Speaker
Description
Despite decades of research, predicting outbursts of X-ray transients, and witnessing their beginning stages, remains very challenging. The cause of these X-ray brightenings is thought to be the sudden increase of accretion of matter onto a black hole or neutron star, initiated by the ionization of hydrogen in the accretion disc. I present recent observational advances in our understanding of how X-ray transients first brighten, focussing on the best case, optical and X-ray monitoring of the early rise of the black hole X-ray binary, Swift J1753.5-0127 in 2023. A delay of ~4 days is measured between a thermal instability developing in the accretion disc, causing heating fronts to begin propagating through the disc (seen by an optical brightening), and the onset of accretion onto the black hole (X-ray brightening). We witness the propagation of the heating wave, as a steady increase in the flux and surface area of the disc, and we constrain the disc viscosity. I demonstrate the ability of optical monitoring to be able to provide a few days (sometimes weeks) lead time to the rise of X-ray transient outbursts. This can help to inform X-ray missions, including possibly reducing the time needed to spot new outbursts in MAXI data.
