- Why in news
- On 2 November 2026, PIB reported that Indian astronomers traced 24 years of X-ray data on NGC 4490 ULX-8 and found it is most likely a stellar-mass black hole accreting near or above the Eddington limit.
- What it is
- Ultraluminous X-ray sources (ULXs) are very bright X-ray emitters outside galaxy centres. They are thought to be neutron stars or black holes pulling matter from a companion star. The Eddington limit is the maximum luminosity at which outward radiation pressure balances inward gravity for an accreting object.
- Source studied
- NGC 4490 ULX-8 lies in the interacting galaxy pair NGC 4490/4485 and is one of the brightest X-ray emitters in nearby galaxies.
- What powers a ULX
- ULXs are thought to host neutron stars or black holes that draw in matter from a companion star. The infalling matter heats up and emits intense X-rays.
- Eddington limit
- It is the maximum luminosity predicted by a balance between gravity pulling matter in and radiation pushing it out. ULXs seem to exceed it.
- Finding
- ULX-8 stayed very luminous with varying brightness. It showed no dramatic switches between accretion states. It is most likely a stellar-mass black hole accreting at or near the Eddington limit.
- Future missions
- ESA's NewAthena and India's proposed XSat will improve ULX studies.
Don’t confuse with ULX vs central supermassive black hole: ULXs are bright X-ray sources outside the galactic centre, powered by stellar-mass compact objects. Supermassive black holes sit at galaxy centres and are far more massive.
Don’t confuse with XSat vs AstroSat: XSat is a proposed next-generation X-ray timing and spectroscopy mission. AstroSat is India's existing multi-wavelength space observatory.
Science and technology: 57 Prelims questions since 2013; most test the concept behind the news, not the news itself. In Mains, 12 news-triggered questions.
Why it matters
- Helps identify the compact objects behind ULXs and how they grow.
- Tests the physics of super-Eddington accretion and gas outflows.
Problems
- Distinguishing neutron stars from black holes in ULXs is hard without direct mass measurements.
Way forward
- Continue long-term multi-mission monitoring with more sensitive observatories such as NewAthena and XSat.
No close past question yet.
No practice question for this one; the card may still help an answer. Why: Concept of ULXs and Eddington limit is textbook science; sci_tech has 57 news-linked Prelims questions, but this research news has few testable facts.
Worth revising until Prelims 2028: most news questions come within two years of the event.