Indian astronomers from the Indian Institute of Astrophysics, Raman Research Institute, IISER Mohali, and IUCAA have published research tracing 24 years of X-ray observations of NGC 4490 ULX-8, one of the brightest X-ray emitters in nearby galaxies. The study analyzed archival and recent observations obtained with Chandra, XMM-Newton, and Swift space-based X-ray telescopes between 2000 and 2024, representing one of the longest X-ray studies of this source to date.
The research revealed that ULX-8 remained an exceptionally luminous X-ray source throughout the 24-year observing period, with its brightness varying substantially over time. The study found no evidence for dramatic transitions between different accretion states that are commonly observed in many black hole X-ray binaries. Instead, the X-ray emission retained characteristics consistent with a persistently active ultraluminous accretion flow.
The results provide strong evidence that ULX-8 is powered by a stellar-mass black hole accreting matter at rates exceeding the classical Eddington limit, which represents how bright an accreting object can be based on the balance between gravity pulling matter inward and radiation pushing it outward. This discovery provides an important laboratory for studying how compact objects can sustain extreme rates of mass accretion and generate enormous amounts of energy.
The research was published in the Monthly Notices of the Royal Astronomical Journal and demonstrates the value of long-term multi-mission monitoring in uncovering the nature of some of the Universe's brightest X-ray sources. The study noted that future observatories such as the European Space Agency's NewAthena mission and India's proposed next-generation timing and spectroscopy mission XSat will provide improved sensitivity and monitoring capabilities for studying ultraluminous X-ray sources.
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