5,604 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
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
X-ray spectral curvature of High Frequency Peaked BL Lacs: a predictor for the TeV flux
Most of the extragalactic sources detected at TeV energies are BL Lac
objects. They belong to the subclass of "high frequency peaked BL Lacs" (HBLs)
exhibiting spectral energy distributions with a lower energy peak in the X-ray
band; this is widely interpreted as synchrotron emission from relativistic
electrons. The X-ray spectra are generally curved, and well described in terms
of a log-parabolic shape. In a previous investigation of TeV HBLs (TBLs) we
found two correlations between their spectral parameters. (1) The synchrotron
peak luminosity L_p increases with its peak energy E_p; (2) the curvature
parameter b decreases as E_p increases. The first is consistent with the
synchrotron scenario, while the second is expected from statistical/stochastic
acceleration mechanisms for the emitting electrons. Here we present an
extensive X-ray analysis of a sample of HBLs observed with XMM-Newton and SWIFT
but undetected at TeV energies (UBLs), to compare their spectral behavior with
that of TBLs. Investigating the distributions of their spectral parameters and
comparing the TBL X-ray spectra with that of UBLs, we develop a criterion to
select the best HBLs candidates for future TeV observations.Comment: 25 pages, 6 figures, Astrophysical Journal publishe
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
Time properties of the the rho-class burst of the microquasar GRS 1915+105 observed with BeppoSAX in April 1999
We present a temporal analysis of a BeppoSAX observation of GRS 1915+105
performed on April 13, 1999 when the source was in the rho class, which is
characterised by quasi-regular bursting activity. The aim of the present work
is to confirm and extend the validity of the results obtained with a BeppoSAX
observation performed on October 2000 on the recurrence time of the burst and
on the hard X-ray delay. We divided the entire data set into several series,
each corresponding to a satellite orbit, and performed the Fourier and wavelet
analysis and the limit cycle mapping technique using the count rate and the
average energy as independent variables. We found that the count rates
correlate with the recurrence time of bursts and with hard X-ray delay,
confirming the results previously obtained. In this observation, however, the
recurrence times are distributed along two parallel branches with a constant
difference of 5.2+/-0.5 s.Comment: Accepted for publication in Section 7. Stellar structure and
evolution of Astronomy and Astrophysic
Spectral Hardening of Large Solar Flares
RHESSI observations are used to quantitatively study the hard X-ray evolution
in 5 large solar flares selected for spectral hardening in the course of the
event. The X-ray bremsstrahlung emission from non-thermal electrons is
characterized by two spectroscopically distinct phases: impulsive and gradual.
The impulsive phase usually consists of several emission spikes following a
soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as
spectral hardening while the flux slowly decreases. Both the soft-hard-soft
(impulsive) phase and the hardening (gradual) phase are well described by
piecewise linear dependence of the photon spectral index on the logarithm of
the hard X-ray flux. The different linear parts of this relation correspond to
different rise and decay phases of emission spikes. The temporal evolution of
the spectra is compared with the configuration and motion of the hard X-ray
sources in RHESSI images. These observations reveal that the two stages of
electron acceleration causing these two different behaviors are closely related
in space and time. The transition between the impulsive and gradual phase is
found to be smooth and progressive rather than abrupt. This suggests that they
arise because of a slow change in a common accelerator rather than being caused
by two independent and distinct acceleration processes. We propose that the
hardening during the decay phase is caused by continuing particle acceleration
with longer trapping in the accelerator before escape.Comment: accepted by Ap
SSC radiation in BL Lac sources, the end of the tether
The synchrotron-self Compton (SSC) radiation process is widely held to
provide a close representation of the double peaked spectral energy
distributions from BL Lac Objects (BL Lacs), which are marked by non-thermal
beamed radiations, highly variable on timescales of days or less. Their
outbursts in the gamma ray relative to the optical/X rays might be surmised to
be enhanced in BL Lacs as these photons are upscattered via the inverse Compton
(IC) process. From the observed correlations among the spectral parameters
during optical/X-ray variations we aim at predicting corresponding correlations
in the gamma-ray band, and the actual relations between the gamma-ray and the
X-ray variability consistent with the SSC emission process. We start from the
homogeneous single-zone SSC source model, with log-parabolic energies
distributions of emitting electron as required by the X-ray data of many
sources. We find relations among spectral parameters of the IC radiation in
both the Thomson (for Low energy BL Lacs) and the Klein-Nishina regimes (mainly
for High energy BL Lacs) and we compute how variability is driven by a smooth
increase of key source parameters, primarily the root mean square electron
energy. The single component SSC source model in the Thomson regime turns out
to be adequate for many LBL sources. However, the simple model meets its limits
with the fast/strong flares recently reported for a few sources in the TeV
range; these require sudden accelerations of emitting electrons in a second
source component.Comment: 12 pages, 2 tables, 8 figure
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