arXiv subjects

G. Ghisellini

Publications and source records attributed to G. Ghisellini.

At least 199 records · Page 11Linked to original sources

arXiv2008

TeV variability in blazars: how fast can it be?

Recent Cerenkov observations of the two BL Lac objects PKS 2155-304 and Mkn 501 revealed TeV flux variability by a factor ~2 in just 3-5 minutes. Even accounting for the effects of relativistic beaming, such short timescales are challenging simple and conventional emitting models, and call for alternative ideas. We explore the possibility that extremely fast variable emission might be produced by particles streaming at ultra-relativistic speeds along magnetic field lines and inverse Compton scattering any radiation field already present. This would produce extremely collimated beams of TeV photons. While the probability for the line of sight to be within such a narrow cone of emission would be negligibly small, one would expect that the process is not confined to a single site, but can take place in many very localised regions, along almost straight magnetic lines. A possible astrophysical setting realising these conditions is magneto-centrifugal acceleration of beams of particles. In this scenario, the variability timescale would not be related to the physical dimension of the emitting volume, but might be determined by either the typical duration of the process responsible for the production of these high energy particle beams or by the coherence length of the magnetic field. It is predicted that even faster TeV variability - with no X-ray counterpart - should be observed by the foreseen more sensitive Cerenkov telescopes.

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arXiv2008

Long lived central engines in Gamma Ray Bursts

The central engine of Gamma Ray Bursts may live much longer than the duration of the prompt emission. Some evidence of it comes from the presence of strong precursors, post-cursors, and X-ray flares in a sizable fraction of bursts. Additional evidence comes from the fact that often the X-ray and the optical afterglow light curves do not track one another, suggesting that they are two different emission components. The typical "steep-flat-steep" behavior of the X-ray light curve can be explained if the same central engine responsible for the main prompt emission continues to be active for a long time, but with a decreasing power. The early X-ray "afterglow" emission is then the extension of the prompt emission, originating at approximately the same location, and is not due to forward shocks. If the bulk Lorentz factor Gamma is decreasing in time, the break ending the shallow phase can be explained, since at early times Gamma is large, and we see only a fraction of the emitting area. Later, when Gamma decreases, we see an increasing fraction of the emitting surface up to the time when Gamma ~ 1/theta_j. This time ends the shallow phase of the X-ray light curve. The origin of the late prompt emission can be the accretion of the fall-back material, with an accretion rate dot M proportional to t^(-5/3). The combination of this late prompt emission with the flux produced by the standard forward shock can explain the great diversity of the optical and the X-ray light curves.

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arXiv2008

Structured jets and VHE emission of blazars and radiogalaxies

Recent observations in the TeV band challenge the simplest models developed to describe the overall emission of blazars and radiogalaxies. In particular, the observation of variable TeV emission from M87 and the fast variability shown by PKS 2155-304 challenge the standard framework. We discuss how the existence of a radial structure in the sub-pc scale jet, with faster a component ("spine" or "needles") embedded in a slower layer can explain the basic phenomenology of these sources.

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arXiv2008

Testing the blazar spectral sequence: X-ray selected blazars

We present simultaneous optical and X-ray data from Swift for a sample of FSRQs selected from the EMSS survey. We present also a complete analysis of Swift and INTEGRAL data on 4 blazars recently discussed as possibly challenging the trends of the "blazar spectral sequence". The SEDs of all these objects are modelled in terms of a general theoretical scheme leading to an estimate of the jets' physical parameters. Our results show that, in the case of the EMSS broad line blazars, X-ray selection does not lead to find sources with synchrotron peaks in the UV/X-ray range, as was the case for X-ray selected BL Lacs. Instead, for a wide range of radio powers all the sources with broad emission lines show similar SEDs, with synchrotron components peaking below the optical/UV range. Of the remaining 4 "anomalous" blazars, two highly luminous sources with broad lines, claimed to possibly emit synchrotron X-rays, are shown to be better described with IC models for their X-ray emission. For one source with weak emission lines (a BL Lac object) a synchrotron peak in the soft X-ray range is confirmed, while for the fourth source, exhibiting lines typical of NLSy1s, no evidence of X-ray emission from a relativistic jet is found. We reexamine the original "blazar spectral sequence" and suggest that the photon ambient, in which the particle acceleration and emission occur, is likely the main factor determining the shape of the blazar SED.

