Optical precursors to X-ray binary outbursts
Abstract
Disc instability models predict that in quiescence, there should be a slow but prominent brightening of the optical flux between outbursts of black hole X-ray binaries (BHXBs). There is also expected to be an optical precursor to outbursts - a brightening at optical wavelengths of the accretion disc, prior to the X-ray outburst. Tracking the X-ray variations of BHXBs in quiescence is generally not possible, so optical monitoring provides the best means to measure the mass accretion rate variability between outbursts. With our regular Faulkes Telescope / Las Cumbres Observatory (LCO) monitoring we are routinely detecting the optical rise of new X-ray binary outbursts before they are detected by X-ray all-sky monitors.We present long term optical monitoring of several black hole X-ray binaries, with the Faulkes Telescopes and LCO. We have detected the slow rise between outbursts in the BHXBs GS 1354-64, H1705-250 and Swift J1357.2-0933, and the optical precursor to the bright 2015 outburst of V404 Cyg.In Swift J1357.2-0933, the quiescent light curve is dominated by high amplitude, short-term (seconds - days) variability spanning 2 mag, with an increasing trend of the mean flux over five years that is steeper than in any other X-ray binary found to date (0.17 mag/yr). Such a steep optical flux rise preceding an outburst is expected according to disk instability models, but thehigh amplitude variability in quiescence is not. We find that a variable optical/infrared jet spectrum is responsible for the brightening in this source.We show that it may be possible to predict when new outbursts are likely to occur, by estimating the accumulation of matter in the disc in quiescence from optical monitoring.Finally, we introduce our new real-time data analysis pipeline, the "X-ray Binary New Early Warning System (XB-NEWS)" which aims to detect and announce new X-ray binary outbursts within a day of first optical detection. This allows us to trigger X-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E2917R