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Astron. Astrophys. 358, L29-L32 (2000)

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3. Results

In principle, from each spectrum the accretion rate through the measured inner radius of the disk could be measured from the values of [FORMULA] and [FORMULA] (see Belloni et al. 1997a) by using the expression from a standard thin accretion disk. However, given the errors on these parameters, this measurement is too uncertain. In order to obtain an improved estimate of the disk accretion rate or, better, a ranking of the observations in terms of accretion rate (since the actual values of the inner disk radii obtained with the multicolor disk-blackbody model are probably underestimates, see Merloni, Fabian & Ross 1999), we plotted the values corresponding to the deepest parts of the X-ray light curves in a [FORMULA] vs. [FORMULA] plane (see Fig. 2). If for each observation the disk accretion rate was constant, the points should lie on the diagonal lines corresponding to a slope -3/4 (as, for a given [FORMULA], [FORMULA] - Belloni et al. 1997a). Their actual distribution is flatter, showing that there is a deviation from the expected law, but it is interesting to note that the distributions lie on parallel curves in the log-log plane. This indicates different values of the disk accretion rate. Lines corresponding to the larger measured radius for each of the four observations are shown in Fig. 2 with their associated accretion rate value. Typical 1[FORMULA] errors are also shown. Although the actual values for the accretion rate are probably not accurate, on the basis of this plot we can rank the observations by accretion rate. It is important to note that the accretion rate measured this way correspond to matter passing through the observed inner radius of the disk only: if some matter leaves the disk before that radius, its presence cannot be detected with this procedure. This estimate of accretion rate can be double checked by considering the length of the state C intervals, which Belloni et al. (1997a,b) interpreted as the viscous time scale of the disk at the edge of the unobservable region which is refilled. The observation from 1999 May 22nd has a smaller inner disk radius (see Fig. 2) than the 1997 ones and a longer re-fill time (Table 1), indicating a lower value of the accretion rate. The 1998 July 10th observation has a much larger inner disk radius than the 1997 ones, by a factor of 1.7 and 2.3, which would correspond to a re-fill time longer by a factor 6.4 and 18 respectively, while it is much shorter, indicating a higher accretion rate.

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© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000