arXiv subjects

F. Tavecchio

Publications and source records attributed to F. Tavecchio.

At least 181 records · Page 10Linked to original sources

arXiv2007

Low energy cut-offs and hard X-ray spectra in high-z radio-loud quasars: the Suzaku view of RBS315

We present the results from the Suzaku observation of the powerful radio-loud quasar RBS315 (z=2.69), for which a previous XMM-Newton observation showed an extremely flat X-ray continuum up to 10 keV (photon index Gamma=1.26) and indications of strong intrinsic absorption (N_H~10^22 cm^{-2} assuming neutral gas). The instrument for hard X-rays HXD/PIN allows us a detection of the source up to 50 keV. The broad-band continuum (0.5-50 keV) can be well modeled with a power-law with slope Gamma=1.5 (definitively softer than the continuum measured by XMM-Newton) above 1 keV with strong deficit of soft photons. The low-energy cut-off can be well fitted either with intrinsic absorption (with column density N_H~10^22 cm^{-2} in the quasar rest frame) or with a break in the continuum, with an extremely hard (Gamma =0.7) power-law below 1 keV. We construct the Spectral Energy Distribution of the source, using also optical-UV measurements obtained through a quasi-simultaneous UVOT/SWIFT observation. The shape of the SED is similar to that of other Flat Spectrum Radio Quasars (FSRQs) with similar power, making this source an excellent candidate for the detection in gamma-rays by GLAST. We model the SED with the synchrotron-Inverse Compton model usually applied to FSRQs, showing that the deficit of soft photons can be naturally interpreted as due to an intrinsic curvature of the spectrum near the low energy end of the IC component rather than to intrinsic absorption, although the latter possibility cannot be ruled out. We propose that in at least a fraction of the radio-loud QSOs at high redshift the cut-off in the soft X-ray band can be explained in a similar way. Further studies are required to distinguish between the two alternatives.

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arXiv2007

Black-body components in Gamma-Ray Bursts spectra?

We study 7 Gamma Ray Bursts (GRBs), detected both by the BATSE instrument, on-board the Compton Gamma Ray Observatory, and by the Wide Field Camera (WFC), on-board BeppoSAX. These bursts have measured spectroscopic redshifts and are a sizeable fraction of the bursts defining the correlation between the peak energy E_peak (i.e. the peak of the vFv spectrum) and the total prompt isotropic energy E_iso (the so called "Amati" relation). Recent theoretical interpretations of this correlation assume that black-body emission dominates the time resolved spectra of GRBs, even if, in the time integrated spectrum, its presence may be hidden by the change of its temperature and by the dilution of a possible non-thermal power law component. We perform a time resolved spectral analysis, and show that the sum of a power-law and a black-body gives acceptable fits to the time dependent spectra within the BATSE energy range, but overpredicts the flux in the WFC X-ray range. Moreover, a fit with a cutoff power-law plus a black-body is consistent with the WFC data, but the black-body component contributes a negligible fraction of the total flux. On the contrary, we find that fitting the spectra with a Band model or a simple cutoff power-law model yields an X-ray flux and spectral slope which well matches the WFC spectra.

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arXiv2007

The Blazar Spectral Sequence and GLAST

The present status and understanding of the "spectral sequence" of blazars is discussed in the perspective of the upcoming GLAST launch. The vast improvement in sensitivity will allow to i) determine more objectively the "average" gamma-ray properties of classes objects ii) probe more deeply the ratio between accretion power and jet power in different systems.

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arXiv2007

Swift follow-up of the gigantic TeV outburst of PKS 2155-304 in 2006

At the end of July 2006, the blazar PKS 2155-304 (z=0.116) underwent a strong outburst observed at TeV energies by HESS (up to 17 Crab flux level at E > 200 GeV). The Swift satellite followed the evolution of the source for about one month. The data analysis - reported in another paper - has shown that, despite the violent activity at TeV energies, the synchrotron energy distribution increased in normalization, but only with a small shift in frequency. In the present work, we fit the broad-band spectrum with a log-parabolic model, to search for indications of intrinsic curvature, which in turn is usually interpreted as a signature of energy-dependent acceleration mechanisms of electrons.

