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F. Tavecchio

Publications and source records attributed to F. Tavecchio.

At least 73 records · Page 4Linked to original sources
arXiv2017

On the detectability of BL Lac objects by IceCube

Since 2010 IceCube observed around 50 high-energy neutrino events of cosmic origin above 60 TeV, but the astrophysical sources of these events are still unknown. We recently proposed high-energy emitting BL Lac (HBL) objects as candidate emitters of high-energy neutrinos. Assuming a direct proportionality between high-energy gamma-ray and very-high energy neutrino fluxes, we calculated the expected neutrino event number in a year for IceCube and the presently under construction Km3NeT. To give a value of the significance of a detection we considered also the background for the single sources. To this aim we derived the through-going muon rate, generated by muon neutrino including the effect of Earth absorption, the density of the Earth and the cross section $νN$. Applying this calculation both to HBL sources and the atmospherical neutrino background, we can calculate the expected significance of the detection by IceCube, showing that our scenario is compatible with a no detection of HBL.

astro-ph.HE↗
arXiv2017

The Fermi blazar sequence

We revisit the blazar sequence exploiting the complete, flux limited sample of blazars with known redshift detected by the Fermi satellite after 4 years of operations (the 3LAC sample). We divide the sources into gamma-ray luminosity bins, collect all the archival data for all blazars, and construct their spectral energy distribution (SED). We describe the average SED of blazars in the same luminosity bin through a very simple, completely phenomenological function consisting of two broken power laws connecting with a power law of fixed slope describing the radio emission. We do that separately for BL Lacs and for flat spectrum radio quasars (FSRQs) and also for all blazars together. The main results are: i) FSRQs display approximately the same SED as the luminosity increases, except for the fact that the relative importance of the high energy peak increases; ii) as a consequence, X-ray spectra of FSRQs become harder for larger luminosities; iii) BL Lacs form indeed a sequence: they become redder (i.e. the peak frequencies becomes smaller) for increasing luminosities, with a steeper gamma-ray slope and a larger dominance of the high energy peak; iv) for all blazars (BL Lacs+FSRQs) these properties becomes more prominent, as the highest luminosity bin is populated mostly by FSRQs and the lowest luminosity bin mostly by BL Lacs. This agrees with the original blazar sequence, although BL Lacs never have an average gamma-ray slope as hard as found in the original sequence. v) At high luminosities, a large fraction of FSRQs shows signs of thermal emission from the accretion disc, contributing in the optical-UV.

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arXiv2016

Gamma Rays From Blazars

Blazars are high-energy engines providing us natural laboratories to study particle acceleration, relativistic plasma processes, magnetic field dynamics, black hole physics. Key informations are provided by observations at high-energy (in particular by Fermi/LAT) and very-high energy (by Cherenkov telescopes). I give a short account of the current status of the field, with particular emphasis on the theoretical challenges connected to the observed ultra-fast variability events and to the emission of flat spectrum radio quasars in the very high energy band.

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arXiv2016

Latest MAGIC discoveries pushing redshift boundaries in VHE Astrophysics

The search for detection of gamma-rays from distant AGNs by Imaging Atmospheric Cherenkov Telescopes (IACTs) is challenging at high redshifts, not only because of lower flux due to the distance of the source, but also due to the consequent absorption of gamma-rays by the extragalactic background light (EBL). Before the MAGIC discoveries reported in this work, the farthest source ever detected in the VHE domain was the blazar PKS 1424+240, at z>0.6. MAGIC, a system of two 17 m of diameter IACTs located in the Canary island of La Palma, has been able to go beyond that limit and push the boundaries for VHE detection to redshifts z~ 1. The two sources detected and analyzed, the blazar QSO B0218+357 and the FSRQ PKS 1441+25 are located at redshift z=0.944 and z=0.939 respectively. QSO B0218+357 is also the first gravitational lensed blazar ever detected in VHE. The activity, triggered by Fermi-LAT in high energy gamma-rays, was followed up by other instruments, such as the KVA telescope in the optical band and the Swift-XRT in X-rays. In the present work we show results on MAGIC analysis on QSO B0218+357 and PKS 1441+25 together with multiwavelength lightcurves. The collected dataset allowed us to test for the first time the present generation of EBL models at such distances.

