Establishing the Spectral Turnover of Blazar PKS 2155-304 as an Outcome of Radiative Losses

TL;DR

This paper models the broad band spectra of blazar PKS 2155-304 using a broken log parabola to explain spectral features and electron escape timescales, addressing discrepancies in spectral index changes.

Contribution

It introduces a novel broken log parabola model for blazar spectra, linking spectral turnover to radiative losses and electron escape processes.

Findings

Spectra are well fitted by a broken log parabola model.

The model explains the spectral index difference observed.

Spectral turnover is attributed to radiative losses.

Abstract

The broad band optical/UV and X-ray spectra of blazars have been often modeled as synchrotron component arising from a broken power-law distribution of electrons. A broken power-law distribution is expected, since the high energy electrons undergo radiative losses effectively. The change in the energy index should then be approximately 1 and corresponds to a spectral index difference of 0.5. However, one of the long outstanding problems has been that the observed index change is significantly different. On the other hand, recent high quality observations of blazars suggest that their local spectra may not be a power-law, instead have a slight curvature and often represented by a log parabola model. Using XMM-Newton observations spanning over 12 years for the BL Lac PKS 2155-304, we show that the optical/UV and X-ray spectra can be well represented by a broken log parabola model. Further, we show that such a spectrum can indicate the energy dependence of the electron escape timescale from the main acceleration zone. This novel approach, besides addressing the observed difference in the photon spectral indices, also tries to explain the spectral turn over in far UV/soft X-rays as a result of the radiative losses.