Awakening the beast: How are X-ray binary outbursts triggered?

Disc instability models predict that for X-ray binaries in quiescence, there should be a brightening of the optical flux prior to an X-ray outburst. With our regular Faulkes Telescope/Las Cumbres Observatory (LCO) monitoring we are routinely detecting the optical rise of new outbursts before they are detected by X-ray all-sky monitors. We have developed a real-time data analysis pipeline, the X-ray Binary New Early Warning System (XB-NEWS) which aims to detect new outbursts in real-time from their optical flux rise. Here, we propose to catch the early stages of a new outburst in X-rays with Chandra, using our optical trigger. We will be able to test the theory that the X-ray flux starts to rise after the optical flux, a key prediction of the disc instability model.