arXiv subjects

F. Tavecchio

Publications and source records attributed to F. Tavecchio.

At least 91 records · Page 5Linked to original sources

arXiv2014

Photon-ALP conversions inside AGN

An intriguing possibility to partially circumvent extragalactic background light (EBL) absorption in very-high-energy (VHE) observations of blazars is that photons convert into axion-like particles (ALPs) $γ\to a$ inside or close to a blazar and reconvert into photons $a \to γ$ in the Milky Way magnetic field. This idea has been put forward in 2008 and has attracted a considerable interest. However, while the probability for the back-conversion $a \to γ$ has been computed in detail (using the maps of the Galatic magnetic field), regretfully no realistic estimate of the probability for the conversion $γ\to a$ inside a blazar has been performed, in spite of the fact that the present-day knowledge allows this task to be accomplished in a reliable fashion. We present a detailed calculation that fills this gap, considering both types of blazars, namely BL Lac objects (BL Lacs) and flat spectrum radio quasars (FSRQ) with their specific structural and environmental properties. We also include the host elliptical galaxy into account. Our somewhat surprising results show that the conversion probability in BL Lacs is strongly dependent on the source parameters -- like the position of the emission region along the jet and the strength of the magnetic field therein -- making it effectivelly unpredictable. On the other hand, the lobes at the termination of FSRQ jets lead to an effective "equipartition" between photons and ALPs due to its chaotic nature, thereby allowing us to make a clear-cut prediction. These results are quite important in view of the planned VHE detectors like the CTA, HAWK and HiSCORE.

astro-ph.HE↗

arXiv2014

Structured jets in BL Lac objects: efficient PeV neutrino factories?

The origin of the high-energy neutrinos (0.1$-$1 PeV range) detected by IceCube remains a mystery. In this work we explore the possibility that efficient neutrino production can occur in structured jets of BL Lac objects, characterized by a fast inner spine surrounded by a slower layer. This scenario has been widely discussed in the framework of the high-energy emission models for BL Lacs and radiogalaxies. One of the relevant consequences of a velocity structure is the enhancement of the inverse Compton emission caused by the radiative coupling of the two zones. We show that a similar boosting could occur for the neutrino output of the spine through the photo-meson reaction of high-energy protons scattering off the amplified soft target photon field of the layer. Assuming the local density and the cosmological evolution of $γ$-ray BL Lac derived from {\it Fermi}-LAT data, we calculate the expected diffuse neutrino intensity, that can match the IceCube data for a reasonable choice of the parameters.

astro-ph.HE↗

arXiv2014

Exploring the blazar zone in High Energy flares of FSRQs

The gamma-ray emission offers a powerful diagnostic tool to probe jets and their surroundings in flat spectrum radio quasars (FSRQ). In particular, sources emitting at high energies (>10 GeV) give us the strongest constraints. This motivates us to start a systematic study of flares with bright emission above 10 GeV, examining archival data of Fermi-LAT gamma-ray telescope. At the same time, we began to trigger Target of Opportunity observations to the Swift observatory at the occurrence of high-energy flares, obtaining a wide coverage of the spectral energy distributions for several FSRQs during flares. Among the others we investigate the SED of a peculiar flare of 3C 454.3, showing a remarkable hard gamma-ray spectrum, quite different from the brightest flares of this source, and a bright flare of CTA 102. We modeled the SED in the framework of the one--zone leptonic model, using also archival optical spectroscopic data to derive the luminosity of the broad lines and thus estimate the disk luminosity, from which the structural parameters of the FSRQ nucleus can be inferred. The model allowed us to evaluate the magnetic field intensity in the blazar zone, and to locate the emitting region of gamma rays in the particular case in which gamma-ray spectra show neither absorption from the BLR, nor the Klein-Nishina curvature expected in leptonic models assuming the BLR as source of seed photons for the External Compton. For FSRQs bright above 10 GeV, we where able to identify short periods lasting less than 1 day characterized by high rate of high energy gamma rays, and hard gamma-ray spectra. We discussed the observed spectra and variability timescales in terms of injection and cooling of energetic particles, arguing that these flares could be triggered by magnetic reconnections events or turbulence in the flow.

