New Delhi: Astronomers have shed light on the changing X-ray behaviour of NGC 4490 ULX-8, one of the brightest X-ray emitters in a nearby galaxy, by analysing X-ray observations collected over a 24-year period.
The findings could help improve understanding of the compact objects that power ultraluminous X-ray sources (ULXs) and how these objects grow in extreme environments.
Ultraluminous X-ray sources (ULXs) are among the brightest X-ray emitters in nearby galaxies, apart from the emission associated with supermassive black holes at their centres.
NGC 4490 ULX-8 is located in the interacting galaxy pair NGC 4490/4485 and has remained an exceptionally luminous X-ray source over the long period of observations.
ULXs are thought to host either neutron stars or black holes that draw in matter from a companion star. As this material falls inward, it heats up and produces intense X-rays.
These sources appear to shine more brightly than expected from the maximum luminosity predicted by a simple balance between gravity pulling matter inward and radiation pushing it outward.
This limit, known as the Eddington limit, is commonly used as a benchmark for how bright an accreting object can be.
Determining the nature of the compact object powering a ULX is therefore crucial for understanding the physics of super-Eddington accretion, powerful outflows of gas and the growth of compact objects in such extreme environments.
A research team from the Indian Institute of Astrophysics (IIA) and the Raman Research Institute, both autonomous institutes of the Department of Science and Technology (DST), along with researchers from other institutions, discovered long-term spectral changes in NGC 4490 ULX-8. The changes point to evolving accretion conditions around the compact object.
The findings provide strong evidence that NGC 4490 ULX-8 is powered by a black hole accreting matter at rates exceeding the classical Eddington limit.
The study also demonstrates the value of long-term, multi-mission monitoring in understanding the nature of some of the Universe’s brightest X-ray sources.
“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.
NGC 4490 ULX-8 Shows Long-Term X-Ray Variations
The study found that NGC 4490 ULX-8 remained an exceptionally luminous X-ray source throughout the 24-year observing period, although its brightness varied substantially over time.
Despite these variations, NGC 4490 ULX-8 showed no evidence of the dramatic transitions between different accretion states that are commonly observed in many black hole X-ray binaries.
Instead, the X-ray emission from NGC 4490 ULX-8 retained characteristics consistent with a persistently active ultraluminous accretion flow.
The findings suggest that the source is most likely powered by a stellar-mass black hole accreting matter at or near the Eddington limit.
The observations make NGC 4490 ULX-8 an important laboratory for studying how compact objects can sustain extreme rates of mass accretion and generate enormous amounts of energy.
Future Observations Could Reveal More About NGC 4490 ULX-8
Future observatories are expected to provide improved capabilities for studying ULXs such as NGC 4490 ULX-8. These include the European Space Agency’s (ESA) NewAthena mission and India’s proposed next-generation timing and spectroscopy mission XSat.
The improved sensitivity and monitoring capabilities of these future missions could enable continued study of NGC 4490 ULX-8 and other ultraluminous X-ray sources.
Continued long-term observations will help reveal 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.
The study was authored by Tarang Vashisht, a student at IISER Mohali; Aru Beri of the Indian Institute of Astrophysics; and researchers from IUCAA and the Raman Research Institute.
The research was published in the Monthly Notices of the Royal Astronomical Society.
