Indian astronomers have identified new clues about the sources of X-ray emission from some of the Universe’s most energetic objects, offering insights into the extreme physical conditions surrounding blazars and the mechanisms that power active galaxies. The study was conducted by researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute under the Department of Science and Technology.
Blazars are active galaxies powered by supermassive black holes at their centres. As matter falls towards these black holes, enormous amounts of energy can be released and powerful jets of relativistic particles can be launched almost directly towards Earth. Because of this orientation, blazars can appear exceptionally bright. TeV blazars are an especially energetic class that produces very-high-energy gamma rays reaching tera-electron-volt energies.
Researchers Riya Bhowmick and Alok C. Gupta studied four classical TeV blazars — Mrk 421, Mrk 501, PG 1553+113 and PKS 2155-304 — using observations from NASA’s NICER and NuSTAR space telescopes. They analysed 13 sets of X-ray observations, with NICER covering lower-energy X-rays and NuSTAR providing observations at higher energies.
Most of the observed X-ray spectra could be explained by the standard model of blazar emission. However, observations of Mrk 421 and Mrk 501 during moderate- to low-activity phases showed an additional component at lower X-ray energies. This suggests that when the powerful jet becomes relatively weaker, radiation from the accretion disk around the black hole may become detectable alongside jet emission. While such evidence had previously been reported for Mrk 421, the observations provide the first indication of a possible similar contribution from the accretion disk in Mrk 501. Further observations will be required to confirm the finding.
The researchers also observed a small additional feature in the X-ray spectrum of Mrk 421. Its precise origin remains uncertain and could be associated with instrumental or background effects. Meanwhile, the observations of PG 1553+113 and PKS 2155-304 were explained by the standard model, with the curved shape of their X-ray spectra potentially reflecting differences in how particles of different energies gain and lose energy through radiation.