149 research outputs found
XMM-Newton observations of HESS J1813-178 reveal a composite Supernova remnant
We present X-ray and 12CO(J=1-0) observations of the very-high-energy (VHE)
gamma-ray source HESS J1813-178 with the aim of understanding the origin of the
gamma-ray emission. Using this dataset we are able to undertake spectral and
morphological studies of the X-ray emission from this object with greater
precision than previous studies. NANTEN 12CO(J=1-0) data are used to search for
correlations of the gamma-ray emission with molecular clouds which could act as
target material for gamma-ray production in a hadronic scenario. The NANTEN
12CO(J=1-0) observations show a giant molecular cloud of mass 2.5 10^5
M_{\sun} at a distance of 4 kpc in the vicinity of HESS J1813-178. Even
though there is no direct positional coincidence, this giant cloud might have
influenced the evolution of the gamma-ray source and its surroundings. The
X-ray data show a highly absorbed non-thermal X-ray emitting object coincident
with the previously known ASCA source AX J1813-178 showing a compact core and
an extended tail towards the north-east, located in the centre of the radio
shell-type Supernova remnant (SNR) G12.82-0.2. This central object shows
morphological and spectral resemblance to a Pulsar Wind Nebula (PWN) and we
therefore consider that the object is very likely to be a composite SNR. We
discuss the scenario in which the gamma-rays originate in the shell of the SNR
and the one in which they originate in the central object. We demonstrate, that
in order to connect the core X-ray emission to the VHE gamma-ray emission
electrons have to be accelerated to energies of at least 1 PeV.Comment: Submitted to A&
Production of Charged Pions, Kaons and Antikaons in Relativistic C+C and C+Au Collisions
Production cross sections of charged pions, kaons and antikaons have been
measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for
different polar emission angles. The kaon and antikaon energy spectra can be
described by Boltzmann distributions whereas the pion spectra exhibit an
additional enhancement at low energies. The pion multiplicity per participating
nucleon M(pi+)/A_part is a factor of about 3 smaller in C+Au than in C+C
collisions at 1.0 AGeV whereas it differs only little for the C and the Au
target at a beam energy of 1.8 AGeV. The K+ multiplicities per participating
nucleon M(K+)/A_part are independent of the target size at 1 AGeV and at 1.8
AGeV. The K- multiplicity per participating nucleon M(K-)/A_part is reduced by
a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This
effect might be caused by the absorption of antikaons in the heavy target
nucleus. Transport model calculations underestimate the K-/K+ ratio for C+C
collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K
mesons are neglected.Comment: 19 pages, 14 figures, accepted for publication in Eur. Phys. J.
Evidence for a Soft Nuclear Equation-of-State from Kaon Production in Heavy Ion Collisions
The production of pions and kaons has been measured in Au+Au collisions at
beam energies from 0.6 to 1.5 AGeV with the Kaon Spectrometer at SIS/GSI. The
K+ meson multiplicity per nucleon is enhanced in Au+Au collisions by factors up
to 6 relative to C+C reactions whereas the corresponding pion ratio is reduced.
