Unravelling the unusually curved X-ray spectrum of RGB J0710+591 using AstroSat observations

TL;DR

This study analyzes AstroSat multi-wavelength data of the blazar RGB J0710+591, revealing an unusually curved X-ray spectrum explained by a power-law electron distribution, and constrains the maximum electron energy and acceleration timescale.

Contribution

First detailed constraint on the photon energy corresponding to maximum electron Lorentz factor in blazar jet acceleration processes.

Findings

Unusually curved X-ray spectrum with high curvature parameter.

Power-law electron distribution explains the spectral shape.

Estimated acceleration timescale based on maximum electron energy.

Abstract

We report the analysis of simultaneous multi-wavelength data of the high energy peaked blazar RGB J0710+591 from the LAXPC, SXT and UVIT instruments on-board AstroSat. The wide band X-ray spectrum (0.35 -- 30 keV) is modelled as synchrotron emission from a non-thermal distribution of high energy electrons. The spectrum is unusually curved, with a curvature parameter $\beta_p \sim 6.4$ for a log parabola particle distribution, or a high energy spectral index $p_2 > 4.5$ for a broken power-law distribution. The spectrum shows more curvature than an earlier quasi-simultaneous analysis of Swift-XRT/NuSTAR data where the parameters were $\beta_p \sim 2.2$ or $p_2 \sim 4$. It has long been known that a power-law electron distribution can be produced from a region where particles are accelerated under Fermi process and the radiative losses in acceleration site decide the maximum attainable Lorentz factor, $\gamma_{max}$. Consequently, this quantity decides the energy at which the spectrum curves steeply. We show that such a distribution provides a more natural explanation for the AstroSat data as well as the earlier XRT/NuSTAR observation, making this as the first well constrained determination of the photon energy corresponding to $\gamma_{max}$. This in turn provides an estimate of the acceleration time-scale as a function of magnetic field and Doppler factor. The UVIT observations are consistent with earlier optical/UV measurements and reconfirm that they plausibly correspond to a different radiative component than the one responsible for the X-ray emission.