arXiv subjects

F. Tavecchio

Publications and source records attributed to F. Tavecchio.

At least 145 records · Page 8Linked to original sources

arXiv2009

Jet and accretion power in the most powerful Fermi blazars

Among the blazars detected by the Fermi satellite, we have selected the 23 blazars that in the three months of survey had an average gamma-ray luminosity above 1e48 erg/s. For 17 out of the 23 sources we found and analysed X-ray and optical-UV data taken by the Swift satellite. With these data, implemented by archival and not simultaneous data, we construct the spectral energy distributions, and interpreted them with a simple one-zone, leptonic, synchrotron and inverse Compton model. When possible, we also compare different high energy states of single sources, like 0528+134 and 3C 454.3, for which multiple good sets of multi-wavelength data are available. In our powerful blazars the high energy emission always dominates the electromagnetic output, and the relatively low level of the synchrotron radiation often does not hide the accretion disk emission. We can then constrain the black hole mass and the disk luminosity. Both are large (i.e. masses equal or greater than 1e9 solar masses and disk luminosities above 0.1 Eddington). By modelling the non-thermal continuum we derive the power that the jet carries in the form of bulk motion of particles and fields. On average, the jet power is found to be slightly larger than the disk luminosity, and proportional to the mass accretion rate.

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arXiv2009

The blazar S5 0014+813: a real or apparent monster?

A strong hard X-ray luminosity from a blazar flags the presence of a very powerful jet. If the jet power is in turn related to the mass accretion rate, the most luminous hard X-ray blazars should pinpoint the largest accretion rates, and therefore the largest black hole masses. These ideas are confirmed by the Swift satellite observations of the blazar S5 0014+813, at the redshift z=3.366. Swift detected this source with all its three instruments, from the optical to the hard X-rays. Through the construction of its spectral energy distribution we are confident that its optical-UV emission is thermal in origin. Associating it to the emission of a standard optically thick geometrically thin accretion disk, we find a black hole mass of 40 billion solar masses, radiating at 40% the Eddington value. The derived mass is among the largest ever found. Super-Eddington slim disks or thick disks with the presence of a collimating funnel can in principle reduce the black hole mass estimate, but tends to produce spectra bluer than observed.

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arXiv2009

Multiwavelength observation of the blazar 1ES1426+428 in May-June 2008

We present the results of the multiwavelength observation of the high frequency-peaked BL Lac object 1ES1426+428 carried in May-June 2008 with the KVA telescope for the optical band, the Swift and Suzaku satellites for the X-ray band and the MAGIC telescope for the very high energy band (E>100 GeV). The optical flux increased by 20% during the observation, while X-ray data show that the source was in an intermediate state of activity. MAGIC did not detect a significant gamma-ray emission from the source, so we report flux Upper Limit at 95% CL for energies above 130 GeV. A Spectral Energy Distribution is also obtained combining the data from the 2008 campaign with the historical ones.

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arXiv2009

Multiwavelength Observations of Mrk 501 in 2008

The well-studied VHE (E >100 GeV) blazar Mrk 501 was observed between March and May 2008 as part of an extensive multiwavelength observation campaign including radio, optical, X-ray and VHE gamma-ray instruments. Mrk 501 was in a low state of activity during the campaign, with a low VHE flux of about 20% the Crab Nebula flux. Nevertheless, significant flux variations could be observed in X-rays as well as $γ$-rays. Overall Mrk 501 showed increased variability when going from radio to gamma-ray energies. The broadband spectral energy distribution during the two different emission states of the campaign was well described by a homogeneous one-zone synchrotron self-Compton model. The high emission state was satisfactorily modeled by increasing the amount of high energy electrons with respect to the low emission state. This parameterization is consistent with the energy-dependent variability trend observed during the campaign.

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arXiv2009

The Next Generation of Cherenkov Telescopes. A White Paper for the Italian National Institute for Astrophysics (INAF)

Motivated by the recent challenging results from TeV astronomy, the VHE INAF community asked a group of them to write this White Paper to summarize the status and future of Cherenkov telescopes for gamma-ray astronomy and the INAF perspectives in this field. This document wants to review both the scientific topics and potential developments of the field as well as to point out both the interests and the capacities (scientific and technical) of the VHE astrophysics community in INAF. It is aimed at identifying the scientific and technological areas where INAF should focus its efforts and resources so that Italian researchers can achieve (or maintain) a leading position in this field.

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arXiv2009

The jet of the BL Lac object PKS 0521 -365 in the near-IR : MAD adaptive optics observations

BL Lac objects are low--power active nuclei exhibiting a variety of peculiar properties that are caused by the presence of a relativistic jet and orientation effects. We present here adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies. The observations were obtained in Ks--band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps. We find a remarkable similarity in the structure of the jet at radio, near-IR, and optical wavelengths. The broad--band emission of the jet knots is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources.

