Gabriele Ghisellini - INAF - Osserv. Astr. di Brera

Gabriele Ghisellini
Are you Gabriele Ghisellini?

Claim your profile, edit publications, add additional information:

Contact Details

Name
Gabriele Ghisellini
Affiliation
INAF - Osserv. Astr. di Brera
City
Milano
Country
Italy

Pubs By Year

External Links

Pub Categories

 
Astrophysics (28)
 
High Energy Astrophysical Phenomena (20)
 
Cosmology and Nongalactic Astrophysics (6)
 
Instrumentation and Methods for Astrophysics (2)
 
Astrophysics of Galaxies (2)
 
Solar and Stellar Astrophysics (1)
 
Nuclear Experiment (1)

Publications Authored By Gabriele Ghisellini

The electromagnetic (EM) follow-up of a gravitational wave (GW) event requires to scan a wide sky region, defined by the so called "skymap", for the detection and identification of a transient counterpart. We propose a novel method that exploits information encoded in the GW signal to construct a "detectability map", which represents the time-dependent ("when") probability to detect the transient at each position of the skymap ("where"). Focusing on the case of a compact binary inspiral which involves at least one neutron star, we model the associated short gamma-ray burst afterglow and macronova emission, using the probability distributions of binary parameters (sky position, distance, orbit inclination, mass ratio) extracted from the GW signal as inputs. Read More

In an effort to understand the cause of the apparent depletion in the number density of radio-loud AGNs at $z>3$, this work investigates the viability of the so-called Cosmic Microwave Background (CMB) quenching mechanism of intrinsically jetted, high-z AGNs, whereby Inverse Compton scattering of CMB photons off electrons within the extended lobes results in a substantial dimming of the lobe synchrotron emission at GHz frequencies, while simultaneously boosting their diffuse X-ray signal. We focus on five $z>3.5$ radio galaxies that have sufficiently deep Chandra exposure (> 50 ks) to warrant a meaningful investigation of any extended X-ray emission. Read More

2016Nov
Affiliations: 1on behalf of the e-ASTROGAM Collaboration, 2on behalf of the e-ASTROGAM Collaboration, 3on behalf of the e-ASTROGAM Collaboration, 4on behalf of the e-ASTROGAM Collaboration, 5on behalf of the e-ASTROGAM Collaboration, 6on behalf of the e-ASTROGAM Collaboration, 7on behalf of the e-ASTROGAM Collaboration, 8on behalf of the e-ASTROGAM Collaboration, 9on behalf of the e-ASTROGAM Collaboration, 10on behalf of the e-ASTROGAM Collaboration, 11on behalf of the e-ASTROGAM Collaboration, 12on behalf of the e-ASTROGAM Collaboration, 13on behalf of the e-ASTROGAM Collaboration, 14on behalf of the e-ASTROGAM Collaboration, 15on behalf of the e-ASTROGAM Collaboration, 16on behalf of the e-ASTROGAM Collaboration, 17on behalf of the e-ASTROGAM Collaboration, 18on behalf of the e-ASTROGAM Collaboration, 19on behalf of the e-ASTROGAM Collaboration, 20on behalf of the e-ASTROGAM Collaboration, 21on behalf of the e-ASTROGAM Collaboration, 22on behalf of the e-ASTROGAM Collaboration, 23on behalf of the e-ASTROGAM Collaboration, 24on behalf of the e-ASTROGAM Collaboration, 25on behalf of the e-ASTROGAM Collaboration, 26on behalf of the e-ASTROGAM Collaboration, 27on behalf of the e-ASTROGAM Collaboration, 28on behalf of the e-ASTROGAM Collaboration, 29on behalf of the e-ASTROGAM Collaboration, 30on behalf of the e-ASTROGAM Collaboration, 31on behalf of the e-ASTROGAM Collaboration, 32on behalf of the e-ASTROGAM Collaboration, 33on behalf of the e-ASTROGAM Collaboration, 34on behalf of the e-ASTROGAM Collaboration, 35on behalf of the e-ASTROGAM Collaboration, 36on behalf of the e-ASTROGAM Collaboration, 37on behalf of the e-ASTROGAM Collaboration, 38on behalf of the e-ASTROGAM Collaboration, 39on behalf of the e-ASTROGAM Collaboration, 40on behalf of the e-ASTROGAM Collaboration, 41on behalf of the e-ASTROGAM Collaboration, 42on behalf of the e-ASTROGAM Collaboration, 43on behalf of the e-ASTROGAM Collaboration, 44on behalf of the e-ASTROGAM Collaboration, 45on behalf of the e-ASTROGAM Collaboration, 46on behalf of the e-ASTROGAM Collaboration, 47on behalf of the e-ASTROGAM Collaboration, 48on behalf of the e-ASTROGAM Collaboration, 49on behalf of the e-ASTROGAM Collaboration, 50on behalf of the e-ASTROGAM Collaboration, 51on behalf of the e-ASTROGAM Collaboration, 52on behalf of the e-ASTROGAM Collaboration, 53on behalf of the e-ASTROGAM Collaboration, 54on behalf of the e-ASTROGAM Collaboration, 55on behalf of the e-ASTROGAM Collaboration, 56on behalf of the e-ASTROGAM Collaboration, 57on behalf of the e-ASTROGAM Collaboration, 58on behalf of the e-ASTROGAM Collaboration, 59on behalf of the e-ASTROGAM Collaboration, 60on behalf of the e-ASTROGAM Collaboration, 61on behalf of the e-ASTROGAM Collaboration, 62on behalf of the e-ASTROGAM Collaboration, 63on behalf of the e-ASTROGAM Collaboration, 64on behalf of the e-ASTROGAM Collaboration, 65on behalf of the e-ASTROGAM Collaboration, 66on behalf of the e-ASTROGAM Collaboration, 67on behalf of the e-ASTROGAM Collaboration, 68on behalf of the e-ASTROGAM Collaboration, 69on behalf of the e-ASTROGAM Collaboration, 70on behalf of the e-ASTROGAM Collaboration

