X-ray Studies of the Black Hole Binary Cygnus X-1 with Suzaku

In order to study X-ray properties of black hole binaries in so-called Low/Hard state, we analyzed 0.5--300 keV data of Cyg X-1, taken with the X-ray Imaging Spectrometer and the Hard X-ray Detector onboard the X-ray satellite Suzaku. The data were acquired on 25 occasions from 2005 to 2009, with a total exposure of ~450 ks. The source was in the Low/Hard state throughout, and the 0.5-300 keV luminosity changed by a factor of 4, corresponding to 2-10% of the Eddington limit for a 10 Mo black hole.

Among the 25 data sets, the first one was already analyzed by Makishima et al. (2008), who successfully reproduced the wide-band spectrum by a linear combination of an emission from a standard accretion disk, soft and hard Comptonization continua, and reprocessed features. Given this, we analyzed the 25 data sets for intensity-related spectral changes, on three different time scales using different analysis methods. One is the source behavior on time scales of days to months, studied via direct comparison among the 25 spectra which are averaged over individual observations. Another is spectral changes on time scales of 1-2 seconds, revealed through ``intensity-sorted spectroscopy''. The other is spectral changes on time scales down to ~0.1 seconds, conducted using ``shot analysis" technique which was originally developed by Negoro et al. (1997) with Ginga. These studies partially incorporated spectral fitting in terms of a thermal Comptonization model. We payed great attention to instrumental problems caused by the source brightness, and occasional ``dipping" episodes which affects the Cyg X-1 spectrum at low energies. The shot analysis incorporated a small fraction of XIS data that were taken in the P-sum mode with a time resolution of 7.8 msec.

Through these consistent analyses of all the 25 data sets, we found that a significant soft X-ray excess develops as the source gets brighter. Comparing results from the different time scales, the soft excess was further decomposed into two different components; a harder one which varies on < 1 sec time scales, and a softer one which varies on >day time scales but remains constant during fast (~1 sec) variations. The former and latter was successfully identified with the soft Compton continuum and directly-visible disk emission, respectively. These results confirm, in a rather model independent manner, the spectrum decomposition employed by Makishima et al. (2008). Furthermore, by inspecting continuum shapes above a few keV, we found that, on all the time scales studied here, the X-ray intensity becomes higher when a larger fraction of disk photons get into the Comptonizing corona, accompanied by a decrease in the Compton y parameter.

We successfully obtained shot profiles with a time resolution of 0.1 sec, in several energy bands covering 10-200 keV. The profiles, which are basically very symmetric with respect to time, in fact involve subtle asymmetry, which becomes rather prominent above 100 keV. Specifically, the y-parameter and the Compton electron temperature both decrease gradually through the rising phase of the shot, but these parameters suddenly return to their time-averaged values immediately (< 0.1 sec) past the shot peak. These results agree with those from Negoro et al. (1997), and extend them into much higher energies. We discuss physical conditions which any successful shot model should explain, and present a few possible examples.