Bulk Compton motion in the luminous quasar 4C04.42?

TL;DR

This study analyzes the broadband X-ray to gamma-ray spectrum of quasar 4C04.42, suggesting the soft X-ray excess may be due to Bulk Compton radiation of cold electrons, with alternative explanations also considered.

Contribution

It proposes that the soft X-ray excess in 4C04.42 can be explained by Bulk Compton radiation, offering a new interpretation for this feature in luminous quasars.

Findings

The spectrum is well fitted by a hard power-law with a soft blackbody component.

The soft X-ray excess can be modeled as Bulk Compton radiation of cold electrons.

Alternative models like broken power-law also fit the data.

Abstract

We present the broadband analysis of the powerful quasar 4C04.42 (z=0.965) observed by XMM and INTEGRAL. The 0.2--200 keV spectrum is well reproduced with a hard power-law component ($\Gamma\sim$1.2), augmented by a soft component below 2 keV (observer frame), which is described by a thermal blackbody with temperature kT$\backsimeq$ 0.15 keV. Alternatively, a broken power-law with E$_{break}$=2 keV and $\Delta\Gamma$=0.4 can equally well describe the data. Using archival data we compile the not-simultaneous Spectral Energy Distribution of the source from radio to gamma-ray frequencies. The SED shows two main components: the low frequency one produced by Synchrotron radiation from the electrons moving in the jet and the high energy one produced through external Compton scattering of the electrons with the photon field of the Broad Line Region. Within this scenario the excess emission in the soft-X ray band can be interpreted as due to Bulk Compton radiation of cold electrons. However, some other processes, briefly discussed in the text, can also reproduce the observed bump.