astro-ph.HE↗

arXiv2014

On the spine-layer scenario for the very high-energy emission of NGC 1275

We discuss the $γ$-ray emission of the radiogalaxy NGC 1275 (the central galaxy of the Perseus Cluster), detected by Fermi-LAT and MAGIC, in the framework of the "spine-layer" scenario, in which the jet is assumed to be characterized by a velocity structure, with a fast spine surrounded by a slower layer. The existence of such a structure in the parsec scale jet of NGC 1275 has been recently proved through VLBI observations. We discuss the constraints that the observed spectral energy distribution imposes to the parameters and we present three alternative models, corresponding to three different choices of the angles between the jet and the line of sight ($θ_{\rm v}=6^{\circ}, 18^{\circ}$ and 25$^{\circ}$). While for the the case with $θ_{\rm v}=6^{\circ}$ we obtain an excellent fit, we consider this solution unlikely, since such small angles seems to be excluded by radio observations of the large-scale jet. For $θ_{\rm v}=25^{\circ}$ the required large intrinsic luminosity of the soft (IR--optical) component of the spine determines a large optical depth for $γ$-rays through the pair production scattering $γγ\rightarrow e^+ e^-$, implying a narrow cut--off at $\sim50$~GeV. We conclude that intermediate angles are required. In this case the low frequency and the high--energy emissions are produced by two separate regions and, in principle, a full variety of correlations is expected. The correlation observed between the optical and the $γ$-ray flux, close to linearity, is likely linked to variations of the emissivity of the spine.

astro-ph.HE↗

arXiv2014

SDSS J114657.79+403708.6: the third most distant blazar at z=5.0

The radio-loud quasar SDSS J114657.79+403708.6 at a redshift z=5.0 is one of the most distant radio-loud objects. The IR-optical luminosity and spectrum suggest that its black hole has a very large mass: M=(5+-1)x 1e9 Msun. The radio-loudness (ratio of the radio to optical flux) of the source is large (around 100), suggesting that the source is viewed at small angles from the jet axis, and could be a blazar. The X-ray observations fully confirm this hypothesis, due to the high level and hardness of the flux. This makes SDSS J114657.79+403708.6 the third most distant blazar known, after Q0906+693 (z=5.47) and B2 1023+25 (z=5.3). Among those, SDSS J114657.79+403708.6 has the largest black hole mass, setting interesting constraints on the mass function of heavy (larger than one billion solar masses) black holes at high redshifts.

astro-ph.HE↗

arXiv2013

Radio-loud AGNs at high redshifts and the cosmic microwave background

We discuss how the interaction between the electrons in a relativistic jet and the Cosmic Microwave Background (CMB) affects the observable properties of radio-loud AGN at early epochs. At high z the magnetic energy density in the radio lobes of powerful radio-loud quasars can be exceeded by the energy density of the CMB (because of its (1+z)^4 dependance). In this case, relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron. Thus, sources sharing the same intrinsic properties have different extended radio and X-ray luminosities when located at different z: more distant sources are less luminous in radio and more luminous in X-rays than their closer counterparts. Instead, in compact regions where the local magnetic field still exceeds the CMB in terms of energy density, synchrotron radiation would be unaffected by the presence of the CMB. Such regions include the compact inner jet and the so-called hot spots in the radio lobes. The decrease in radio luminosity is larger in misaligned sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux limited radio surveys, and therefore they are possibly under-represented in the associated samples. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB photons is less important at low radio frequencies. Therefore objects not detected so far in current surveys at a few GHz could be picked up by low frequency deep surveys, such as LOFAR and SKA. Until then, we can estimate the number of high redshift radio-loud AGNs through the census of their aligned proxies, i.e., blazars. Indeed, their observed radio emission arises in the inner and strongly magnetized compact core of the relativistic jet, and not affected by inverse Compton scattering off CMB photons.

astro-ph.CO↗

arXiv2013

Basic properties of Narrow-Line Seyfert 1 Galaxies with Relativistic Jets

We present the preliminary results of a survey performed with Swift to observe a sample of radio-loud Narrow-Line Seyfert 1 Galaxies (RLNLS1s). Optical-to-X-ray data from Swift are complemented with gamma-ray observations from Fermi/LAT and radio measurements available in the literature. The comparison with a sample of bright Fermi blazars indicates that RLNLS1s seem to be the low-power tail of the distribution.

astro-ph.HE↗

arXiv2013

Studying flux variability of the BL Lac object 1ES0806+524 with MAGIC in a multi-wavelength context

