4,756 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
Application of the MST clustering to the high energy gamma-ray sky. III - New detections of gamma-ray emission from blazars
We present the results of a photon cluster search in the gamma-ray sky
observed by the Fermi Large Area Telescope, using the new Pass 8 dataset, at
energies higher than 10 GeV. By means of the Minimum Spanning Tree (MST)
algorithm, we found 25 clusters associated with catalogued blazars not
previously known as gamma-ray emitters. The properties of these sources are
discussed.Comment: 10 pages, 3 figures. Accepted for publication in Astrophysics & Space
Scienc
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
FR0CAT: a FIRST catalog of FR0 radio galaxies
With the aim of exploring the properties of the class of FR0 radio galaxies,
we selected a sample of 108 compact radio sources, called FR0CAT, by combining
observations from the NVSS, FIRST, and SDSS surveys. The catalog includes
sources with z, with a radio size 5 kpc, and with an
optical spectrum characteristic of low-excitation galaxies. Their 1.4-GHz radio
luminosities range erg/s. The
FR0CAT hosts are mostly (86%) luminous () red
early-type galaxies with black hole masses : similar to the hosts of FRI radio galaxies, but they are on
average a factor 1.6 less massive. The number density of FR0CAT sources
is 5 times higher than that of FRIs, and thus they represent the dominant
population of radio sources in the local Universe. Different scenarios are
considered to account for the smaller sizes and larger abundance of FR0s with
respect to FRIs. An age-size scenario that considers FR0s as young radio
galaxies that will all eventually evolve into extended radio sources cannot be
reconciled with the large space density of FR0s. However, the radio activity
recurrence, with the duration of the active phase covering a wide range of
values and with short active periods strongly favored with respect to longer
ones, might account for their large density number. Alternatively, the jet
properties of FR0s might be intrinsically different from those of the FRIs, the
former class having lower bulk Lorentz factors, possibly due to lower black
hole spins. Our study indicates that FR0s and FRI/IIs can be interpreted as two
extremes of a continuous population of radio sources that is characterized by a
broad distribution of sizes and luminosities of their extended radio emission,
but shares a single class of host galaxies.Comment: 11 pages, 6 figures, accepted for publication on A&
FRICAT: A FIRST catalog of FRI radio galaxies
We built a catalog of 219 FRI radio galaxies (FRIs), called FRICAT, selected
from a published sample and obtained by combining observations from the NVSS,
FIRST, and SDSS surveys. We included in the catalog the sources with an
edge-darkened radio morphology, redshift , and extending (at the
sensitivity of the FIRST images) to a radius larger than 30 kpc from the
center of the host. We also selected an additional sample (sFRICAT) of 14
smaller (10 30 kpc) FRIs, limiting to . The hosts of the FRICAT
sources are all luminous (), red early-type
galaxies with black hole masses in the range ; the spectroscopic classification based on the optical
emission line ratios indicates that they are all low excitation galaxies.
Sources in the FRICAT are then indistinguishable from the FRIs belonging to the
Third Cambridge Catalogue of Radio Sources (3C) on the basis of their optical
properties. Conversely, while the 3C-FRIs show a strong positive trend between
radio and [OIII] emission line luminosity, these two quantities are unrelated
in the FRICAT sources; at a given line luminosity, they show radio luminosities
spanning about two orders of magnitude and extending to much lower ratios
between radio and line power than 3C-FRIs. Our main conclusion is that the
3C-FRIs just represent the tip of the iceberg of a much larger and diverse
population of FRIs.Comment: 34 pages, 8 figures, 1 table, 1 appendix,accepted for publication in
A&A, pre-proof versio
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
The optical to gamma-ray emission of the Crab pulsar: a multicomponent model
We present a multicomponent model to explain the features of the pulsed
emission and spectrum of the Crab Pulsar, on the basis of X and gamma-ray
observations obtained with BeppoSAX, INTEGRAL and CGRO. This model explains the
evolution of the pulse shape and of the phase-resolved spectra, ranging from
the optical/UV to the GeV energy band, on the assumption that the observed
emission is due to more components. The first component, C_O, is assumed to
have the pulsed double-peaked profile observed at the optical frequencies,
while the second component, C_X, is dominant in the interpeak and second peak
phase regions. The spectra of these components are modelled with log-parabolic
laws and their spectral energy distributions have peak energies at 12.2 and 178
keV, respectively. To explain the properties of the pulsed emission in the
MeV-GeV band, we introduce two more components, C_Ogamma and C_Xgamma, with
phase distributions similar to those of C_O and C_X and log-parabolic spectra
with the same curvature but peak energies at about 300 MeV and 2 GeV. This
multicomponent model is able to reproduce both the broadband phase-resolved
spectral behaviour and the changes of the pulse shape with energy. We also
propose some possible physical interpretations in which C_O and C_X are emitted
by secondary pairs via a synchrotron mechanism while C_Ogamma and C_Xgamma can
originate either from Compton scattered or primary curvature photons.Comment: 14 pages, 16 figures; accepted by Astronomy and Astrophysic
Application of the MST clustering to the high energy gamma-ray sky. I - New possible detection of high-energy gamma-ray emission associated with BL Lac objects
In this paper we show an application of the Minimum Spanning Tree (MST)
clustering method to the high-energy gamma-ray sky observed at energies higher
than 10 GeV in 6.3 years by the Fermi-Large Area Telescope. We report the
detection of 19 new high-energy gamma-ray clusters with good selection
parameters whose centroid coordinates were found matching the positions of
known BL Lac objects in the 5th Edition of the Roma-BZCAT catalogue. A brief
summary of the properties of these sources is presented.Comment: 11 pages, 7 figures. Accepted for publication in Astrophysics & Space
Scienc
CLaSPS: a new methodology for Knowledge extraction from complex astronomical dataset
In this paper we present the Clustering-Labels-Score Patterns Spotter
(CLaSPS), a new methodology for the determination of correlations among
astronomical observables in complex datasets, based on the application of
distinct unsupervised clustering techniques. The novelty in CLaSPS is the
criterion used for the selection of the optimal clusterings, based on a
quantitative measure of the degree of correlation between the cluster
memberships and the distribution of a set of observables, the labels, not
employed for the clustering. In this paper we discuss the applications of
CLaSPS to two simple astronomical datasets, both composed of extragalactic
sources with photometric observations at different wavelengths from large area
surveys. The first dataset, CSC+, is composed of optical quasars
spectroscopically selected in the SDSS data, observed in the X-rays by Chandra
and with multi-wavelength observations in the near-infrared, optical and
ultraviolet spectral intervals. One of the results of the application of CLaSPS
to the CSC+ is the re-identification of a well-known correlation between the
alphaOX parameter and the near ultraviolet color, in a subset of CSC+ sources
with relatively small values of the near-ultraviolet colors. The other dataset
consists of a sample of blazars for which photometric observations in the
optical, mid and near infrared are available, complemented for a subset of the
sources, by Fermi gamma-ray data. The main results of the application of CLaSPS
to such datasets have been the discovery of a strong correlation between the
multi-wavelength color distribution of blazars and their optical spectral
classification in BL Lacs and Flat Spectrum Radio Quasars and a peculiar
pattern followed by blazars in the WISE mid-infrared colors space. This pattern
and its physical interpretation have been discussed in details in other papers
by one of the authors.Comment: 18 pages, 9 figures, accepted for publication in Ap
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
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