e-ASTROGAM (`enhanced ASTROGAM') is a breakthrough Observatory mission dedicated to the study of the non-thermal Universe in the photon energy range from 0.3 MeV to 3 GeV. The mission is based on an advanced space-proven detector technology, with unprecedented sensitivity, angular and energy resolution, combined with polarimetric capability. Read More

One of the most outstanding results of the Chandra X-ray Observatory was the discovery that AGN jets are bright X-ray emitters on very large scales, up to hundreds of kpc. Of these, the powerful and beamed jets of Flat Spectrum Radio Quasars are particularly interesting, as the X-ray emission cannot be explained by an extrapolation of the lower frequency synchrotron spectrum. Instead, the most common model invokes inverse Compton scattering of photons of the Cosmic Microwave Background (EC/CMB) as the mechanism responsible for the high energy emission. Read More

It has been proposed that blazar jets are structured, with a fast spine surrounded by a slower sheath or layer. This structured jet model explains some properties of their emission and morphology. Because of their relative motion, the radiation produced by one component is seen amplified by the other, thus enhancing the inverse Compton emission of both. Read More

2016Sep
Affiliations: 1INAF - Osservatorio Astronomico di Brera

I discuss the spectral energy distribution (SED) of all blazars with redshift detected by the {\it Fermi} satellite and listed in the 3LAC catalog. I will update the so called "blazar sequence" from the phenomenological point of view, with no theory or modelling. I will show that: i) pure data show that jet and accretion power are related; ii) the updated blazar sequence maintains the properties of the old version, albeit with a less pronounced dominance of the $\gamma$--ray emission; iii) at low bolometric luminosities, two different type of objects have the same high energy power: low black hole mass flat spectrum radio quasars and high mass BL Lacs. Read More

2016Sep
Affiliations: 1INAF - Osservatorio Astronomico di Brera

Fast Radio Bursts (FRBs), with a typical duration of 1 ms and 1 Jy flux density at GHz frequencies, have brightness temperatures exceeding 1e33 K, requiring a coherent emission process. This can be achieved by bunching particles in volumes smaller than the typical wavelength, but this may be challenging. Alternatively, we can have maser emission. Read More

2016Mar
Affiliations: 1INAF-Brera Observ, 2Univ. di Milano Bicocca

At redshift larger than 3 there is a disagreement between the number of blazars (whose jet is pointing at us) and the number of expected parents (whose jet is pointing elsewhere). Now we strengthen this claim because (i) the number of blazars identified within the SDSS+FIRST survey footprint increased, demanding a more numerous parent population, and (ii) the detected blazars have a radio flux large enough to be above the FIRST flux limit even if the jet is slightly misaligned. The foreseen number of these slightly misaligned jets, in principle detectable, is much larger than the radio-detected sources in the FIRST+SDSS survey (at redshift larger than 4). Read More

