153 research outputs found
High-energy and very high-energy gamma-ray emission from the magnetar SGR 1900+14 neighbourhood
Magnetar wind nebulae (MWNe), created by new-born millisecond magnetars, and
magnetar giant flares are PeVatron candidates and even potential sources of
ultra high energy () cosmic rays (UHECRs). Nonthermal
high-energy (HE, ) and very high-energy (VHE, ) -ray emission from magnetars' neighbourhoods should be
a promising signature of acceleration processes. We investigate a possibility
of explaining HE and VHE -ray emission from the vicinity of the
magnetar SGR 1900+14 by cosmic rays accelerated in a Supernova remnant of a
magnetar-related Supernova and/or in a MWN. Simulation of the observed HE (the
extended Fermi-LAT source 4FGL J1908.6+0915e) and VHE (the extended H.E.S.S.
source candidate HOTS J1907+091 and the point-like HAWC TeV source 3HWC
J1907+085) -ray emission, spatially coincident with the magnetar SGR
1900+14, was carried out in the framework of hadronic (pp collisions with a
subsequent pion decay) and leptonic (inverse Compton scattering of low energy
background photons by ultrarelativistic electrons) models. We show that under
reasonable assumptions about parameters of the circumstellar medium the
observed -ray emission of Fermi-LAT 4FGL J1908.6+0915e, H.E.S.S.
HOTSJ1907+091 and 3HWC J1907+085 sources may be explained or at least
considerably contributed by a (still undetected) magnetar-connected Hypernova
remnant and/or a MWN created by new-born millisecond magnetar with a large
reserve of rotational energy .Comment: 13 pages, 7 figure
A dip in the UHECR spectrum and the transition from galactic to extragalactic cosmic rays
The dip is a feature in the diffuse spectrum of ultra-high energy (UHE)
protons caused by electron-positron pair production on the cosmic microwave
background (CMB) radiation. For a power-law generation spectrum , the
calculated position and shape of the dip is confirmed with high accuracy by the
spectra observed by the Akeno-AGASA, HiRes, Yakutsk and Fly's Eye detectors.
When the particle energies, measured in these detectors, are calibrated by the
dip, their fluxes agree with a remarkable accuracy. The predicted shape of the
dip is quite robust. The dip is only modified strongly when the fraction of
nuclei heavier than protons is high at injection, which imposes some
restrictions on the mechanisms of acceleration operating in UHECR sources. The
existence of the dip, confirmed by observations, implies that the transition
from galactic to extragalactic cosmic rays occurs at E \lsim 1\times 10^{18}
eV. We show that at energies lower than a characteristic value eV, the spectrum of extragalactic cosmic rays
flattens in all cases of interest, and it provides a natural transition to a
steeper galactic cosmic ray spectrum. This transition occurs at some energy
below , corresponding to the position of the so-called second knee.
We discuss extensively the constraints on this model imposed by current
knowledge of acceleration processes and sources of UHECR and compare it with
the traditional model of transition at the ankle.Comment: Version Accepted for Publication in Astroparticle Physics (minor
changes
A Bow Shock Nebula Around a Compact X-Ray Source in the Supernova Remnant IC443
We present spectra and high resolution images of the hard X-ray feature along
the southern edge of the supernova remnant IC443. Data from the Chandra X-ray
Observatory reveal a comet-shaped nebula of hard emission, which contains a
softer point source at its apex. We also present 20cm, 6cm, and 3.5cm images
from the Very Large Array that clearly show the cometary nebula. Based on the
radio and X-ray morphology and spectrum, and the radio polarization properties,
we argue that this object is a synchrotron nebula powered by the compact source
that is physically associated with IC443. The spectrum of the soft point source
is adequately but not uniquely fit by a black body model (kT=0.71 +/- 0.08 keV,
L=(6.5 +/- 0.9) * 10^31 erg/s). The cometary morphology of the nebula is the
result of the supersonic motion of the neutron star (V_NS=250 +/- 50 km/s),
which causes the relativistic wind of the pulsar to terminate in a bow shock
and trail behind as a synchrotron tail. This velocity is consistent with an age
of 30,000 years for the SNR and its associated neutron star.Comment: 9 pages, 5 figures, accepted for publication in the ApJ Letter
Anomalous Fluctuations in Observations of Q0957+561 A,B: Smoking Gun of a Cosmic String?
