for the observed spectrum.Comment: 25 pages including 8 figures. To appear in Astronomy and Astrophysic

Multi-frequency, Multi-Epoch Study of Mrk 501: Hints for a two-component nature of the emission

Since the detection of very high energy (VHE) $\gamma$-rays from Mrk 501, its broad band emission of radiation was mostly and quite effectively modeled using one zone emission scenario. However, broadband spectral and flux variability studies enabled by the multiwavelength campaigns carried out during the recent years have revealed rather complex behavior of Mrk 501. The observed emission from Mrk 501 could be due to a complex superposition of multiple emission zones. Moreover new evidences of detection of very hard intrinsic $\gamma$-ray spectra obtained from {\it Fermi}--LAT observations have challenged the theories about origin of VHE $\gamma$-rays. Our studies based on {\it Fermi}--LAT data indicate the existence of two separate components in the spectrum, one for low energy $\gamma$-rays and the other for high energy $\gamma$-rays. Using multiwaveband data from several ground and space based instruments, in addition to HAGAR data, the spectral energy distribution of Mrk~501 is obtained for various flux states observed during 2011. In the present work, this observed broadband spectral energy distribution is reproduced with a leptonic, multi-zone Synchrotron Self-Compton model.Comment: Published in Astrophysical Journal (ApJ