If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles $\theta_{\rm view} \gtrsim \theta_{\rm jet} + \Gamma^{-1}$, the pulse broadening significantly smears out the light curve variability. Read More

Gamma-ray bursts (GRBs) are some of the most extreme events in the Universe. As well as providing a natural laboratory for investigating fundamental physical processes, they might trace the cosmic star formation rate up to extreme redshifts and probe the composition of the intergalactic medium over most of the Universe's history. Radio observations of GRBs play a key part in determining their physical properties, but currently they are largely limited to follow-up observations of $\gamma$-ray-detected objects. Read More

High energy observations of extreme BL Lac objects, such as 1ES 0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. However, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates starting from the BL Lac sample of Plotkin et al. Read More

Relativistic jets in active galactic nuclei (AGN) are among the most powerful astrophysical objects discovered to date. Indeed, jetted AGN studies have been considered a prominent science case for SKA, and were included in several different chapters of the previous SKA Science Book (Carilli & Rawlings 2004). Most of the fundamental questions about the physics of relativistic jets still remain unanswered, and await high-sensitivity radio instruments such as SKA to solve them. Read More

GRB 130925A is a peculiar event characterized by an extremely long gamma-ray duration ($\approx$7 ks), as well as dramatic flaring in the X-rays for $\approx$20 ks. After this period, its X-ray afterglow shows an atypical soft spectrum with photon index $\Gamma$$\sim$4, as observed by Swift and Chandra, until $\approx 10^7$ s, when XMM-Newton observations uncover a harder spectral shape with $\Gamma$$\sim$2.5, commonly observed in GRB afterglows. Read More

We selected all radio-quiet AGN in the latest release of Sloan digital sky survey quasar catalog, with redshift in the range 0.56-0.73. Read More

2012Feb

Contents: Some Fundamental definitions; Bremsstrahlung and black body; Beaming; Synchrotron emission and absorption; Compton scattering; Synchrotron Self-Compton; Pairs; Active Galactic Nuclei. Read More

We investigate the relative occurrence of radio--loud and radio-quiet quasars in the first billion years of the Universe, powered by black holes heavier than one billion solar masses. We consider the sample of high-redshfit blazars detected in the hard X-ray band in the 3-years all sky survey performed by the Burst Alert Telescope (BAT) onboard the Swift satellite. All the black holes powering these blazars exceed a billion solar mass, with accretion luminosities close to the Eddington limit. Read More

The Gamma-ray Burst Monitor (GBM) on board Fermi allows to study the spectra of Gamma Ray Bursts (GRBs) over an unprecedented wide energy range (8 keV - 35 MeV). We compare the spectral properties of short and long GRBs detected by the GBM (up to March 2010) with those of GRBs detected by the BATSE instrument on board the CGRO. GBM and BATSE long bursts have similar distributions of fluence (F), Epeak and peak flux (P) but GBM bursts have a slightly harder low-energy spectral index \alpha with respect to BATSE GRBs. Read More

2010Oct
Affiliations: 1INAF - Ossrv. Astron.di Brera

The recent years witnessed a dramatic improvement in our knowledge of the phenomenology and physics of Gamma Ray Bursts (GRBs). However, our "pillars of knowledge" remain a few, while many aspects remain obscure and not understood. There is no general agreement on the radiation mechanism of the prompt emission, nor on the process able to convert the bulk motion of the fireball into random energy of the emitting leptons. Read More

2010Feb

The new high energy data coming mainly from the Fermi and Swift satellites and from the ground based Cerenkov telescopes are making possible to study not only the energetics of blazar jets, but also their connection to the associated accretion disks. Furthermore, the black hole mass of the most powerful objects can be constrained through IR-optical emission, originating in the accretion disks. For the first time, we can evaluate jet and accretion powers in units of the Eddington luminosity for a large number of blazars. Read More

2009Feb

Although blazars are thought to emit most of their luminosity in the gamma-ray band, there are subclasses of them very prominent in hard X-rays. These are the best candidates to be studied by Simbol-X. They are at the extremes of the blazar sequence, having very small or very high jet powers. Read More

