152 research outputs found
Analysis of GeV-band gamma-ray emission from SNR RX J1713.7-3946
RX J1713.7-3946 is the brightest shell-type Supernova remnant (SNR) of the
TeV gamma-ray sky. Earlier Fermi-LAT results on low-energy gamma-ray emission
suggested that, despite large uncertainties in the background determination,
the spectrum is inconsistent with a hadronic origin. We update the GeV-band
spectra using improved estimates for the diffuse galactic gamma-ray emission
and more than doubled data volume. We further investigate the viability of
hadronic emission models for RX J1713.7-3946. We produced a high-resolution map
of the diffuse Galactic gamma-ray background corrected for HI self-absorption
and used it in the analysis of more than 5~years worth of Fermi-LAT data. We
used hydrodynamic scaling relations and a kinetic transport equation to
calculate the acceleration and propagation of cosmic-rays in SNR. We then
determined spectra of hadronic gamma-ray emission from RX J1713.7-3946,
separately for the SNR interior and the cosmic-ray precursor region of the
forward shock, and computed flux variations that would allow to test the model
with observations. We find that RX J1713.7-3946 is now detected by Fermi-LAT
with very high statistical significance, and the source morphology is best
described by that seen in the TeV band. The measured spectrum of RX
J1713.7-3946 is hard with index gamma=1.53 +/- 0.07, and the integral flux
above 500 MeV is F = (5.5 +/- 1.1)e-9 photons/cm^2/s. We demonstrate that
scenarios based on hadronic emission from the cosmic-ray precursor region are
acceptable for RX J1713.7-3946, and we predict a secular flux increase at a few
hundred GeV at the level of around 15% over 10 years, which may be detectable
with the upcoming CTA observatory.Comment: 9 pages, accepted for publication in Astronomy & Astrophysic
Stochastic re-acceleration and magnetic-field damping in Tycho's supernova remnant
A number of studies suggest that shock acceleration with particle feedback
and very efficient magnetic-field amplification combined with Alfv\'{e}nic
drift are needed to explain the rather soft radio spectrum and the narrow rims
observed for Tycho's SNR. We show that the broadband spectrum of Tycho's SNR
can alternatively be well explained when accounting for stochastic acceleration
as a secondary process. The re-acceleration of particles in the turbulent
region immediately downstream of the shock should be efficient enough to impact
particle spectra over several decades in energy. The so-called Alfv\'{e}nic
drift and particle feedback on the shock structure are not required in this
scenario. Additionally, we investigate whether synchrotron losses or
magnetic-field damping play a more profound role in the formation of the
non-thermal filaments. We solve the full particle transport equation in
test-particle mode using hydrodynamic simulations of the SNR plasma flow. The
background magnetic field is either computed from the induction equation or
follows analytic profiles, depending on the model considered. Fast-mode waves
in the downstream region provide the diffusion of particles in momentum space.
We show that the broadband spectrum of Tycho can be well explained if
magnetic-field damping and stochastic re-acceleration of particles are taken
into account. Although not as efficient as standard DSA, stochastic
acceleration leaves its imprint on the particle spectra, which is especially
notable in the emission at radio wavelengths. We find a lower limit for the
post-shock magnetic-field strength , implying
efficient amplification even for the magnetic-field damping scenario. For the
formation of the filaments in the radio range magnetic-field damping is
necessary, while the X-ray filaments are shaped by both the synchrotron losses
and magnetic-field damping.Comment: 14 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Predicted gamma-ray image of SN 1006 due to inverse Compton emission
We propose a method to synthesize the inverse Compton (IC) γ-ray image of a supernova remnant starting from the radio (or hard X-ray) map and using results of the spatially resolved X-ray spectral analysis. The method is successfully applied to SN 1006. We found that synthesized IC γ-ray images of SN 1006 show morphology in nice agreement with that reported by the High Energy Stereoscopic System (HESS) collaboration. The good correlation found between the observed very high energy γ-ray and X-ray/radio appearance can be considered as evidence of the fact that the γ-ray emission of SN 1006 observed by HESS is leptonic in origin, although a hadronic origin may not be excluded.Fil: Petruk, O.. Institute for Applied Problems in Mechanics and Mathematics; UcraniaFil: Bocchino, F.. Istituto Nazionale Di AstrofÃsica. Osservatorio Astronómico Di Palermo; ItaliaFil: Miceli, M.. Istituto Nazionale Di AstrofÃsica. Osservatorio Astronómico Di Palermo; ItaliaFil: Dubner, Gloria Mabel. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; ArgentinaFil: Castelletti, Gabriela Marta. Consejo Nacional de Investigaciónes CientÃficas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de AstronomÃa y FÃsica del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de AstronomÃa y FÃsica del Espacio; ArgentinaFil: Orlando, S.. Istituto Nazionale Di AstrofÃsica. Osservatorio Astronómico Di Palermo; ItaliaFil: Iakubovskyi, D.. Bogolyubov Institute for Theoretical Physics; UcraniaFil: Telezhinsky, I.. Kiev National Taras Shevchenko University; Ucrani
A New Model For Vela Jr. Supernova Remnant
We consider Vela Jr. as being the old Supernova Remnant (SNR) at the
beginning of the transition from adiabatic to radiative stage of evolution.