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arXiv2007

An Infrared Study of the Large-scale Jet in Quasar PKS 1136-135

We present Spitzer IRAC imaging of the large-scale jet in the quasar PKS 1136-135 at wavelengths of 3.6 and 5.8 um, combined with previous VLA, HST, and Chandra observations. We clearly detect infrared emission from the jet, resulting in the most detailed multifrequency data among the jets in lobe-dominated quasars. The spectral energy distributions of the jet knots have significant variations along the jet, like the archetypal jet in 3C 273. The infrared measurements with IRAC are consistent with the previous idea that the jet has two spectral components, namely (1) the low-energy synchrotron spectrum extending from radio to infrared, and (2) the high-energy component responsible for the X-ray flux. The optical fluxes may be a mixture of the two components. We consider three radiation models for the high-energy component: inverse Compton scattering off CMB photons by radio-emitting electrons in a highly relativistic jet, synchrotron radiation by a second distinct electron population, and synchrotron radiation by ultra high energy protons. Each hypothesis leads to important insights into and constraints on particle acceleration in the jet, as well as the basic physical properties of the jet such as bulk velocity, transporting power, and particle contents.

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arXiv2007

Chandra and HST observations of gamma-ray blazars: comparing jet emission at small and large scales

We present new Chandra and HST data for four gamma-ray blazars selected on the basis of radio morphology with the aim of revealing X-ray and optical emission from their jets at large scales. All the sources have been detected. Spectral Energy Distributions of the large scale jets are obtained as well as new X-ray spectra for the blazar cores. Modeling for each object the core (sub-pc scale) and large-scale (>100 kpc) jet SEDs, we derive the properties of the same jet at the two scales. The comparison of speeds and powers at different scales supports a simple scenario for the dynamics and propagation of high power relativistic jets.

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arXiv2007

X-ray/UV/Optical follow-up of the blazar PKS 2155-304 after the giant TeV flares of July 2006

We present all the publicly available data, from optical/UV wavelengths (UVOT) to X-rays (XRT, BAT), obtained from Swift observations of the blazar PKS 2155-304, performed in response to the rapid alert sent out after the strong TeV activity (up to 17 Crab flux level at E > 200 GeV) at the end of July 2006. The X-ray flux increased by a factor of 5 in the 0.3-10 keV energy band and by a factor of 1.5 at optical/UV wavelengths, with roughly one day of delay. The comparison of the spectral energy distribution built with data quasi-simultaneous to the TeV detections shows an increase of the overall normalization with respect to archival data, but only a small shift of the frequency of the synchrotron peak that remains consistent with the values reported in past observations when the TeV activity was much weaker.

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arXiv2007

Wind circumburst density profile: a linear E_p-E_gamma correlation

Ghirlanda et al. (2004) derived the collimation-corrected energy E_gamma for a sample of 15 bursts under the assumption of a homogeneous circumburst density profile. They found a correlation (the so-called Ghirlanda correlation) between E_gamma and the rest frame peak energy of the nuF_nu prompt spectrum (E_p). Nava et al. (2006) showed that, assuming a circumburst density distribuited with a r^-2 wind profile, the Ghirlanda correlation remains tight and becomes linear. This implies that: i) it remains linear also in the comoving frame, no matter the distribution of bulk Lorentz factors, ii) it entails that different bursts have the same number of relevant photons. We have updated these findings including recently detected bursts (21 in total), stressing the two important implications.

