2,526 research outputs found
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
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
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 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
Unveiling the nature of the unidentified gamma-ray sources III: gamma-ray blazar-like counterparts at low radio frequencies
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.Comment: 15 pages, 13 figures, 3 tables, ApJS accepted for publication
(version pre-proof corrections
Optical spectroscopic observations of gamma-ray blazar candidates II. The 2013 KPNO campaign in the Northern Hemisphere
We recently started a systematic search of low-energy counterparts of the
unidentified gamma-ray sources (UGSs) listed in the Fermi-Large Area Telescope
(LAT) First Source Catalog (1FGL) and the Fermi-LAT 2-Year Source Catalog
(2FGL).} The main goal of our investigation is to find active galaxies
belonging to the blazar class that lie within the positional uncertainty region
of the UGSs and thus could be their potential low-energy counterparts. To
achieve our aims, we first adopted several procedures based on the peculiar
observational properties of blazars in the radio and in the IR. Then we carried
out a follow-up spectroscopic campaign in the optical band to verify the nature
of the candidates selected as potential counterparts of the UGSs. Here we
present the results of the observations carried out in 2013 in the Northern
Hemisphere at Kitt Peak National Observatory (KPNO). Optical spectroscopy is
crucial to confirm the nature of the sources and can be used to estimate their
redshifts; it will also allow us to test the robustness of our methods when the
whole campaign is completed. Here we present the optical spectroscopic
observations of 39 sources. Within our sample we found that 6 sources are
blazars, candidates to be low-energy counterparts of the UGSs listed in the
2FGL. We confirm that an additional 8 sources, previously classified as active
galaxies of uncertain type and associated in the 2FGL, are also all BL Lac
objects. Moreover, we also present 20 new spectra for known blazars listed in
the Multi-frequency Catalogue of Blazars as having an uncertain redshift and/or
being classified as BL Lac candidates. We conclude that our methods for
selecting gamma-ray blazar candidates allows us to discover new blazars and
increase the list of potential low-energy counterparts for the Fermi UGSs.Comment: 27 pages, 39 figures, 1 table, A&A accepted for publication
(pre-proof version
Exploring the bulk of the BL Lac object population:1. parsec-scale radio structures
Context. The advent of Fermi is changing our understanding on the radio and
gamma-ray emission in Active Galactic Nuclei. Contrary to pre-Fermi ideas, BL
Lac objects are found to be the most abundant emitters in the gamma-ray band.
However, since they are relatively weak radio sources, most of their
parsec-scale structure and their multi-frequency properties are poorly
understood and/or have not been investigated in a systematically fashion. Aims.
Our main goal is to analyze the radio and gamma-ray emission properties of a
sample of 42 BL Lacs selected, for the first time in the literature, with no
constraint on their radio and gamma-ray flux densities/emission. Methods.
Thanks to new Very Long Baseline Array observations at 8 and 15 GHz for the
whole sample, we present here fundamental parameters such as radio flux
densities, spectral index information, and parsec-scale structure. Moreover, we
search for gamma-ray counterparts using data reported in the Second Catalog of
Fermi Gamma-ray sources. Results. Parsec-scale radio emission is observed in
the majority of the sources at both frequencies. Gamma-ray counterparts are
found for 14/42 sources. Conclusions. The comparison between our results in
radio and gamma-ray bands points out the presence of a large number of faint BL
Lacs showing "non classical" properties such as low source compactness, core
dominance, no gamma-ray emission and steep radio spectral indexes. A deeper
multiwavelength analysis will be needed.Comment: 19 pages, 6 figures, 6 tables, accepted for publication in A&
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