Instrumental selection effects can act upon the estimates of the peak energy Ep, the fluence F and the peak flux P of GRBs. If this were the case, then the correlations involving the corresponding rest frame quantities (i.e. Read More

2008Jul
Affiliations: 1INAF-Osservatorio Astronomico di Brera, 2INAF-Osservatorio Astronomico di Brera, 3INAF-Osservatorio Astronomico di Brera, 4U.N.A.M. - Mexico
Category: Astrophysics

The spectral-energy and (luminosity) correlations in long GRBs are being hotly debated to establish, first of all, their reality against possible selection effects. These are best studied in the observer planes, namely the peak energy E_peak_obs vs the fluence F or the peak flux P. In a recent paper we started to attack this problem considering all GRBs with known z and spectral properties. Read More

2007Jul
Affiliations: 1INAF - Trieste Astronomical Observatory, 2INAF - IASF, Bologna, 3INAF - Brera Astronomical Observatory
Category: Astrophysics

The blazars 3C 454.3, PKS 0537-441 and PKS 2155-304 are traditionally known to be among the most active sources of this class. They emit at all frequencies, up to the gamma-rays, and are good probes of multiwavelength nuclear variability. Read More

2007May
Affiliations: 1Oss. Astr. di Brera, Italy
Category: Astrophysics

The discovery that the bolometric energetics (and/or peak luminosity) of Gamma Ray Bursts correlates with their spectral properties has allowed to standardize the burst energetics to such a degree to enable their use for constraining the cosmological parameters, in the same way as SN Ia. With respect to SN Ia, there is the advantage of having sources free from extinction problems, and easily detectable also at large redshifts. On the other hand, these spectral-energy correlations are not yet understood, and bursts with a complete set of information (to standardize their energetics) are still few (two dozens). Read More

2006Nov
Affiliations: 1SISSA, Trieste, Italy, 2INAF-OAB, Merate, Italy, 3IoA, Cambridge, UK
Category: Astrophysics

We study the time dependent spectra produced via the bulk Compton process by a cold, relativistic shell of plasma moving (and accelerating) along the jet of a blazar, scattering on external photons emitted by the accretion disc and reprocessed in the broad line region. Bulk Comptonization of disc photons is shown to yield a spectral component contributing in the far UV band, and would then be currently unobservable. On the contrary, the bulk Comptonization of broad line photons may yield a significant feature in the soft X-ray band. Read More

2005Apr
Affiliations: 1JILA, University of Colorado;, 2Oss. Astr. Brera, Italy, 3Oss. Astr. Brera, Italy
Category: Astrophysics

The giant flare observed on Dec. 27th 2004 from SGR 1806-20 has revived the idea that a fraction of short (<2 s) Gamma Ray Bursts (GRBs) is due to giant flares from Soft Gamma Ray Repeaters located in nearby galaxies. One of the distinguishing characteristics of these events is the thermal (black body) spectrum with temperatures ranging from ~50 to ~180 keV, with the highest temperature observed for the initial 0. Read More

Polarimetry of Gamma-Ray Burst (GRB) afterglows in the last few years has been considered one of the most effective tool to probe the geometry, energetic, dynamics and the environment of GRBs. We report some of the most recent results and discuss their implications and future perspectives. Read More

The extragalactic gamma-ray sky is dominated by two classes of sources: Gamma-Ray Bursts (GRBs) and radio loud active galactic nuclei whose jets are pointing at us (blazars). We believe that the radiation we receive from them originates from the transformation of bulk relativistic energy into random energy. Although the mechanisms to produce, collimate and accelerate the jets in these sources are uncertain, it is fruitful to compare the characteristics of both classes of sources in search of enlightening similarities. Read More

We consider internal shocks as the main dissipation mechanism responsible for the emission in blazars and show that it can satisfactorily account for the properties of all blazars. In particular, we extend previous work (Spada et al. 2001) on powerful objects, to intermediate (BL Lac) and low power sources (Mkn 421), in order to reproduce the whole of the blazar sequence. Read More

2003Sep
Affiliations: 1IoA Cambridge;, 2IoA Cambridge;, 3OAB Milano, 4IoA Cambridge;
Category: Astrophysics