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arXiv2006

The unexpected clustering of the optical afterglow luminosities

We studied the behaviour of the optical afterglow lightcurves of a sample of 24 Gamma--Ray Bursts (GRBs) with known redshift and published estimates of the optical extinction in the source frame, detected before the SWIFT satellite launch. We found an unexpected clustering of the optical luminosities at 12 hours in the source frame. The distribution of the optical luminosities is narrower than the distribution of X-ray luminosities at the same time. Few (3) bursts stand apart from the main optical distribution, being fainter by a factor of about 15. We also analysed the optical luminosities of the SWIFT burst with known redshift finding that the luminosity distribution is similar to the pre SWIFT GRBs one, even if they have a different mean redshift. These results can suggest the existence of a family of intrinsically optically under--luminous dark GRBs.

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arXiv2006

Puzzled by GRB 060218

We study the optical-UV/X-ray spectral energy distribution of GRB 060218 during the prompt phase and during what seems to be the afterglow phase. The results are puzzling, since if the opt-UV and the X-ray emission belong to a single backbody (BB), then its luminosity is too large, and this BB cannot be interpreted as the signature of the shock breakout of the supernova. There are also serious problems in associating the emission expected by the supernova shock breakout with either the opt-UV or the X-ray emission. In the former case we derive too small ejecta velocities; in the latter case, on the contrary, the required velocity is too large, corresponding to the large radius of a BB required to peak close to the UV band. We then present what we think is the most conservative alternative explanation, namely a synchrotron spectrum, self-absorbed in the opt-UV and extending up to the X-ray band, where we observe the emission of the most energetic electrons, which are responsible for the exponential roll-over of the spectrum. The obtained fit can explain the entire spectrum except the BB observed in the X-rays, which must be a separate component. The puzzling feature of this interpretation is that the same model is required to explain the spectrum also at later times, up to 1e5 s, because the opt-UV emission remains constant in shape and also (approximately) in normalisation. In this case the observed X-ray flux is produced by self-Compton emission. Thus the prompt emission phase should last for ~1e5 s or more. Finally, we show that the BB observed in X-rays, up to 7000 seconds, can be photospheric emission from the cocoon or stellar material, energized by the GRB jet at radii comparable to the stellar radius (i.e. 1e10-1e11 cm), not very far from where this material becomes transparent (e.g. 1e12 cm).

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arXiv2006

Are GRB 980425 and GRB 031203 real outliers or twins of GRB 060218?

GRB 980425 and GRB 031203 are apparently two outliers with respect to the correlation between the isotropic equivalent energy E_iso emitted in the prompt radiation phase and the peak frequency E_peak of the spectrum in a vF(v) representation (the so-called Amati relation). We discuss if these two bursts are really different from the others or if their location in the E_iso-E_peak plane is the result of other effects, such as viewing them off-axis, or through a scattering screen, or a misinterpretation of their spectral properties. The latter case seems particularly interesting after GRB 060218, that, unlike GRB 031203 and GRB 980425, had a prompt emission detected both in hard and soft X-rays which lasted ~2800 seconds. This allowed to determine its E_peak and total emitted energy. Although it shares with GRB 031203 the total energetics, it is not an outlier with respect to the Amati correlation. We then investigate if a hard-to-soft spectral evolution in GRB 031203 and GRB 980425, consistent with all the observed properties, can give rise to a time integrated spectrum with an E_peak consistent with the Amati relation.

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arXiv2006

The jet/disk connection in AGN: Chandra and XMM observations of three powerful radio-loud quasars

The connection between the accretion process that powers AGN and the formation of jets is still poorly understood. Here we tackle this issue using new, deep \chandra and \xmm observations of the cores of three powerful radio loud quasars: 1136--135, 1150+497 (\chandra), and 0723+679 (\xmm), in the redshift range $z$=0.3--0.8. These sources are known from our previous \chandra snapshot survey to have kpc-scale X-ray jets. In 1136--135 and 1150+497, evidence is found for the presence of diffuse thermal X-ray emission around the cores, on scales of 40--50 kpc and with luminosity L$_{0.3-2 keV} \sim 10^{43}$ \lum, suggesting thermal emission from the host galaxy or a galaxy group. The X-ray continua of the cores in the three sources are described by an upward-curved (concave) broken power law, with photon indices $Γ_{soft} \sim 1.8-2.1$ and $Γ_{hard} \sim 1.7$ below and above $\approx 2$ keV, respectively. There is evidence for an unresolved \feka line with EW $\sim$ 70 eV in the three quasars. The Spectral Energy Distributions of the sources can be well described by a mix of jet and disk emission, with the jet dominating the radio and hard X-rays (via synchrotron and external Compton) and the disk dominating the optical/UV through soft X-rays. The ratio of the jet-to-disk powers is $\sim 1$, consistent with those derived for a number of gamma-ray emitting blazars. This indicates that near equality of accretion and jet power may be common in powerful radio-loud AGN.

