Blazar X-ray study hints at accretion disk glow when jets quiet down

Astronomers studying four of the most energetic blazars have found that X-ray emission from these objects may not come from their jets alone. In observations of Mrk 421 and Mrk 501 during relatively quiet phases, an extra component appeared at lower X-ray energies, pointing to a possible contribution from the accretion disk around the central black hole.

Blazars are active galaxies powered by supermassive black holes. As matter falls inward, vast amounts of energy are released and powerful jets of relativistic particles are launched almost directly toward Earth, making these objects appear exceptionally bright. A particularly extreme subset, known as TeV blazars, produces very-high-energy gamma rays reaching tera-electron volt energies.

Because the jets point almost straight at Earth, their emission usually dominates the X-ray spectrum, masking radiation from other regions such as the accretion disk. To probe this, researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute under the Department of Science and Technology, examined four classical TeV blazars: Mrk 421, Mrk 501, PG 1553+113 and PKS 2155-304.

Riya Bhowmick and Alok C. Gupta analysed 13 sets of X-ray observations using NASA's NICER and NuSTAR space telescopes. NICER captured lower-energy X-rays while NuSTAR covered a higher-energy range, giving a broader view of the emission.

Most spectra matched the standard blazar model. However, a few observations of Mrk 421 and Mrk 501 showed an additional low-energy component, both during moderate-to-low activity states. This suggests that when the jets weaken, emission from the accretion disk can become detectable alongside jet emission. While such evidence had been reported earlier for Mrk 421, this is the first indication of a similar contribution in Mrk 501, though further observations are needed to confirm it.

Mrk 421 also showed a small extra peak in its X-ray spectrum, whose origin remains unclear and may be linked to instrumental or background effects. The other two blazars, PG 1553+113 and PKS 2155-304, were well explained by the standard model, with the curved shape of their spectra possibly arising because particles of different energies gain and lose energy at different rates.

Published in The Astrophysical Journal, the findings indicate that blazar X-ray spectra are not governed by a single radiation source. Observing these objects in both active and quiet states could help clarify how jets and accretion flow each contribute, and improve understanding of how active galaxies are powered and how black holes interact with their surroundings.