arXiv subjects

A. Paggi

Publications and source records attributed to A. Paggi.

85 records · Page 5Linked to original sources

arXiv2013

Unveiling the nature of the unidentified gamma-ray sources

One of the main scientific objectives of the recent Fermi mission is investigating the origin of the unidentified gamma-ray sources (UGSs). Despite the large improvements of Fermi in the gamma-ray source localization with respect to the past gamma-ray missions, about 1/3 of the gamma-ray objects detected do not have yet an assigned counterpart a low energies. We recently developed a new association method to identify if there is a gamma-ray blazar candidate within the positional uncertainty region of a generic gamma-ray source. This method is entirely based on the discovery that blazars, the largest known class of gamma-ray sources, can be recognized and separated from other extragalactic sources on the basis of their infrared colors. Here we summarize the results obtained by applying our association procedure to the unidentified gamma-ray sources (UGSs) and to the active galaxies of uncertain type (AGUs) listed in the Second Fermi Large Area Telescope (LAT) catalog.

astro-ph.HE↗

arXiv2013

Unveiling the nature of the unidentified gamma-ray sources I: a new method for the association of gamma-ray blazars

We present a new method for identifying blazar candidates by examining the locus, i.e. the region occupied by the Fermi gamma-ray blazars in the three-dimensional color space defined by the WISE infrared colors. This method is a refinement of our previous approach that made use of the two-dimensional projection of the distribution of WISE gamma-ray emitting blazars (the Strip) in the three WISE color-color planes (Massaro et al. 2012a). In this paper, we define the three-dimensional locus by means of a Principal Component (PCs) analysis of the colors distribution of a large sample of blazars composed by all the ROMA-BZCAT sources with counterparts in the WISE All-Sky Catalog and associated to gamma-ray source in the second Fermi LAT catalog (the WISE Fermi Blazars sample, WFB). Our new procedure yields a total completeness of c~81% and total efficiency of e~97%. We also obtain local estimates of the efficiency and completeness as functions of the WISE colors and galactic coordinates of the candidate blazars. The catalog of all WISE candidate blazars associated to the WFB sample is also presented, complemented by archival multi-frequency information for the alternative associations. Finally, we apply the new association procedure to all gamma-ray blazars in the 2FGL and provide a catalog containing all the gamma-ray candidates blazars selected according to our procedure.

astro-ph.HE↗

arXiv2013

A new method for the extraction of mid-infrared gamma-ray emitting candidate blazars

astro-ph.HE↗

arXiv2012

A new flaring high energy gamma-ray source

We report the detection of a new gamma-ray source in the Fermi-LAT sky using a source detection tool based on the Minimal Spanning Tree algorithm. The source, not reported in previous LAT catalogues but very recently observed in the X-rays and optical bands, is characterized by an increasing gamma-ray activity in 2012 June-September, reaching a weekly peak flux of (3.3+-0.6)*10^-7 photons cm^-2 s^-1. A search for a possible counterpart provides indication that it can be associated with the radio source NVSS J141828+354250 whose optical SDSS colours are typical of a blazar.

astro-ph.HE↗

arXiv2012

Searching for gamma-ray blazar candidates among the unidentified INTEGRAL sources

The identification of low-energy counterparts for gamma-ray sources is one of the biggest challenge in modern gamma-ray astronomy. Recently, we developed and successfully applied a new association method to recognize gamma-ray blazar candidates that could be possible counterparts for the unidentified gamma-ray sources above 100 MeV in the second Fermi Large Area Telescope (LAT) catalog (2FGL). This method is based on the Infrared (IR) colors of the recent Wide-Field Infrared Survey Explorer (WISE) all-sky survey. In this letter we applied our new association method to the case of unidentified INTEGRAL sources (UISs) listed in the fourth soft gamma-ray source catalog (4IC). Only 86 UISs out of the 113 can be analyzed, due to the sky coverage of the WISE Preliminary data release. Among these 86 UISs, we found that 18 appear to have a gamma-ray blazar candidate within their positional error region. Finally, we analyzed the SWIFT archival data available for 10 out these 18 gamma-ray blazar candidates, and we found that 7 out of 10 are clearly detected in soft X-rays and/or in the optical-ultraviolet band. We cannot confirm the associations between the UISs and the selected gamma-ray blazar candidates due to the discrepancies between the INTEGRAL and the soft X-ray spectra. However, the discovery of the soft X-ray counterparts for the selected gamma-ray blazar candidates adds an important clue to help understand their origin and to confirm their blazar nature.