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arXiv2006

Simultaneous X-ray and optical observations of S5 0716+714 after the outburst of March 2004

At the end of March 2004, the blazar S5 0716+714 underwent an optical outburst that prompted for quasi-simultaneous target-of-opportunity observations with the INTEGRAL and XMM-Newton satellites. In this paper, we report the results of the XMM-Newton and INTEGRAL OMC data analysis. The X-ray spectrum is well-represented by a concave broken power-law model, with the break at about 2 keV. In the framework of the synchrotron self-Compton model, the softer part of the spectrum, which is described by a power law of index $α\simeq 1.8$ ($f_ν\propto ν^{-α}$), is probably due to synchrotron emission, while the harder part of the spectrum, which has $α\simeq 1$, is due to inverse Compton emission. The blazar shows the long and short-term variability typical of low-frequency peaked BL Lac (LBL): the former is manifested by a gradual decrease in the optical flux from the peak as observed by ground telescopes at the end of March 2004, while the latter is characterized by soft X-ray and optical flares on time scales from a few thousand seconds to few hours. We can follow spectral variations on sub-hour time scales and study their correlation with the flux variability. We find evidence that the peak energy of the time-resolved spectra is increasing with flux. The modeling of the spectral energy distribution compared with archival observations suggests that the long-term variability (from outburst to quiescence or viceversa) could be due to a change in the injected power, while the short-term variability (flares) could be explained with changes in the slope of the distribution of the electrons.

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arXiv2006

Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope

We have performed infrared imaging of the jet of the quasar 3C 273 at wavelengths 3.6 and 5.8 microns with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. When combined with the radio, optical and X-ray measurements, the IRAC photometry clearly shows that the optical emission is dominated by the high-energy component of the jet, not by the radio synchrotron component, as had been assumed to date. The high-energy component may be due to a second synchrotron component or to IC scattering of ambient photons. In the former case, we argue that the acceleration of protons exceeding 10^16 eV or possibly even to 10^19 eV would be taking place in the jet. In contrast, the IC model, into which highly relativistic Doppler beaming has to be incorporated, requires very low-energy electrons (~ 1 MeV). The present polarization data in the radio and optical would favor the former interpretation in the case of the 3C 273 jet. Sensitive and detailed measurements of optical polarization are important to establish the radiation mechanism responsible for the high-energy emission. The present study offers new clues as to the controversial origin of the X-ray emission seen in many quasar jets.

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arXiv2006

A Short Hard X-ray Flare from the Blazar NRAO 530 Observed by INTEGRAL

We report about a short flare from the blazar NRAO 530 occurred on 17 February 2004 and detected serendipitously by the IBIS/ISGRI detector on board INTEGRAL. In the 20-40 keV energy range, the source, that is otherwise below the detection limit, is detected at a level of ~2 x 10^-10 erg cm^-2 s^-1 during a time interval of less than 2000 s, which is about a factor 2 above the detection threshold. At other wavelengths, only nearly-simultaneous radio data are available (1 observation at 2 cm on 11 February 2004), indicating a moderate increase of the polarization. This appears to be the shortest time variability episode ever detected in a high luminosity blazar at hard X-rays, unless the blazar is contaminated by the presence of an unknown unresolved rapidly varying source.

