International Journal of Modern Physics D

The Swift mission, launched on 20 November 2004, is detecting ~ 100 gamma-ray bursts (GRBs) each year, and immediately (within ~ 90 s) starting X-ray and UV/optical observations of the afterglow. It has already collected an impressive database including prompt emission to higher sensitivities than BATSE, uniform monitoring of afterglows, and rapid follow-up by other observatories notified through the Gamma-ray bursts Coordinates Network (GCN). The X-ray afterglows have been found to have complex temporal shapes including tails emission from the prompt phase and bright flares. X-ray and optical afterglow detections from short bursts have led to accurate localizations. It is found that they can occur in non-star forming galaxies or regions, whereas long GRBs are strongly concentrated within star forming regions. This is consistent with the NS merger model. Swift has greatly increased the redshift range of GRB detection. The highest redshift GRBs, at z ~ 5-6, are approaching the era of reionization. Ground-based deep optical spectroscopy of high redshift bursts is giving metallicity measurements and other information on the source environment to much greater distance than other techniques. The localization of GRB 060218 in a nearby galaxy, and association with SN 2006aj, added a valuable member to the class of GRBs with detected supernova. The prospects for future progress are excellent given the > 10 year orbital lifetime of the Swift satellite.

Gamma-ray bursts are capable of accelerating cosmic rays up to GZK energies E_p ~ 10²⁰ eV, which can lead to a flux at Earth comparable to that observed by large EAS arrays such as Auger. The semi-relativistic outflows inferred in GRB-related hypernovae are also likely sources of somewhat lower energy cosmic rays. Leptonic processes, such as synchrotron and inverse Compton, as well as hadronic processes, can lead to GeV-TeV gamma-rays measurable by GLAST, AGILE, or ACTs, providing useful probes of the burst physics and model parameters. Photo-meson interactions also produce neutrinos at energies ranging from sub-TeV to EeV, which will be probed with forthcoming experiments such as IceCube, ANITA and KM3NeT. This would provide information about the fundamental interaction physics, the acceleration mechanism, the nature of the sources and their environment.

The evidence is reviewed that the primary form of energy that escapes to infinity from gamma-ray bursts (GRBs) is gamma-rays, and/or Poynting flux, and that the kinetic energy in ultrarelativistic baryons is a secondary component resulting from acceleration of baryons by radiation pressure near or beyond the photosphere. This could account for several observed characteristics of observed GRB spectra and light curves, such as the typical peak photon energy, the correlation of this peak with apparent GRB energy, and the profiles and spectral lagging of GRB subpulses.

We present observations of the early afterglow emission of GRB 060418. Thanks to the simultaneous coverage at optical, X-ray and gamma-ray wavelengths, we can detect and separate the external shock emission (visible in the optical and late X-ray data) and the central engine activity (early X and gamma rays). The two components are clearly distinguished based on temporal and spectral properties. The detection of the afterglow onset (in the optical) allows the determination of the fundamental fireball properties, namely its bulk Lorentz factor and total energy. The early time X-ray flare closely resembles the prompt emission gamma-ray pulses in its temporal profile, being wider at low energies and showing lags between the hard and soft bands. This provides a strong suggestion that X-ray flares are a continuation of the prompt emission.

The ESA γ-ray astronomy satellite INTEGRAL localized 46 gamma-ray bursts between October 2002 and July 2007. Spectral lags have been determined from the burst time profiles for about half the sample, despite the extreme faintness of many INTEGRAL bursts. INTEGRAL's sensitivity across the energy range of interest for the prompt burst emission allows us to investigate the hypothesis of a low-luminosity, nearby population of bursts. The azimuthal distribution of double-scatter events in SPI's germanium detectors, in conjunction with a sophisticated spacecraft and instrument mass model, have been used to place limits on the percentage of polarization in the prompt emission of the brightest INTEGRAL burst, GRB 041219a.

GRB 070311 was a long burst that triggered INTEGRAL. We present prompt γ-ray, early NIR/optical, late optical and X-ray data on this burst and its afterglow. Interestingly, the H-band light curve acquired with REM exhibits two pulses at 80 and 140 s after the peak of the γ-ray burst, with possible evidence for a contemporaneous faint γ-ray tail. The late optical and X-ray afterglow underwent a rebrightening between 3 × 10⁴ and 2 × 10⁵ s after the burst with energy comparable with that of the prompt emission extrapolated in the X-ray band. After fitting the early γ-ray and optical light curves, we modelled the time profile of the late rebrightening as the time-rescaled version of the prompt γ-ray pulse over an underlying power law. This result supports a common origin for both prompt and late X-ray/optical afterglow rebrightening of GRB 070311 within the external shock scenario.

