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.

The transition between BL Lac objects and flat spectrum radio quasars

We analyze the behavior of the parsec-scale jet of the quasar 3C 454.3 during pronounced flaring in 2005-2008. Three major disturbances propagated down the jet along different trajectories with Lorentz factors Γ > 10. The disturbances show a clear connection with millimeter-wave outbursts, in 2005 May/June, 2007 July, and 2007 December. High-amplitude optical events in the R-band light curve precede peaks of the millimeter-wave outbursts by 15-50 days. Each optical outburst is accompanied by an increase in X-ray activity. We associate the optical outbursts with propagation of the superluminal knots and derive the location of sites of energy dissipation in the form of radiation. The most prominent and long lasting of these, in 2005 May, occurred closer to the black hole, while the outbursts with a shorter duration in 2005 autumn and in 2007 might be connected with the passage of a disturbance through the millimeter-wave core of the jet. The optical outbursts, which coincide with the passage of superluminal radio knots through the core, are accompanied by systematic rotation of the position angle of optical linear polarization. Such rotation appears to be a common feature during the early stages of flares in blazars. We find correlations between optical variations and those at X-ray and γ-ray energies. We conclude that the emergence of a superluminal knot from the core yields a series of optical and high-energy outbursts, and that the millimeter-wave core lies at the end of the jet's acceleration and collimation zone. We infer that the X-ray emission is produced via inverse Compton scattering by relativistic electrons of photons both from within the jet (synchrotron self-Compton) and external to the jet (external Compton, or EC); which one dominates depends on the physical parameters of the jet. A broken power-law model of the γ-ray spectrum reflects a steepening of the synchrotron emission spectrum from near-IR to soft UV wavelengths. We propose that the γ-ray emission is dominated by the EC mechanism, with the sheath of the jet supplying seed photons for γ-ray events that occur near the millimeter-wave core.

Flaring behavior of the quasar 3c 454.3 across the electromagnetic spectrum

We extend a method for modeling synchrotron and synchrotron self-Compton radiations in blazar jets to include external Compton (EC) processes. The basic model assumption is that the blazar radio through soft X-ray flux is nonthermal synchrotron radiation emitted by isotropically distributed electrons in the randomly directed magnetic field of outflowing relativistic blazar jet plasma. Thus, the electron distribution is given by the synchrotron spectrum, depending only on the Doppler factor ?D and the mean magnetic field B, given that the comoving emission region size scale R' b c?D tv /(1 + z), where tv is the variability time and z is the source redshift. Generalizing the approach of Georganopoulos, Kirk, & Mastichiadis to arbitrary anisotropic target radiation fields, we use the electron spectrum implied by the synchrotron component to derive accurate Compton-scattered ?-ray spectra throughout the Thomson and Klein-Nishina regimes for EC scattering processes. We derive and calculate accurate ?-ray spectra produced by relativistic electrons that Compton-scatter (1) a point source of radiation located radially behind the jet, (2) photons from a thermal Shakura-Sunyaev accretion disk, and (3) target photons from the central source scattered by a spherically symmetric shell of broad-line region gas. The calculations of broadband spectral energy distributions from the radio through ?-ray regimes are presented, which include self-consistent ?? absorption on the same radiation fields that provide target photons for Compton scattering. The application of this baseline flat spectrum radio/?-ray quasar model is considered in view of data from ?-ray telescopes and contemporaneous multiwavelength campaigns.

https://iopscience.iop.org/article/10.1088/0004-637X/692/1/32/pdf

Gamma-Ray Studies of Blazars: Synchro-Compton Analysis of Flat Spectrum Radio Quasars

We analyze the multi-frequency behavior of the quasar 3C 454.3 during three prominent γ-ray outbursts: 2009 Autumn, 2010 Spring, and 2010 Autumn. The data reveal a repeating pattern, including a triple flare structure, in the properties of each γ-ray outburst, which implies similar mechanism(s) and location for all three events. The multi-frequency behavior indicates that the lower frequency events are co-spatial with the γ-ray outbursts, although the γ-ray emission varies on the shortest timescales. We determine that the variability from UV to IR wavelengths during an outburst results from a single synchrotron component whose properties do not change significantly over the different outbursts. Despite a general increase in the degree of optical linear polarization during an outburst, the polarization drops significantly at the peak of the γ-ray event, which suggests that both shocks and turbulent processes are involved. We detect two disturbances (knots) with superluminal apparent speeds in the parsec-scale jet associated with the outbursts in 2009 Autumn and 2010 Autumn. The kinematic properties of the knots can explain the difference in amplitudes of the γ-ray events, while their millimeter-wave polarization is related to the optical polarization during the outbursts. We interpret the multi-frequency behavior within models involving either a system of standing conical shocks or magnetic reconnection events located in the parsec-scale millimeter-wave core of the jet. We argue that γ-ray outbursts with variability timescales as short as ~3 hr can occur on parsec scales if flares take place in localized regions such as turbulent cells.