The recent claim by Coburn & Boggs to have detected a very high degree of linear polarization in the prompt emission of GRB 021206 has stimulated interest in how much polarization could arise in gamma-ray bursts from synchrotron emission. Alternatively, as Shaviv & Dar have shown, GRB polarization could be produced by inverse Compton scattering in the point-source limit. We discuss polarization from a fireball that upscatters a soft radiation field. Read More

Beppo}SAX contributed substantially to our understanding of the physics of blazars. This has been made possible mainly by its wide energy range and especially by its high energy detector. Together with the information coming from still higher energies we know at last the entire spectral energy distribution (SED) of blazars. Read More

We reanalyze the XMM--Newton data of GRB 011211 showing that the spectral features, interpreted by Reeves et al. (2002, 2003) as due thermal emission from a collisionally ionized plasma, can be also reproduced by a reflection model (with ionization parameter $\xi\sim 10^2$). We discuss the implications of this interpretation, estimating the total mass required in the simplified case of a funnel geometry. Read More

Gamma-Ray Burst afterglow polarization measurements, in spite of their intrinsic difficulties, have been carried out for a number of events that begins to be adequate to draw some general statistical conclusions. Although the presence of some degree of intrinsic polarization seems to be well established, there are still open problems regarding the polarization time evolution and the possible contribution of polarization induced by dust in the host galaxies. Read More

2002Oct

We consider the evidence for very hard low energy spectra during the prompt phase of Gamma-Ray Bursts (GRB). In particular we examine the spectral evolution of GRB 980306 together with the detailed analysis of some other bursts already presented in the literature (GRB 911118, GRB 910807, GRB 910927 and GRB 970111), and check for the significance of their hardness (i.e. Read More

2002Jul
Affiliations: 1Osservatorio Astronomico di Brera, 2Osservatorio Astronomico di Brera, 3Osservatorio Astronomico di Brera
Category: Astrophysics

We summarize the results of BeppoSAX ToO observation of blazars that were known to be in a high state from observations carried out in the optical or X-ray or TeV bands.In some of the observed sources, two spectral components were detected, which are interpreted as synchrotron and inverse Compton emission, respectively. Fast variability was detected in three sources (ON 231, BL Lac and S5 0716+714), but always only for the synchrotron component. Read More

2002Feb

We report the discovery of a transient and fading hard X-ray emission in the BATSE lightcurves of a sample of short gamma-ray bursts. We have summed each of the four channel BATSE light curves of 76 short bursts to uncover the average overall temporal and spectral evolution of a possible transient signal following the prompt flux. We found an excess emission peaking ~30 s after the prompt one, detectable for ~100 s. Read More

Despite great observational and theoretical effort, the burst progenitor is still a mysterious object. It is generally accepted that one of the best ways to unveil its nature is the study of the properties of the close environment in which the explosion takes place. We discuss the potentiality and feasibility of time resolved X-ray spectroscopy, focusing on the prompt gamma-ray phase. Read More

2001Oct
Affiliations: 1IoA Cambridge;, 2IoA Cambridge;, 3Oss. Astr. Brera
Category: Astrophysics

We report the discovery of a transient and fading hard X-ray emission in the BATSE lightcurves of a sample of short gamma-ray bursts. We have summed each of the four channel BATSE light curves of 76 short bursts to uncover the average overall temporal and spectral evolution of a possible transient signal following the prompt flux. We found an excess emission peaking ~30 s after the prompt one, detectable for ~100 s. Read More

The main goal of the REM project is the observation of prompt afterglow of Gamma-Ray Burst (GRB) events. Such observations at Near InfraRed (NIR) wavelengths are even very promising, since they allow to monitor high-z Ly-$\alpha$ absorbed bursts as well as events occurring in dusty star-forming regions. In addition to GRB science, a large amount of time ($\sim 40%$) will be available for different scientific targets: among these the study of variability of stellar objects open exciting new perspectives. Read More

2001Jun

The recent detection of a transient absorption feature in the X-ray prompt emission of GRB 990705 showed the importance of such observations in the understanding of gamma-ray bursts and their progenitors. We investigate the time dependence of photoionization edges during the prompt emission of bursts in different environments. We show that their variability can be used to infer the density and geometry of the surrounding medium, giving important clues to unveil the nature of the burst progenitor. Read More

2001Mar
Affiliations: 1Oss. Astr. Arcetri;, 2Oss. Astr. Brera;, 3Oss. Astr. Brera;, 4SISSA/ISAS
Category: Astrophysics