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arXiv2009

Structure of the Accretion Flow in Broad-Line Radio Galaxies: The Case of 3C390.3

We present XMM and Suzaku observations of the Broad-Line Radio Galaxy (BLRG) 3C390.3. The Fe Ka line has a width FWHM ~ 8,800 km/s, consistent within a factor two with the width of the double-peaked H_alpha line, suggesting an origin from the Broad Line Region. The data show for the first time a weak, broad bump extending from 5 to 7 keV. When fitted with a Gaussian, its centroid energy is 6.6 keV in the source's rest-frame with FWHM of 43,000 km/s and EW of 50 eV; its most likely interpretation is emission from He-like Fe (Fe XXV), suggesting the presence of an ionized medium in the inner regions of 3C390.3. The broad-band 0.5-100 keV continuum is well described by a single power law with photon index Gamma=1.6 and cutoff energy 157 keV, plus cold reflection with strength R=0.5. In addition, ionized reflection is required to account for the 6.6 keV bump in the broad-band continuum, yielding an ionization parameter xi ~ 2700 ergs cm s^-1; the inner radius of the ionized reflector is constrained to be larger than 20 r_G, although this result depends on the assumed emissivity profile of the disk. If true, we argue that the lack of broad Fe K emission from within 20 r_G indicates that the innermost regions of the disk in 3C390.3 are obscured and/or poorly illuminated. While the SED of 3C390.3 is generally dominated by accretion-related continuum, during accretion low states the jet can significantly contribute in the optical to X-ray bands via synchrotron self-Compton emission. (Abridged)

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arXiv2009

Fermi/LAT discovery of gamma-ray emission from a relativistic jet in the narrow-line quasar PMN J0948+0022

We report the discovery by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope of high-energy gamma-ray emission from the peculiar quasar PMN J0948+0022 (z=0.5846). The optical spectrum of this object exhibits rather narrow Hbeta (FWHM(Hbeta) ~ 1500 km s^-1), weak forbidden lines and is therefore classified as a narrow-line type I quasar. This class of objects is thought to have relatively small black hole mass and to accrete at high Eddington ratio. The radio loudness and variability of the compact radio core indicates the presence of a relativistic jet. Quasi simultaneous radio-optical-X-ray and gamma-ray observations are presented. Both radio and gamma-ray emission (observed over 5-months) are strongly variable. The simultaneous optical and X-ray data from Swift show a blue continuum attributed to the accretion disk and a hard X-ray spectrum attributed to the jet. The resulting broad band spectral energy distribution (SED) and, in particular, the gamma-ray spectrum measured by Fermi are similar to those of more powerful FSRQ. A comparison of the radio and gamma-ray characteristics of PMN J0948+0022 with the other blazars detected by LAT shows that this source has a relatively low radio and gamma-ray power, with respect to other FSRQ. The physical parameters obtained from modelling the SED also fall at the low power end of the FSRQ parameter region discussed in Celotti & Ghisellini (2008). We suggest that the similarity of the SED of PMN J0948+0022 to that of more massive and more powerful quasars can be understood in a scenario in which the SED properties depend on the Eddington ratio rather than on the absolute power.

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arXiv2009

Canonical high power blazars

The jets of powerful blazars propagate within regions relatively dense of radiation produced externally to the jet. This radiation is a key ingredient to understand the origin of the high energy emission of blazars, from the X-ray to the gamma-ray energy band. These external radiation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region, by assuming rather "standard" (i.e. "canonical") prescriptions for the accretion disk and its X-ray corona, the profile of the jet magnetic field and the external radiation. The magnetic energy density of a "canonical" jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad line region is particularly interesting, since it accounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X-ray spectrum. Furthermore it makes the high power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds.

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arXiv2009

The Fermi blazars divide

Flat Spectrum Radio Quasars (FSRQs) and BL Lac objects detected in the first three months of the Fermi survey neatly separate in the gamma-ray spectral index vs gamma-ray luminosity plane. BL Lac objects are less luminous and have harder spectra than broad line blazars. We suggest that this division has its origin in the different accretion regimes of the two classes of objects. Using the gamma-ray luminosity as a proxy for the observed bolometric one we show that the boundary between the two subclasses of blazars can be associated with the threshold between the regimes of optically thick accretion disks and of radiatively inefficient accretion flows, which lies at an accretion rate of the order of 0.01 the Eddington rate. The spectral separation in hard (BL Lacs) and soft (FSRQs) objects can then result from the different radiative cooling suffered by the relativistic electrons in jets propagating in different ambients. We argue that the bulk of the most luminous blazars alread detected by Fermi should be characterised by large black hole masses, around 10^9 solar masses, and predict that lowering the gamma-ray flux threshold the region of the alpha_gamma-L_gamma plane corresponding to steep spectral indices and lower luminosities will be progressively populated by FSRQs with lower mass black holes, while the region of hard spectra and large luminosities will remain forbidden.

