Astron. Astrophys. 347, L23-L26 (1999)

5. Discussion

A time averaged energy spectrum of Cyg X-1 (e.g. (Ebisawa et al. 1992), (Gierlinski et al. 1997)) as well as frequency resolved spectra at low frequency (Fig. 1) show a clear deviation from a single slope power law spectrum. This deviation is commonly ascribed to the reflection from an optically thick media located in the vicinity of the production site of primary hard X-ray radiation. In a simple case of reflection from an optically thick cold neutral medium the main reflection features are well known ((Basko et al., 1974), (George & Fabian, 1991)) - a narrow unshifted iron line at 6.4 keV with an equivalent width of eV, an absorption edge at keV and a reflected continuum peaked at keV. However, the real spectra of compact X-ray binaries show considerably more complicated behavior. In particular the centroid energy of the line is often different from 6.4 keV, the line width in many cases is as large as keV and a broad "smeared edge" above 7-8 keV is observed instead of a sharp absorption edge at keV ((Ebisawa et al. 1992)). This departure from a simple reflection model hints at a complicated ionization state and/or motion (e.g. Keplerian motion in the disk) of the reflecting media.

It is obvious, that the simple spectral model used for fitting the data in the previous section is neither adequate nor completely justified from the physical point of view. The apparent line centroid energy and width vary with frequency. In particular the best fit centroid energy shifts below 6.0 keV and the width of the line exceeds 1 keV as the frequency increases. However, due to the lack of a satisfactory realistic model of reflection in an X-ray binary, especially applicable to the frequency resolved spectral data, we restricted ourselves to the simple two component model described in the previous section. The main purpose of using this spectral model was to quantify the major effect which is clearly seen in Fig. 1, namely the suppression of the reflection features in the energy spectrum with the increase of frequency.

The results of the analysis presented in this paper show that the relative amplitude of the reflected component variations is lower on the short time scales ( 50-100 msec), than that of the primary X-ray radiation. A similar effect was found by Oosterbroek et al. (1996) for GS 2023+338 but the characteristic time scale in this case was much longer, 200 sec. GS 2023+338 is peculiar in many ways. In particular the source exhibited strong ( cm^-2) and highly variable (on the time scales of sec) low energy absorption (e.g. (Zycki et al., 1999)) and, most importantly, extremely strong Fe line with equivalent width of up to 1.4 keV ((Oosterbroek et al. 1996)), That could mean that geometry of the reprocessing media in GS2023+338 is different from that in Cyg X-1. Recently we found the suppression of the reflected component variations in the low spectral state of another black hole candidate, GX339-4 (Revnivtsev, Gilfanov & Churazov, 1999). In this case the characteristic time scale (50-100 msec) was similar to that in Cyg X-1. We, therefore, can tentatively conclude that such behavior might be a common feature of the black hole binaries in the low spectral state.

A most straightforward explanation of this effect would be in terms of a finite light crossing time of the reflector , caused by a finite spatial extend of the reflecting media. From Fig. 2 one can see that the equivalent width of the iron line drops by a factor of two at the frequency Hz. This value gives us a rough estimate of the characteristic response time of the reflector msec and the characteristic size of the reflecting media cm which would correspond to for a black hole. We note, however that if the primary continuum originates within the sphere in the inner zone of the accretion disk, then only a small fraction of the emitted hard radiation has the possibility to be reflected from the accretion disk regions with (in the case of a flat disk) and it can hardly provide the observed eV equivalent width of the iron line. Therefore, the assumption that high frequency variations of the reflected component are caused by the finite light crossing time of the reflector implies interesting constraints on the geometry of the source of the primary continuum and the reflector.

An alternative explanation might be that the short time scale, 50-100 msec, variations appear in the geometrically different, likely inner, part of the accretion flow and give a rise to significantly weaker, if any, reflected emission than the longer time scale events, presumably originating in the outer regions. This might be caused, for instance, by a smaller solid angle of the reflector as seen by the short time scale events and/or due to the screening of the reflector from the short time scale events by the outer parts of the accretion flow.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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