Astronomers studying some of the Universe’s most energetic objects, known as blazars, have found new clues about the sources of their X-ray emissions, which could improve understanding of the extreme physical conditions around supermassive black holes and how active galaxies are powered.
Researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institute under the Department of Science and Technology (DST), 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, researchers Riya Bhowmick and Alok C. Gupta analysed the X-ray emissions from the objects to investigate the mechanisms behind their energetic activity.
Blazars are active galaxies powered by supermassive black holes at their centres. As matter falls towards the black hole, large amounts of energy can be released and powerful jets of relativistic particles can be launched almost directly towards Earth, making the objects appear exceptionally bright.
A particularly energetic class of these objects, known as TeV blazars, produces very-high-energy gamma rays reaching tera-electron volt energies.
Because their powerful jets are directed nearly towards Earth, the X-ray spectra of blazars are generally dominated by jet emission. This makes it difficult to detect radiation from other regions of the galaxy, including the accretion disk surrounding the black hole.
The researchers analysed 13 sets of X-ray observations of the four TeV blazars. NICER provided observations at lower X-ray energies, while NuSTAR covered a higher-energy range, allowing the researchers to examine a broader spectrum of emissions.
Most of the X-ray spectra were explained by the standard model of blazar emission. However, some observations of Mrk 421 and Mrk 501 showed an additional component at lower X-ray energies. Both objects were observed in moderate- to low-activity states during these periods.
The finding suggests that when the jets of these blazars become relatively weaker, radiation from the accretion disk may become detectable alongside jet emission.
While such evidence has 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 observations of Mrk 421 also showed a small additional Gaussian feature, or excess of X-ray radiation at a particular energy. Its exact origin remains unclear and could be related to instrumental or background effects.
The X-ray observations of PG 1553+113 and PKS 2155-304, meanwhile, were well explained by the standard model. The curved shape of their X-ray spectra could be linked to differences in the rates at which particles of different energies gain energy and lose it through radiation.
The study, published recently in The Astrophysical Journal, provides further insight into the mechanisms producing X-rays in powerful blazars. It indicates that their X-ray emission may not always originate from a single source and that radiation from the accretion flow can become detectable when jet emission weakens.
The researchers said observations of blazars during both highly active and relatively quiet phases could help astronomers better understand the respective contributions of the jet and accretion flow.
Further observations using different types of telescopes could help establish the role of the accretion disk in X-ray emission during moderate- to low-activity states and improve understanding of how supermassive black holes interact with their surrounding environment.