According to our model, Vela Jr. is situated outside Vela SNR at the distance
of 600 pc and its age is 17500 yr. We model the high energy fluxes from Vela
Jr. and its broadband spectrum. We find our results compatible with
experimental data in radio waves, X- and gamma-rays. Our hydrodynamical model
of Vela Jr. explains the observed TeV gamma-ray flux by hadronic mechanism. The
proposed model does not contradict to the low density environment of the SNR
and does not need extreme fraction of the explosion energy to be transferred to
Cosmic Rays.Comment: Accepted for publication in Astroparticle Physic
Time-Dependent Escape of Cosmic Rays from Supernova Remnants, and their Interaction with Dense Media
Context. Supernova remnants (SNRs) are thought to be the main source of
Galactic cosmic rays (CRs) up to the "knee" in CR spectrum. During the
evolution of a SNR, the bulk of the CRs are confined inside the SNR shell. The
highest-energy particles leave the system continuously, while the remaining
adiabatically cooled particles are released when the SNR has expanded
sufficiently and decelerated so that the magnetic field at the shock is no
longer able to confine them. Particles escaping from the parent system may
interact with nearby molecular clouds, producing -rays in the process
via pion decay. The soft gamma-ray spectra observed for a number of SNRs
interacting with molecular clouds, however, challenge current theories of
non-linear particle acceleration that predict harder spectra. Aims. We study
how the spectrum of escaped particles depends on the time-dependent
acceleration history in both Type Ia and core-collapse SNRs, as well as on
different assumptions about the diffusion coefficient in the vicinity of the
SNR. Methods. We solve the CR transport equation in a test-particle approach
combined with numerical simulations of SNR evolution. Results. We extend our
method for calculating the cosmic-ray acceleration in SNRs to trace the escaped
particles in a large volume around SNRs. We calculate the evolution of the
spectra of CRs that have escaped from a SNR into a molecular cloud or dense
shell for two diffusion models. We find a strong confinement of CRs in a close
region around the SNR, and a strong dilution effect for CRs that were able to
propagate out as far as a few SNR radii.Comment: 11 pages, 8 figures. Accepted for publication in A&
Aspect angle for interstellar magnetic field in SN 1006
A number of important processes taking place around strong shocks in
supernova remnants (SNRs) depend on the shock obliquity. The measured
synchrotron flux is a function of the aspect angle between interstellar
magnetic field (ISMF) and the line of sight. Thus a model of non-thermal
emission from SNRs should account for the orientation of the ambient magnetic
field. We develop a new method for the estimation of the aspect angle, based on
the comparison between observed and synthesized radio maps of SNRs, making
different assumptions about the dependence of electron injection efficiency on
the shock obliquity. The method uses the azimuthal profile of radio surface
brightness as a probe for orientation of ambient magnetic field because it is
almost insensitive to the downstream distribution of magnetic field and
emitting electrons. We apply our method to a new radio image of SN 1006
produced on the basis of archival VLA and Parkes data. The image recovers
emission from all spatial structures with angular scales from few arcseconds to
15 arcmin. We explore different models of injection efficiency and find the
following best-fitting values for the aspect angle of SN 1006: phi=70 +/- 4.2
deg if the injection is isotropic, phi=64 +/- 2.8 deg for quasi-perpendicular
injection (SNR has an equatorial belt in both cases) and phi=11 +/- 0.8 deg for
quasi-parallel injection (polar-cap model of SNR). In the last case, SN 1006 is
expected to have a centrally-peaked morphology contrary to what is observed.