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arXiv2006

XMM-Newton observations of a sample of gamma-ray loud active galactic nuclei

(abridged) We performed a homogeneous and systematic analysis of simultaneous X-ray and optical/UV properties of a group of 15 gamma-ray loud AGN, using observations performed with XMM-Newton. The sample is composed of 13 blazars (6 BL Lac and 7 Flat-Spectrum Radio Quasar) and 2 radio galaxies, that are associated with detections at energies >100 MeV. The data for 7 of them are analyzed here for the first time, including the first X-ray observation of PKS 1406-706. Then, the spectral characteristics of the sources in the present sample have been compared with those in previous catalogs of blazars and other AGN, in order to search for any difference or long term change. All the selected sources appear to follow the classic "blazar sequence" and the spectral energy distributions (SED) built with the present X-ray and optical/UV data and completed with historical data, confirm the findings of previous studies on this type of sources. Some sources display interesting features worth noting: four of them, namely AO 0235+164, PKS 1127-145, S5 0836+710 and PKS 1830-211 show the presence of an intervening absorption system along the line of sight, but only the latter is known to be surely gravitationally lensed. AO 0235+164 was detected during an outburst and its SED shows a clear shift of the synchrotron peak. 3C 273 shows a change in the state with respect to the previous BeppoSAX observations, that can be interpreted as an increase of the Seyfert-like component and a corresponding decline of the jet emission. This is consistent with the monitoring at radio wavelengths performed during the same years. PKS 1406-706 is detected with a flux higher than in the past, but with a corresponding low optical flux. Moreover, although it is classified as FSRQ, the SED can be modelled with a simple synchrotron self-Compton model.

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arXiv2006

Long-Term X-Ray and TeV variability of Mrk 501

We present X-ray observations of the blazar Mrk501 obtained with RXTE during 1997, 1998, 1999, 2000, and 2004. The goal of this study is twofold: 1) characterize the long-term X-ray flux and spectral variability of the source with a model-independent analysis, and 2) investigate the X-ray and TeV correlation on long timescales. The spectral and temporal variability properties appear to be markedly different compared to those of non-jet-dominated radio-loud and radio-quiet AGN monitored with RXTE. To investigate the X-ray/TeV correlation on long timescales we compared RXTE data with HEGRA and Whipple historical light curves. We confirm the presence of a direct correlation, which appears to be stronger when the source is brighter. The analysis of individual flares shows that the X-ray\TeV correlated activity is heterogeneous. However, more sensitive TeV observations are necessary to confirm these findings, and to put tighter constraints on jet models (abridged).

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arXiv2006

Stochastic particle acceleration and synchrotron self--Compton radiation in TeV blazars

We analyse the influence of the stochastic particle acceleration for the evolution of the electron spectrum. We assume that all investigated spectra are generated inside a spherical, homogeneous source and also analyse the synchrotron and inverse Compton emission generated by such an object. The stochastic acceleration is treated as the diffusion of the particle momentum and is described by the momentum-diffusion equation. We investigate the stationary and time dependent solutions of the equation for several different evolutionary scenarios. The scenarios are divided into two general classes. First, we analyse a few cases without injection or escape of the particles during the evolution. Then we investigate the scenarios where we assume continuous injection and simultaneous escape of the particles. In the case of no injection and escape the acceleration process, competing with the radiative cooling, only modifies the initial particle spectrum. The competition leads to a thermal or quasi-thermal distribution of the particle energy. In the case of the injection and simultaneous escape the resulting spectra depend mostly on the energy distribution of the injected particles. In the simplest case, where the particles are injected at the lowest possible energies, the competition between the acceleration and the escape forms a power-law energy distribution. We apply our modeling to the high energy activity of the blazar Mrk 501 observed in April 1997. Calculating the evolution of the electron spectrum self-consistently we can reproduce the observed spectra well with a number of free parameters that is comparable to or less than in the "classic stationary" one--zone synchrotron self-Compton scenario.

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