24 years of X-rays point to a black hole

IIA and RRI astronomers tracked the ultraluminous X-ray source NGC 4490 ULX-8 across 24 years of Chandra, XMM-Newton and Swift data and conclude it is most likely a stellar-mass black hole accreting at or above the Eddington limit.

What happened

• Astronomers explained the changing X-ray behaviour of NGC 4490 ULX-8, one of the brightest X-ray emitters in a nearby galaxy, by tracking it over 24 years.
• The team from IIA and RRI, with IUCAA and IISER Mohali, used Chandra, XMM-Newton and Swift observations from 2000 to 2024.
• ULX-8 remained exceptionally luminous throughout, with large brightness variations but no dramatic transitions between accretion states typical of black hole X-ray binaries.
• The results point to a stellar-mass black hole accreting at rates at or exceeding the Eddington limit.
• The study appeared in MNRAS; ESA's NewAthena and India's proposed XSat are named as future tools for studying ULXs.

For Prelims

• ULX: an ultraluminous X-ray source - an off-nucleus point source brighter than the Eddington limit allows for a typical stellar-mass object; powered by a neutron star or black hole pulling matter from a companion.
• Eddington limit: the luminosity at which outward radiation pressure balances gravity on infalling gas; exceeding it ('super-Eddington') requires special geometry or very high accretion.
• Stellar-mass vs supermassive: stellar-mass black holes form from collapsed stars (a few to tens of solar masses); supermassive ones sit in galactic centres - the release separates ULXs from those.
• Accretion states: black hole X-ray binaries usually switch between 'hard' and 'soft' states as accretion changes; ULX-8's lack of such switching is the key observational result.
• Instruments: Chandra (NASA, 1999), XMM-Newton (ESA, 1999) and Swift (NASA, 2004) are space X-ray observatories; X-rays are absorbed by Earth's atmosphere, so they must be observed from space.
• IIA and RRI: the Indian Institute of Astrophysics (Bengaluru) and the Raman Research Institute (Bengaluru) are autonomous institutes of the Department of Science and Technology.
• India's X-ray capability: AstroSat (2015) was India's first multi-wavelength space observatory and XPoSat (2024) studies X-ray polarisation; XSat is a proposed timing and spectroscopy mission.
• NGC 4490/4485: an interacting pair of galaxies; interactions trigger star formation, which produces the massive binaries that become ULXs.

For Mains

Syllabus: GS3.13 · GS3.11 · Linkage L1