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arXiv2008

Ultra-High Energy Cosmic Rays, Spiral galaxies and Magnetars

We measure the correlation between the arrival directions of the highest energy cosmic rays detected by the Pierre Auger Observatory with the position of the galaxies in the HI Parkes All Sky Survey (HIPASS) catalogue, weighted for their HI flux and Auger exposure. The use of this absorption-free catalogue, complete also along the galactic plane, allows us to use all the Auger events. The correlation is significant, being 86.2% for the entire sample of HI galaxies, and becoming 99% when considering the richest galaxies in HI content, or 98% with those lying between 40-55. We interpret this result as the evidence that spiral galaxies are the hosts of the producers of UHECR and we briefly discuss classical (i.e energetic and distant) long Gamma Ray Burst (GRBs), short GRBs, as well as newly born or late flaring magnetars as possible sources of the Auger events. With the caveat that these events are still very few, and that the theoretical uncertainties are conspicuous, we found that newly born magnetars are the best candidates. If so, they could also be associated with sub-energetic, spectrally soft, nearby, long GRBs. We finally discuss why there is a clustering of Auger events in the direction on the radio-galaxy Cen A and an absence of events in the direction of the radio-galaxy M87.

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arXiv2008

Precursors in Swift Gamma Ray Bursts with redshift

We study a sample of Gamma-Ray Bursts detected by the Swift satellite with known redshift which show a precursor in the Swift-BAT light curve. We analyze the spectra of the precursors and compare them with the time integrated spectra of the prompt emission. We find neither a correlation between the two slopes nor a tendency for the precursors spectra to be systematically harder or softer than the prompt ones. The energetics of the precursors are large: on average, they are just a factor of a few less energetic (in the source rest frame energy range 15-150 keV) than the entire bursts. These properties do not depend upon the quiescent time between the end of the precursor and the start of the main event. These results suggest that what has been called a "precursor" is not a phenomenon distinct from the main event, but is tightly connected with it, even if, in some case, the quiescent time intervals can be longer than 100 seconds.

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arXiv2008

Extragalactic jets: a new perspective

The power carried by the jet of blazars is large, compared to the luminosity produced by their accretion disk, and is probably in the form of kinetic energy of a normal electron-proton plasma. The Poynting flux is modest, as suggested by the inconspicuous synchrotron luminosity when compared to the high energy (hard X-rays and gamma-ray) one, assumed to be produced by the inverse Compton process. It is suggested that the jet power and the SED (Spectral Energy Distribution) of its emission are linked to the mass of the black hole M and the accretion rate dot M. This corresponds to a new "blazar sequence" based on M and dot M instead of only the observed blazar luminosity. These ideas can be tested quite easily once the AGILE and especially the GLAST satellite observations, coupled with information in the optical/X-ray band from Swift, will allow the knowledge of the entire SED of hundreds blazars.

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arXiv2008

XIAO: a soft X-ray telescope for the SVOM Mission

The study of Gamma-ray bursts (GRBs) is a key field to expand our understanding of several astrophysical and cosmological phenomena. SVOM is a Chinese-French Mission which will permit to detect and rapidly locate GRBs, in particular those at high redshift, and to study their multiwavelength emission. The SVOM satellite, to be launched in 2013, will carry wide field instruments operating in the X/gamma-ray band and narrow field optical and soft X-ray telescopes. Here we describe a small soft X-ray telescope (XIAO) proposed as an Italian contribution to the SVOM mission. Thanks to a grazing incidence X-ray telescope with effective area of ~120 cm^2 and a short focal length, coupled to a very compact, low noise, fast read out CCD camera, XIAO can substantially contribute to the overall SVOM capabilities for both GRB and non-GRB science.