We present optical observations of the short GRB 070707, discovered by INTEGRAL. The afterglow light-curve shows a very steep decay after ≈ 2 days from the trigger, which cannot be accounted for by jetted or high latitude emission. Thanks to very deep ESO–VLT observations, we detect a very faint host galaxy with R = 27.3 ± 0.13. This result confirms that short GRBs can be associated with very faint galaxies, and suggests that some associations with bright objects in the close vicinity of other short GRBs should be treated with caution.

Recent observations have shown an apparent association of short duration gamma-ray bursts with a variety of host galaxies at moderate redshifts. However, a statistical analysis with a large sample of BATSE GRBs indicates that at least some fraction of short GRBs originates in the local universe. We have considered pre-Swift well-localized short GRBs to examine a possible association of these bursts with galaxies at low redhsifts. We have used BATSE/IPN localizations of short bursts and SDSS DR5/PSCz catalogs as galaxies surveys.

In the case of high energy collimated outflows it is important to know the density and structure of material through which jets propagate. Here we present a simple study of the environment of GRBs where the relativistic ejecta travel during the event. We use observed spectral lines in the afterglow of gamma-ray bursts and by constructing the curve of growth for most redshifted lines, we extract the column densities and Doppler parameter. This can give us the overall picture of abundances in the vicinity of GRBs. Also, we compare this result with the spectral lines with lower redshift to examine the dependence of elemental abundances on the distance from GRB event.

We describe a model within the "quark-nova" scenario to interpret the recent observations of early X-ray afterglows of long gamma-ray bursts (GRBs) with the Swift satellite. This is a three-stage model within the context of a core-collapse supernova. STAGE 1 is an accreting (proto-) neutron star leading to a possible delay between the core collapse and the GRB. STAGE 2 is accretion onto a quark star, launching an ultrarelativistic jet generating the prompt GRB. This jet also creates the afterglow as the jet interacts with the surrounding medium creating an external shock. Slower shells ejected from the quark star (during accretion), can re-energize the external shock leading to a flatter segment in the X-ray afterglow. STAGE 3, which occurs only if the quark star collapses to form a black hole, consists of an accreting black hole. The jet launched in this accretion process interacts with the preceding quark star jet, and could generate the flaring activity frequently seen in early X-ray afterglows. Alternatively, a STAGE 2b can occur in our model if the quark star does not collapse to a black hole. The quark star in this case can then spin down due to magnetic braking, and the spin down energy may lead to flattening in the X-ray afterglow as well. This model seems to account for both the energies and the timescales of GRBs, in addition to the newly discovered early X-ray afterglow features.

Using a generalized method, we have tested an estimator of luminosity/redshift proposed by Zhang et al. (2006a), in which relative spectral lag (RSL) is tightly related with luminosity and redshift, respectively. The derived luminosities from the new estimator are found to be comparable with the observed ones. For two different GRB samples, we have examined that there are no outliers violated the luminosity relation within 3 σ confidence level. Several potential outliers for other luminosity relations are otherwise found to match the estimator well. These results suggest that the luminosity/redshift relation could be a potential tool for cosmological studies.

Ongoing experimental efforts to detect cosmic sources of high energy neutrinos are guided by the expectation that astrophysical accelerators of cosmic ray protons also generate high energy neutrinos through their interactions with ambient matter and/or photons. However the predicted neutrino flux is reduced if cosmic ray sources accelerate not only protons but also a significant number of heavier nuclei, as is indicated by recent air shower data. I consider two plausible extragalactic class of sources, active galactic nuclei and gamma-ray bursts, and demand consistency with the observed cosmic ray composition and energy spectrum at Earth after allowing for propagation through intergalactic radiation fields. This allows me to calculate the degree of photo-disintegration and pion production expected to occur in these sources, and hence the neutrino fluxes from them.

