Researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute of the Department of Science and Technology, conducted a study on four classical TeV blazars - Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304 - that emit very high-energy radiation. Using 13 sets of X-ray observations from NASA's NICER and NuSTAR space telescopes, the team analyzed X-ray radiation to better understand how these energetic galaxies behave, with NICER covering lower-energy X-rays and NuSTAR covering higher-energy ranges.

The study, published in The Astrophysical Journal, revealed that while most X-ray spectra could be explained by standard blazar emission models, some observations of Mrk 421 and Mrk 501 showed an additional component at lower X-ray energies during moderate- to low-active states. This suggests that emission from the accretion disk contributed to the X-ray spectra alongside emission from the jet when these blazars were relatively less active. While similar evidence has been reported earlier for Mrk 421, this represents the first indication of a possible similar contribution from Mrk 501.

The observations of Mrk 421 also showed a small additional Gaussian feature representing a slight excess of X-ray radiation at a particular energy, though its exact origin remains unclear and may be related to instrument or background effects. The X-ray observations of the other two blazars, PG 1553+113 and PKS 2155-304, were well explained by the standard model, with their curved X-ray spectra potentially caused by particles of different energies gaining energy at different rates and losing energy through radiation.

The findings provide a better understanding of how powerful blazars produce X-rays, demonstrating that X-ray spectra are not simply dominated by a single source of radiation. The research suggests that when emission from powerful jets becomes weaker, emission from the accretion flow may become detectable, and observing these objects during both active and quiet phases can help astronomers understand how these two radiation sources contribute. Further observations using different types of telescopes are needed to confirm the accretion disk's contribution in X-ray emission during moderate-low active states.

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