Testing the blazar spectral sequence: X-ray selected blazars

TL;DR

This study analyzes the spectral energy distributions of X-ray selected blazars, confirming the blazar spectral sequence and clarifying the emission mechanisms in various types of blazars through simultaneous optical and X-ray observations.

Contribution

It provides a comprehensive analysis of blazar SEDs, challenging previous notions about X-ray selected blazars and emphasizing the role of the photon environment in shaping their spectra.

Findings

X-ray selection does not find sources with synchrotron peaks in UV/X-ray range.

Most broad line blazars have SEDs peaking below optical/UV.

Some luminous blazars are better described by IC models for X-ray emission.

Abstract

We present simultaneous optical and X-ray data from Swift for a sample of FSRQs selected from the EMSS survey. We present also a complete analysis of Swift and INTEGRAL data on 4 blazars recently discussed as possibly challenging the trends of the "blazar spectral sequence". The SEDs of all these objects are modelled in terms of a general theoretical scheme leading to an estimate of the jets' physical parameters. Our results show that, in the case of the EMSS broad line blazars, X-ray selection does not lead to find sources with synchrotron peaks in the UV/X-ray range, as was the case for X-ray selected BL Lacs. Instead, for a wide range of radio powers all the sources with broad emission lines show similar SEDs, with synchrotron components peaking below the optical/UV range. Of the remaining 4 "anomalous" blazars, two highly luminous sources with broad lines, claimed to possibly emit synchrotron X-rays, are shown to be better described with IC models for their X-ray emission. For one source with weak emission lines (a BL Lac object) a synchrotron peak in the soft X-ray range is confirmed, while for the fourth source, exhibiting lines typical of NLSy1s, no evidence of X-ray emission from a relativistic jet is found. We reexamine the original "blazar spectral sequence" and suggest that the photon ambient, in which the particle acceleration and emission occur, is likely the main factor determining the shape of the blazar SED.