The variety of extreme blazars in the AstroSat view

Abstract

Context. Among the blazar class, extreme blazars have exceptionally hard intrinsic X-ray/TeV spectra, and extreme peak energies in their spectral energy distribution (SED). Observational evidence suggests that the non-thermal emission from extreme blazars is typically non-variable. All these unique features present a challenging case for the blazar emission models, especially for the sources with hard TeV spectra.

Aims. We aim to explore the X-ray and GeV observational features of a variety of extreme blazars, including extreme-TeV, extremeSynchrotron and regular High-frequency-peaked BL Lac objects (HBLs). Furthermore, we aim to test the applicability of various blazar emission models that could explain the very hard TeV spectra.

Methods. We conduct a detailed spectral analysis of X-ray data collected with AstroSat and Swift-XRT, along with quasi-simultaneous γ-ray data from Fermi-LAT, for five sources; 1ES 0120+340, RGB J0710+591, 1ES 1101-232, 1ES 1741+196 and 1ES 2322-409. To model the SEDs, we employ three approaches: 1) a steady-state one-zone synchrotron-self-Compton (SSC) code, 2) another leptonic scenario of co-accelerated electrons and protons on multiple shocks, applied only on the extreme-TeVsources and 3) a one-zone hadro-leptonic (OneHaLe) code. The hadro-leptonic code is used twice to explain the γ-ray emission process: proton synchrotron and synchrotron emission of secondary pairs.

Results. Our X-ray analysis provides well-constrained estimates of the synchrotron peak energies for both 1ES0120+340 and 1ES1741+196. These findings categorize them as extreme-Synchrotron sources, as they consistently exhibit peak energies above 1 keV in different flux states. The multi-epoch X-ray and GeV data reveal spectral and flux variabilities in RGB J0710+591 and 1ES 1741+196, even on time scales of days to weeks. As anticipated, the one-zone SSC model adequately reproduces the SEDs of regular HBLs but encounters difficulties in explaining the hardest TeV emission. Hadronic models offer a reasonable fit to the hard TeV spectrum, though with the trade-off of requiring extreme jet powers. On the other hand, the lepto-hadronic scenario faces additional challenges in fitting the GeV spectra of extreme-TeV sources. Finally, the e-p co-acceleration scenario naturally accounts for the observed hard electron distributions and effectively matches the hardest TeV spectrum of RGB J0710+591 and 1ES 1101-232.