We discuss results of 2D simulations of magnetorotational (MR) mechanism of core collapse supernova explosions. Due to the nonuniform collapse, the collapsed core rotates differentially. In the presence of an initial poloidal magnetic field its toroidal component appears and grows with time. Increased magnetic pressure leads to the formation of a compression wave which moves outwards. It transforms into the fast MHD shock wave (supernova shock wave). The shape of the MR supernova explosion qualitatively depends on the configuration of the initial magnetic field. For a dipole-like initial magnetic field, the supernova explosion develops mainly along the rotational axis, forming a mildly collimated jet. A quadrupole-like initial magnetic field leads to the explosion developing mainly along the equatorial plane. The magnetorotational instability was found in our simulations. The supernova explosion energy grows with an increase of the initial core mass and rotational energy of the core, and corresponds to the observational data.

I outline a mechanism, akin to Weibel instabilities of interpenetrating beams, in which the neighboring current sheets in a striped wind from an oblique rotator interact through a two stream-like mechanism (a Weibel instability in flatland), to create an anomalous resistivity that heats the sheets and causes the magnetic field to diffusively annihilate in the wind upstream of the termination shock. The heating has consequences for observable unpulsed emission from pulsars.

The origin of γ-rays emitted by blazars is widely attributed to radiative cooling of the most energetic particles in a relativistic flow, which itself is thought to originate from the surroundings of a supermassive black hole. The linkage between the accreting gas, the black hole, jet formation, and the observed radiation from the jet, are still very difficult to establish. The most energetic γ-rays are likely to be produced close to the comptonizing particles acceleration zone, and are therefore a unique probe of the underlying physical mechanisms at play. This report contains a review of the status of the field of Very High Energy (VHE) γ-ray astronomy, in the light of measurements from the current generation of Atmospheric Čerenkov Telescopes (ACTs), which have dramatically changed our view of blazars over the last years, and given us new insights about the blazar phenomenon.

The high-frequency-peaked BL Lac PKS 2155-304 is one of the brightest and best-studied VHE gamma-ray sources in the southern hemisphere. Since 2002, the High Energy Stereoscopic System (H.E.S.S.) has monitored this source and found it to be in an unusually high state in July 2006. On the nights of 28 and 30 July, two major outbursts occurred, with peak fluxes ~ 80 times the usual values and well-resolved structures varying on time scales of ~ 200 s. Here, we report on spectral variability studies of VHE data of the first flare and show first results of the H.E.S.S. observations during the second flare night.

On the night of 29–30 July 2006, a major γ-ray outburst occurred from PKS 2155-304, which was observed simultaneously with H.E.S.S., Chandra and the Bronberg optical telescopes. An overall 6–8 hours of simultaneous uninterrupted coverage was obtained. Here we report the first preliminary results of this exceptional dataset. The source showed correlated variability between the X-ray and VHE bands, with no lags. The spectra evolve with similar patterns. Huge VHE variations (~ 22×) are accompanied only by small-amplitude X-ray and optical variations (factor 2 and 15% respectively). The source has shown for the first time in an HBL a large Compton dominance (L_C/L_S ~ 10), and a cubic relation between VHE and X-ray flux variations, during a decaying phase. These results challenge all known "standard" scenarios for the blazar emission.

The present status of the blazar spectral sequence is discussed, including new findings about blazars selected with different criteria than the original complete radio-samples. Despite extensive searches of blazars "breaking" the sequence, the original idea proposed 10 years ago still seems to hold. On the other hand, the forthcoming launch of the GLAST satellite will provide a new selection band for blazars and blazar related populations as well as fantastic progress on the spectra and variability behaviour of presently known blazars. The order of magnitude increase in sensitivity of GLAST will allow detection of γ-rays from jets with lower power and/or lower beaming factor, thus sampling a much wider population.

In almost a decade of operations, Chandra has observed and discovered the X-ray counterparts of several tens of extragalactic radio jets, with a variety of morphologies and SEDs. Here we take a fresh approach and assume that we do not know their relationship. We then search for phenomenological trends. While admittedly naive, this perspective allow us to start identifying key quantities towards a large-scale jet "unitarian view", just as was previously done for their compact, pc-scale cousins, the blazars. Similarly to the latter, we find that key parameters governing the properties of Chandra jets are their powers and the mass flux, the latter parameterizing the jet interaction with the ambient gas.

