11 research outputs found
The low-frequency radio catalog of flat spectrum sources
A well known property of the gamma-ray sources detected by COS-B in the
1970s, by the Compton Gamma-ray Observatory in the 1990s and recently by the
Fermi observations is the presence of radio counterparts, in particular for
those associated to extragalactic objects. This observational evidence is the
basis of the radio-gamma-ray connection established for the class of active
galactic nuclei known as blazars. In particular, the main spectral property of
the radio counterparts associated with gamma-ray blazars is that they show a
flat spectrum in the GHz frequency range. Our recent analysis dedicated to
search blazar-like candidates as potential counterparts for the unidentified
gamma-ray sources (UGSs) allowed us to extend the radio-gamma-ray connection in
the MHz regime. We also showed that below 1 GHz blazars maintain flat radio
spectra. Thus on the basis of these new results, we assembled a low-frequency
radio catalog of flat spectrum sources built by combining the radio
observations of the Westerbork Northern Sky Survey (WENSS) and of the
Westerbork in the southern hemisphere (WISH) catalog with those of the NRAO
Very Large Array Sky survey (NVSS). This could be used in the future to search
for new, unknown blazar-like counterparts of the gamma-ray sources. First we
found NVSS counterparts of WSRT radio sources and then we selected flat
spectrum radio sources according to a new spectral criterion specifically
defined for radio observations performed below 1 GHz. We also described the
main properties of the catalog listing 28358 radio sources and their logN-logS
distributions. Finally a comparison with with the Green Bank 6-cm radio source
catalog has been performed to investigate the spectral shape of the
low-frequency flat spectrum radio sources at higher frequencies.Comment: 10 pages, 10 figures, 1 table, ApJS published in 2014 (pre-proof
version uploaded
Optical spectroscopic observations of blazars and gamma-ray blazar candidates in the Sloan Digital Sky Survey Data Release Nine
We present an analysis of the optical spectra available in the Sloan Digital
Sky survey data release nine (SDSS DR9) for the blazars listed in the
ROMA-BZCAT and for the gamma-ray blazar candidates selected according to their
IR colors. First, we adopt a statistical approach based on MonteCarlo
simulations to find the optical counterparts of the blazarslisted in the
ROMA-BZCAT catalog. Then we crossmatched the SDSS spectroscopic catalog with
our selected samples of blazars and gamma-ray blazar candidates searching for
those with optical spectra available to classify our blazar-like sources and,
whenever possible, to confirm their redshifts. Our main objectives are
determining the classification of uncertain blazars listed in the ROMA-BZCAT
and discovering new gamma-ray blazars. For the ROMA-BZCAT sources we
investigated a sample of 84 blazars confirming the classification for 20 of
them and obtaining 18 new redshift estimates. For the gamma-ray blazars,
indicated as potential counterparts of unassociated Fermi sources or with
uncertain nature, we established the blazar-like nature of 8 out the 27 sources
analyzed and confirmed 14 classifications.Comment: 7 pages, 2 figures, 4 tables, AJ published in 2014 (pre-proof
version
Unveiling the nature of the unidentified gamma-ray sources IV: the catalog of potential X-ray counterparts
A significant fraction ( %) of the high-energy gamma-ray sources
listed in the second LAT (2FGL) catalog are still of unknown
origin, being not yet associated with counterparts at lower energies. In order
to investigate the nature of these enigmatic sources, we present here an
extensive search of X-ray sources lying in the positional uncertainty region of
a selected sample of these Unidentified Gamma-ray Sources (UGSs) that makes use
of all available observations performed by the X-ray Telescope
before March 31, 2013, available for 205 UGSs. To detect the fainter sources,
we merged all the observations covering the LAT positional
uncertainty region at 95 % level of confidence of each UGSs. This yields a
catalog of 357 X-ray sources, finding {candidate} X-ray counterparts for % of the selected sample. In particular, 25 % of the UGSs feature a single
X-ray source within their positional uncertainty region while 45 % have
multiple X-ray sources. For each X-ray source we also looked in the
corresponding UVOT merged images for optical and ultraviolet
counterparts, also performing source photometry. We found ultraviolet-optical
correspondences for % of the X-ray sources. We searched several major
radio, infrared, optical and ultraviolet surveys for possible counterparts
within the positional error of the sources in the X-ray catalog to obtain
additional information on their nature. Applying the kernel density estimator
technique to infrared colors of WISE counterparts of our X-ray sources we
select 6 -ray blazar candidates. In addition, comparing our results
with previous analyses, we select 11 additional -ray blazar candidates.Comment: 19 pages, 10 figures, 6 tables. Accepted for publication on Ap
UNVEILING THE NATURE OF THE UNIDENTIFIED GAMMA-RAY SOURCES V: ANALYSIS OF THE RADIO CANDIDATES WITH THE KERNEL DENSITY ESTIMATION
Nearly one-third of the γ-ray sources detected by Fermi are still unidentified, despite significant recent progress in this effort. On the other hand, all the γ-ray extragalactic sources associated in the second Fermi-LAT catalog have a radio counterpart. Motivated by this observational evidence we investigate all the radio sources of the major radio surveys that lie within the positional uncertainty region of the unidentified γ-ray sources (UGSs) at 95 % level of confidence. First we search for their infrared counterparts in the all-sky survey performed by the Wide-field Infrared Survey Explorer (WISE) and then we analyze their IR colors in comparison with those of the known γ-ray blazars. We propose a new approach, based on a 2-dimensional kernel density estimation (KDE) technique in the single [3.4]-[4.6]-[12] µm WISE color-color plot, replacing the constraint imposed in our previous investigations on the detection at 22µm of each potential IR counterpart of the UGSs with associated radioemission. The main goal of this analysisis to find distant γ-rayblazar candidates that, being too faint at 22µm, are not detected by WISE and thus are not selected by our purely IR based methods. We find fifty-five UGS’s likely correspond to radio sources with blazar-like IR signatures. Additional eleven UGSs having, blazar-like IR colors, have been found within the sample of sources found with deep recent ATCA observations
BL LAC CANDIDATES FOR TEV OBSERVATIONS
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 observationswith ground based Cherenkov telescopes
UNVEILING THE NATURE OF THE UNIDENTIFIED GAMMA-RAYS SOURCES VIII: COMPUTING THE ASSOCIATION PROBABILITY
Despite the significant improvements of the Fermi satellite on the source localization with respect to the previous γ-ray missions, the positional uncertainties of the Fermi sources are still large, making the search for potential low-energy counterparts a challenging task. In the Fermi source catalogs (i.e., 1FGL and 2FGL, respectively) for each counterpart associated with an high-energy source a corresponding value of the association probability was provided. Thus several methods based on the source position or on the logN-logS distribution of potential counterparts were developed to derive the association probabilities. Recently, we discovered a tight connection between the infrared (IR) surveys and the γ-ray sky that allowed us to create several lists of γ-ray blazar-like sources, potential counterparts of Fermi objects. Here we complete our previous analyses presenting a new approach based on Montecarlo simulations to determine the association probability for γ-ray blazarlike sources selected on the basis of their peculiar IR colors. We also describe a different version of the likelihood ratio technique with some improvements based on the IR-γ-ray connection. Both methods are compared with the 2FGL associations to asses their reliability. We found reliable counterparts for 39 previously unidentified γ-ray sources listed in the 2FGL and 5 new γ-ray blazar candidates out of 20 sources associated for a subsample of the 1FGL not detected in the 2FGL. Both methods are also able to associate radio loud narrow line Seyfert 1 showing blazar-like IR colors