Therefore, our analysis provides some indication against the quasi-parallel
injection model.Comment: Accepted for publication in MNRA
Gamma-ray Observations Under Bright Moonlight with VERITAS
Imaging atmospheric Cherenkov telescopes (IACTs) are equipped with sensitive
photomultiplier tube (PMT) cameras. Exposure to high levels of background
illumination degrades the efficiency of and potentially destroys these
photo-detectors over time, so IACTs cannot be operated in the same
configuration in the presence of bright moonlight as under dark skies. Since
September 2012, observations have been carried out with the VERITAS IACTs under
bright moonlight (defined as about three times the night-sky-background (NSB)
of a dark extragalactic field, typically occurring when Moon illumination >
35%) in two observing modes, firstly by reducing the voltage applied to the
PMTs and, secondly, with the addition of ultra-violet (UV) bandpass filters to
the cameras. This has allowed observations at up to about 30 times previous NSB
levels (around 80% Moon illumination), resulting in 30% more observing time
between the two modes over the course of a year. These additional observations
have already allowed for the detection of a flare from the 1ES 1727+502 and for
an observing program targeting a measurement of the cosmic-ray positron
fraction. We provide details of these new observing modes and their performance
relative to the standard VERITAS observations
The populations of hard X- and {\gamma}-ray sources: a correlation study and new possible identifications
We present the results of our analysis devoted to the research of sources
emitting in the energy bands surveyed by both the Swift-BAT and the Fermi-LAT
telescopes. We cross-correlate the Fermi-LAT 1-year point source catalogue
(1FGL) of {\gamma}-ray sources and the second Palermo BAT catalogue (2PBC) of
hard X-ray sources, establishing a correspondence between sources when their
error boxes overlap. We also extract the significance value in the BAT 15-150
keV map, obtained using a dedicated software for the reduction of BAT data, in
the direction of the 1FGL sources and take into account those above the
significance threshold {\sigma} = 3. We obtain a sample of common sources
emitting in both the hard X- and the {\gamma}-ray energy bands and evaluate its
content in galactic and extragalactic objects. We assess the fraction of
unidentified sources and describe in greater detail the properties of two of
them, 1FGL J0137.8+5814 and 1FGL J2056.7+4938, supporting their classification
as blazars after the analysis of their broad-band spectral energy distribution.
We discuss the blazar content of the collected 1FGL-2PBC sources: we build its
redshift distibution and compare it with that of the whole blazar population as
reported in the second edition of the BZCAT blazar catalogue.Comment: 12 pages, 5 figures, 5 table
Gamma-ray observations of Tycho's SNR with VERITAS and Fermi
High-energy gamma-ray emission from supernova remnants (SNRs) has provided a
unique perspective for studies of Galactic cosmic-ray acceleration. Tycho's SNR
is a particularly good target because it is a young, type Ia SNR that is
well-studied over a wide range of energies and located in a relatively clean
environment. Since the detection of gamma-ray emission from Tycho's SNR by
VERITAS and Fermi-LAT, there have been several theoretical models proposed to
explain its broadband emission and high-energy morphology. We report on an
update to the gamma-ray measurements of Tycho's SNR with 147 hours of VERITAS
and 84 months of Fermi-LAT observations, which represents about a factor of two
increase in exposure over previously published data. About half of the VERITAS
data benefited from a camera upgrade, which has made it possible to extend the
TeV measurements toward lower energies. The TeV spectral index measured by
VERITAS is consistent with previous results, but the expanded energy range
softens a straight power-law fit. At energies higher than 400 GeV, the
power-law index is . It
is also softer than the spectral index in the GeV energy range, , measured by this study using
Fermi--LAT data. The centroid position of the gamma-ray emission is coincident
with the center of the remnant, as well as with the centroid measurement of
Fermi--LAT above 1 GeV. The results are consistent with an SNR shell origin of
the emission, as many models assume. The updated spectrum points to a lower
maximum particle energy than has been suggested previously.Comment: Accepted for publication in The Astrophysical Journa
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