The jet-disc connection in AGN

We present our latest results on the connection between accretion rate and relativistic jet power in AGN, by using a large sample which includes mostly blazars, but contains also some radio--galaxies. The jet power can be traced by $\gamma$--ray luminosity in the case of blazars, and radio luminosity for both classes. The accretion disc luminosity is instead traced by the broad emission lines. Among blazars, we find a correlation between broad line emission and the $\gamma$--ray or radio luminosities, suggesting a direct tight connection between jet power and accretion rate. We confirm that the observational differences between blazar subclasses reflect differences in the accretion regime, but with blazars only we cannot properly access the low--accretion regime. By introducing radio--galaxies, we succeed in observing the fingerprint of the transition between radiatively efficient and inefficient accretion discs in the jetted AGN family. The transition occurs at the standard critical value $L_{\rm d}/L_{\rm Edd}\sim10^{-2}$ and it appears smooth. Below this value, the ionizing luminosity emitted by the accretion structure drops significantly.Comment: 11 pages, 6 figures. Accepted for publication in MNRA

Synchrotron and Compton Components and their Variability in BL Lac Objects

BL Lacertae objects are extreme extragalactic sources characterized by the emission of strong and rapidly variable nonthermal radiation over the entire electromagnetic spectrum. Synchrotron emission followed by inverse Compton scattering in a relativistic beaming scenario is generally thought to be the mechanism powering these objects. ...Comment: 4 pages, TeX plus 3 figures. Proceedings of the conference "X-ray Astronomy 1999", September 6-10,1999, Bologn

Constraints on the Physical Parameters of TeV Blazars

We consider the constraints on the physical parameters of a homogeneous SSC model that can be derived from the spectral shape and variability of TeV blazars. Assuming that the relativistic electron spectrum is a broken power law, where the break energy $\gamma_b$ is a free parameter, we write the analytical formulae that allow to connect the physical parameters of the model to observable quantities. The constraints can be summarized in a plane where the coordinates are the Doppler factor and the magnetic field. The consistency between the break energy and the balance between cooling and escape and the interpretation of the soft photon lags measured in some sources as radiative cooling times are treated as additional independent constraints. We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk 421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in ApJ, Dec 199

High redshift Fermi blazars observed by GROND and Swift

We observed 5 gamma-ray loud blazars at redshift greater than 2 with the X-Ray Telescope (XRT) and the UltraViolet and Optical Telescope (UVOT) onboard the Swift satellite, and the Gamma-Ray burst Optical Near-Infrared Detector (GROND) instrument. These observations were quasi simultaneous, usually within a few hours. For 4 of these blazars the near-IR to UV data show the presence of an accretion disc, and we could reliably estimate its accretion rate and black hole mass. One of them, PKS 1348+007, was found in an extraordinarily high IR-optical state, almost two orders of magnitude brighter than at the epoch of the Sloan Digital Sky Survey observations. For all the 5 quasars the physical parameters of the jet emitting zone, derived by applying a one-zone emission model, are similar to that found for the bulk of other gamma-ray loud quasars. With our observations we have X-ray data for the full sample of blazars at z>2 present in the Fermi 2-yrs (2LAC) catalog. This allows to have a rather complete view of the spectral energy distribution of all high-redshift Fermi blazars, and to draw some conclusions about their properties, and especially about the relation between the accretion rate and the jet power.Comment: 11 pages, 6 figures, submitted to MNRA

BeppoSAX Observations of 1-Jy BL Lacertae Objects - II

We present new BeppoSAX LECS and MECS observations, covering the energy range 0.1 - 10 keV (observer's frame), of four BL Lacertae objects selected from the 1 Jy sample. All sources display a flat (alpha_x ~ 0.7) X-ray spectrum, which we interpret as inverse Compton emission. One object shows evidence for a low-energy steepening (Delta alpha_x ~ 0.9) which is likely due to the synchrotron component merging into the inverse Compton one around ~ 2 keV. A variable synchrotron tail would explain why the ROSAT spectra of our sources are typically steeper than the BeppoSAX ones (Delta alpha_x} ~ 0.7). The broad-band spectral energy distributions fully confirm this picture and model fits using a synchrotron inverse Compton model allow us to derive the physical parameters (intrinsic power, magnetic field, etc.) of our sources. By combining the results of this paper with those previously obtained on other sources we present a detailed study of the BeppoSAX properties of a well-defined sub-sample of 14 X-ray bright (f_x (0.1 - 10 keV) > 3 x 10^{-12} erg/cm^2/s) 1-Jy BL Lacs. We find a very tight proportionality between nearly simultaneous radio and X-ray powers for the 1-Jy sources in which the X-ray band is dominated by inverse Compton emission, which points to a strong link between X-ray and radio emission components in these objects.Comment: 13 pages, 6 figures. Accepted for publication in MNRAS. Postscript file also available at http://www.stsci.edu/~padovani/xrayspectra_papers.htm

High Energy Break and Reflection Features in the Seyfert Galaxy MCG+8-11-11

We present the results from ASCA and OSSE simultaneous observations of the Seyfert 1.5 galaxy MCG+8-11-11 performed in August-September 1995. The ASCA observations indicate a modest flux increase (20%) in 3 days, possibly correlated to a softening of the 0.6-9 keV spectrum. The spectrum is well described by a hard power law (Gamma=1.64) absorbed by a column density slightly larger than the Galactic value, with an iron line at 6.4 keV of EW=400 eV. The simultaneous OSSE data are characterized by a much softer power law with photon index Gamma=3.0, strongly suggesting the presence of a spectral break in the hard X/soft gamma-ray band. A joint fit to OSSE and ASCA data clearly shows an exponential cut-off at about 300 keV, and strong reflection component. MCG+8-11-11 features a spectral break in the underlying continuum unambiguously. This, together with the inferred low compactness of this source, favours thermal or quasi-thermal electron Comptonization in a structured Corona as the leading process of high energy radiation production.Comment: 13 pages, + 4 figure.ps AAS LateX [11pt,aasms4]{article} To be published in ApJ, Main Journa

High-energy neutrinos from FR0 radio-galaxies?

