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

4. Discussion

The results of our analysis indicate that, at least for observations of class and (which have many similar traits), we have a way to estimate the disk accretion rate during an instability event, when the inner disk radius grows from its "minimum" value of 30 km and slowly moves back to it. Although we know that the measured value is only an underestimate, it is natural to associate this minimum value with the innermost stable orbit. It is interesting to compare these values, or at least their ranking, with the rate of ejection in the jets. As we mentioned above, the accretion rate measured through this procedure is associated to matter flowing through the observable inner edge of a geometrically thin accretion disk. Some of the accreting gas must leave the accretion disk to form the jet, unless it is entirely composed of pairs generated by photon-photon interactions. and how this happens is basically unknown. There are two extreme possibilities: either matter ejected in the jet leaves the accretion disk before entering the innermost regions, thus not contributing to our measured disk accretion rate (case 1), or it leaves it after passing through our measured inner disk radius, in which case it is a fraction of the accretion rate we measure (case 2). In case 1, if the fraction of matter in the jet is constant and the total external accretion rate (disk+jet) is variable, we expect a positive correlation between disk accretion rate (from X rays) and disk ejection rate (from the infrared). If the fraction is variable and the total is constant, these quantities should be anticorrelated. In case 2, if the fraction of matter in the jet is constant, we expect a positive correlation, while the constant total is in this case not possible as the total would be what we measure, which is not observed to be constant. If both fraction and total vary, the situation is complicated. Of course, there is a spectrum of intermediate possibilities, where the jet production is connected to the inner region of the disk in a way that would not allow to dissociate the two processes. With the paucity of existing data, we limit ourselves to the extreme cases. Notice that measuring an anti-correlation would be an indication against case 2.

Table 1 also lists an estimate of the mass ejection rate . This is based upon an equipartition calculation for one proton for each electron, negligible kinetic energy associated with the repeated ejection events, and an average over the repetition period of the oscillations. Note that there is a systematic uncertainty in these numbers due to lack of knowledge of the intrinsic electron spectrum which corresponds to the observed flat-spectrum radio-infrared emission. However, unless the spectral form of the distribution changes between observations then the effect is the same for all data sets and the ranking remains the same. Of course we may be observing synchrotron emission from a pair plasma with no baryonic content, in which case the amount of power being supplied to the jet, , makes more useful comparison with the accretion rate; this value is also listed in Table 1. For more details of how these quantities are calculated, see Fender & Pooley (2000). Either way, there appears to be an anticorrelation between accretion rate inferred from the X-ray spectral fits and the outflow rate of mass/energy in the jet. The low number of points in our sample prevents us from saying something more firm. Notice that an anticorrelation is also suggested by the strong flat-spectrum radio emission observed during long `plateau' intervals; periods when Belloni et al. (2000) estimate that the accretion rate must be very low. We also note that the faint infrared flares reported by Eikenberry et al. (2000) do not appear to be different from the others in other respects, as the X-ray light curves are too undersampled to allow a detailed correlation.

If future observations show that disk accretion rate and jet ejection rate are indeed anti-correlated, the following scenario could be speculated. A fraction of the accreting gas leaves the geometrically thin accretion disk before reaching the inner edge (from which it would fall into the black hole) and goes into a hot corona. The details are not known, but our results indicate that this does not happen after the inner edge. As the disk refills, the inner radius moves inwards, more soft photons from the disk reach the corona, which causes its Comptonization emission to soften gradually. At the end of the instability period, when the disk is refilled down to the innermost stable orbit, this "reservoir" of hot gas is expelled to produce the jet, resulting in the observed infrared / mm / radio emission, causing the power-law component to steepen dramatically and to cause the sudden change in the X-ray count rate and spectral parameters. Notice that, as we remarked earlier, the distributions of points in Fig. 2 are flatter than the expected curve for a constant disk accretion rate according to a standard thin disk: in other words, as the inner disk radius decreases, the disk accretion rate seems to decrease as well. This could mean that the process that re-routes some gas from the disk to the corona becomes more efficient closer to the central object, and therefore the fraction of matter going into the corona increases as the disk refills.

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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