Speaker
Description
Low-mass X-ray binaries spend most of their lifetimes in quiescence, where accretion occurs at extremely low luminosities and the optical emission is often interpreted as a combination of the donor-star contribution and non-stellar light. A0620–00, the archetypal quiescent black-hole transient, is the best-studied system in this regime. Since its 1975 outburst, the source has remained in X-ray quiescence. Its optical behaviour alternates between passive and active states. In the passive state, the light curve is dominated by the Roche-lobe-filling donor star, whose tidally distorted shape produces the orbital ellipsoidal modulation. In the active state, the source is brighter and more irregular, indicating an enhanced non-stellar contribution.
I will present a long-term monitoring study of A0620–00, in which we identify a superorbital modulation with P = 261.9 ± 9.4 days and a peak-to-peak amplitude of ~0.2 mag. We find that the relative occurrence of the passive and active quiescent states is modulated by the superorbital period, with passive states preferentially found near superorbital minimum and active states more common near superorbital maximum. This suggests that a long-term process affecting the non-stellar optical component organises the optical states of the system, even at luminosities of order 10⁻⁸ $L_{\rm Edd}$. I will discuss how retrograde nodal precession of a misaligned hot inner flow provides the most self-consistent explanation. In this picture, the alternation between quiescent states may be linked to the cyclic geometric reorientation of the hot inner flow, which changes the projected visibility of its optical emission and is reflected in their phase-dependent occurrence.