When MAXI J0637−430, a binary system containing a black hole, began to fade after a major outburst, something unexpected happened: the outer regions of its disc became more illuminated, rather than less. A study published in Nature Astronomy, led by Montserrat Armas Padilla, a researcher at the Instituto de Astrofísica de Canarias, has revealed that this behaviour is due to changes in the shape of the inner part of the disc, which can cast a shadow over the regions furthest from the black hole. During the brightest phase of the outburst, this structure had swollen, blocking some of the intense light generated near the event horizon. As the brightness decreased, the disc deflated, the shadow disappeared and the outer regions were exposed to the radiation. The finding reveals that accretion discs can change shape at much lower luminosities than previously thought.

MAXI J0637−430 is a binary system in which a black hole feeds on matter from a companion star. Before crossing the event horizon, this material accumulates to form an accretion disc. The innermost regions of the disc reach extreme temperatures and emit mainly X-rays, while the outer regions can be studied at visible and infrared wavelengths.

From time to time, these systems undergo major outbursts, during which the amount of matter falling towards the black hole increases dramatically and their brightness can increase millions of times.

MAXI J0637−430 was discovered during one such outburst. The surprise came as the system began to fade. One would naturally expect that, as the system dims and produces fewer X-rays, the outer regions of the disc would receive progressively less radiation. Yet the observations showed exactly the opposite.

At the peak of the outburst, the spectrum obtained with the X-shooter instrument on the Very Large Telescope (VLT) showed strong hydrogen and helium emission lines, characteristic of a relatively cool, weakly irradiated outer disc. Just two weeks later, when the X-ray luminosity had fallen by roughly half, these lines had almost disappeared, and the spectrum indicated a much hotter, strongly irradiated outer disc.

“It was a surprise, exactly the opposite of what we expected,” explains Montserrat Armas Padilla, a researcher at the Instituto de Astrofísica de Canarias (IAC) and lead author of the study. “If the X-ray source was fading, we would have expected the outer disc to receive less radiation. Instead, the observations suggested that it was receiving more. The key had to lie in how the geometry of the disc itself was changing.”

A disappearing shadow

The team’s proposed explanation is easy to picture. During the brightest phase of the outburst, the inner part of the disc puffs up, increasing its vertical thickness. This structure acts as a screen, blocking some of the intense radiation produced near the black hole, casting a shadow over the outer regions and keeping them relatively cool. As the outburst fades, the pressure exerted by radiation within the inner disc also decreases, allowing the disc to become thinner again. Once the shadow disappears, the outer regions are exposed to X-rays coming from close to the black hole.

“It is like dimming a lamp while removing its shade: even though the bulb emits less light, an area that was previously in shadow becomes more brightly illuminated. That is precisely what we think is happening in MAXI J0637−430,” explains Teo Muñoz Darias, researcher at the IAC and co-author of the paper.

Visible and infrared light reveal the effects of this change in the outer regions of the disc. X-rays, meanwhile, allow researchers to probe the regions much closer to the black hole. Examining data obtained with the NICER, Swift and NuSTAR space missions, the team found signs in the X-ray spectra that the disc’s inner structure was also changing as the system faded. The pieces fell into place. “The surprise was not just that the disc changed shape, but when it did so,” explains Armas Padilla.

According to classical models, accretion discs should remain relatively thin at moderate luminosities, only becoming substantially thicker as they approach the so-called Eddington limit: the luminosity at which the outward pressure of radiation can counteract gravity and hinder the infall of matter. In MAXI J0637−430, however, this change appears to occur when the system is emitting at only around 10–20 per cent of that limit.

This matters because the shape of the disc is far from a minor detail. Many estimates of black hole properties depend on assumptions about how matter is distributed in the innermost regions of the disc. If the disc is thicker than previously assumed, this could affect estimates of the black hole’s spin, for example — one of its most fundamental properties, alongside its mass.

Eight researchers from the IAC are taking part in the study: Montserrat Armas Padilla, who has led the work, Teo Muñoz-Darias, Daniel Mata Sánchez, Enzo Saavedra, Alessandra Ambrifi, Jorge Casares, Manuel A. P. Torres e Idaira Yanes-Rizo. Furthermore, it forms part of an international collaboration involving institutions from Spain, the Netherlands, Japan, Germany, Italy, Chile, the United Kingdom and the United Arab Emirates.

To carry out the research, the team combined observations in visible and infrared light – obtained mainly using the X-shooter instrument, installed on the European Southern Observatory’s (ESO) Very Large Telescope (VLT) in Chile – with X-ray data from the NICER, Swift and NuSTAR space observatories. This combination of techniques made it possible to simultaneously track the evolution of the inner and outer regions of the accretion disc and to study how its structure changes during the outburst.

Article: Montserrat Armas Padilla et al., “A self-shadowing black hole accretion disk at sub-Eddington luminosities”, Nature Astronomy, 2026. DOI: https://doi.org/10.1038/s41550-026-03005-2

Contact:
Montserrat Armas Padilla  m.armaspadilla [at] iac.es (m[dot]armaspadilla[at]iac[dot]es)