4,129 research outputs found
The WISE gamma-ray strip parametrization: the nature of the gamma-ray Active Galactic Nuclei of Uncertain type
Despite the large number of discoveries made recently by Fermi, the origins
of the so called unidentified gamma-ray sources remain unknown. The large
number of these sources suggests that among them there could be a population
that significantly contributes to the isotropic gamma-ray background and is
therefore crucial to understand their nature. The first step toward a complete
comprehension of the unidentified gamma-ray source population is to identify
those that can be associated with blazars, the most numerous class of
extragalactic sources in the gamma-ray sky. Recently, we discovered that
blazars can be recognized and separated from other extragalactic sources using
the infrared (IR) WISE satellite colors. The blazar population delineates a
remarkable and distinctive region of the IR color-color space, the WISE blazar
strip. In particular, the subregion delineated by the gamma-ray emitting
blazars is even narrower and we named it as the WISE Gamma-ray Strip (WGS). In
this paper we parametrize the WGS on the basis of a single parameter s that we
then use to determine if gamma-ray Active Galactic Nuclei of the uncertain type
(AGUs) detected by Fermi are consistent with the WGS and so can be considered
blazar candidates. We find that 54 AGUs out of a set 60 analyzed have IR colors
consistent with the WGS; only 6 AGUs are outliers. This result implies that a
very high percentage (i.e., in this sample about 90%) of the AGUs detected by
Fermi are indeed blazar candidates.Comment: 22 pages, 13 figures, Astrophysical Journal in pres
The Long Term Optical Variability of the BL Lac object S5 0716+714: Evidence for a Precessing Jet
We present the historic light curve of the BL Lac object S5 0716+714,
spanning the time interval from 1953 to 2003, built using Asiago archive plates
and our recent CCD observations, together with literature data. The source
shows an evident long term variability, over which well known short term
variations are superposed. In particular, in the period from 1961 to 1983 the
mean brightness of S5 0716+714 remained significantly fainter than that
observed after 1994. Assuming a constant variation rate of the mean magnitude
we can estimate a value of about 0.11 magnitude/year. The simultaneous
occurrence of decreasing ejection velocities of superluminal moving components
in the jet reported by Bach et al. (2005) suggests that both phenomena are
related to the change of the direction of the jet to the line of sight from
about 5 to 0.7 degrees for an approximately constant bulk Lorentz factor of
about 12. A simple explanation is that of a precessing relativistic jet, which
should presently be close to the smallest orientation angle. One can therefore
expect in the next ten years a decrease of the mean brightness of about 1
magnitude.Comment: to appear on The Astronomical Journal, 17 pages, 7 figures. Fig.2 is
given as a separated jpg fil
Infrared Colors of the gamma-ray detected blazars
Blazars constitute the most enigmatic class of extragalactic gamma-ray
sources, and their observational features have been ascribed to a relativistic
jet closely aligned to the line of sight. They are generally divided in two
main classes: the BL Lac objects (BL Lacs) and the Flat Spectrum Radio Quasars
(FSRQs). In the case of BL Lacs the double bumped spectral energy distribution
(SED) is generally described by the Synchrotron Self Compton (SSC) emission,
while for the FSRQs it is interpreted as due to External Compton (EC) emission.
Recently, we showed that in the [3.4]-[4.6]-[12] micron color- color diagram
the blazar population covers a distinct region (i.e., the WISE blazar Strip,
WBS), clearly separated from the other extragalactic sources that are dominated
by thermal emission. In this paper we investigate the relation between the
infrared and gamma-ray emission for a subset of confirmed blazars from the
literature, associated with Fermi sources, for which WISE archival observations
are available. This sample is a proper subset of the sample of sources used
previously, and the availability of Fermi data is critical to constrain the
models on the emission mechanisms for the blazars. We found that the selected
blazars also lie on the WISE blazar Strip covering a narrower region of the
infrared color-color planes than the overall blazars population. We then found
an evident correlation between the IR and gamma-ray spectral indices expected
in the SSC and EC frameworks. Finally, we determined the ratio between their
gamma-ray and infrared fluxes, a surrogate of the ratio of powers between the
inverse Compton and the synchrotron SED components, and used such parameter to
test different emitting scenarios blazars.Comment: 15 pages, 14 figure, accepted for publication in ApJ, to appear in
2012 March 20 editio
Focusing on the extended X-ray emission in 3C 459 with a Chandra follow-up observation
6 pages, 4 figures. Reproduced with permission from Astronomy & Astrophysics. © 2019 ESO.Aims. We investigated the X-ray emission properties of the powerful radio galaxy 3C 459 revealed by a recent Chandra follow-up observation carried out in October 2014 with a 62 ks exposure. Methods. We performed an X-ray spectral analysis from a few selected regions on an image obtained from this observation and also compared the X-ray image with a 4.9 GHz VLA radio map available in the literature. Results. The dominant contribution comes from the radio core but significant X-ray emission is detected at larger angular separations from it, surrounding both radio jets and lobes. According to a scenario in which the extended X-ray emission is due to a plasma collisionally heated by jet-driven shocks and not magnetically dominated, we estimated its temperature to be ∼0.8 keV. This hot gas cocoon could be responsible for the radio depolarization observed in 3C 459, as recently proposed also for 3C 171 and 3C 305. On the other hand, our spectral analysis and the presence of an oxygen K edge, blueshifted at 1.23 keV, cannot exclude the possibility that the X-ray radiation originating from the inner regions of the radio galaxy could be intercepted by some outflow of absorbing material intervening along the line of sight, as already found in some BAL quasars.Peer reviewe
Optical and Radio monitoring of S5 1803+74
The optical (BVRI) and radio (8.4 GHz) light curves of S5 1803+784 on a time
span of nearly 6 years are presented and discussed. The optical light curve
showed an overall variation greater than 3 mag, and the largest changes occured
in three strong flares. No periodicity was found in the light curve on time
scales up to a year. The variability in the radio band is very different, and
shows moderate oscillations around an average constant flux density rather than
relevant flares, with a maximum amplitude of 30%, without a simultaneous
correspondence between optical and radio luminosity. The optical spectral
energy distribution was always well fitted by a power law. The spectral index
shows small variations and there is indication of a positive correlation with
the source luminosity. Possible explanations of the source behaviour are
discussed in the framework of current models.Comment: 25 pages, 12 figure
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
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
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