The sources responsible for the emission of high-energy ($\gtrsim$ 100 TeV) neutrinos detected by IceCube are still unknown. Among the possible candidates, active galactic nuclei with relativistic jets are often examined, since the outflowing plasma seems to offer the ideal environment to accelerate the required parent high-energy cosmic rays. The non-detection of single point sources or -- almost equivalently -- the absence, in the IceCube events, of multiplets originating from the same sky position, constrains the cosmic density and the neutrino output of these sources, pointing to a numerous population of faint sources. Here we explore the possibility that FR0 radiogalaxies, the population of compact sources recently identified in large radio and optical surveys and representing the bulk of radio-loud AGN population, can represent suitable candidates for neutrino emission. Modeling the spectral energy distribution of a FR0 radiogalaxy recently associated to a $\gamma$-ray source detected by the Large Area Telescope onboard Fermi, we derive the physical parameters of its jet, in particular the power carried by it. We consider the possible mechanisms of neutrino production, concluding that $p\gamma$ reactions in the jet between protons and ambient radiation is too inefficient to sustain the required output. We propose an alternative scenario, in which protons, accelerated in the jet, escape from it and diffuse in the host galaxy, producing neutrinos as a result of $pp$ scattering with the interstellar gas, in strict analogy with the processes taking place in star-forming galaxies.Comment: 5 pages, 3 figures, accepted for publication in MNRA

Multi-Wavelength Variability of the Synchrotron Self-Compton Model for Blazar Emission

Motivated by recent reports of strongly correlated radio and X-ray variability in 3C279 (Grandi, etal 1995), we have computed the relative amplitudes of variations in the synchrotron flux at $\nu$ and the self-Compton X-ray flux at 1 keV ($R(\nu)$) for a homogeneous sphere of relativistic electrons orbiting in a tangled magnetic field. Relative to synchrotron self-Compton scattering without induced Compton scattering, stimulated scattering reduces the amplitude of $R(\nu)$ by as much as an order of magnitude when \tau_T \gtwid 1. When $\tau_T$ varies in a fixed magnetic field, $R_{\tau}$ increases monotonically from 0.01 at $\nu_o$, the self-absorption turnover frequency, to $0.5$ at $100 \nu_o$. The relative amplitudes of the correlated fluctuations in the radio-mm and X-ray fluxes from 3C279 are consistent with the synchrotron self-Compton model if $\tau_T$ varies in a fixed magnetic field and induced Compton scattering is the dominant source of radio opacity. The variation amplitudes are are too small to be produced by the passage of a shock through the synchrotron emission region unless the magnetic field is perpendicular to the shock front.Comment: 21 pages, 4 fig

Modeling the flaring activity of the high z, hard X-ray selected blazar IGR J22517+2217

We present new Suzaku and Fermi data, and re-analyzed archival hard X-ray data from INTEGRAL and Swift-BAT survey, to investigate the physical properties of the luminous, high-redshift, hard X-ray selected blazar IGR J22517+2217, through the modelization of its broad band spectral energy distribution (SED) in two different activity states. Through the analysis of the new Suzaku data and the flux selected data from archival hard X-ray observations, we build the source SED in two different states, one for the newly discovered flare occurred in 2005 and one for the following quiescent period. Both SEDs are strongly dominated by the high energy hump peaked at 10^20 -10^22 Hz, that is at least two orders of magnitude higher than the low energy (synchrotron) one at 10^11 -10^14 Hz, and varies by a factor of 10 between the two states. In both states the high energy hump is modeled as inverse Compton emission between relativistic electrons and seed photons produced externally to the jet, while the synchrotron self-Compton component is found to be negligible. In our model the observed variability can be accounted for by a variation of the total number of emitting electrons, and by a dissipation region radius changing from within to outside the broad line region as the luminosity increases. In its flaring activity, IGR J22517+2217 shows one of the most powerful jet among the population of extreme, hard X-ray selected, high redshift blazar observed so far.Comment: Accepted for publication in MNRA

Emission and power of blazar jets

Through the modelling of the Spectral Energy Distribution of blazars we can infer the physical parameters required to originate the flux we see. Then we can estimate the power of blazar jets in the form of matter and fields. These estimates are rather robust for all classes of blazars, although they are in part dependent of the chosen model (i.e. leptonic rather than adronic). The indication is that, in almost all cases, the carried Poynting flux is not dominant, while protons should carry most of the power. In emission line blazars the jet has a comparable, and often larger, power than the luminosity of the accretion disk. This is even more true for line-less BL Lacs. If the jet is structured at the sub-pc scale, with a fast spine surrounded by a slower layer, then one component sees the radiation of the other boosted, and this interplay enhances the Inverse Compton flux of both. Since the layer emission is less beamed, it can be seen also at large viewing angles, making radio-galaxies very interesting GLAST candidates. Such structures need not be stable components, and can form and disappear rapidly. Ultrafast TeV variability is challenging all existing models, suggesting that at least parts of the jets are moving with large bulk Lorentz factors and at extremely small viewing angles. However, these fast "bullets" are not necessarily challenging our main ideas about the energetics and the composition of the bulk of the jet.Comment: 6 pages, 5 figures, invited talk at the workshop: High Energy Phenomena in Relativistic Outflows (HEPRO), Dublin, 24-28 September 200