The ratio of the K+ meson excitation functions for Au+Au and C+C collisions
increases with decreasing beam energy. This behavior is expected for a soft
nuclear equation-of-state.Comment: 14 pages, 2 figures, accepted for publication in Phys. Rev. Let
Enhanced Out-of-plane Emission of K+ Mesons observed in Au+Au Collisions at 1 AGeV
The azimuthal angular distribution of K+ mesons has been measured in Au + Au
collisions at 1 AGeV. In peripheral and semi-central collisions, K+ mesons
preferentially are emitted perpendicular to the reaction plane. The strength of
the azimuthal anisotropy of K+ emission is comparable to the one of pions. No
in-plane flow was found for K+ mesons near projectile and target rapidity.Comment: Accepted for publication in Phys. Rev.Let
Probing the local environment of the supernova remnant HESS J1731-347 with CO and CS observations
The shell-type supernova remnant HESS J1731 â 347 emits TeV gamma-rays, and is a key object for the study of the cosmic ray acceleration potential of supernova remnants. We use 0.5â1 arcmin Mopra CO/CS(1â0) data in conjunction with HI data to calculate column densities towards the HESS J1731 â 347 region. We trace gas within at least four Galactic arms, typically tracing total (atomic+molecular) line-of-sight H column densities of 2â3Ă 10ÂČÂČ cmâ»ÂČ. Assuming standard X-factor values and that most of the HI/CO emission seen towards HESS J1731 â 347 is on the near-side of the Galaxy, X-ray absorption column densities are consistent with HI+CO-derived column densities foreground to, but not beyond, the ScutumâCrux Galactic arm, suggesting a kinematic distance of âŒ3.2 kpc for HESS J1731 â 347. At this kinematic distance, we also find dense, infrared-dark gas traced by CS(1â0) emission coincident with the north of HESS J1731 â 347, the nearby HII region G353.43â0.37 and the nearby unidentified gamma-ray source HESS J1729 â 345. This dense gas lends weight to the idea that HESS J1729 â 345 and HESS J1731 â 347 are connected, perhaps via escaping cosmic-rays.N. Maxted, M. Burton, C. Braiding, G. Rowell, H. Sano, F. Voisin, M. Capasso, G. PĂŒhlhofer and Y. Fuku
Shock Acceleration of Cosmic Rays - a critical review
Motivated by recent unsuccessful efforts to detect the predicted flux of TeV
gamma-rays from supernova remnants, we present a critical examination of the
theory on which these predictions are based. Three crucial problems are
identified: injection, maximum achievable particle energy and spectral index.
In each case significant new advances in understanding have been achieved,
which cast doubt on prevailing paradigms such as Bohm diffusion and
single-fluid MHD. This indicates that more realistic analytical models, backed
by more sophisticated numerical techniques should be employed to obtain
reliable predictions. Preliminary work on incorporating the effects of
anomalous transport suggest that the resulting spectrum should be significantly
softer than that predicted by conventional theory.Comment: 8 pages, invited review presented at the 17th ECRS, Lodz, July 2000;
to appear in Journal of Physics G: Nuclear and Particle Physic
Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT
Studying the temporal variability of BL Lac objects at the highest energies
provides unique insights into the extreme physical processes occurring in
relativistic jets and in the vicinity of super-massive black holes. To this
end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in
the high (HE, 100 MeV 200 GeV)
gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE
light curve in the quiescent state is consistent with a log-normal behavior.
The VHE variability in this state is well described by flicker noise
(power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales
larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives
consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10
days) compatible with the VHE findings. The HE and VHE power spectral densities
show a scale invariance across the probed time ranges. A direct linear
correlation between the VHE and HE fluxes could neither be excluded nor firmly
established. These long-term-variability properties are discussed and compared
to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during
VHE-flaring states. The difference in power spectral noise behavior at VHE
energies during quiescent and flaring states provides evidence that these
states are influenced by different physical processes, while the compatibility
of the HE and VHE long-term results is suggestive of a common physical link as
it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure
Swift/XRT follow-up observation on IGR J17177-3656 at soft X-rays
Following the discovery of IGR J17177-3656 (Frankowski et al. 2011, ATel #3223) on 2011-03-15, we requested a Swift ToO, which was executed on 2011-03-16 from 18:39:24 to 20:41:01 UT with a total exposure of ~3.9 ks. 2266 photons have been extracted from the source region between 0.2 and 10 keV, corresponding to a net count rate of ~0.59 cts/s. Using the online Swift tool we find an enhanced position of RA = 259.428 (17h17m43s), Dec = -36.934 deg (-36:56:03s) (J2000), with a 90% confidence level error of 2.1 arcsec, consistent with the position measurements made by IBIS/ISGRI and JEM-X (ATel #3223)
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