Bulk Comptonization: new hints from the luminous blazar 4C+25.05

TL;DR

This paper analyzes the X-ray spectrum of the luminous blazar 4C+25.05, revealing a soft excess consistent with bulk Comptonization, and infers a jet Lorentz factor of about 11.7, providing new insights into blazar emission mechanisms.

Contribution

It presents the first detailed X-ray spectral analysis of 4C+25.05, identifying a soft excess compatible with bulk Comptonization, and estimates the jet's bulk Lorentz factor, advancing understanding of blazar jet physics.

Findings

Soft X-ray excess modeled as blackbody with kT ≈ 0.66 keV

Bulk Lorentz factor of jet estimated at ~11.7

Supports bulk Comptonization as a common blazar feature

Abstract

Blazars are often characterized by a spectral break at soft X-rays, whose origin is still debated. While most sources show a flattening, some exhibit a blackbody-like soft excess with temperatures of the order of $\sim$0.1 keV, similar to low-luminosity, non-jetted Seyferts. Here we present the analysis of the simultaneous XMM-Newton and NuSTAR observation of the luminous FSRQ 4C+25.05 ($z=2.368$). The observed 0.3-30 keV spectrum is best described by the sum of a hard X-ray power law ($\Gamma = 1.38_{-0.03}^{+0.05}$) and a soft component, approximated by a blackbody with $kT_{\rm BB} = 0.66_{-0.04}^{+0.05}$ keV (rest frame). If the spectrum of 4C+25.05 is interpreted in the context of bulk Comptonization by cold electrons of broad-line region photons emitted in the direction of the jet, such an unusual temperature implies a bulk Lorentz factor of the jet of $\Gamma_{\rm bulk}\sim 11.7$. Bulk Comptonization is expected to be ubiquitous on physical grounds, yet no clear signature of it has been found so far, possibly due to its transient nature and the lack of high-quality, broad-band X-ray spectra.