Research Highlight

Distinguishing a black hole from a neutron star

doi:10.1038/nindia.2020.150 Published online 28 September 2020

Astrophysicists have detected distinct signs in X-ray emissions from black holes with masses five to 20 times that of the Sun1. These signs, the researchers say, separate a black hole from a neutron star.

Neutron stars, with hard surfaces, give off extra X-ray emissions that black holes with no hard surfaces lack, the researchers found.

The scientists, including researchers from the Tata Institute of Fundamental Research (TIFR) in Mumbai, India, analysed observational data generated by NASA’s spacecraft the Rossi X-Ray Timing Explorer between 1995 and 2012 from 11 black holes and 13 neutron stars.

Both black holes and neutron stars have an X-ray emission component called corona, made up of high-energy electrons.

With scientists from the Max Planck Institute for Astrophysics in Germany and the Space Research Institute of Russian Academy of Sciences, the TIFR researchers found that the extra emission from the hard surface of a neutron star cools down the corona, resulting in lower electron temperature. Black holes without such extra X-ray emissions have higher electron temperatures.

Stellar-mass black holes, they say, can bend the space-time around them more powerfully than a supermassive black hole. Such black holes can even merge, emitting gravitational waves.

Stellar-mass black holes are therefore indispensable to probe some extreme events in the universe, says TIFR astrophysicist Sudip Bhattacharyya.


1. Banerjee, S. et al. Observing imprints of black hole event horizon on X-ray spectra. Mon. NotRoy.Astron. Soc. (2020) doi: 10.1093/mnras/staa2788