We consider the emission processes in the large-scale jets of powerful quasars based on the results obtained with the VLA, Spitzer, Hubble, and Chandra. We show that two archetypal jets, 3C 273 and PKS 1136–135, have two distinct spectral components on large-scales: (1) the low-energy (LE) synchrotron spectrum extending from radio to infrared, and (2) the high-energy (HE) component arising from optical and extending to X-rays. The X-ray emission in quasar jets is often attributed to inverse-Compton scattering of cosmic microwave background (CMB) photons by radio-emitting electrons in a highly relativistic jet. However, recent data prefer synchrotron radiation by a second distinct electron population as the origin of the HE component. We anticipate that optical polarimetry with Hubble will establish the synchrotron nature of the HE component. Gamma-ray observations with GLAST (renamed as the Fermi Gamma-ray Space Telescope), as well as future TeV observations, are expected to place important constraints on the jet models.

We present the results from multiwavelength campaigns of three powerful gamma-ray quasars, PKS 1510-089, RBS 315 and Swift J0746.3+2548, recently organized with Suzaku. The Suzaku observation provided one of the highest S/N X-ray spectra ever reported between 0.3 and 50 keV. For these quasars, the X-ray spectrum is well represented by an extremely hard power-law with photon index Γ ≃ 1.2, but is augmented by an additional soft component apparently below 1 keV for PKS 1510-089, whereas a strong deficit of soft photons is observed in RBS 315. We model the broadband spectra of these powerful quasars and argue that the power of the jet is dominated by protons but with the number of electrons/positrons exceeding the number of protons by a factor ≃ 10. We also argue that an extremely hard X-ray spectra may result from a double power-law form of the injected electrons, with the break energy γ_br ≃ 1000 corresponding to the anticipated threshold of diffusive shock acceleration.

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 on the chosen model (i.e. leptonic rather than hadronic). 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.

The quasar 3C 279 was the target of an extensive multiwavelength monitoring campaign from January through April 2006, including an optical-IR-radio Whole Earth Blazar Telescope (WEBT) campaign and Target of Opportunity X-ray and soft γ-ray observations with Chandra and INTEGRAL in mid-January 2006, with additional X-ray coverage by RXTE and Swift XRT as well as independent very-high-energy (VHE) γ-ray observations by MAGIC, which led to the first-ever reported tentative detection of a quasar at VHE γ-rays. In this paper we summarize the results of the WEBT campaign.

The source exhibited substantial variability of optical flux and spectral shape, with a characteristic time scale of a few days. The variability patterns throughout the optical BVRI bands were very closely correlated with each other, while there was no obvious correlation between the optical and radio variability. In intriguing contrast to other (in particular, BL Lac type) blazars, we find a lag of shorter-wavelength behind longer-wavelength variability throughout the RVB wavelength ranges, with a time delay increasing with increasing frequency. Spectral hardening during flares appears delayed with respect to a rising optical flux. This, in combination with the very steep IR-optical continuum spectral index of α_o ~ 1.5 – 2.0, may indicate a highly oblique magnetic field configuration near the base of the jet, leading to inefficient particle acceleration and a very steep electron injection spectrum. An alternative explanation through a slow (time scale of several days) acceleration mechanism would require an unusually low magnetic field of B < 0.2 G, about an order of magnitude lower than inferred from previous analyses of simultaneous SEDs of 3C 279 and other FSRQs with similar properties.

Very-high-energy (VHE; E > 100 GeV) γ-rays from distant sources suffer attenuation through pair-production with low energy photons from the diffuse extragalactic photon fields in the ultraviolet (UV) to far-infrared (FIR) (commonly referred to as extragalactic background light; EBL). When modeling the intrinsic spectra of the VHE γ-ray sources it is crucial to correctly account for the attenuation. Unfortunately, direct measurements of the EBL are difficult and the knowledge about the EBL over certain wavelength ranges is poor. To calculate the EBL attenuation, usually predictions from theoretical models are used. Recently, the limits on the EBL from direct and indirect methods have narrowed down the possible EBL range and many of the previous models are in conflict with these limits. We propose a new generic EBL density (not a complete model), which is in compliance with the new EBL limits. EBL evolution with redshift is included in the calculation in a very simple but effective ad hoc way. Properties of this generic EBL are discussed.