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arXiv2015

Extreme BL Lacs: probes for cosmology and UHECR candidates

High-energy observations of extreme BL Lac objects, such as 1ES0229+200 or 1ES 0347-121, recently focused interest both for blazar and jet physics and for the implication on the extragalactic background light and intergalactic magnetic field estimate. Moreover, their enigmatic properties have been interpreted in a scenario in which their primary high- energy output is through a beam of high-energy hadrons. However, despite their possible important role in all these topics, the number of these extreme highly peaked BL Lac objects (EHBL) is still rather small. Aiming at increase their number, we selected a group of EHBL candidates considering those undetected (or only barely detected) by the LAT onboard Fermi and characterized by a high X-ray versus radio flux ratio. We assembled the multi-wavelength spectral energy distribution of the resulting 9 sources, using available archival data of Swift, GALEX, and Fermi satellites, confirming their nature. Through a simple one-zone synchrotron self-Compton model we estimate the expected very high energy flux, finding that in the majority of cases it is within the reach of present generation of Cherenkov arrays or of the forthcoming CTA.

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arXiv2015

Optical spectroscopy of SDSS J004054.65-0915268: three possible scenarios for the classification. A z \sim 5 BL Lacertae, blue FSRQ or a weak emission line quasar

The haunt of high redshift BL Lacerate objects is day by day more compelling, to firmly understand their intrinsic nature and evolution. SDSS J004054.65-0915268 is, at the moment, one of the most distant BL Lac candidate at z \sim 5 (Plotkin et al 2010). We present a new optical-near IR spectrum obtained with ALFOSC-NOT with a new, custom designed dispersive grating aimed to detect broad emission lines that could disprove this classification. In the obtained spectra we do not detect any emission features and we provide an upper limit to the luminosity of the C IV broad emission line. Therefore, the nature of the object is then discussed, building the overall spectral energy distribution and fitting it with three different models. Our fits, based on the SED modeling with different possible scenarios, cannot rule out the possibility that this source is indeed a BL Lac object although, the absence of optical variability and lack of strong radio flux, they seems to suggest that the observed optical emission originate from a thermalized accretion disk.

astro-ph.HE↗
arXiv2015

On the magnetization of BL Lac jets

The current paradigm foresees that relativistic jets are launched as magnetically dominated flows, whose magnetic power is progressively converted to kinetic power of of the matter of the jet, until equipartition is reached. Therefore, at the end of the acceleration phase, the jet should still carry a substantial fraction ($\approx$ half) of its power in the form of a Poynting flux. It has been also argued that, in these conditions, the best candidate particle acceleration mechanism is efficient reconnection of magnetic field lines, for which it is predicted that magnetic field and accelerated relativistic electron energy densities are in equipartition.Through the modeling of the jet non--thermal emission, we explore if equipartition is indeed possible in BL Lac objects, i.e. low-power blazars with weak or absent broad emission lines. We find that one-zone models (for which only one region is involved in the production of the radiation we observe) the particle energy density is largely dominating (by 1-2 orders of magnitude) over the magnetic one. As a consequence, the jet kinetic power largely exceeds the magnetic power. Instead, if the jet is structured (i.e. made by a fast spine surrounded by a slower layer), the amplification of the IC emission due to the radiative interplay between the two components allows us to reproduce the emission in equipartition conditions.