/pdf/a-tight-connection-between-gamma-ray-outbursts-and-parsec-9o1a3fjzjx.pdf

A tight connection between gamma-ray outbursts and parsec-scale jet activity in the quasar 3c 454.3

This is the first report of Fermi Gamma-ray Space Telescope observations of the quasar 3C 454.3, which has been undergoing pronounced long-term outbursts since 2000. The data from the Large Area Telescope (LAT), covering 2008 July 7 - October 6, indicate strong, highly variable gamma-ray emission with an average flux of ~3 x 10^{-6} photons cm^{-2} s^{-1}, for energies above 100 MeV. The gamma-ray flux is variable, with strong, distinct, symmetrically-shaped flares for which the flux increases by a factor of several on a time scale of about three days. This variability indicates a compact emission region, and the requirement that the source is optically thin to pair-production implies relativistic beaming with Doppler factor delta > 8, consistent with the values inferred from VLBI observations of superluminal expansion (delta ~ 25). The observed gamma-ray spectrum is not consistent with a simple power-law, but instead steepens strongly above ~2 GeV, and is well described by a broken power-law with photon indices of ~2.3 and ~3.5 below and above the break, respectively. This is the first direct observation of a break in the spectrum of a high luminosity blazar above 100 MeV, and it is likely direct evidence for an intrinsic break in the energy distribution of the radiating particles. Alternatively, the spectral softening above 2 GeV could be due to gamma-ray absorption via photon-photon pair production on the soft X-ray photon field of the host AGN, but such an interpretation would require the dissipation region to be located very close (less than 100 gravitational radii) to the black hole, which would be inconsistent with the X-ray spectrum of the source.

http://iopscience.iop.org/article/10.1088/0004-637X/699/1/817/pdf

Early Fermi Gamma-ray Space Telescope Observations of the Quasar 3C 454.3

Context. The quasar-type blazar 3C 454.3 was observed to undergo an unprecedented optical outburst in spring 2005, affecting the source brightness from the near-IR to the X-ray frequencies. This was first followed by a millimetric and then by a radio outburst, which peaked in February 2006. Aims. In this paper we report on follow-up observations to study the multiwavelength emission in the post-outburst phase. Methods. Radio, near-infrared, and optical monitoring was performed by the Whole Earth Blazar Telescope (WEBT) collaboration in the 2006-2007 observing season. XMM-Newton observations on July 2-3 and December 18-19, 2006 added information on the X-ray and UV states of the source. Results. The source was in a faint state. The radio flux at the higher frequencies showed a fast decreasing trend, which represents the tail of the big radio outburst. It was followed by a quiescent state, common at all radio frequencies. In contrast, moderate activity characterized the near-IR and optical light curves, with a progressive increase of the variability amplitude with increasing wavelength. We ascribe this redder-when-brighter behaviour to the presence of a ""little blue bump"" due to line emission from the broad line region, which is clearly visible in the source spectral energy distribution (SED) during faint states. Moreover, the data from the XMM- Newton Optical Monitor reveal a rise of the SED in the ultraviolet, suggesting the existence of a "" big blue bump"" due to thermal emission from the accretion disc. The X-ray spectra are well fitted with a power- law model with photoelectric absorption, possibly larger than the Galactic one. However, the comparison with previous X-ray observations would imply that the amount of absorbing matter is variable. Alternatively, the intrinsic X-ray spectrum presents a curvature, which may depend on the X-ray brightness. In this case, two scenarios are possible. i) There is no extra absorption, and the X-ray spectrum hardens at low energies, the hardening being more evident in bright states; ii) there is a constant amount of extra absorption, likely in the quasar environment, and the X-ray spectrum softens at low energies, at least in faint X-ray states. This softening might be the result of a flux contribution by the high-frequency tail of the big blue bump.

/pdf/webt-and-xmm-newton-observations-of-3c-454-3-during-the-post-2lgqjm1s8u.pdf

I Stanev

Papers

WEBT and XMM-Newton observations of 3C 454.3 during the post-outburst phase Detection of the little and big blue bumps ⋆