Cygnus X-1: A Case for a Magnetic Accretion Disk?

We analyse a large number ($> 500$) pointed RXTE observations of Cyg X-1 and model the spectrum of each one. A subset of the observations for which there is simultaneous reliable measure of the hardness ratio by the All Sky Monitor, shows that the sample covers nearly all the spectral shapes of Cyg X-1. The relative strength, width of the Iron line and the reflection parameter are in general correlated with the high energy photon spectral index $\Gamma$. This is broadly consi...

It is shown that the energy dependence of the time lags in Cygnus X-1 excludes any significant contribution of the standard reflected component to the observed lags. The conclusion is valid in the 0.1--10 Hz frequency range where time lags have been detected with high enough significance. In fact the data hint that reflected component is working in opposite direction, reducing the lags at energies where contribution of the reflected component is significant. We argue that t...

We present the results of multi-year gamma-ray observations by the AGILE satellite of the black hole binary system Cygnus X-1. In a previous investigation we focused on gamma-ray observations of Cygnus X-1 in the hard state during the period mid-2007/2009. Here we present the results of the gamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried out for which includes a remarkably prolonged `soft state' phase (June 2010 -- May 2011). Previous 1--10 MeV obse...

We present X-ray/gamma-ray observations of Cyg X-1 in the soft state during 1996 May-June. We analyze ASCA, RXTE and OSSE data. The spectrum consists of soft X-ray blackbody emission of an optically thick accretion disk in the vicinity of a black hole and a power law with an energy index alpha around 1.2-1.5 extending to at least several hundred keV. In the spectra, we find the presence of strong Compton reflection, which probably comes from the disk.

We evaluate 0.03-20 Hz power spectra of the bright black hole binary Cyg X-1 obtained from non-deconvolved INTEGRAL-ISGRI event data. The ISGRI power spectra are compared to contemporary RXTE-PCA ones in the same hard X-ray energy band of 15-70 keV. They agree well in shape. Since the ISGRI power spectrum of Cyg X-1 is not background corrected it lies about an order of magnitude below the PCA values. In 2003 a soft outburst of Cyg X-1 occurred. From the RXTE-ASM and Ryle ra...

We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state based on four observations with NuSTAR. Despite the basic similarity of the observed spectra, there is clear spectral variability between epochs. To investigate this variability, we construct a model incorporating both the standard disk-corona continuum and relativistic reflection from the accretion disk, based on prior NuSTAR work on Cygnus X-1, and apply this model to each epoch independently. We f...

We execute a detailed spectral-timing study of Cygnus X-1 in the low/hard, intermediate and high/soft states with the Hard X-ray Modulation Telescope observations. The broad band energy spectra fit well with the "truncated disk model" with the inner boundary of the accretion disk stays within $\sim$10 \textit R$_{\rm g}$ and moves inward as the source softens. Through studying of PDS, rms and Fourier-frequency component resolved spectroscopy, we find that the X-ray variations...

The different electron distributions in the hard and soft spectral states (HS and SS) of BH binaries could be caused by kinetic processes and changing because of varying physical conditions in the corona. In presence of a magnetic field in the corona, the electron distribution can appear thermal, even when acceleration mechanisms would produce non thermal distributions. This is due to fast and efficient thermalization through synchrotron self-absorption. We have analyzed data...

We present the results from the RXTE observations of Cygnus X-1 in its high state. In the energy range of 2-200 keV, the observed X-ray spectrum can be described by a model consisting of a soft blackbody component and a broken power-law with a high energy cutoff. The low energy spectrum (below about 11 keV) varies significantly from observation to observation while the high energy portion changes little. The X-ray flux varies on all timescales down to milliseconds. The power ...

We present the time variability characteristics of Cygnus X-1 in its two spectral states. The observations were carried out using the Pointed Proportional Counters (PPC) on-board the Indian X-ray Astronomy Experiment (IXAE). The details of the instrument characteristics, the observation strategy, and the background modeling methods are described. In the soft state of Cyg X-1, we confirm the general trend of the Power Density Spectrum (PDS) obtained using the Proportional Coun...