Black hole keeps blowing gas long after its peak
Very Large Telescope measurements show Swift J1727.8-1613 still driving winds and jets once its activity had fallen to roughly one percent of peak, the University of Warwick says.
Astronomers at the University of Warwick said on July 30 that the black hole in the binary system Swift J1727.8-1613 kept expelling gas even after its activity dropped to about one percent of peak. The measurements were made with the European Southern Observatory’s Very Large Telescope (VLT), and the paper appeared in Monthly Notices of the Royal Astronomical Society, DOI 10.1093/mnras/stag1175. The result sits badly with the familiar picture of a black hole as a pit that keeps whatever falls toward it.
Swift J1727.8-1613 is a system in which a black hole pulls matter off a nearby companion star. That gas does not drop straight in. It first winds up in an accretion disc, a hot ring that radiates in X-rays, and part of it leaves again in two forms: a jet, a narrow stream along the rotation axis, and a wind, a broader and slower outflow off the disc itself.
The system flared in 2023 and became one of the brightest X-ray sources in the sky, which put it on the schedule of observatory after observatory. Outbursts like this get watched hardest while they are at their brightest, because the late phase gives a faint signal and needs a big telescope to be measured at all. That late phase is exactly what the Warwick team went after.
How the measurement was done
The team used X-Shooter on the VLT, a spectrograph that covers the ultraviolet, visible and near-infrared range in a single exposure. It collected 13 optical spectra across 11 epochs during the 2023 outburst, one of the more detailed optical records assembled for a system of this kind. The spectra carry lines from gas moving away from the disc, and those lines are what the estimate of the outflow is built on.
The expectation was that the outflow would fade along with the brightness. It did not. Jets and winds went on pushing matter out of the system once activity had fallen to roughly one percent of peak.
“People often imagine black holes simply swallowing everything around them. What we’re seeing is a much more complex process. Matter falls in, the system processes it, and a surprising amount is expelled again,” said Dr Noel Castro Segura, a postdoctoral fellow at the University of Warwick and lead author of the paper, in the university’s press release.
How much does the black hole actually keep?
The team estimates the wind carries away at least about 10 to the minus ninth solar masses per year. Modelling of the X-ray spectrum puts the accretion rate in the same order of magnitude, meaning the amount of gas leaving the system is comparable to the amount reaching the black hole. What the paper delivers is that ratio rather than an absolute figure, and both numbers carry measurement uncertainty.
“If black holes can continue shedding material even after their largest outbursts, it means they may be much less efficient eaters than we previously assumed. A significant fraction of the meal may never reach the black hole at all, changing our understanding of how binary stars in galaxies evolve,” Castro Segura said.
The paper has 24 co-authors, among them researchers from the University of Cape Town. One of them is doctoral researcher Kyle Solomons, who worked on the late outburst phase.
“We usually gravitate towards the dramatic fireworks when a black hole outburst begins, but our observations show that the finale can be just as intense. Even as the system’s X-ray emission dropped to a fraction of its peak, it still had enough power to generate a massive expulsion of gas,” Solomons said in the Warwick press release.
What changes in the accounting
If the same behaviour turns up in other systems, the bookkeeping of how binary star systems evolve in galaxies has to be redone: how much mass the black hole really gains, how much goes back into the surrounding gas, and how far the orbit shifts because of it. We wrote about black holes recently over a rogue one that a piece of software found in archived data rather than an astronomer at a telescope. That story was about how black holes get found at all. This one is about what they do once we already have them in view.
What the paper does not prove
One outburst of one system was measured. That does not establish that every stellar-mass black hole behaves this way, and it says nothing about supermassive black holes in galactic centres, where the timescales are longer by several orders of magnitude. The accretion rate estimate depends on the X-ray spectral model and moves with assumptions about disc geometry, so the comparison with the outflow links two numbers rather than closing a measured budget. Confirmation needs the same kind of late-phase monitoring on several more outbursts, and only a handful of those come along each year.