astro-ph.CO↗

arXiv2009

The changing look of PKS 2149-306

Aims: We study the blazar nature of the high-redshift Flat-Spectrum Radio Quasar PKS 2149-306 (z = 2.345) by investigating its long-term behavior. Methods: We analyzed all publicly available optical-to-X-ray observations performed by XMM-Newton, Swift, and INTEGRAL. Conclusions: PKS 2149-306 is one of four blazars at z>2 that have been observed in the hard-X-ray regime with both the BAT and ISGRI instruments. Observations acquired almost 1 year apart in the 60-300 keV energy band in the object rest frame, exhibit no noticeable change in spectral slope associated with a flux variation of more than a factor of two. Swift data appear to show a roll-off below ~1 keV, which becomes increasingly evident during a ~3-day time-frame, that can be explained as the natural spectral break caused by the Inverse Compton onset. The broad-band spectra allow us to identify two different states. The SED modeling suggests that they can be interpreted by only a change in the bulk Lorentz factor of the jet.

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arXiv2009

3C66B as a TeV radiogalaxy

The MAGIC collaboration reported the detection of a new VHE source, MAGIC J0223+430, located close to the position of the blazar 3C66A, considered a candidate TeV blazar since a long time. A careful analysis showed that the events with energies above 150 GeV are centered on the position of the FRI radiogalaxy 3C66B (at 6 arcmin from 3C66A), with a probability of 95.4% (85.4% including systematic uncertainties) that the source is not related to 3C66A. We present a model for the possible emission of 3C66B based on the structured jet model already used to interpret the TeV emission of the radiogalaxy M87. The model requires parameters similar to those used for M87 but a larger luminosity for the layer, to account for the more luminous TeV emission. We also show that the spectrum obtained by MAGIC can be interpreted as the combined emission of 3C66B, dominating above ~200 GeV, and of 3C66A. The high-energy emission from the latter source, being strongly attenuated by the interaction with the extragalactic background light, can only contribute at low energies. If we were to see the jet emission of 3C 66B at small viewing angles we would see a spectral energy distribution closely resembling the one of S5 0716+714, a typical blazar.

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arXiv2009

Discovery of very high energy gamma-rays from the flat spectrum radio quasar 3C 279 with the MAGIC telescope

3C 279 is one of the best studied flat spectrum radio quasars located at a comparatively large redshift of z = 0.536. Observations in the very high energy band of such distant sources were impossible until recently due to the expected steep energy spectrum and the strong gamma-ray attenuation by the extragalactic background light photon field, which conspire to make the source visible only with a low energy threshold. Here the detection of a significant gamma-ray signal from 3C 279 at very high energies (E > 75 GeV) during a flare in early 2006 is reported. Implications of its energy spectrum on the current understanding of the extragalactic background light and very high energy gamma-ray emission mechanism models are discussed.

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arXiv2009

Blazar nuclei in radio-loud narrow-line Seyfert 1?

It has been suggested that some radio-loud narrow-line Seyfert 1 contain relativistic jets, on the basis of their flat-spectrum radio nuclei and studies on variability. We present preliminary results of an ongoing investigation of the X-ray and multiwavelength properties of 5 radio-loud NLS1 based on archival data from Swift and XMM-Newton. Some sources present interesting characteristics, very uncharacteristic for a radio-quiet narrow-line Seyfert 1, such as very hard X-ray spectra, and correlated optical and ultraviolet variability. However, none of the studied sources show conclusive evidence for relativistic jets. gamma-ray observations with Fermi are strongly recommended to definitely decide on the presence or not of relativistic jets.

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arXiv2008

Intrinsic absorption in 3C 279 at GeV-TeV energies and consequences for estimates of the EBL

We revisit the limits of the level of the extragalactic background light (EBL) recently reported by the MAGIC collaboration based on the observed gamma-ray spectrum of the quasar 3C279, considering the impact of absorption of high-energy gamma-ray photons inside the broad line region (BLR) of the quasar. We use the photoionization code CLOUDY to calculate the expected optical-UV radiation field inside the BLR and the optical depth to gamma-rays for a relatively extended set of the parameters. We found that the absorption of gamma-ray photons, though important for the estimate of the true radiative output of the source, does not produce an important hardening of the spectrum of 3C279 in the energy band accessible by MAGIC, supporting the method used to infer the upper limits to the level of the EBL.

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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

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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