TESS spots the rise of a black hole X-ray binary system

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Reduced TESS light curve of the AT 2019wey outburst observed in Sector 19. Gray dots are individual measurements from difference imaging on the TESS full-frame images, with the red being the power-law rise model. The red line is the best-fit piecewise power-law function fit to the rising light curve. The green shaded region indicates the best-fit MAXI X-ray evolution found by Y. Yao et al. (2021b). Credit: arXiv (2026). DOI: 10.48550/arxiv.2603.24683

Designed to hunt for new alien worlds, NASA's Transiting Exoplanet Survey Satellite (TESS) has serendipitously observed the rising outburst of a black hole X-ray binary known as AT 2019wey. The observations, which may help us better understand the nature of this system, were presented March 25 on the arXiv pre-print server.

Not only a planet hunt

Launched into space in 2018, TESS is conducting a survey of about 200,000 bright stars near the sun with the aim of searching for transiting exoplanets. Currently, during its extended mission, it also investigates all types of objects that change in brightness, from nearby asteroids to pulsating stars and distant galaxies containing supernovae.

During an exoplanet search in late 2019, TESS serendipitously identified one such brightness-changing object. It turned out that the planet-hunting telescope caught AT 2019wey—a black hole X-ray binary (BHXRB) first detected by the Zwicky Transient Facility (ZTF) and the ATLAS optical survey. In general, BHXRBs are binary systems consisting of a black hole orbited by a stellar companion, typically a low-mass, evolved star.

"The source was observed by NASA's TESS as it entered outburst, capturing the rise with high photometric precision and nearly uninterrupted 27-day coverage," reports a team of astronomers led by Alyana Jusino of the City College of New York.

Spotting the outburst from the beginning

AT 2019wey was observed by TESS in Sector 19, with full-frame images obtained using a 30-minute cadence. The telescope managed to capture the initial stages of the outburst—from two days prior to 25 days after the outburst. All in all, TESS delivered the highest time resolution optical rising phase observations of any known BHXRB system.

The observations found that the outburst of AT 2019wey started on Nov. 26, 2019 and had a power-law rise index of approximately 0.74. It turned out that the derived outburst onset time precedes the first detection of AT 2019wey by ZTF and ATLAS.

The astronomers compared the results with the data from the Monitor of All-sky X-ray Image (MAXI) telescope on the International Space Station (ISS). They found that the X-ray brightening of AT 2019wey also began prior to the first detection from ZTF and TESS.

According to the authors of the paper, this suggests an inside-out outburst scenario for AT 2019wey, in which thermal instability begins in the inner accretion disk of the binary and then moves outward.

Given that previous ZTF observations of AT 2019wey identified a possible periodic high frequency modulation (with a 1.3-hour period) on the fading phase of the light curve, Jusino's team tried to confirm this finding with TESS. However, they found no significant periodic modulations exceeding an amplitude of 0.48 mJy and corresponding to periods from one hour to 14 days.

Written for you by our author Tomasz Nowakowski, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

Alyana Jusino et al, The rise of the black hole X-ray binary AT2019wey observed with TESS, arXiv (2026). DOI: 10.48550/arxiv.2603.24683

Journal information: arXiv

Key concepts

Transient & explosive astronomical phenomenaTime domain astronomyX-ray astronomyLight curvesSpace telescopes

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