F. Tavecchio

Constraints on the Physical Parameters of TeV Blazars

We consider the constraints that can be derived from the spectral shape and variability of TeV blazars on the homogeneous synchrotron self-Compton (SSC) model. Assuming that the relativistic electron spectrum is a broken power law, in which the break energy is a free parameter, we write analytical formulae that allow us to connect observable quantities to the physical parameters of the model. We also give approximate analytic formulae for the case of Compton scattering occurring in the Klein-Nishina regime, which is of particular relevance for TeV Blazars. In particular we find that, even in the latter regime, a power-law component can be present at the highest energies. Further restrictions in the parameter space are set on the assumption that the break energy results from a balance between cooling and escape and that the soft photon lags measured in some sources derive from radiative cooling of high-energy particles. The constraints can be summarized as allowed regions in the Doppler factor-magnetic field parameter space and are in principle sufficient to determine the model parameters and their uncertainties unequivocally. We apply the method to three well-studied sources: Mrk 421, PKS 2155-304, and Mrk 501. For Mrk 421 the available data are sufficient to constrain the model fully. The additional restrictions are found to be consistent with the model parameters, supporting the proposed interpretation. In the case of PKS 2155-304, not yet detected in the TeV band, we estimate the peak frequency of the Compton component (40 GeV) and the expected TeV flux. The derived physical parameters are similar for the two sources, with a relatively large value of the Doppler factor (δ ~ 25) and a magnetic field of B 0.2 G. For Mrk 501 we consider both the historical low state and the flaring state observed in 1997 April. In the first case, consistency between the various assumptions is reached for δ 10 and B 0.3 G. For the high state, a similar value of the Doppler factor and a somewhat larger value of the magnetic field are indicated, while the extremely large frequency of the observed synchrotron peak requires continuous injection/reacceleration of relativistic particles.

General physical properties of bright Fermi blazars

We studied all blazars of known redshift detected by the Fermi satellite during its first 3-month survey. For the majority of them, pointed Swift observations ensure a good multiwavelength coverage, enabling us to reliably construct their spectral energy distributions (SEDs). We model the SEDs using a one-zone leptonic model and study the distributions of the derived interesting physical parameters as a function of the observed γ-ray luminosity. We confirm previous findings concerning the relation of the physical parameters with source luminosity which are at the origin of the blazar sequence. The SEDs allow to estimate the luminosity of the accretion disc for the majority of broad emitting line blazars, while for the lineless BL Lac objects in the sample upper limits can be derived. We find a positive correlation between the jet power and the luminosity of the accretion disc in broad-line blazars. In these objects, we argue that the jet must be proton dominated, and that the total jet power is of the same order of (or slightly larger than) the disc luminosity. We discuss two alternative scenarios to explain this result.

Structured jets in TeV BL Lac objects and radiogalaxies - Implications for the observed properties

TeV BL Lacertae objects require extreme relativistic bulk motions in the gamma-ray emission region, but at the VLBI scale their radio knots hardly move. The same sources show evidence, in radio, of a structure made of a fast spine plus a slow layer. We propose that this structure exists even on the spatial scale of regions responsible for the gamma-ray emission. One component sees the (beamed) radiation produced by the other, and this enhances the inverse Compton emission of both components. In addition, this allows the magnetic field to be nearly in equipartition with the emitting particles. The inverse Compton emission of the spine is anisotropic in its frame, possibly producing a deceleration of the spine by the Compton rocket effect. In this scenario, the slow layer is also a relatively strong high-energy emitter, and thus radiogalaxies become potentially detectable by GLAST.

Canonical high-power blazars

The jets of powerful blazars propagate within regions relatively dense of radiation produced externally to the jet. This radiation is a key ingredient to u nderstand the origin of the high energy emission of blazars, from the X‐ray to the γ‐ray energy band. The main components contributing to the external radiation field are the accreti on disk emission, including its X‐ray corona, the broad line region, the infrared emitting torus a nd the cosmic background radiation. Their importance changes as a function of the distance from the black hole and of the value of the bulk Lorentz factor of the jet. These external ra diation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region, by assuming ra ther “standard” (i.e. “canonical”) prescriptions for the accretion disk and its X‐ray corona, t he profile of the jet magnetic field and the external radiation. We confirm that most of the dissip ation, if producing the γ‐ray flux we see, must occur at hundreds of Schwarzschild radii fro m the black hole, to avoid the γ‐γ! e ± process, and the consequent re‐emission by the produced pairs. The magnetic energy density of a “canonical” jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad line region is particularly interesting, since it acc ounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X‐ray spectrum. Furthermore it makes the high power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds.

The X-Ray Jet of PKS 0637–752: Inverse Compton Radiation from the Cosmic Microwave Background?