astro-ph.HE↗

arXiv2012

X-ray and TeV emissions from High Frequency Peaked BL Lacs

The majority of the extragalactic sources yet detected at TeV photon energies belong to the class of "high frequency peaked BL Lacs" (HBLs) that exhibit a spectral energy distribution with a lower peak in the X-ray band. Such spectra are well described in terms of a log-parabolic shape with a considerable curvature, and widely interpreted as synchrotron emission from ultrarelativistic electrons outflowing in a relativistic jet; these are expected to radiate also in gamma-rays by the inverse Compton process. Recently we have compared the X-ray spectral parameter distributions of TeV detected HBLs (TBLs) with those undetected (UBLs), and found that the distributions of the peak energies E_p are similarly symmetric around a value of a few keVs for both subclasses, while the X-ray spectra are broader for TBLs than for UBLs. Here we propose an acceleration scenario to interpret both the E_p and the spectral curvature distributions in terms of a coherent and a stochastic acceleration mechanisms, respectively. We show how the curvature parameter b< 0.3 - 0.7 of the synchrotron X rays, that depends only on the latter acceleration component, can be related to the inverse Compton luminosity in gamma-rays, so introducing a link between the X-ray and the TeV observations of HBLs.

astro-ph.HE↗

arXiv2012

X-ray spectral curvature of High Frequency Peaked BL Lacs: a predictor for the TeV flux

Most of the extragalactic sources detected at TeV energies are BL Lac objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs) exhibiting spectral energy distributions with a lower energy peak in the X-ray band; this is widely interpreted as synchrotron emission from relativistic electrons. The X-ray spectra are generally curved, and well described in terms of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we found two correlations between their spectral parameters. (1) The synchrotron peak luminosity L_p increases with its peak energy E_p; (2) the curvature parameter b decreases as E_p increases. The first is consistent with the synchrotron scenario, while the second is expected from statistical/stochastic acceleration mechanisms for the emitting electrons. Here we present an extensive X-ray analysis of a sample of HBLs observed with XMM-Newton and SWIFT but undetected at TeV energies (UBLs), to compare their spectral behavior with that of TBLs. Investigating the distributions of their spectral parameters and comparing the TBL X-ray spectra with that of UBLs, we develop a criterion to select the best HBLs candidates for future TeV observations.

astro-ph.HE↗

arXiv2010

Gamma Ray Bursts in the Fermi era: the spectral energy distribution of the prompt emission

Gamma Ray Bursts (GRBs) show evidence of different light curves, duration, afterglows, host galaxies and they explode within a wide redshift range. However, their spectral energy distributions (SEDs) appear to be very similar showing a curved shape. Band et al. (1993) proposed a phenomenological description of the integrated spectral shape for the GRB prompt emission, the so called Band function. In this letter we suggest an alternative scenario to explain the curved shape of GRB SEDs: the log-parabolic model. In comparison with the Band spectral shape our model is statistically favored because it fits the GRB spectra with one parameter less than the Band function and it is motivated by a theoretical acceleration scenario. The new Fermi observations of GRBs will be crucial to disentangle between these two models.

astro-ph.HE↗

arXiv2009

Power for dry BL Lacertae objects

Is it significant that the intrinsic outputs of several BL Lacs are observed to level off at values of about 10^46 erg s^-1? In searching for an answer, we compare gamma-ray observations by the AGILE satellite of the BL Lac S5 0716+714 with those of Mrk 421 and Mrk 501; the former are particularly marked by intense flares up to fluxes of 2 x 10^-6 photons cm^-2 s^-1 in the 0.1-10 GeV energy range. These "dry" BL Lacs show evidence of neither thermal disk emissions nor emission lines signaling any accreting or surrounding gas; the spectral distributions of their pure non-thermal radiations are effectively represented by the synchrotron self-Compton process. With source parameters correspondingly derived and tuned with simultaneous multiwavelength observations, we find for S5 0716+714 a total jet power of about 3 x 10^45 erg s^-1, which makes it one of the brightest dry BL Lacs so far detected in gamma rays. We evaluate the mass of the associated Kerr hole to be around 5 x 10^8 M_sun, implying that the source is significantly gauged in terms of the maximal power around 4 x 10^45 erg s^-1 extractable via the Blandford-Znajek electrodynamical mechanism; other dry BL Lacs observed in gamma rays remain well below that threshold. These findings and those forthcoming from Fermi-LAT will provide a powerful test of electrodynamics in the surroundings of the hole, that are dominated by GR effects.

