Speaker
Description
V4641 Sgr is a peculiar black hole X-ray binary with unusual X-ray outburst behaviour, and bright superluminal radio jets which are misaligned with the very high-energy and ultrahigh-energy gamma ray emission. This system may be considered as a low mass X-ray binary due to mass transfer via Roche lobe overflow, despite harbouring a B-type subgiant companion star, which provides a rare opportunity to study the companion star of such systems. During X-ray quiescence, V4641 Sgr is observed to transition between passive and active optical states. Here, we present 18 years of optical photometric data from the Las Cumbres Observatory 2-m and 1-m telescopes in 3 bands during quiescent and outburst states. We measure a refined orbital period from the modulation of the subgiant companion, which dominates the quiescent emission. By subtracting the modulation in each band, the spectral and variability properties of the non-stellar flux are constrained during the elusive faint "mini-outbursts" of the system, of which we detect several during the observing campaign. These are combined with new multi-wavelength (infrared and X-ray) data to construct spectral energy distributions and optical-IR/X-ray flux correlations. We also probe short-term optical variability and flickering during these mini-outbursts. The results help to constrain the disc and jet contributions to the optical/IR flux, and shed light on the nature of the rare mini-outbursts from accreting black holes.