We present results of multi-wavelength (MWL) observations of the high-frequency-peaked BL Lacertae (HBL) object 1ES 0806+524 (z=0.138). Triggered by a high optical state, very high energy (VHE; E > 100 GeV) observations were carried out with the MAGIC stereoscopic system from January to March 2011. During the observations a relatively short VHE gamma-ray flare was detected that lasted no longer than one night. To complement the VHE observations, simultaneous MWL data were collected in high energy gamma-rays using the \textit{Fermi Large Area Telescope (HE, 300 MeV - 100 GeV), in the X-ray and UV band with the \textit{Swift} satellite, in the optical R--band through observations with the KVA telescope and in the radio band using the OVRO telescope. This constitutes the first time that such a broad band coverage has been obtained for this source. We study the source properties through the characterization of the spectral energy distribution (SED) and its evolution through two different VHE flux states. The SED can be modeled with a simple one-zone SSC model, resulting in parameters that are comparable to those obtained for other HBLs.

astro-ph.HE↗

arXiv2013

MAGIC discovery and multiwavelength observations of the BL Lac 1ES 1727+502

Blazars, active galactic nuclei whose jet axis is pointed towards the observer, constitute the most numerous class of extragalactic very high energy (VHE, E > 100\, GeV) gamma-ray emitters. The MAGIC experiment, a system of two Imaging Atmospheric Cherenkov Telescopes located in the Canary Island of La Palma (Northern hemisphere), with an energy threshold of 50 GeV, is a well suited experiment for observations of such objects. Here we present the discovery of the BL Lac 1ES 1727+502 (z = 0.055) as VHE source. This object was identified as a promising TeV candidate based on archival data and the observation that lead to this detection was not triggered by any high state alert in other wavebands. The MAGIC observations are complemented by other observations are lower frequencies: optical data from the KVA telescope, UV, optical and X-ray archival data taken with the instruments on board the Swift satellite and high energy (HE, 300 MeV < E < 100 GeV) data from the \textit{Fermi}-LAT instrument. We studied the spectral energy distribution of 1ES 1727+502 and interpreted it with a one-zone synchrotron self-Compton model with parameters that are typical for this class of sources.

astro-ph.HE↗

arXiv2013

The ASTRI Mini-Array Science Case

ASTRI is a Flagship Project financed by the Italian Ministry of Education, University and Research, and led by INAF, the Italian National Institute of Astrophysics. Within this framework, INAF is currently developing an end-to-end prototype of a Small Size Telescope in a dual-mirror configuration (SST-2M) for the Cherenkov Telescope Array (CTA), scheduled to start data acquisition in 2014. Although the ASTRI SST-2M prototype is mainly a technological demonstrator, it will perform scientific observations of the Crab Nebula, Mrk 421 and Mrk 501 at E>1 TeV. A remarkable improvement in terms of performance could come from the operation, in 2016, of a SST-2M mini-array, composed of a few SST-2M telescopes to be placed at final CTA Southern Site. The SST mini-array will be able to study in great detail relatively bright sources (a few x 10E-12 erg/cm2/s at 10 TeV) with angular resolution of a few arcmin and energy resolution of about 10-15%. Thanks to the stereo approach, it will be possible to verify the wide field of view (FoV) performance through the detections of very high-energy showers with core located at a distance up to 500 m, to compare the mini-array performance with the Monte Carlo expectations by means of deep observations of selected targets, and to perform the first CTA science at the beginning of the mini-array operations. Prominent sources such as extreme blazars, nearby well-known BL Lac objects and radio-galaxies, galactic pulsar wind nebulae, supernovae remnants, micro-quasars, and the Galactic Center can be observed in a previously unexplored energy range, in order to investigate the electron acceleration and cooling, relativistic and non relativistic shocks, the search for cosmic-ray (CR) Pevatrons, the study of the CR propagation, and the impact of the extragalactic background light on the spectra of the sources.

astro-ph.HE↗

arXiv2013

The far emission region of the gamma-ray blazar PKS B1424-418

We report multi-wavelength (IR-O, UV, X-ray, gamma-ray) data of the flat spectrum radio quasar (FSRQ) PKS B1424-418 (z=1.52) gathered during an active phase in 2013 April . As for a few other cases of FSRQs reported in literature, the detection by the Large Area Telescope onboard Fermi of gamma rays with energy above 10 GeV indicates that the emission likely occurs beyond the highly opaque (tau ~10) broad line region. This conclusion is strengthened by a model fit to the spectral energy distribution, which allows us to locate the emission region even beyond the distance generally assumed for the dusty torus. The consequent large size (~1 pc) inferred for the emission region cannot account for the observed daily-scale variability of the gamma-ray flux. We discuss the possibility that short-term variability results from fast magnetic reconnection events, as proposed in recent works.

