Unidentifed gamma-ray sources: hunting gamma-ray blazars

One of the main scientific objectives of the ongoing Fermi mission is unveiling the nature of the unidentified gamma-ray sources (UGSs). Despite the large 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 to low energy counterparts. Recently, using the Wide-Field Infrared Survey Explorer (WISE) 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 reognize 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 to the UGS sample of the second Fermi gamma-ray catalog (2FGL). We found that our method associates at least one gamma-ray blazar candidate as a counterpart each of 156 out of 313 UGSs analyzed. These new low-energy candidates have the same IR properties as the blazars associated to gamma-ray sources in the 2FGL catalog.Comment: 24 pages, 4 figures, Accepted for publication on the Astrophysical Journa

Two-sided radio emission in ON231 (W Comae)

Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show remarkable new features in the source structure compared to those previously published. The images were obtained from observations made with the European VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical outburst occurred in Spring 1998. The up-to-date B band historic light curve of ON231 is also presented together with the R band luminosity evolution in the period 1994--1999. We identify the source core in the radio images with the brightest component having the flattest spectrum. A consequence of this assumption is the existence of a two--sided emission in ON231 not detected in previous VLBI images. A further new feature is a large bend in the jet at about 10 mas from the core. The emission extends for about 20 mas after the bend, which might be due to strong interaction with the environment surrounding the nucleus. We suggest some possible interpretations to relate the changes in the source structure with the optical and radio flux density variation in the frame of the unification model.Comment: 8 pages, 8 figure

New blazars from the cross-match of recent multi-frequency catalogs

Blazars are radio-loud active galactic nuclei well known for their non thermal emission spanning a wide range of frequencies. The Roma-BZCAT is, to date, the most comprehensive list of these sources. We performed the cross-match of several catalogs obtained from recent surveys at different frequencies to search for new blazars. We cross-matched the 1$^{st}$ Swift-XRT Point Source catalog with the spectroscopic sample of the 9$^{th}$ Data Release of the Sloan Digital Sky Survey. Then, we performed further cross-matches with the catalogs corresponding to the Faint Images of the Radio Sky at Twenty cm survey and to the AllWISE Data release, focusing on sources with infrared colors similar to those of confirmed $\gamma$-ray blazars included in the Second Fermi-LAT catalog. As a result, we obtained a preliminary list of objects with all the elements needed for a proper blazar classification according to the prescriptions of the Roma-BZCAT. We carefully investigated additional properties such as their morphology and the slope of their spectral energy distribution in the radio domain, the features shown in their optical spectrum, and the luminosity in the soft X rays to exclude generic active galactic nuclei and focus on authentic blazar-like sources. At the end of our screening we obtained a list of 15 objects with firmly established blazar properties.Comment: 8 pages, 3 figures, 1 table. Accepted for publication in Astrophysics and Space Science on 2015 April 25. Corrected typo in Section

Optical variability of the BL Lacertae object GC 0109+224. Multiband behaviour and time scales from a 7-years monitoring campaign

We present the most continuous data base of optical $BVR_{c}I_{c}$ observations ever published on the BL Lacertae object GC 0109+224, collected mainly by the robotic telescope of the Perugia University Observatory in the period November 1994-February 2002. These observations have been complemented by data from the Torino Observatory, collected in the period July 1995-January 1999, and Mt. Maidanak Observatory (December 2000). GC 0109+224 showed rapid optical variations and six major outbursts were observed at the beginning and end of 1996, in fall 1998, at the beginning and at the end of 2000, and at the beginning of 2002. Fast and large-amplitude drops characterized its flux behaviour. The $R_c$ magnitude ranged from 13.3 (16.16 mJy) to 16.46 (0.8 mJy), with a mean value of 14.9 (3.38 mJy). In the periods where we collected multi-filter observations, we analyzed colour and spectral indexes, and the variability patterns during some flares. The long-term behaviour seems approximatively achromatic, but during some isolated outbursts we found evidence of the typical loop-like hysteresis behaviour, suggesting that rapid optical variability is dominated by non-thermal cooling of a single emitting particle population. We performed also a statistical analysis of the data, through the discrete correlation function (DCF), the structure function (SF), and the Lomb-Scargle periodogram, to identify characteristic times scales, from days to months, in the light curves, and to quantify the mode of variability. We also include the reconstruction of the historical light curve and a photometric calibration of comparison stars, to favour further extensive optical monitoring of this interesting blazar.Comment: 13 pages, 11 PS figures, 1 EPS figure, 3 tables, accepted by Astronomy and Astrophysics. Uses A&A documentclass aa.cls, and the package graphicx.st