A growing number of jets in active galactic nuclei (AGN) show clear signs of helical magnetic (B) fields: Faraday-rotation gradients across the VLBI jets, extended regions of jet with orthogonal B fields, transverse polarization and total-intensity structures characteristic of helical fields, interknot polarization implying underlying orthogonal B fields, and a predominance of orthogonal B fields in the VLBI cores. In addition, a link has now been found between the circular polarization detected in AGN cores and the presence of helical jet B fields within these cores. This now abundant evidence compels us to take very seriously the idea that many, possibly all, AGN jets have helical B fields. As a whole, the recent observational results considered here suggest that we must look at AGN jets as fundamentally electromagnetic, current-carrying structures if we wish to fully understand their nature. This provides an overall framework for interpreting various observed phenomena associated with the relativistic jets of AGN, including high-energy phenomena. Superposed on the structure of the underlying helical B field may be the effects of relativistic shocks and interaction with the surrounding medium in some cases; these may dominate observed phenomena locally, while it is the "intrinsic" helical B field of the jet itself that determines the global observed characteristics of the jet.

We present the results of parsec-scale circular polarization measurements based on Very Long Baseline Array data for a number of radio-bright, core-dominated active galactic nuclei obtained simultaneously at 22 and 15 GHz. The degrees of circular polarization m_c for the VLBI core region at 15 GHz are similar to values reported earlier at this wavelength, with typical values of a few tenths of a percent. The origin of this polarization is almost certainly the conversion of linear to circular polarization during the propagation of the radiation through a magnetised plasma. We find that m_c is as often higher as lower at the higher frequency, for reasons that are not clear. Our results confirm the earlier finding that the sign of the circular polarization at a given observing frequency is generally consistent across epochs separated by several years or more, suggesting stability of the magnetic field orientation in the innermost jets.

The continua of highly active radio-loud AGN are dominated by synchrotron radiation over virtually the entire spectrum. Despite this fact, it has often been thought that no correlation between optical and radio polarization should be observed, based on the idea that the synchrotron radiation at very different wavelengths is emitted at very different regions in the jet. However, a clear correlation between the Faraday-rotation-corrected VLBI core radio polarization angle and the nearly simultaneously measured optical polarization angle was found by Gabuzda et al., suggesting that the optical and compact radio polarizations are more closely related than had been thought. We have obtained new data in order to investigate this question. Our preliminary results show that, although the optical and radio core polarizations of BL Lac objects are usually aligned, the behavior displayed by quasars is more complex, and some may involve additional phenomenology, such as internal or very high core Faraday rotation.

We re-examine the differences observed in the pc-scale magnetic field geometry of high and low optical polarization quasars (HPQs, LPRQs) using the MOJAVE sample. We find that, as previously reported, HPQ jets exhibit predominantly transverse B fields while LPRQ jets tend to display longitudinal B fields. We attempt to understand these results along with the different B field geometry observed in the low and high energy peaked BL Lacs (LBLs, HBLs) using a simple picture wherein the spinning central black holes in these AGNs influence the speed and strength of the jet components (spine, sheath). Higher spin rates in HPQs compared to LPRQs and in LBLs compared to HBLs could explain the different total radio powers, VLBI jet speeds, and the observed B field geometry in these AGN classes.

Multi-frequency (4.6, 5, 5.5, 8, 8.8, 13, 15, 22 & 43 GHz) polarization observations of six "blazars" were obtained on the American Very Long Baseline Array (VLBA) over a 24-hr period on 2 July 2006. Observing at several frequencies, separated by short and long intervals, enabled reliable determination of the distribution of Faraday rotation on a range of scales. In all cases the magnitude of the RM increases in the higher frequency observations, implying that the electron density and/or magnetic field strength is increasing as we get closer to the central engine. After correcting for Faraday rotation, the polarization orientation in the jet is either parallel or perpendicular to the jet direction. A transverse rotation measure (RM) gradient was detected in the jet of 0954+658, providing evidence for the presence of a helical magnetic field surrounding the jet. For three of the sources (0954+658, 1418+546, 2200+420), the sign of the RM in the core region changes in different frequency-intervals, indicating that the line-of-sight component of the magnetic field is changing with distance from the base of the jet. We suggest an explanation for this in terms of bends in a relativistic jet surrounded by a helical magnetic field; where there is no clear evidence for pc-scale bends, the same effect can be explained by an accelerating/decelerating jet.

