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.
We present the analysis of nine radio sources belonging to the Third Cambridge Revised catalog (3CR) observed with \(Chandra\) during Cycle 20 in the redshift range between 1.5 and 2.5. This study ...completes the 3CR \(Chandra\) Snapshot Survey thus guaranteeing the X-ray coverage of all 3CR sources identified to date. This sample lists two compact steep spectrum sources, four radio galaxies and three quasars. We detected X-ray emission from all nuclei, with the only exception of 3C 326.1 and 3C 454.1 and from radio lobes in 6 out of 9 sources at level of confidence larger than \(\sim\)5\(\sigma\). We measured X-ray fluxes and luminosities for all nuclei and lobes in the soft (0.5 - 1 keV), medium (1 - 2 keV) and hard (2 - 7 keV) X-ray bands. Since the discovered X-ray extended emission is spatially coincident with the radio structure in all cases, its origin could be due to Inverse Compton scattering of the Cosmic Microwave Background (IC/CMB) occurring in radio lobes.
We have investigated a group of unassociated radio sources included in the 3CR cat- alogue to increase the multi-frequency information on them and possibly obtain an identification. We have carried ...out an observational campaign with the Swift satellite to observe with the UVOT and the XRT telescopes the field of view of 21 bright NVSS sources within the positional uncertainty region of the 3CR sources. Furthermore, we have searched in the recent AllWISE Source Catalogue for infrared sources matching the position of these NVSS sources. We have detected significant emission in the soft X-ray band for nine of the investigated NVSS sources. To all of them, and in four cases with no soft X-ray association, we have associated a WISE infrared counterpart. Eight of these infrared candidates have not been proposed earlier in the literature. In the five remaining cases our candidate matches one among a few optical candidates suggested for the same 3CR source in previous studies. No source has been detected in the UVOT filters at the position of the NVSS objects, confirming the scenario that all of them are heavily obscured. With this in mind, a spectroscopic campaign, preferably in the infrared band, will be necessary to establish the nature of the sources that we have finally identified.
We present a catalog of radio-loud candidate gamma-ray emitting blazars with WISE mid-infrared colors similar to the colors of confirmed gamma-ray blazars. The catalog is assembled from WISE sources ...detected in all four WISE filters, with colors compatible with the three-dimensional locus of the WISE gamma-ray emitting blazars, and which can be spatially cross-matched with radio sources from either one of the three radio surveys: NVSS, FIRST and/or SUMSS. Our initial WISE selection uses a slightly modified version of previously successful algorithms. We then select only the radio-loud sources using a measure of the radio-to-IR flux, the q22 parameter, which is analogous to the q24 parameter known in the literature but which instead uses the WISE band-four flux at 22 micron. Our final catalog contains 7855 sources classified as BL Lacs, FSRQs or mixed candidate blazars; 1295 of these sources can be spatially re-associated with confirmed blazars. We describe the properties of the final catalog of WISE blazar-like radio-loud sources and consider possible contaminants. Finally, we discuss why this large catalog of candidate gamma-ray emitting blazars represents a new and useful resource to address the problem of finding low energy counterparts to currently unidentified high-energy sources.
Blazars are a highly-variable, radio-loud subclass of active galactic nuclei (AGN). In order to better understand such objects we must be able to easily identify candidate blazars from the growing ...population of unidentified sources. Working towards this goal we attempt to identify new gamma-ray blazar candidates from a sample of 102 previously unidentified sources. These sources are selected from Astronomer's Telegrams and the literature on the basis of non-periodic variability and multi-wavelength behavior. We then attempt to associate these objects to an IR counterpart in the WISE all-sky survey. We are able to identify sixteen candidate sources whose IR colors are consistent with those of the blazar population. Of those sixteen, thirteen sources have IR colors indicative of being gamma-ray emitting blazar candidates. These sources all possess archival multi-wavelength observations that support their blazar-like nature.
Blazars are the most extreme class of active galactic nuclei (AGNs). Despite a previous investigation at 102 MHz for a small sample of BL Lacs and our recent analysis of blazars detected in the ...Westerbork Northern Sky Survey (WENSS), a systematic study of the blazar spectral properties at frequencies below 100 MHz has been never carried out. In this paper, we present the first analysis of the radio spectral behavior of blazars based on the recent Very Large Array Low-Frequency Sky Survey (VLSS) at 74 MHz. We search for blazar counterparts in the VLSS catalog confirming that they are detected at 74 MHz. We then show that blazars present radio flat spectra {(i.e., radio spectral indices ~0.5)} when evaluated also about an order of magnitude in frequency lower than previous analyses. Finally, we discuss the implications of our findings in the context of the blazars - radio galaxies connection since the low frequency radio data provide a new diagnostic tool to verify the expectations of the unification scenario for radio-loud active galaxies.
About one third of the gamma-ray sources listed in the second Fermi LAT catalog (2FGL) have no firmly established counterpart at lower energies so being classified as unidentified gamma-ray sources ...(UGSs). Here we propose a new approach to find candidate counterparts for the UGSs based on the 325 MHz radio survey performed with Westerbork Synthesis Radio Telescope (WSRT) in the northern hemisphere. First we investigate the low-frequency radio properties of blazars, the largest known population of gamma-ray sources; then we search for sources with similar radio properties combining the information derived from the Westerbork Northern Sky Survey (WENSS) with those of the NRAO VLA Sky survey (NVSS). We present a list of candidate counterparts for 32 UGSs with at least one counterpart in the WENSS. We also performed an extensive research in literature to look for infrared and optical counterparts of the gamma-ray blazar candidates selected with the low-frequency radio observations to confirm their nature. On the basis of our multifrequency research we identify 23 new gamma-ray blazar candidates out of 32 UGSs investigated. Comparison with previous results on the UGSs are also presented. Finally, we speculate on the advantages on the use of the low-frequency radio observations to associate UGSs and to search for gamma-ray pulsar candidates.
This paper describes the effect of wind turbine mechanical characteristic of construction and grid parameters on the transient voltage stability of a fixed speed wind turbine (FSWT) connected to a ...simple grid. The model of FSWT has been developed in Matlab/Simulink. Using this model, a three-phase fault is applied close to the wind turbine (WT) and cleared by disconnecting the faulted line. The effects of mechanical construction of WT and of grid parameters on the voltage stability of this simple case are evaluated and discussed. The considered parameters are the inertia coefficient of the WT, the short-circuit power at the connection bus, the hub-generator resonant frequency, the wind speed and the reactive power compensation. For each parameter, the terminal voltage and the generator rotor speed for different fault-clearing time are drawn. At last, the terminal voltage and the generator rotor speed obtained for a fifth-order model of the induction generator are compared with those obtained for a third order model. The differences between the two models are discussed. The obtained results will help to understand the transient stability phenomena in wind farms.
BL Lac objects are the most numerous class of extragalactic TeV-detected sources. One of the biggest difficulties in investigating their TeV emission resides in their limited number, since only 47 BL ...Lacs are known as TeV emitters. In this paper, we propose new criteria to select TeV BL Lac candidates based on the infrared (IR) and X-ray observations. We apply our selection criteria to the BL Lac objects listed in the ROMA-BZCAT catalog so identifying 41 potential TeV emitters. We then consider a search over a more extended sample combining the ROSAT bright source catalog and the WISE all-sky survey revealing 54 additional candidates for TeV observations. Our investigation also led to a tentative classification of 16 unidentified X-ray sources as BL Lac candidates. This analysis provides new interesting BL Lac targets for future observations with ground based Cherenkov telescopes.
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