We propose that the X-ray emission detected by Chandra from the 100 kpc-scale jet of PKS 0637-752 is produced through inverse Compton scattering of the cosmic microwave background (CMB). We analyze the physical state of the jet and show that inverse Compton scattering from the CMB is consistent with the equipartition for a moderate beaming of the emission, with a Doppler factor δ ~ 10. The power transported by the jet is then similar to that of other powerful blazars, Lj ~ 1048 ergs s-1, and the jet has low radiative efficiency. The radiative cooling times of the electrons are a few thousand years, compatible with the size of the knot. The low-energy cutoff of the electron distribution is constrained to be γmin ~ 10, the first such constraint from spectral considerations. A parallel analysis for the synchrotron self-Compton model yields far less reasonable physical conditions.

The blazar sequence: a new perspective

We revisit the so called “blazar sequence”, which connects the observed bolometric luminosity to the shape of the spectral energy distribution (S ED) of blazars. We propose that the power of the jet and the SED of its emission are linked to the two main parameters of the accretion process, namely the mass of the black hole and the accretion rate. We assume: i) that the jet kinetic power is proportional to the mass accretion r ate; ii) that most of the jet dissipation takes place at a distance proportional to the black hole mass; iii) that the broad line region exists only above a critical value of the disk luminosity, in Eddington units, and iv) that the radius of the broad line region scales as the square root of th e ionising disk luminosity. These assumptions, motivated by existing observations or by reasonable theoretical considerations, are sufficient to uniquely determine the SED of all blazars. T his framework accounts for the existence of “blue quasars”, i.e. objects with broad emissi on lines but with SEDs resembling those of low luminosity high energy peaked BL Lac objects, as well as the existence of relatively low luminosity “red” quasars. Implications on the possible evolution of blazars are briefly discussed. This scenario can be tested quite easily o nce the AGILE and especially the GLAST satellite observations, coupled with information in the op tical/X‐ray band from Swift, will allow the knowledge of the entire SED of hundreds (and possibly thousands) blazars.

The power of relativistic jets is larger than the luminosity of their accretion disks

Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central supermassive black hole, as well as from the magnetic field near the event horizon. The physical mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous samples used prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power, as measured through the γ-ray luminosity, and accretion luminosity, as measured by the broad emission lines, with the jet power dominating the disk luminosity, in agreement with numerical simulations. This implies that the magnetic field threading the black hole horizon reaches the maximum value sustainable by the accreting matter.

The Fermi blazars' divide

Flat-spectrum radio quasars (FSRQs) and BL Lac objects detected in the first three months of the Fermi survey neatly separate in the γ-ray spectral index versus γ-ray luminosity plane. BL Lac objects are less luminous and have harder spectra than broad-line blazars. We suggest that this division has its origin in the different accretion regimes of the two classes of objects. Using the γ-ray luminosity as a proxy for the observed bolometric one, we show that the boundary between the two subclasses of blazars can be associated with the threshold between the regimes of optically thick accretion discs and of radiatively inefficient accretion flows, which lies at an accretion rate of the order of 10−2 the Eddington rate. The spectral separation in hard (BL Lacs) and soft (FSRQs) objects can then result from the different radiative cooling suffered by the relativistic electrons in jets propagating in different ambients. We argue that the bulk of the most luminous blazars already detected by Fermi should be characterized by large black hole masses, around 109 solar masses, and predict that lowering the γ-ray flux threshold the region of the αγ–Lγ plane corresponding to steep spectral indices and lower luminosities will be progressively populated by FSRQs with lower mass black holes, while the region of hard spectra and large luminosities will remain forbidden.

The transition between BL Lac objects and flat spectrum radio quasars

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.

A Survey of Extended Radio Jets with Chandra and the Hubble Space Telescope

We present the results from an X-ray and optical survey of a sample of 17 radio jets in AGN performed with Chandra and HST. The sample was selected from the radio and is unbiased toward detection at shorter wavelengths, but preferentially it includes beamed sources. We find that X-ray emission is common on kpc-scales, with over half radio jets exhibiting at least one X-ray knot on the Chandra images. The distributions of the radio-to-X-ray and radio-to-optical spectral indices for the detected jets are similar to the limits for the non-detections,suggesting all bright radio jets have X-ray counterparts which will be visible in longer observations. Comparing the radio and X-ray morphologies shows that the majority of the X-ray jets have structures that closely map the radio. Analysis of the SED of the jet knots suggest the knots in which the X-ray and radio morphologies track each other produce X-rays by IC scattering of the Cosmic Microwave Background. The remaining knots produce X-rays by the synchrotron process. Spectral changes are detected along the jets, with the ratio of the X-ray-to-radio and optical-to-radio flux densities decreasing from the inner to the outer regions. This suggests the presence of an additional contribution to the X-ray flux in the jets inner part, either from synchrotron or IC of the stellar light. Alternatively, in a pure IC/CMB scenario, the plasma decelerates as it flows from the inner to the outer regions. Finally, the X-ray spectral indices for the brightest knots are flat, indicating that the bulk of the luminosity of the jets is emitted at GeV energies, and raising the interesting possibility of future detections with GLAST.