Engineering topics

Paggi, A

Publications and source records attributed to Paggi, A.

7 records

DOE OSTI2013

UNVEILING THE NATURE OF UNIDENTIFIED GAMMA-RAY SOURCES. II. RADIO, INFRARED, AND OPTICAL COUNTERPARTS OF THE GAMMA-RAY BLAZAR CANDIDATES

A significant fraction ({approx}30%) of the high-energy gamma-ray sources listed in the second Fermi Large Area Telescope catalog (2FGL) are still of unknown origin, being not yet associated with counterparts at 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 2FGL source. This method is entirely based on the discovery that blazars have distinct infrared colors with respect to other extragalactic sources found, thanks to the Wide-field Infrared Survey Explorer (WISE) all-sky observations. Several improvements have also been performed to increase the efficiency of our method in recognizing {gamma}-ray blazar candidates. In this paper we applied our method to two different samples, the first constituted by unidentified {gamma}-ray sources (UGSs), and the second by active galaxies of uncertain type, both listed in the 2FGL. We present a catalog of IR counterparts for {approx}20% of the UGSs investigated. Then, we also compare our results for the associated sources with those present in the literature. In addition, we illustrate the extensive archival research carried out to identify the radio, infrared, optical, and X-ray counterparts of the WISE-selected, {gamma}-ray blazar candidates. Finally, we discuss the future developments of our method based on ground-based follow-up observations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

ON THE NATURE OF THE GAMMA-RAY SOURCE 2FGL J1823.8+4312: THE DISCOVERY OF A NEW CLASS OF EXTRAGALACTIC X-RAY SOURCES

One of the unsolved mysteries of gamma-ray astronomy concerns the nature of unidentified gamma-ray sources. Recently, using the Second Fermi LAT source catalog (2FGL) and the Wide-field Infrared Survey Explorer (WISE) archive, we discovered that the WISE counterparts of gamma-ray blazars, a class of active galactic nuclei, delineate a region (the WISE Gamma-ray Strip) in three-dimensional infrared color space well separated from the locus of other astronomical objects. Based on this result, we built an association procedure to recognize if there are WISE blazar candidates within the positional uncertainty region of the unidentified gamma-ray sources. Here we report on our analysis of 2FGL J1823.8+4312, a gamma-ray active galactic nucleus of uncertain type associated with the X-ray source 1RXS J182418.7+430954 according to the 2FGL, to verify whether it is a blazar. Applying our association method we found two sources with IR colors typical of gamma-ray blazars, located within the 99.9% confidence region of 2FGL J1823.8+4312: WISE J182352.33+431452.5 and WISE J182409.25+431404.7. We then searched in the Chandra, NVSS, and SDSS archival observations for their counterparts. We discovered that WISE J182352.33+431452.5, our preferred gamma-ray blazar candidate according to our WISE association procedure, is detected in the optical and in the X-rays but not in the radio, making it extremely unusual if it is a blazar. Given its enigmatic spectral energy distribution, we considered the possibility that it is a 'radio-faint blazar' or the prototype of a new class of extragalactic sources; our conclusion is independent of whether WISE J182352.33+431452.5 is the actual counterpart of 2FGL J1823.8+4312.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

UNIDENTIFIED {gamma}-RAY SOURCES: HUNTING {gamma}-RAY BLAZARS

One of the main scientific objectives of the ongoing Fermi mission is unveiling the nature of unidentified {gamma}-ray sources (UGSs). Despite the major improvements of Fermi in the localization of {gamma}-ray sources with respect to the past {gamma}-ray missions, about one-third of the Fermi-detected objects are still not associated with low-energy counterparts. Recently, using the Wide-field Infrared Survey Explorer survey, we discovered that blazars, the rarest class of active galactic nuclei and the largest population of {gamma}-ray sources, can be recognized and separated from other extragalactic sources on the basis of their infrared (IR) colors. Based on this result, we designed an association method for the {gamma}-ray sources to recognize if there is a blazar candidate within the positional uncertainty region of a generic {gamma}-ray source. With this new IR diagnostic tool, we searched for {gamma}-ray blazar candidates associated with the UGS sample of the second Fermi {gamma}-ray LAT catalog (2FGL). We found that our method associates at least one {gamma}-ray blazar candidate as a counterpart to each of 156 out of 313 UGSs analyzed. These new low-energy candidates have the same IR properties as the blazars associated with {gamma}-ray sources in the 2FGL catalog.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

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 challenges 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 Catalog. This method is based on the infrared 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. 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 Swift archival data available for 10 out of 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 us understand their origin and to confirm their blazar nature.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

X-RAY AND TeV EMISSIONS FROM HIGH-FREQUENCY-PEAKED BL LAC OBJECTS

The majority of the extragalactic sources yet detected at TeV photon energies belong to the class of 'high-frequency-peaked BL Lac objects' (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 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 through 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{sub 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{sub p} and the spectral curvature distributions in terms of a coherent and a stochastic acceleration mechanisms, respectively. We show how the curvature parameter b {approx_equal} 0.3-0.7 of the synchrotron X-rays, which depends only on the latter acceleration component, can be related to the inverse Compton luminosity in {gamma}-rays, thus introducing a link between the X-ray and the TeV observations of HBLs.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

X-RAY SPECTRAL CURVATURE OF HIGH-FREQUENCY-PEAKED BL LAC OBJECTS: 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 Lac objects (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{sub p} increases with its peak energy E{sub p} and (2) the curvature parameter b decreases as E{sub 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 HBL candidates for future TeV observations.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2011

FLARING PATTERNS IN BLAZARS

Blazars radiate from relativistic jets launched by a supermassive black hole along our line of sight; the subclass of flat spectrum radio quasars exhibits broad emission lines, a telltale sign of a gas-rich environment and high accretion rate, contrary to the other subclass of the BL Lacertae objects. We show that this dichotomy of the sources in physical properties is enhanced in their flaring activity. The BL Lac flares yielded spectral evidence of being driven by further acceleration of highly relativistic electrons in the jet. Here, we discuss spectral fits of multi-{lambda} data concerning strong flares of the two flat spectrum radio quasars 3C 454.3 and 3C 279 recently detected in {gamma}-rays by the AGILE and Fermi satellites. We find that optimal spectral fits are provided by external Compton radiation enhanced by increasing production of thermal seed photons by growing accretion. We find such flares to trace patterns on the jet-power-electron-energy plane that diverge from those followed by flaring BL Lac objects and discuss why these occur.

79 ASTRONOMY AND ASTROPHYSICS↗