Indian astronomers from the Indian Institute of Astrophysics and Raman Research Institute, both autonomous institutes of the Department of Science and Technology, 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 source showed no evidence for dramatic transitions between different accretion states commonly observed in black hole X-ray binaries, instead maintaining characteristics consistent with a persistently active ultraluminous accretion flow. The long-term spectral changes point to evolving accretion conditions around the compact object, providing strong evidence that ULX-8 is powered by a stellar-mass black hole accreting matter at rates exceeding the classical Eddington limit.
The findings contribute to understanding the physics of super-Eddington accretion, powerful outflows of gas, and the growth of compact objects in extreme environments. The work demonstrates the value of long-term multi-mission monitoring in uncovering the nature of some of the Universe's brightest X-ray sources. Future observatories including the European Space Agency's NewAthena mission and India's proposed XSat mission will provide improved sensitivity and monitoring capabilities for continued study of these extreme systems.
The study was authored by Tarang Vashisht (IISER Mohali), Aru Beri (Indian Institute of Astrophysics), and researchers from IUCAA and Raman Research Institute, and published in the Monthly Notices of the Royal Astronomical Journal. This content may not be financially relevant.