IIA Team Traces Black Hole Behind Bright X-ray Source ULX-8

Indian astronomers have explained the shifting X-ray behaviour of NGC 4490 ULX-8, one of the brightest X-ray emitters in a nearby galaxy, by following it for nearly a quarter of a century. Their work points to a stellar-mass black hole that is swallowing matter at or beyond the classical limit on how fast such an object can feed.

What an ultraluminous X-ray source is

Ultraluminous X-ray sources, or ULXs, are the brightest X-ray emitters in nearby galaxies apart from the supermassive black holes at galactic centres. A ULX is thought to hold either a neutron star or a black hole that pulls gas off a companion star. As the gas spirals in it heats up and pours out X-rays. Oddly, ULXs look brighter than the Eddington limit allows. That limit is the brightness at which the outward push of radiation balances the inward pull of gravity, and it is the usual yardstick for how luminous an accreting object can be.

How the study was done

A team from the Indian Institute of Astrophysics and the Raman Research Institute, both autonomous institutes of the Department of Science and Technology, worked with IUCAA and other institutions. They combined archival and recent observations of ULX-8 in the interacting galaxy pair NGC 4490/4485, taken by the Chandra, XMM-Newton and Swift space telescopes between 2000 and 2024. Aru Beri of the Indian Institute of Astrophysics, a co-author, described it as one of the longest X-ray studies of this source so far.

What they found

• The source stayed exceptionally luminous through the whole observing span, though its brightness swung widely.
• It showed none of the sharp switches between accretion states that are common in black hole X-ray binaries.
• Its emission kept the marks of a persistently active ultraluminous accretion flow.
• Long-term spectral changes indicate that conditions around the compact object are evolving.

Taken together, the evidence favours a black hole feeding at super-Eddington rates, making the system a natural laboratory for studying how compact objects sustain such extremes and release so much energy.

Who wrote it and what comes next

The paper was written by Tarang Vashisht, a student at IISER Mohali, with Aru Beri and researchers from IUCAA and the Raman Research Institute, and appeared in Monthly Notices of the Royal Astronomical Society. Sharper instruments are on the way: the European Space Agency's NewAthena mission and India's proposed next-generation timing and spectroscopy mission XSat are both expected to improve sensitivity and monitoring of such sources.