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arXiv2008

Infrared to X-ray observations of PKS 2155-304 in a low state

AIMS: Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy gamma-rays, through the investigation of the prototypical blazar PKS 2155-304, which shows complex behaviour. METHODS: We analyze simultaneous infrared-to-X-ray observations obtained with XMM-Newton and REM on November 7, 2006, when the source was in a low X-ray state. We perform a comparative analysis of these results with those obtained from previous observations in different brightness states. RESULTS: We found that the peak of the synchrotron emission moved from ultraviolet to optical wavelengths and the X-ray spectrum is best fit with a broken power law model with Gamma_2 ~ 2.4 harder than Gamma_1 ~ 2.6 and a break at about 3.5 keV. This suggests that the soft X-rays (E < 3.5 keV) are related to the high-energy tail of the synchrotron emission, while the hard X-rays (E > 3.5 keV) are from the energy region between the synchrotron and inverse-Compton humps. The different variability at energies below and above the break strengthens this hypothesis. Our results also stress the importance of monitoring this source at both low and high energies to better characterize its variability behaviour.

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arXiv2008

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 (SED) 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 rate; 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 the ionising disk luminosity. These assumptions, motivated by existing observations or by reasonable theoretical considerations, are sufficient to uniquely determine the SED of all blazars. This framework accounts for the existence of "blue quasars", i.e. objects with broad emission 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 once the AGILE and especially the GLAST satellite observations, coupled with information in the optical/X-ray band from Swift, will allow the knowledge of the entire SED of hundreds (and possibly thousands) blazars.

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arXiv2008

The Epeak-Eiso plane of long Gamma Ray Bursts and selection effects

We study the distribution of long Gamma Ray Bursts in the Ep-Eiso and in the Ep,obs-Fluence planes through an updated sample of 76 bursts, with measured redshift and spectral parameters, detected up to September 2007. We confirm the existence of a strong rest frame correlation Ep ~ Eiso^0.54+-0.01. Contrary to previous studies, no sign of evolution with redshift of the Ep-Eiso correlation (either its slope and normalisation) is found. The 76 bursts define a strong Ep,obs-Fluence correlation in the observer frame (Ep,obs ~ F^0.32+-0.05) with redshifts evenly distributed along this correlation. We study possible instrumental selection effects in the observer frame Ep,obs-Fluence plane. In particular, we concentrate on the minimum peak flux necessary to trigger a given GRB detector (trigger threshold) and the minimum fluence a burst must have to determine the value of Ep,obs (spectral analysis threshold). We find that the latter dominates in the Ep,obs-Fluence plane over the former. Our analysis shows, however, that these instrumental selection effects do not dominate for bursts detected before the launch of the Swift satellite, while the spectral analysis threshold is the dominant truncation effect of the Swift GRB sample (27 out of 76 events). This suggests that the Ep,obs-Fluence correlation defined by the pre--Swift sample could be affected by other, still not understood, selection effects. Besides we caution about the conclusions on the existence of the Ep,obs-Fluence correlation based on our Swift sample alone.

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arXiv2008

Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality

We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical extinction detected by the Swift satellite fully confirm earlier results concerning the distribution of the optical afterglow luminosity at 12 hours after trigger (rest frame time). This distribution is bimodal and relatively narrow, especially for the high luminosity branch. This is intriguing, given that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is unexpected in the common scenario explaining the GRB afterglow. We investigate if the observed distribution can be the result of selection effects affecting a unimodal parent luminosity distribution, and find that either the distribution is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a substantial amount of grey dust. In both cases we suggest that most dark bursts should belong to the underluminous optical family.

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arXiv2008

Gamma-Ray Bursts, new cosmological beacons

Long Gamma-Ray Bursts (GRBs) are the brightest electromagnetic explosions in the Universe, associated to the death of massive stars. As such, GRBs are potential tracers of the evolution of the cosmic massive star formation, metallicity, and Initial Mass Function. GRBs also proved to be appealing cosmological distance indicators. This opens a unique opportunity to constrain the cosmic expansion history up to redshifts 5-6. A brief review on both subjects is presented here.

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arXiv2008

The Spectral Sequence of Blazars - Status and Perspectives

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 to detect gamma-rays from jets with lower power and/or lower beaming factor, thus sampling a much wider population.

