Abstract
Abstract We use the distributions of spectral indices (α v ) of a large homogenous sample of Fermi-detected blazars to re-investigate the relationship between flat spectrum radio quasars (FSRQs) and subclasses of BL Lac objects (BL Lacs). We compute the broadband synchrotron and Compton spectral indices from radio-to-X-ray and X-ray to y-ray bands, respectively. Analyses of our data show continuity in the distributions of the spectral indices from FSRQs to HSP through LSP and ISP subclasses of BL Lacs. We find from y-ray luminosity distribution that the jetted radio galaxies form the low-luminosity tail of the distribution, which is suggestive that the sequence can be extended to the young jetted galaxy populations. We observe a significant difference in the shape of Compton and synchrotron spectra: significant anti-correlation (r ∼−0.80) exists between the broadband Compton and synchrotron spectral indices. Furthermore, the broadband spectral indices vary significantly with redshift (z) at low redshift (z < 0.3) and remain fairly constant at high (z ≥ 0.3) redshift. The trend of the variations suggests a form of evolutionary connection between subclasses of blazars. Thus, while selection effect may be significant at low redshift, evolutionary sequence can also be important. Our results are not only consistent with a unified scheme for blazars and their young jetted galaxy counterparts but also suggest that the broadband spectral sequence of blazars is not a secondary effect of redshift dependence.
Highlights
Blazars are exciting subclass of active galactic nuclei (AGNs) that are thought to be very bright and inconsistent sources of high energy (Abdo et al 2009, 2010a; Yefimov 2011)
From the third catalogue of Active Galactic Nuclei detected by the Fermi-Large Area Telescope (Fermi-LAT), compiled by Ackerman et al (2015) and Acero et al (2015), we select a sample of 1081 blazars with clear optical identification: 461 flat spectrum radio quasars (FSRQs) and 620 BL Lac objects (BL Lacs)
Most objects in current sample have been detected in various earlier surveys, which enabled a characterization of their spectral energy distributions using empirical relationships between low and high energy flux (Acero et al 2015; Ackerman et al 2015)
Summary
Blazars are exciting subclass of active galactic nuclei (AGNs) that are thought to be very bright and inconsistent sources of high energy (Abdo et al 2009, 2010a; Yefimov 2011) They reportedly have other outstanding observational features like large and variable luminosity, continuous spectral energy distributions (SEDs), apparent superluminal motions, high and variable polarization and very active GeV to TeV γ-ray emissions (Andruchow et al 2005; Abdo et al 2010b; Yang et al 2018). The typical spectral energy distribution (SED) of blazars exhibits double peaks, usually attributed to synchrotron and inverse Compton radiation from the relativistic jets, which dominate in blazars due to the alignment of the jet axis close to the line of sight (Böttcher 2007). BL Lacs are conveniently classified into high-synchrotron peaked (HSP), with synchrotron peak frequency in the range log νspyenak (Hz) > 15, intermediate synchrotron peaked (ISP) with ≤ log νspyenak (Hz) ≤ and low synchrotron peaked (LSP) with log νspyenak (Hz) < 14, (e.g. Abdo et al 2010a; Ackerman et al 2011)
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