We report the detection of anomalous brightness fluctuations in the multiple
image Q0957+561 A,B gravitational lens system, and consider whether such
anomalies have a plausible interpretation within the framework of cosmic string
theory. We study a simple model of gravitational lensing by an asymmetrical
rotating string. An explicit form of the lens equation is obtained and
approximate relations for magnification are derived. We show that such a model
with typical parameters of the GUT string can quantitatively reproduce the
observed pattern of brightness fluctuations. On the other hand, explanation
involving a binary star system as an alternative cause requires an unacceptably
large massive object at a small distance. We also discuss possible
observational manifestations of cosmic strings within our lens model.Comment: Published in Astronomy and Astrophysics. 7 pages, 6 figure
Pulsar PSR B0656+14, the Monogem Ring, and the Origin of the `Knee' in the Primary Cosmic Ray Spectrum
The Monogem ring is a bright, diffuse, 25-degree-diameter supernova remnant
easily visible in soft X-ray images of the sky. Projected within the ring is a
young radio pulsar, PSR B0656+14. An association between the remnant and pulsar
has been considered, but was seemingly ruled out by the direction and magnitude
of the pulsar proper motion and by a distance estimate that placed the pulsar
twice as far from Earth as the remnant. Here we show that in fact the pulsar
was born very close to the center of the expanding remnant, both in distance
and projection. The inferred pulsar and remnant ages are in good agreement. The
conclusion that the pulsar and remnant were born in the same supernova
explosion is nearly inescapable. The remnant distance and age are in remarkable
concordance with the predictions of a model for the primary cosmic ray energy
spectrum in which the `knee' feature is produced by a single dominant source.Comment: 4 pages, to appear in the Astrophys. J. Lett. Full size color figure
can be found at http://www.thorsett.org/researc
Dark Matter Search Perspectives with GAMMA-400
GAMMA-400 is a future high-energy gamma-ray telescope, designed to measure
the fluxes of gamma-rays and cosmic-ray electrons + positrons, which can be
produced by annihilation or decay of dark matter particles, and to survey the
celestial sphere in order to study point and extended sources of gamma-rays,
measure energy spectra of Galactic and extragalactic diffuse gamma-ray
emission, gamma-ray bursts, and gamma-ray emission from the Sun. GAMMA-400
covers the energy range from 100 MeV to ~3000 GeV. Its angular resolution is
~0.01 deg(Eg > 100 GeV), and the energy resolution ~1% (Eg > 10 GeV). GAMMA-400
is planned to be launched on the Russian space platform Navigator in 2019. The
GAMMA-400 perspectives in the search for dark matter in various scenarios are
presented in this paperComment: 4 pages, 4 figures, submitted to the Proceedings of the International
Cosmic-Ray Conference 2013, Brazil, Rio de Janeir
Seeking String Theory in the Cosmos
We review the existence, formation and properties of cosmic strings in string
theory, the wide variety of observational techniques that are being employed to
detect them, and the constraints that current observations impose on string
theory models.Comment: 25 pages; contribution for String Cosmology issue of Classical and
Quantum Gravity. References added and other improvements. Matches journal
versio
Pulsar Wind Nebulae in Evolved Supernova Remnants
For pulsars similar to the one in the Crab Nebula, most of the energy input
to the surrounding wind nebula occurs on a timescale of less than 1000 years;
during this time, the nebula expands into freely expanding supernova ejecta. On
a timescale 10,000 years, the interaction of the supernova with the surrounding
medium drives a reverse shock front toward the center of the remnant, where it
crushes the PWN (pulsar wind nebula). One- and two-dimensional, two-fluid
simulations of the crushing and re-expansion phases of a PWN show that (1)
these phases are subject to Rayleigh-Taylor instabilities that result in the
mixing of thermal and nonthermal fluids, and (2) asymmetries in the surrounding
interstellar medium give rise to asymmetries in the position of the PWN
relative to the pulsar and explosion site. These effects are expected to be
observable in the radio emission from evolved PWN because of the long lifetimes
of radio emitting electrons. The scenario can explain the chaotic and
asymmetric appearance of the Vela X PWN relative to the Vela pulsar without
recourse to a directed flow from the vicinity of the pulsar. The displacement
of the radio nebulae in G327.1--1.1, MSH15--56 (G326.3--1.8), G0.9+0.1, and W44
relative to the X-ray nebulae may be due to this mechanism. On timescales much
greater than the nebular crushing time, the initial PWN may be mixed with
thermal gas and become unobservable, so that even the radio emission is
dominated by recently injected particles.Comment: 22 pages, 9 figures; submitted to Ap
Understanding the origin and impact of relativistic cosmic particles with very-high-energy gamma-rays
This white paper briefly summarizes the importance of the study of relativistic cosmic rays, both as a constituent of our Universe, and through their impact on stellar and galactic evolution. The focus is on what can be learned over the coming decade through ground-based gamma-ray observations over the 20 GeV to 300 TeV range
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