Astronomers have traced the changing X-ray behaviour of NGC 4490 ULX-8 through observations spanning 24 years, offering new insights into the compact object powering this exceptionally bright source.
Astronomers have investigated the long-term X-ray behaviour of NGC 4490 ULX-8, one of the brightest X-ray sources in a nearby galaxy, by analysing observations collected between 2000 and 2024. The findings could help scientists understand the compact objects powering ultraluminous X-ray sources (ULXs) and how these objects grow under extreme conditions.
ULXs are among the brightest X-ray emitters in nearby galaxies, apart from the emission produced by supermassive black holes at galactic centres. They are thought to contain either neutron stars or black holes that draw matter from a companion star. As this material moves inward, it heats up and releases intense X-rays.
These sources appear brighter than the maximum luminosity expected from the balance between gravity pulling matter inward and radiation pushing it outward. This threshold is known as the Eddington limit and serves as a benchmark for the brightness of an accreting object.
Understanding the compact object powering a ULX is important for studying super-Eddington accretion, powerful gas outflows and the growth of compact objects in extreme environments.
Researchers Track 24 Years of X-ray Observations
A research team from the Indian Institute of Astrophysics (IIA) and the Raman Research Institute, both autonomous institutes under the Department of Science and Technology (DST), along with researchers from other institutions, identified long-term changes in the X-ray spectrum of ULX-8. These changes point to evolving accretion conditions around the compact object.
The team analysed archival and recent observations of ULX-8 in the interacting galaxy pair NGC 4490/4485. The data came from three space-based X-ray telescopes: Chandra, XMM-Newton and Swift, covering the period from 2000 to 2024.
“We analysed archival and recent observations of ULX-8 in the interacting galaxy pair NGC 4490/4485, obtained with Chandra, XMM-Newton, and Swift space-based X-ray telescopes between 2000 and 2024, enabling one of the longest X-ray studies of this source to date”, explained Aru Beri, a faculty member at IIA and a co-author of the study.
Findings Point to a Black Hole Powering ULX-8
The observations showed that ULX-8 remained an exceptionally luminous X-ray source throughout the 24-year period, although its brightness varied substantially over time.
Despite these variations, researchers found no evidence of the dramatic transitions between different accretion states commonly seen in many black hole X-ray binaries. Instead, the source retained X-ray emission characteristics consistent with a persistently active ultraluminous accretion flow.
The findings suggest that ULX-8 is most likely powered by a stellar-mass black hole accreting matter at or near the Eddington limit. The results provide strong evidence that the source is powered by a black hole accreting at rates exceeding the classical Eddington limit.
According to the researchers, ULX-8 offers an important opportunity to study how compact objects sustain extreme rates of mass accretion and generate enormous amounts of energy. The study also demonstrates the importance of long-term observations using multiple space missions to investigate the nature of bright X-ray sources.
Future Missions Could Improve ULX Observations
Future observatories are expected to provide better sensitivity and monitoring capabilities for studying ULXs. These include the European Space Agency’s (ESA) NewAthena mission and India’s proposed next-generation timing and spectroscopy mission, XSat.
Continued long-term observations could help scientists understand how these extreme systems evolve and provide deeper insights into the physics of super-Eddington accretion and the nature of some of the brightest X-ray sources in the nearby Universe.
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