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arXiv2015

On the detectability of Lorentz invariance violation through anomalous multi-TeV $γ$-ray spectra of blazars

Cosmic opacity for very high-energy gamma rays ($E>10$ TeV) due to the interaction with the extragalactic background light can be strongly reduced because of possible Lorentz-violating terms in the particle dispersion relations expected, e.g., in several versions of quantum gravity theories. We discuss the possibility to use very high energy observations of blazars to detect anomalies of the cosmic opacity induced by LIV, considering in particular the possibility to use -- besides the bright and close-by BL Lac Mkn 501 -- extreme BL Lac objects. We derive the modified expression for the optical depth of $γ$ rays considering also the redshift dependence and we apply it to derive the expected high-energy spectrum above 10 TeV of Mkn 501 in high and low state and the extreme BL Lac 1ES 0229+200. We find that, besides the nearby and well studied BL Lac Mkn 501 -- especially in high state --, suitable targets are extreme BL Lac objects, characterized by quite hard TeV intrinsic spectra likely extending at the energies relevant to detect LIV features.

astro-ph.HE↗
arXiv2015

CMB quenching of high-redshift radio-loud AGNs

The very existence of more than a dozen of high-redshift (z>4) blazars indicates that a much larger population of misaligned powerful jetted AGN was already in place when the Universe was <1.5 Gyr old. Such parent population proved to be very elusive, and escaped direct detection in radio surveys so far. High redshift blazars themselves seem to be failing in producing extended radio-lobes, raising questions about the connection between such class and the vaster population of radio-galaxies. We show that the interaction of the jet electrons with the intense cosmic microwave background (CMB) radiation explains the lack of extended radio emission in high redshift blazars and in their parent population, helping to explain the apparently missing misaligned counterparts of high redshift blazars. On the other hand, the emission from the more compact and more magnetised hot spots are less affected by the enhanced CMB energy density. By modelling the spectral energy distribution of blazar lobes and hot spots we find that most of them should be detectable by low frequency deep radio observations, e.g., by LOw-Frequency ARray for radio astronomy (LOFAR) and by relatively deep X-ray observations with good angular resolution, e.g., by the Chandra satellite. At high redshifts, the emission of a misaligned relativistic jet, being de-beamed, is missed by current large sky area surveys. The isotropic flux produced in the hot spots can be below ~1 mJy and the isotropic lobe radio emission is quenched by the CMB cooling. Consequently, even sources with very powerful jets can go undetected in current radio surveys, and misclassified as radio-quiet AGNs.

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arXiv2015

Unveiling the nature of the gamma-ray emitting AGN PKS 0521-36

PKS 0521-36 is an Active Galactic Nucleus (AGN) with uncertain classification. We investigate the properties of this source from radio to gamma rays. The broad emission lines in the optical and UV bands and steep radio spectrum indicate a possible classification as an intermediate object between broad-line radio galaxies (BLRG) and steep spectrum radio quasars (SSRQ). On pc-scales PKS 0521-36 shows a knotty structure similar to misaligned AGN. The core dominance and the gamma-ray properties are similar to those estimated for other SSRQ and BLRG detected in gamma rays, suggesting an intermediate viewing angle with respect to the observer. In this context the flaring activity detected from this source by Fermi-LAT between 2010 June and 2012 February is very intriguing. We discuss the gamma-ray emission of this source in the framework of the structured jet scenario, comparing the spectral energy distribution (SED) of the flaring state in 2010 June with that of a low state. We present three alternative models corresponding to three different choices of the viewing angles theta_v = 6, 15, and 20 deg. We obtain a good fit for the the first two cases, but the SED obtained with theta_v = 15 deg if observed at a small angle does not resemble that of a typical blazar since the synchrotron emission should dominate by a large factor (about 100) the inverse Compton component. This suggests that a viewing angle between 6 and 15 deg is preferred, with the rapid variability observed during gamma-ray flares favouring a smaller angle. However, we cannot rule out that PKS 0521-36 is the misaligned counterpart of a synchrotron-dominated blazar.

astro-ph.HE↗
arXiv2015

Short Timescale Photometric and Polarimetric Behavior of two BL Lacertae Type Objects