We present a 200 ks Chandra ACIS-I image of Cygnus A, and discuss a long linear feature seen in its counterlobe. This feature has a nonthermal spectrum and lies on the line connecting the brighter hotspot on the approaching side and the nucleus. We therefore conclude that this feature is (or was) a jet. However, the outer part of this X-ray jet does not trace the current counterjet observed in radio. No X-ray counterpart is observed on the jet side. Using light-travel time effects we conclude that this X-ray 50 kpc linear feature is a relic jet that contains enough low-energy plasma (γ ~ 10³) to inverse-Compton scatter cosmic microwave background photons, producing emission in the X-rays.

The radio galaxy M87 has recently been found to be a rapidly variable TeV emitting source. We analyze the implications of the observed TeV characteristics and show that it proves challenging to account for them within conventional acceleration and emission models. We discuss a new pulsar-type scenario for the origin of variable, very high energy (VHE) emission close to the central supermassive black hole and show that magneto-centrifugally accelerated electrons could efficiently Compton upscatter sub-mm ADAF disk photons to the TeV regime, leading to VHE characteristics close to those observed. This suggests, conversely, that VHE observations of highly under-luminous AGNs could provide an important diagnostic tool for probing the conditions prevalent in the inner accretion disk of these sources.

M 87 is the first extragalactic source detected in the TeV γ-ray domain that is not a blazar, its large scale jet not being aligned to the line of sight. We present here a multi-blob synchrotron self-Compton model accounting explicitly for large viewing angles and moderate Lorentz factors as inferred from magnetohydrodynamic simulations of jet formation, motivated by the detection of M 87 at very high energies (VHE; E > 100 GeV). Predictions are presented for the very high-energy emission of active galactic nuclei with extended optical or X-ray jet, which could be misaligned blazars but still show some moderate beaming. We include predictions for 3C 273, Cen A and PKS 0521–36.

Fast variability now observed in VHE gamma-rays from active galactic nuclei (PKS 2155–304, M87, Mkn 501) seems to require very small TeV emitting zones, even in the presence of a significant relativistic beaming. We explore the possibility to accelerate particles up to VHE energies in such small compact regions around massive black holes, taking into account the two places in the black hole surroundings where efficient acceleration can be expected during the accretion-ejection process, namely turbulent low-luminosity accretion disks and rotating magnetospheres.

In powerful cosmic nonthermal radiation sources with dominant magnetic-field self generation, the generation of magnetic fields at almost equipartition strength by relativistic plasma instabilities operates as fast as the acceleration or injection of ultra-high energy radiating electrons and hadrons in these sources. Consequently, the magnetic field strength becomes time-dependent and adjusts itself to the actual kinetic energy density of the radiating electrons in these sources. This coupling of the magnetic field and the magnetic field energy density to the kinetic energy of the radiating particles changes both the intrinsic temporal evolution of the relativistic particle energy spectrum after injection and the synchrotron and synchrotron self-Compton emissivities.

We consider some implications of the rapid X-ray and TeV variability observed in M87 and the TeV blazars. We outline a model for jet focusing and demonstrate that modest radiative cooling can lead to recollimation of a relativistic jet in a nozzle having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine and may explain recent observations of the HST-1 knot in M87. Possible applications of this model to TeV blazars are discussed. We also discuss a scenario for the very rapid TeV flares observed with H.E.S.S. and MAGIC in some blazars, that accommodates the relatively small Doppler factors inferred from radio observations.

Supercriticality of the same kind as that in a nuclear pile can take place in high-energy astrophysical objects producing a number of impressive effects. For example, it could cause an explosive release of the energy of a cloud of ultrarelativistic protons into radiation. More certainly, supercriticality should be responsible for energy dissipation of very energetic relativistic fluids such as ultrarelativistic shocks in gamma-ray bursts and jets in active galactic nuclei (AGNs). In this case, the photon breeding process operates. It is a kind of converter mechanism with the high-energy photons and e⁺e^- pairs converting into each other via pair production and inverse Compton scattering. Under certain conditions, which should be satisfied in powerful AGNs, the photon breeding mechanism becomes supercritical: the high-energy photons breed exponentially until their feedback on the fluid changes its velocity pattern. Then the system comes to a self-adjusting near-critical steady state. Monte-Carlo simulations with detailed treatment of particle propagation and interactions demonstrate that a jet with a Lorentz factor Γ ≈ 20 can radiate away up to a half of its total energy, and for Γ = 40 the radiation efficiency can be up to 80 per cent. Outer layers of the jet decelerate down to a moderate Lorentz factor 2–4, while the spine of the jet has a final Lorentz factor in the range 10–20 independent of the initial Γ. Such sharp deceleration under the impact of radiation must cause a number of interesting phenomena such as formation of internal shocks and an early generation of turbulence.

