The rise of the black hole X-ray binary AT2019wey observed with TESS

TL;DR

This paper presents high-cadence optical observations of the black hole X-ray binary AT2019wey using TESS, revealing the earliest optical rise and providing insights into its outburst onset and early behavior.

Contribution

It provides the highest time resolution optical rising phase data for a BHXRB and analyzes the outburst onset timing relative to X-ray brightening.

Findings

Optical rise began after faint X-ray brightening in MAXI data.

Outburst onset time determined as MJD 58817.86±0.09.

No significant high-frequency periodic modulation detected.

Abstract

Black hole X-ray binaries (BHXRBs) have traditionally been discovered by X-ray surveys with cadences of hours to days. However, large optical time-domain surveys now provide novel avenues for early detection and insights into their elusive outburst triggering mechanisms. We present early-time light curves of the BHXRB AT 2019wey serendipitously observed by the Transiting Exoplanet Survey Satellite (TESS). The TESS images are sampled at 30 minute cadence from $\approx2$ d prior to $\approx25$ d after outburst, providing the highest time resolution optical rising phase observations of any known BHXRB. We fit a piece-wise power law to the rising light curve, finding an outburst onset time of MJD $58817.86\pm0.09$ and power-law rise index $n=0.74\pm0.04$. The onset time precedes all ground-based optical detections, and suggests that the optical rise began after the start of the faint X-ray brightening in MAXI data. We search for periodic high frequency modulation and detect none exceeding amplitude $\approx0.48$ mJy at periods of $\gtrsim1$ h at 90% confidence.