Details of the extreme physics of stellar mass black holes, supermassive galactic black holes, or even neutron stars (hyperdense remnants of stellar supernovae) used to be well-nigh impossible to come by. That’s in part because for decades space-based observations in the high-energy x-ray spectrum were in their infancy; basically crude and imprecise.
That began to change with the advent of NASA’s Chandra and the European Space Agency’s XMM-Newton x-ray space observatories. But with the 2023 launch of Japan’s X-Ray Imaging and Spectroscopy Mission satellite, high resolution x-ray astronomy is proving its ability to rewrite much of what we know about the extreme, high-energy universe.
In late 2024, XRISM (led by the Japan Aerospace Exploration Agency in partnership with NASA and the European Space Agency) used the satellite’s high-resolution Resolve instrument to gather data from the neutron star GX13+1. Located some 23,000 light years away towards the galactic center, NASA says GX13+1 is what’s known as an x-ray binary neutron star system — a collapsed, compact star at the end of its evolution accreting material from a more normal star.
In the process, this material is superheated to such high temperatures that the neutron star becomes a bright source of x-ray emission.
Yet a few days before the XRISM team’s observations were due to take place, GX13+1 unexpectedly got brighter – reaching or even exceeding a theoretical ceiling known as the Eddington limit, says ESA. At the Eddington limit, the amount of high-energy light being produced is essentially enough to transform almost all of the infalling matter into a cosmic wind, the space agency notes.
An international team of collaborators detail their findings in a recent paper appearing in the journal Nature.
As for how GX13+1 first formed?
The more massive star underwent a supernova and left a neutron star remnant, Joseph Neilsen, a Villanova University astrophysicist and one of the Nature paper’s corresponding authors, tells me via email. The upshot is that matter spirals from the companion star and then onto the neutron star in the form of an accretion disk, Neilsen tells me. Friction in the disk then causes the gas to slowly trickle inward and heat up which leads to the bright x-ray emission, he says.
During a rare event during which the system brightened beyond the Eddington limit, the resulting data revealed unexpectedly dense but surprisingly slow winds, says the Japan Aerospace Exploration Agency. This challenges current models of how matter and energy interact in extreme environments, says JAXA.
Winds around supermassive black holes like those found at the centers of most full-scale spiral galaxies, for instance, speed along at clips that can reach some 200 million km per hour. In contrast, GX13+1’s winds barely clocked in at one million km per hour.
There was so much wind that it started to become opaque, like a thick fog, says Neilsen. We realized that if we could still see the accretion disk through this wind, it must be even brighter than it looks, he says.
In fact, the team inferred that GX 13+1 was undergoing a “super-Eddington phase,” says Villanova University. Such a phase occurs when the neutron star shines so brightly that the radiation pressure from its surface can counteract gravity, leading to a powerful ejection of any infalling material, the university notes.
One reason we talk about X-ray binaries as laboratories for studying accretion onto compact objects is because they exhibit the same accretion process as supermassive black holes but — because they're so much smaller — they vary a lot faster, says Neilsen.
These thick winds can hide supermassive galactic black holes from view.
So, if we want to know how black hole activity contributes to the growth and evolution of the universe, understanding "obscured” sources like this is really important, says Neilsen.
There’s a lot of buzz about large supermassive black holes in the early universe, but to get so big that fast, black holes have to accrete lots of material in just a few hundred thousand years, says Neilsen. That means that super-Eddington accretion is a very important part of understanding how black holes grow over cosmic time, he says.
The bottom line?
The findings could reshape our understanding of how energy and matter interact in extreme environments, influencing everything from how stars form to how galaxies grow, Matteo Guainazzi, ESA’s XRISM project scientist, says in a statement.