Photon breeding in relativistic jets involves multiplication of high-energy photons propagating from the jet into the external environment and back, with the conversion into electron-positron pairs. The exponential growth of the energy density of these photons is a supercritical process powered by the bulk energy of the jet. The efficient deceleration of the jet outer layers creates a structured jet morphology with a fast spine and slow sheath. In initially fast and high-power jets even the spine can be decelerated efficiently leading to very high radiative efficiencies of conversion of the jet bulk energy into radiation. The decelerating, structured jets have angular distribution of radiation significantly broader than that predicted by a simple blob model with a constant Lorentz factor. This reconciles the discrepancy between the high Doppler factors determined by the fits to the spectra of TeV blazars and the low apparent velocities observed at VLBI scales as well as the low jet Lorentz factors required by the observed statistics and luminosity ratio of Fanaroff-Riley I radio galaxies and BL Lac objects. Photon breeding produces a population of high-energy leptons in agreement with the constraints on the electron injection function required by spectral fits of the TeV blazars. Relativistic pairs created outside the jet and emitting gamma-rays by the inverse Compton process might explain the relatively high level of TeV emission from the misaligned jet in the radio galaxies. The mechanism reproduces basic spectral features observed in blazars including the blazar sequence (shift of the spectral peaks towards lower energies with increasing luminosity). The mechanism is very robust and can operate in various environments characterized by the high photon density.

We present detailed calculations of the electron thermalization by synchrotron self-absorption accounting for cooling by Compton scattering. For the first time, we solve the coupled kinetic equations for electrons and photons without any approximations on the relevant cross-sections and compute self-consistently the resulting electron and photon distributions. The results presented in this contribution may be applied to the magnetized coronae around galactic black hole accretion disks.

Adopting the smoothed particle hydrodynamics (SPH) numerical method, we performed a grid of evolving models of a 3D, axially symmetric, physically viscous accretion disc around a black hole (BH) in an AGN. In such disc models, the role of the specific angular momentum λ and of the physical turbulent viscosity parameter α, according to the Shakura and Sunyaev prescription, are examined. One or two shock fronts develop in the radial inviscid flow, according to the assigned initial kinematic and thermodynamic conditions. Couples of (α, λ) values determine radial periodical oscillations in the shock front. An outflow can develop from the subsonic post shock region, close to the black hole, in some cases. This provides evidence for a link between the accretion disc and the fueling of a jet, through the presence of shock fronts in an accretion disc close to the centrifugal barrier.

The "supercritical pile" is a very economical GRB model that provides for the efficient conversion of the energy stored in the protons of a relativistic blast wave (RBW) into radiation and at the same time produces — in the prompt GRB phase, even in the absence of any particle acceleration — a spectral peak at an energy ~ 1 MeV. We extend this model to include also the evolution of the RBW Lorentz factor Γ and thus follow the spectral and temporal features of this model into the GRB early afterglow stage. One of the novel features of the present treatment is the inclusion of the feedback of the GRB produced radiation on the evolution of Γ with radius. This way one can obtain afterglow light curves with steep decays followed by a relatively flatter flux stage, as observed in a large number of bursts.

A new mechanism for the production and acceleration of relativistic outflows in active galactic nuclei is presented. When copious pair production occurs in a hot accretion plasma, optically thin to absorption but optically thick to scattering, the free energy per electron or positron can exceed its rest mass. Then, if pairs can escape, they produce a powerful outflow of a large bulk Lorentz factor. We show analytically and numerically that this picture holds for spherically symmetric flows of pure pairs and radiation. We further simulate the generation process of a pair outflow from a hot plasma with normal proton-electron composition. Using a multifluid approximation and a Monte Carlo method of radiative transfer, we obtain spherically symmetric steady solutions of radiation and pair outflows, taking Coulomb friction into account. We show that a powerful and moderately relativistic flow is realized and discuss several possibilities to realize a highly relativistic flow.