astro-ph.HE↗

arXiv2013

Fermi LAT detection of extraordinary variability in the gamma-ray emission of the blazar PKS 1510-089

We have reanalyzed the giant outburst of the blazar PKS 1510-089 (z=0.36) that occurred on 2011 October-November. The gamma-ray flux in the 0.1-100 GeV energy range exceeded the value of 10^-5 ph/cm2/s for several days. The peak flux was reached on 2011 October 19, with a value of 4.4 x 10^-5 ph/cm2/s, which in turn corresponds to a luminosity of 2 x 10^49 erg/s. A very short timescale variability was measured. Particularly on 2011 October 18, the flux-doubling time was as short as 20 minutes. This is the shortest variability ever detected in the MeV-GeV energy band. We compared our analysis with two other outbursts observed in 2009 March and 2012 February-March, when the blazar was also detected by HESS and MAGIC to infer information about the emission at hundreds of GeV.

astro-ph.HE↗

arXiv2013

The red blazar PMN J2345-1555 becomes blue

The Flat Spectrum Radio Quasar PMN J2345-1555 is a bright gamma-ray source, that recently underwent a flaring episode in the IR, UV and gamma-ray bands. The flux changed quasi simultaneously at different frequencies, suggesting that it was produced by a single population of emitting particles, hence by a single and well localized region of the jet. While the overall Spectral Energy Distribution (SED) before the flare was typical of powerful blazars (namely two broad humps peaking in the far IR and below 100 MeV bands, respectively), during the flare the peaks moved to the optical-UV and to energies larger than 1 GeV, to resemble low power BL Lac objects, even if the observed bolometric luminosity increased by more than one order of magnitude. We interpret this behavior as due to a change of the location of the emission region in the jet, from within the broad line region, to just outside. The corresponding decrease of the radiation energy density as seen in the comoving frame of the jet allowed the relativistic electrons to be accelerated to higher energies, and thus produce a "bluer" SED.

astro-ph.HE↗

arXiv2013

VHE gamma-ray emission from the FSRQs observed by the MAGIC telescopes

Among more than fifty blazars detected in very high energy (VHE, E>100GeV) gamma-rays, only three belong to the subclass of Flat Spectrum Radio Quasars (FSRQs): PKS 1510-089, PKS 1222+216 and 3C 279. The detection of FSRQs in the VHE range is challenging, mainly because of their steep soft spectra in the GeV-TeV regime. MAGIC has observed and detected all FSRQs known to be VHE emitters up to now and found that they exhibit very different behavior. The 2010 discovery of PKS 1222+216 (z = 0.432) with the fast variability observed, challenges simple one-zone emission models and more complicated scenarios have been proposed. 3C 279 is the most distant VHE gamma-ray emitting AGN (z = 0.536), which was discovered by MAGIC in 2006 and detected again in 2007. In 2011 MAGIC observed 3C 279 two times: first during a monitoring campaign and later observations were triggered by a flare detected with Fermi-LAT. We present the MAGIC results and the multiwavelength behavior during this flaring epoch. Finally, we report the 2012 detection of PKS 1510-089 (z = 0.36), together with its simultaneous multiwavelength data from optical to gamma-rays.

astro-ph.HE↗

arXiv2012

Radio-to-gamma-ray Monitoring of the Narrow-Line Seyfert 1 Galaxy PMN J0948+0022 from 2008 to 2011

We present more than three years of observations at different frequencies, from radio to high-energy gamma-rays, of the Narrow-Line Seyfert 1 (NLS1) Galaxy PMN J0948+0022 (z=0.585). This source is the first NLS1 detected at energies above 100 MeV and therefore can be considered the prototype of this emerging new class of gamma-ray emitting active galactic nuclei (AGN). The observations performed from 2008 August 1 to 2011 December 31 confirmed that PMN J0948+0022 generates a powerful relativistic jet, able to develop an isotropic luminosity at gamma-rays of the order of 10^48 erg s^-1, at the level of powerful quasars. The evolution of the radiation emission of this source in 2009 and 2010 followed the canonical expectations of relativistic jets, with correlated multiwavelength variability (gamma-rays followed by radio emission after a few months), but it was difficult to retrieve a similar pattern in the light curves of 2011. The comparison of gamma-ray spectra before and including 2011 data suggested that there was a softening of the high-energy spectral slope. We selected five specific epochs to be studied by modelling the broad-band spectrum, characterised by an outburst at gamma-rays or very low/high flux at other wavelengths. The observed variability can largely be explained either by changes in the injected power, the bulk Lorentz factor of the jet or the electron spectrum. The characteristic time scale of doubling/halving flux ranges from a few days to a few months, depending on the frequency and the sampling rate. The shortest doubling time scale at gamma-rays is 2.3+-0.5 days. These small values underline the need of highly-sampled multiwavelength campaigns to better understand the physics of these sources.