astro-ph.CO↗

arXiv2009

1st Roman Young Researchers Meeting Proceedings

During the last few decades scientists have been able to test the bases of the physics paradigms, where the quantum mechanics has to match the cosmological scales. Between the extremes of this scenario, biological phenomena and their complexity take place, challenging the laws we observe in the atomic and sub-atomic world. In order to explore the details of this world, new huge experimental facilities are under construction. These projects involve people coming from several countries and give physicists the opportunity to work together with chemists, biologists and other scientists. The Roman Young Researchers Meeting is a conference, organised by Ph. D. students and young postdocs connected to the Roman area. It is aimed primarily at graduate students and post-docs, working in physics. The 1st conference has been held on the 21st of July 2009 at the University of Roma Tor Vergata. It was organised in three sessions, devoted to Astrophysics and Cosmology, Soft and Condensed Matter Physics and Theoretical and Particle Physics. In this proceeding we collect the contributions which have been presented and discussed during the meeting, according to the specific topics treated.

physics.gen-ph↗

arXiv2009

Powerful high energy emission of the remarkable BL Lac object S5 0716+714

BL Lac objects of the intermediate subclass (IBLs) are known to emit a substantial fraction of their power in the energy range 0.1--10 GeV. Detecting gamma-ray emission from such sources provides therefore a direct probe of the emission mechanisms and of the underlying powerhouse. The AGILE gamma-ray satellite detected the remarkable IBL S5 0716+714 (z \simeq 0.3) during a high state in the period from 2007 September - October, marked by two very intense flares reaching peak fluxes of 200\times10^{-8} ph / cm^2 s above 100 MeV, with simultaneous optical and X-ray observations. We present here a theoretical model for the two major flares and discuss the overall energetics of the source. We conclude that 0716+714 is among the brightest BL Lac's ever detected at gamma-ray energies. Because of its high power and lack of signs for ongoing accretion or surrounding gas, the source is an ideal candidate to test the maximal power extractable from a rotating supermassive black hole via the pure Blandford-Znajek (BZ) mechanism. We find that during the 2007 gamma-ray flares our source approached or just exceeded the upper limit set by BZ for a black hole of mass 10^9 M_sun

astro-ph.HE↗

arXiv2009

Gamma Rays from Annihilations at the Galactic Center in a Physical Dark Matter Distribution

We discuss the gamma-ray signal to be expected from dark matter (DM)annihilations at the Galactic Center. To describe the DM distribution in the Galactic halo we base on the Jeans equation for self-gravitating, anisotropic equilibria. In solving the Jeans equation, we adopt the specific correlation between the density ρ(r) and the velocity dispersion σ^2_r(r) expressed by the powerlaw behavior of the DM `entropy' K= σ_r^2/ρ^{2/3} ~ r^αwith α~ 1.25-1.3. Indicated (among others) by several recent N-body simulations, this correlation is privileged by the form of the radial pressure term in the Jeans equation, and yields a main body profile consistent with the classic self-similar development of DM halos. In addition, we require the Jeans solutions to satisfy regular boundary conditions both at the center (finite pressure, round gravitational potential) and in the outskirts (finite overall mass). With these building blocks we derive physical solutions, dubbed `α-profiles'. We find the one with α=1.25, suitable for the Galaxy halo, to be intrinsically flatter at the center relative to the empirical NFW formula, yet steeper than the empirical Einasto profile. So on scales of 10^{-1} deg it yields annihilation fluxes lower by a factor 5 than the former yet higher by a factor 10 than the latter; such fluxes will eventually fall within the reach of the Fermi satellite. We show the effectiveness of the α-profile in relieving the astrophysical uncertainties related to the macroscopic DM distribution, and discuss its expected performance as a tool instrumental to interpret the upcoming gamma-ray data in terms of DM annihilation.

astro-ph.HE↗

arXiv2009

SSC radiation in BL Lac sources, the end of the tether

The synchrotron-self Compton (SSC) radiation process is widely held to provide a close representation of the double peaked spectral energy distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal beamed radiations, highly variable on timescales of days or less. Their outbursts in the gamma ray relative to the optical/X rays might be surmised to be enhanced in BL Lacs as these photons are upscattered via the inverse Compton (IC) process. From the observed correlations among the spectral parameters during optical/X-ray variations we aim at predicting corresponding correlations in the gamma-ray band, and the actual relations between the gamma-ray and the X-ray variability consistent with the SSC emission process. We start from the homogeneous single-zone SSC source model, with log-parabolic energies distributions of emitting electron as required by the X-ray data of many sources. We find relations among spectral parameters of the IC radiation in both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly for High energy BL Lacs) and we compute how variability is driven by a smooth increase of key source parameters, primarily the root mean square electron energy. The single component SSC source model in the Thomson regime turns out to be adequate for many LBL sources. However, the simple model meets its limits with the fast/strong flares recently reported for a few sources in the TeV range; these require sudden accelerations of emitting electrons in a second source component.

astro-ph.CO↗