Blazars are astrophysical sources whose emission is dominated by non-thermal processes, typically interpreted as synchrotron and inverse Compton emission. Although the general picture is rather robust and consistent with observations, many aspects are still unexplored. Polarimetric monitoring can offer a wealth of information about the physical processes in blazars. Models with largely different physical ingredients can often provide almost indistinguishable predictions for the total flux, but usually are characterized by markedly different polarization properties. We explore, with a pilot study, the possibility to derive structural information about the emitting regions of blazars by means of a joint analysis of rapid variability of the total and polarized flux at optical wavelengths. Short timescale (from tens of seconds to a couple of minutes) optical linear polarimetry and photometry for two blazars, BL Lacertae and PKS 1424+240, was carried out with the PAOLO polarimeter at the 3.6m Telescopio Nazionale Galileo. Several hours of almost continuous observations were obtained for both sources. Our intense monitoring allowed us to draw strongly different scenarios for BL Lacertae and PKS 1424+240, with the former characterized by intense variability on time-scales from hours to a few minutes and the latter practically constant in total flux. Essentially the same behavior is observed for the polarized flux and the position angle. The variability time-scales turned out to be as short as a few minutes, although involving only a few percent variation of the flux. The polarization variability time-scale is generally consistent with the total flux variability. Total and polarized flux appear to be essentially uncorrelated. However, even during our relatively short monitoring, different regimes can be singled out. (abridged)

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arXiv2015

No axions from the Sun

Preliminary evidence of solar axions in XMM-Newton observations has quite recently been claimed by Fraser et al. as an interpretation of their detection of a seasonally-modulated excess of the X-ray background. Within such an interpretation, these authors also estimate the axion mass to be $m_a \simeq 2.3 \cdot 10^{- 6}$ eV. Since an axion with this mass behaves as a cold dark matter particle, according to the proposed interpretation the considered detection directly concerns cold dark matter as well. So, the suggested interpretation would lead to a revolutionary discovery if confirmed. Unfortunately, we have identified three distinct problems in this interpretation of the observed result of Fraser et al. which ultimately imply that the detected signal - while extremely interesting in itself - cannot have any relation with hypothetical axions produced by the Sun. Thus, a physically consistent interpretation of the observed seasonally-modulated X-ray excess still remains an exciting challenge.

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arXiv2015

Blazar candidates beyond redshift 4 observed by Swift

We have selected SDSS J222032.50+002537.5 and SDSS J142048.01+120545.9 as best blazar candidates out of a complete sample of extremely radio-loud quasars at z>4, with highly massive black holes. We observed them and a third serendipitous candidate with similar features (PMN J2134-0419) in the X-rays with the Swift/XRT telescope, to confirm their blazar nature. We observed strong and hard X-ray fluxes (i.e. $α_X<0.6$, where $F(ν)\proptoν^{-α_X}$ in the 0.3-10keV observed energy range, ~1-40keV rest frame) in all three cases. This allowed us to classify our candidates as real blazars, being characterized by large Lorentz factors (~13) and very small viewing angles (~3deg). All three sources have black hole masses exceeding 10^9Msun and their classification provides intriguing constraints on supermassive black hole formation and evolution models. We confirm our earlier suggestion that there are different formation epochs of extremely massive black holes hosted in jetted (z~4) and non-jetted systems (z~2.5).

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arXiv2015

Fermi/LAT broad emission line blazars

We study the broad emission line blazars detected in the gamma-ray band by the Large Area Telescope onboard the Fermi satellite and with the optical spectrum studied by Shaw et al. (2012, 2013). The observed broad line strength provides a measure of the ionizing luminosity of the accretion disk, while the gamma-luminosity is a proxy for the bolometric non-thermal beamed jet emission. The resulting sample, composed by 217 blazars, is the best suited to study the connection between accretion and jet properties. We compare the broad emission line properties of these blazars with those of radio-quiet and radio-loud quasars present in the Sloan Digital Sky Survey, to asses differences and similarities of the disc luminosity and the virial black hole mass. For most sources, we could derive the black hole mass by reproducing the IR-optical-UV data with a standard accretion disc spectrum, and we compared the black hole masses derived with the two methods. The distributions of the masses estimated in the two ways agree satisfactorily. We then apply a simple, one-zone, leptonic model to all the 217 objects of our sample. The knowledge of the black hole mass and disc luminosity helps to constrain the jet parameters. On average they are similar to what found by previous studies of smaller samples of sources.