astro-ph.HE↗

arXiv2012

Evidence for an axion-like particle from PKS 1222+216?

The surprising discovery by MAGIC of an intense, rapidly varying emission in the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216 represents a challenge for all interpretative scenarios. Indeed, in order to avoid absorption of γrays in the dense ultraviolet radiation field of the broad line region (BLR), one is forced to invoke some unconventional astrophysical picture, like for instance the existence of a very compact (r\sim 10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet base. We offer the investigation of a scenario based on the standard blazar model for PKS 1222+216 where γrays are produced close to the central engine, but we add the new assumption that inside the source photons can oscillate into axion-like particles (ALPs), which are a generic prediction of several extensions of the Standard Model of elementary particle interactions. As a result, a considerable fraction of very-high-energy photons can escape absorption from the BLR through the mechanism of photon-ALP oscillations much in the same way as they largely avoid absorption from extragalactic background light when propagating over cosmic distances in the presence of large-scale magnetic fields in the nG range. In addition we show that the above MAGIC observations and the simultaneous Fermi/LAT observations in the energy range 0.3 - 3 GeV can both be explained by a standard spectral energy distribution for experimentally allowed values of the model parameters. In particular, we need a very light ALP just like in the case of photon-ALP oscillations in cosmic space. Moreover, we find it quite tantalizing that the most favorable value of the photon-ALP coupling happens to be the same in both situations. Although our ALPs cannot contribute to the cold dark matter, they are a viable candidate for the quintessential dark energy. [abridged]

astro-ph.HE↗

arXiv2012

High redshift Fermi blazars observed by GROND and Swift

We observed 5 gamma-ray loud blazars at redshift greater than 2 with the X-Ray Telescope (XRT) and the UltraViolet and Optical Telescope (UVOT) onboard the Swift satellite, and the Gamma-Ray burst Optical Near-Infrared Detector (GROND) instrument. These observations were quasi simultaneous, usually within a few hours. For 4 of these blazars the near-IR to UV data show the presence of an accretion disc, and we could reliably estimate its accretion rate and black hole mass. One of them, PKS 1348+007, was found in an extraordinarily high IR-optical state, almost two orders of magnitude brighter than at the epoch of the Sloan Digital Sky Survey observations. For all the 5 quasars the physical parameters of the jet emitting zone, derived by applying a one-zone emission model, are similar to that found for the bulk of other gamma-ray loud quasars. With our observations we have X-ray data for the full sample of blazars at z>2 present in the Fermi 2-yrs (2LAC) catalog. This allows to have a rather complete view of the spectral energy distribution of all high-redshift Fermi blazars, and to draw some conclusions about their properties, and especially about the relation between the accretion rate and the jet power.

astro-ph.CO↗

arXiv2012

"Flat" broad line region and gamma-ray absorption in blazars

We study the impact of the geometry of the broad line region (BLR) on the expected absorption, through the γγ\rightarrow e^{\pm} process, of gamma rays produced in the relativistic jet of flat spectrum radio quasars (FSRQ). We consider "flat" (or "disky") BLR models, and use BLR spectra calculated with the photoionization code CLOUDY, already used to investigate the emission and the absorption of high-energy photons in FSRQ. We characterize the energy-dependent optical depth of the process, τ(E), for different accretion disk luminosities, aperture angles of the BLR (α, as measured from the equatorial plane), and initial injection eighths of the high-energy photons, $R_{\rm o}$. We study in particular how the change of these parameters influences the spectral break at GeV energies, predicted if the emission occurs within the BLR. We found a well defined relation between the break energy and the post-break slope, both uniquely determined by α. We finally find that even a rather disk-like BLR (α\sim 25^\circ) corresponds to important absorption (τ>1) of photons above few tens of GeV produced within the BLR. We therefore conclude that the VHE emission detected from FSRQs occurs beyond the BLR.

astro-ph.HE↗