# Optical precursors to X-ray binary outbursts

## TL;DR

This paper demonstrates that optical monitoring can detect the early stages of X-ray binary outbursts before X-ray detection, using a new real-time pipeline to enable prompt multi-wavelength follow-up and better understanding of outburst mechanisms.

## Contribution

The authors introduce a new real-time data analysis pipeline, XB-NEWS, that detects X-ray binary outbursts within a day of optical rise, improving early warning capabilities.

## Key findings

- Optical precursors to X-ray outbursts are detectable before X-ray signals.
- The XB-NEWS pipeline enables rapid detection and announcement of outbursts.
- Initial observations of MAXI J1820+070 show early optical rise detection.

## Abstract

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. Tracking the X-ray variations of X-ray binaries in quiescence is generally not possible, so optical monitoring provides the best means to measure the mass accretion rate variability between outbursts, and to identify the beginning stages of new 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 examples of detections of an optical rise in X-ray binaries prior to X-ray detection. We also present initial optical monitoring of the new black hole transient MAXI J1820+070 (ASASSN-18ey) with the Faulkes, LCO telescopes and Al Sadeem Observatory in Abu Dhabi, UAE. 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 will allow us to trigger X-ray and multi-wavelength campaigns during the very early stages of outbursts, to constrain the outburst triggering mechanism.