Optical characterization of WISE selected blazar candidates

Context. Over the last decade more than five thousand γ-ray sources have been detected by the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope. Given the positional uncertainty of the telescope, nearly 30% of these sources remain without an obvious counterpart at lower energies. This has motivated the release of new catalogs of γ-ray counterpart candidates and several follow up campaigns in the last decade. Aims. Recently, two new catalogs of blazar candidates were released. These are the improved and expanded version of the WISE Blazar-Like Radio-Loud Sources (WIBRaLS2) catalog and the Kernel Density Estimation selected candidate BL Lacs (KDEBLLACS) catalog, both selecting blazar-like sources based on their infrared colors from the Wide-field Infrared Survey Explorer (WISE). In this work we characterize these two catalogs, clarifying the true nature of their sources based on their optical spectra from SDSS data release 15, thus testing their efficiency in selecting true blazars. Methods. We first selected all WIBRaLS2 and KDEBLLACS sources with available optical spectra in the footprint of Sloan Digital Sky Survey data release 15. We then analyzed these spectra to verify the nature of each selected candidate and to measure the fraction of the catalogs composed by spectroscopically confirmed blazars. Finally, we evaluated the impact of selection effects, especially those related to optical colors of WIBRaLS2/KDEBLLACS sources and their optical magnitude distributions. Results. We found that at least ∼30% of each catalog is made up of confirmed blazars, with quasars being the major contaminants in the case of WIBRaLS2 (≈58%) and normal galaxies in the case of KDEBLLACS (≈38.2%). The spectral analysis also allowed us to identify the nature of 11 blazar candidates of uncertain type (BCUs) from the Fermi-LAT fourth Point Source Catalog (4FGL) and to find 25 new BL Lac objects.Fil: de Menezes, Raniere. Universidade Do Sao Paulo. Instituto Astronomia, Geofísica E Ciencias Atmosfericas. Departamento de Astronomia; Brasil. Università degli Studi di Torino; ItaliaFil: Peña Herazo, Harold A.. Instituto Nacional de Astrofísica; México. Università di Torino; Italia. Istituto Nazionale Di Fisica Nucleare.; Italia. Osservatorio Astrofisico di Torino; ItaliaFil: Marchesini, Ezequiel Joaquín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Istituto Nazionale Di Fisica Nucleare.; Italia. Università di Torino; ItaliaFil: D´Abrusco, Raffaele. Harvard-Smithsonian Center for Astrophysics; Estados UnidosFil: Masetti, Nicola. Osservatorio di Astrofisica e Scienza dello Spazio; Italia. Universidad Andrés Bello; ChileFil: Nemmen, Rodrigo. Universidade Do Sao Paulo. Instituto Astronomia, Geofísica E Ciencias Atmosfericas. Departamento de Astronomia; BrasilFil: Massaro, Francesco. Università di Torino; Italia. Istituto Nazionale Di Fisica Nucleare.; Italia. Osservatorio Astrofisico di Torino; Italia. Consorzio Interuniversitario per la Fisica Spaziale ; ItaliaFil: Ricci, Federica. Pontificia Universidad Católica de Chile; ChileFil: Landoni, Marco. Osservatorio Astronomico di Brera; ItaliaFil: Paggi, Alessandro. Università di Torino; ItaliaFil: Smith, Howard A.. Harvard-Smithsonian Center for Astrophysics; Estados Unido

Optical Spectroscopic Observations of Gamma-Ray Blazar Candidates. XII. Follow-up Observations from SOAR, Blanco, NTT, and OAN-SPM

Roughly one third of the sources in the Fermi-LAT catalogs are listed as unidentified/unassociated γ-ray sources (UGS), i.e., they lack a low-energy counterpart. In addition, there is a growing population of blazars of uncertain type (BCUs). Spectroscopic observations are crucial to confirm the blazar nature of the UGSs candidate counterparts and BCUs. Hence, in 2013 we started an optical spectroscopic campaign to carry out the identifications and classifications. In this paper, as a continuation of the campaign we report the spectra of 39 sources: the sample comprises 37 sources classified as BCUs, one source classified as a BL Lac in the Fourth Source Catalog of the Fermi-LAT (4FGL), and one source classified as UGS. We classify 19 of the sources in the sample as BL Lacs, 13 as blazars with nonnegligible host-galaxy emission, six as Flat Spectrum Radio Quasars, and one as a normal elliptical galaxy. The source listed as BL Lac in the 4FGL seems to be a blazar with nonnegligible host-galaxy emission in our observations, most likely due to an ongoing quiescent state. We classified the UGS source as a BL Lac. Six out of the 39 sources were previously reported in the campaign; in general, both the classifications and redshifts are in agreement, except for one of them with no redshift reported before. Altogether, we provided reliable redshift estimates to 21 out of the 39 sources. Finally, we describe the statistics of the data collected in our campaign so far

Extended X-ray emission around FR II radio galaxies: hotspots, lobes and galaxy clusters

We present a systematic analysis of the extended X-ray emission discovered around 35 FR II radio galaxies from the revised Third Cambridge catalog (3CR) Chandra Snapshot Survey with redshifts between 0.05 to 0.9. We aimed to (i) test for the presence of extended X-ray emission around FR II radio galaxies, (ii) investigate if the extended emission origin is due to Inverse Compton scattering of seed photons arising from the Cosmic Microwave Background (IC/CMB) or to thermal emission from an intracluster medium (ICM) and (iii) test the impact of this extended emission on hotspot detection. We investigated the nature of the extended X-ray emission by studying its morphology and compared our results with low-frequency radio observations (i.e., ∼150 MHz), in the TGSS and LOFAR archives, as well as with optical images from Pan-STARRS. In addition, we optimized a search for X-ray counterparts of hotspots in 3CR FR II radio galaxies. We found statistically significant extended emission (>3σ confidence level) along the radio axis for ∼90%, and in the perpendicular direction for ∼60% of our sample. We confirmed the detection of 7 hotspots in the 0.5 - 3 keV. In the cases where the emission in the direction perpendicular to the radio axis is comparable to that along the radio axis, we suggest that the underlying radiative process is thermal emission from ICM. Otherwise, the dominant radiative process is likely non-thermal IC/CMB emission from lobes. We found that non-thermal IC/CMB is the dominant process in ∼70% of the sources in our sample, while thermal emission from the ICM dominates in ∼15% of them