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arXiv2008

The power of blazar jets

We estimate the power of relativistic, extragalactic jets by modelling the spectral energy distribution of a large number of blazars. We adopt a simple one-zone, homogeneous, leptonic synchrotron and inverse Compton model, taking into account seed photons originating both locally in the jet and externally. The blazars under study have an often dominant high energy component, which, if interpreted as due to inverse Compton radiation, limits the value of the magnetic field within the emission region. As a consequence, the corresponding Poynting flux cannot be energetically dominant. Also the bulk kinetic power in relativistic leptons is often smaller than the dissipated luminosity. This suggests that the typical jet should comprise an energetically dominant proton component. If there is one proton per relativistic electrons, jets radiate around 2-10 per cent of their power in high power blazars and 3-30 per cent in less powerful BL Lacs.

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arXiv2008

The spectrum of the Broad Line Region and the high-energy emission of powerful blazars

High-energy emission (from the X-ray through the gamma-ray band) of Flat Spectrum Radio Quasars is widely associated with the inverse Compton (IC) scattering of ambient photons, produced either by the accretion disk or by the Broad Line Region, by high-energy electrons in a relativistic jet. In the modelling of the IC spectrum one usually adopts a simple black-body approximation for the external radiation field, though the real shape is probably more complex. The knowledge of the detailed spectrum of the external radiation field would allow to better characterize the soft-medium X-ray IC spectrum, which is crucial to address several issues related to the study of these sources. Here we present a first step in this direction, calculating the IC spectra expected by considering a realistic spectrum for the external radiation energy density produced by the BLR, as calculated with the photoionization code CLOUDY. We find that, under a wide range of the physical parameters characterizing the BLR clouds, the IC spectrum calculated with the black-body approximation reproduces quite well the exact spectrum for energies above few keV. In the soft energy band, instead, the IC emission calculated using the BLR emission shows a complex shape, with a moderate excess with respect to the approximate spectrum, which becomes more important for decreasing values of the peak frequency of the photoionizing continuum. We also show that the high-energy spectrum shows a marked steepening, due to the energy dependence of the scattering cross section, above a characteristic energy of 10-20 GeV, quasi independent on the Lorentz factor of the jet.

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arXiv2008

Rapid variability in TeV blazars: the case of PKS 2155-304

Recent Cherenkov observations of BL Lac objects showed that the TeV flux of PKS 2155-304 changed by a factor 2 in just 3-5 minutes. This fast variability can be accounted for if the emitting region is moving with a bulk Lorentz factor Gamma~50 and a similar relativistic Doppler factor. If this Gamma is adopted, several models can fit the data, but, irrespective of the chosen model, the jet is matter dominated. The Doppler factor requires viewing angles of the order of 1 degree or less: if the entire jet is as narrow as this, then we have problems with current unification schemes. This suggests that there are small active regions, inside a larger jet, moving faster than the rest of the plasma, occasionally pointing at us. Coordinated X-ray/TeV variability can discriminate between the different scenarios.

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arXiv2008

Simultaneous multiwavelength observations of the blazar 1ES1959+650 at a low TeV flux

We present the results from a multiwavelength campaign on the TeV blazar 1ES 1959+650, performed in May, 2006. Data from the optical, UV, soft- and hard-X-ray and very high energy (VHE) gamma-ray (E > 100 GeV) bands were obtained with the SUZAKU and SWIFT satellites, with the MAGIC telescope and other ground based facilities. The source spectral energy distribution (SED), derived from SUZAKU and MAGIC observations at the end of May 2006, shows the usual double hump shape, with the synchrotron peak at a higher flux level than the Compton peak. With respect to historical values, during our campaign the source exhibited a relatively high state in X-rays and optical, while in the VHE band it was at one of the lowest level so far recorded. We also monitored the source for flux-spectral variability on a time window of 10 days in the optical-UV and X-ray bands and 7 days in the VHE band. The source varies more in the X-ray, than in the optical band, with the 2-10 keV X-ray flux varying by a factor of ~2. The synchrotron peak is located in the X-ray band and moves to higher energies as the source gets brighter, with the X-ray fluxes above it varying more rapidly than the X-ray fluxes at lower energies. The variability behaviour observed in the X-ray band cannot be produced by emitting regions varying independently, and suggests instead some sort of ``standing shock'' scenario. The overall SED is well represented by an homogeneous one-zone synchrotron inverse Compton emission model, from which we derive physical parameters that are typical of high energy peaked blazars.

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