The central engine causing the production of jets in radio sources may work intermittently, accelerating shells of plasma with different mass, energy and velocity. Faster but later shells can then catch up slower earlier ones. In the resulting collisions shocks develop, converting some of the ordered bulk kinetic energy into magnetic field and random energy of the electrons which then radiate. Read More

2001Mar
Affiliations: 1Oss. Astr. Brera;, 2Oss. Astr. Brera;, 3Univ. dell'Insubria, 4Oss. Astr. Brera;
Category: Astrophysics

The nature of Gamma-Ray Burst progenitors is still a debated issue, but consensus is growing on the association of GRBs with massive stars. Furthermore, current models for the reionization of the universe consider massive Pop--III stars as the sources of the ionizing photons. There could then be a natural link between GRBs and reionization. Read More

2001Mar
Affiliations: 1OAB, Merate, Italy;, 2S.I.S.S.A., Trieste, Italy
Category: Astrophysics

The recent discovery, by the Chandra satellite, that jets of blazars are strong X-ray emitters at large scales (0.1-1 Mpc) bears support to the hypothesis that (also) on these scales the emitting plasma is moving at highly relativistic speeds. In this case in fact the emission via inverse Compton scattering off cosmic background photons is enhanced and the resulting predicted X-ray spectrum well accounts for the otherwise puzzling observations. Read More

2000Nov
Affiliations: 1Universita' di Roma 3, 2Osservatorio Astronomico di Brera, 3Osservatorio Astronomico di Brera, 4Osservatorio Astronomico di Roma, 5Osservatorio Astronomico di Roma
Category: Astrophysics

We consider models for the generation of the emission line recently discovered in the X-ray afterglow spectrum of several bursts, and especially of GRB 991216 observed by Chandra. These observations suggest the presence of 0.1-1 solar masses of iron in the vicinity of the bursts. Read More

2000Nov
Affiliations: 1Osserv. Astr. di Brera, Merate, Italy
Category: Astrophysics

The radiation observed by blazars is believed to originate from the transformation of bulk kinetic energy of relativistic jets into random energy. A simple way to achieve this is to have an intermittent central power source, producing shells of plasma with different bulk Lorentz factors. These shells will collide at some distance from the center, producing shocks and then radiation. Read More

We explore the viability of the unification of BL Lacs and FR I radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are studied in the radio-optical luminosity plane: we calculate the predicted luminosity of BL Lacs when observed off-axis, in the frame of a simple one-zone model. We find that the bulk Lorentz factors required in order to account for the observed luminosities are significantly smaller than those implied by other, both observational and theoretical, considerations. Read More

2000Jul
Affiliations: 1OAB, Italy, 2OAB, Italy, 3OAB, Italy, 4OAB, Italy, 5OAL, Portugal
Category: Astrophysics

The bright X-ray selected BL Lac object 1ES1101--232 shows a flat X-ray spectrum, making it detectable with high statistics over the wide BeppoSAX energy range. We have observed it in two different epochs with BeppoSAX, and found a variation of the flux of about 30% that can be explained by a change in the spectral index above the synchrotron peak. We present here the data and infer limits on the strength of the magnetic field based on models of emission for High-frequency peaked BL Lacs. Read More

2000Jun
Affiliations: 1Oss. Astr. Arcetri;, 2Oss. Astr. Brera;, 3Oss. Astr. Brera;, 4SISSA
Category: Astrophysics

The development of instabilities leading to the formation of internal shocks is expected in the relativistic outflows of both gamma-ray bursts and blazars. The shocks heat the expanding ejecta, generate a tangled magnetic field and accelerate leptons to relativistic energies. While this scenario has been largely considered for the origin of the spectrum and the fast variability in gamma-ray bursts, here we consider it in the contest of relativistic jets of blazars. Read More

We explore the viability of the unification of BL Lacs and FR I radio galaxies by comparing the core emission of radio galaxies with those of BL Lacs of similar extended radio power, taking advantage of the newly measured optical nuclear luminosity of FR I sources. The spectral properties of complete samples are also studied in the radio-optical luminosity plane: starting from the Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted luminosity of FR I nuclei in the frame of a simple one--zone model, by properly taking into account the relativistic transformations. We find that the bulk Lorentz factors required by the spread in the observed luminosities in all bands are significantly smaller than those implied by other, both observational and theoretical, considerations. Read More