astro-ph.HE↗
arXiv2014

The power of relativistic jets is larger than the luminosity of their accretion disks

Theoretical models for the production of relativistic jets from active galactic nuclei predict that jet power arises from the spin and mass of the central black hole, as well as the magnetic field near the event horizon. The physical mechanism mechanism underlying the contribution from the magnetic field is the torque exerted on the rotating black hole by the field amplified by the accreting material. If the squared magnetic field is proportional to the accretion rate, then there will be a correlation between jet power and accretion luminosity. There is evidence for such a correlation, but inadequate knowledge of the accretion luminosity of the limited and inhomogeneous used samples prevented a firm conclusion. Here we report an analysis of archival observations of a sample of blazars (quasars whose jets point towards Earth) that overcomes previous limitations. We find a clear correlation between jet power as measured through the gamma-ray luminosity, and accretion luminosity as measured by the broad emission lines, with the jet power dominating over the disk luminosity, in agreement with numerical simulations. This implies that the magnetic field threading the black hole horizon reaches the maximum value sustainable by the accreting matter.

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arXiv2014

High-energy cosmic neutrinos from spine-sheath BL Lac jets

We recently proposed that structured (spine-sheath) jets associated to BL Lac objects offer a suitable environment for the production of the extragalactic high-energy ($E>100$ TeV) neutrino recently revealed by IceCube. Our previous analysis was limited to low-power BL Lac objects. We extend our preliminary study to the entire BL Lac population. We assume that the power of cosmic rays as well as the radiative luminosity of the sheath depend linearly on the the jet power. In turn, we assume that the latter is well traced by the $γ$-ray luminosity. We exploit the BL Lac $γ$-ray luminosity function and its cosmic evolution as recently inferred from Fermi-LAT data to derive the expected neutrino cumulative intensity from the entire BL Lac population. When considering only the low-power BL Lacs, a large cosmic ray power for each source is required to account for the neutrino flux. Instead, if BL Lacs of all powers produce neutrinos, the power demand decreases, and the required cosmic ray power becomes of the same order of the radiative jet power. We also discuss the prospects for the direct association of IceCube events with BL Lacs, providing an estimate of the expected counts for the most promising sources.

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arXiv2014

An active state of the BL Lac Object Markarian 421 detected by INTEGRAL in April 2013

Multiwavelength variability of blazars offers indirect insight into their powerful engines and on the mechanisms through which energy is propagated from the centre down the jet. The BL Lac object Mkn 421 is a TeV emitter, a bright blazar at all wavelengths, and therefore an excellent target for variability studies. Mkn 421 was observed by INTEGRAL and Fermi-LAT in an active state on 16-21 April 2013. Well sampled optical, soft, and hard X-ray light curves show the presence of two flares. The average flux in the 20-100 keV range is 9.1e-11 erg/s/cm2 (~4.5 mCrab) and the nuclear average apparent magnitude, corrected for Galactic extinction, is V ~12.2. In the time-resolved X-ray spectra (3.5-60 keV), which are described by broken power laws and, marginally better, by log-parabolic laws, we see a hardening that correlates with flux increase, as expected in refreshed energy injections in a population of electrons that later cool via synchrotron radiation. The hardness ratios between the JEM-X fluxes in two different bands and between the JEM-X and IBIS/ISGRI fluxes confirm this trend. During the observation, the variability level increases monotonically from the optical to the hard X-rays, while the large LAT errors do not allow a significant assessment of the MeV-GeV variability. The cross-correlation analysis during the onset of the most prominent flare suggests a monotonically increasing delay of the lower frequency emission with respect to that at higher frequency, with a maximum time-lag of about 70 minutes, that is however not well constrained. The spectral energy distributions from the optical to the TeV domain are satisfactorily described by homogeneous models of blazar emission based on synchrotron radiation and synchrotron self-Compton scattering, except in the state corresponding to the LAT softest spectrum and highest flux.

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