The transition between BL Lac objects and flat spectrum radio quasars

By G. Ghisellini, F. Tavecchio, Luigi Foschini, G. Ghirlanda. 2010

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We study the BL Lac objects detected in the one year all sky survey of the Fermi satellite, with a energy spectral slope α in the [0.1‐100 GeV] band greater than 1.2. In the α vs γ‐ray luminosity plane, these BL Lacs occupy the region populated by Flat Spectrum Radio Quasars (FSRQs). Studying the properties of their spectral energy d istributions (SED) and of their emitting lines, we find that several of these BL Lacs have a SED similar to FSRQs and that they do have broad lines of large equivalent width, and should be reclassified as FSRQs even adopting the current phenomenological definition (i.e. equ ivalent width EW of the emitting line greater than 5 ˚ A). In other cases, even if the EW width is small, the emitting lines can be as luminous as in quasars, and again their SED is similar to the SED of FSRQs. Sources classified as BL Lacs with a SED appearing as intermediate bet ween BL Lacs and FSRQs also have relatively weak broad emission lines and small EW, and can be considered as transition sources. These properties are confirmed also by model fitting , that allows to derive the relevant intrinsic jet parameters and the jet power. This study leads us to propose a physical distinction between the two classes of blazars, based on the luminosity of the broad line region measured in Eddington units. The dividing line is of the order of LBLR/LEdd � 5 × 10 4 , in good agreement with the idea that the presence of strong emitting lines is related to a transition in the accretion regime, becoming radiatively inefficient bel ow a disk luminosity of the order of one per cent of the Eddington one.

Subfields: Astronomy and Astrophysics, Instrumentation, Nuclear and High Energy Physics

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Citations in the corpus

On the fundamental dichotomy in the local radio-AGN population (2012, background): “…(Kollmeier et al. 2006; Trump et al. 2009a, 2011), and indications that a switch between flatspectrum radio quasars and BL Lac objects (which are believed to be beamed versions of the radiatively-inefficient sources) also occurs at that Eddington rate (Wu et al. 2011; Ghisellini et al. 2011).”
Properties of flat-spectrum radio-loud Narrow-Line Seyfert 1 Galaxies (2014, background): “We noted some BL Lacs with strong accretion disk, in the region occupied by FSRQs: these are the so-called “intruders” (Ghisellini et al. 2011, Giommi et al. 2012).”
SPECTROSCOPY OF BROAD-LINE BLAZARS FROM 1LAC (2012, background): “Ghisellini et al. (2011) has proposed that FSRQ are more active accretors than BL Lacs.”
A simplified view of blazars: clearing the fog around long‐standing selection effects (2011, background): “…itself, the prototype of the class, which displays at times moderately strong, broad lines (e.g. Vermeulen et al. 1996; Capetti et al. 2010; Ghisellini et al. 2011) and 3C 279, a well-studied FSRQ, which can appear nearly featureless in a bright state (Pian et al. 1999); (ii) different…”
The Fermi blazar sequence (2017, background): “An alternative way, proposed by Ghisellini et al. (2011), is to measure the luminosity LBLR of all broad lines in ⋆ E–mail: gabriele.ghisellini@brera.inaf.it units of the Eddington luminosity (LEDD), and call FSRQs the sources whose LBLR >∼ 10−3LEdd, and BL Lacs the others.”