A 0620 - 00 revisited: a black-hole transient case-study

Abstract

For the first time we have performed a detailed study of the X-ray, optical and infra-red light curves and an intercomparison between these wavelengths (including radio) of the 1975/1976 outburst of the famous black-hole transient A 0620 - 00 (Nova Mon 1975, V616 Mon). We also investigated the optical behaviour up to a year after the main outburst. This study enabled us to find some new features, which have not been discussed before. During the various stages of the outburst of A 0620 - 00 we found the X-rays lag the optical on the order of ∼5 to ∼20 days. Moreover, we found evidence that the activity associated with the secondary maximum started even earlier in the infra-red. This suggests that most of the processes associated with the outburst occur in the outer parts of the accretion disk. Although various drops in intensity (lasting on the order of a day or more) in the optical and X-ray outburst light curves of A 0620 - 00 have been reported before, we identified some new ones. One such X-ray ‘dip’ only appeared in the soft X-rays (1.5–6 keV) whereas at higher energies (≳6 keV) the intensity slightly increases. This shows that the X-ray spectrum pivots near ∼6 keV around that time. In the optical we found evidence for another local maximum around that time (so-called ‘intermediate’ maximum). The ‘intermediate’ maximum appears ∼30 days after the secondary maximum, which is close to the viscous time scale of an irradiated accretion disk. We suggest this feature to be an ‘echo’ of the secondary maximum. At the end of the outburst another local maximum occurs. Since the time difference between the peak of the tertiary maximum and this local maximum is also on the order of the viscous time scale we suggest that this feature is an ‘echo’ of the tertiary maximum. We find that drops in the optical and X-ray intensity before and during the rise to tertiary maximum are also present in various other short period soft X-ray transients (SXTs). They always occur ∼ 150 days after the start of the outburst. Although the X-ray spectrum of A 0620 - 00 gradually softens during the decay from outburst, we find for the first time that it starts to harden again ∼ 100–150 days after the start of the outburst, similar to that seen in GS 2000+25 and GS 1124 - 68. This suggests we witness the transition from the so-called high to low state identified in black-hole X-ray binaries. We note that the time of spectral hardening in A 0620 - 00, GS 2000+25 and GS 1124 - 68 is simultaneous with the occurrence of the drops in optical and X-ray intensity. We also show that the optical outburst amplitude and the shape of the optical outburst light curve of A 0620 - 00 closely resembles those of the cataclysmic variable AL Com (where the compact star is a white dwarf). This strengthens the similarity in outburst and quiescent properties of the SXTs and ‘Tremendous Outburst Amplitude Dwarf novae’ (TOADs) or WZ Sge stars, and shows that in general the optical outburst light curves of both groups are governed by the disk properties and not by the compact object. Since irradiation provides a natural mechanism to prolong the outburst of SXTs, we suggest this could be of influence as well during TOAD outbursts.