Synchrotron X-ray diagnostics of cutoff shape of nonthermal electron spectrum at young supernova remnants. (arXiv:1301.7499v1 [astro-ph.HE])

Synchrotron X-ray diagnostics of cutoff shape of nonthermal electron spectrum at young supernova remnants. (arXiv:1301.7499v1 [astro-ph.HE]):
Context: The synchrotron X-rays can be a useful tool to investigate the
electron acceleration at young supernova remnants (SNRs). Aims: At present,
since the magnetic field configuration around the shocks of SNRs is uncertain,
it is not clear whether the electron acceleration is limited by SNR age,
synchrotron cooling, or even escape from the acceleration region. We study if
the acceleration mechanism can be constrained by the cutoff shape of the
electron spectrum around the maximum energy. Methods: We derive analytical
formulae of the cutoff shape in each case where the maximum electron energy is
determined by SNR age, synchrotron cooling and escape from the shock. They are
related to the energy dependence of the electron diffusion coefficient. Next,
we discuss whether information on the cutoff shape is provided by near future
observations which gives simply the photon indices and the flux ratios in the
soft and hard X-ray bands. Results: If the power-law index of the electron
spectrum is independently determined by other observations, then we can
constrain the cutoff shape by comparing theoretical predictions of the photon
indices and/or the flux ratios with observed data which will be measured by
NuStar and/or Astro-H. Such study is helpful in understanding the acceleration
mechanism. In particular, it will bring us another independent constraint on
the magnetic field strength around the shocks of SNRs.

Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region. (arXiv:1210.2108v1 [astro-ph.HE])

Nonthermal X-rays from low-energy cosmic rays: Application to the 6.4 keV line emission from the Arches cluster region. (arXiv:1210.2108v1 [astro-ph.HE]):
The iron line at 6.4 keV provides a valuable spectral diagnostic in several
fields of X-ray astronomy. It often results from the reprocessing of external
X-rays by a neutral or low-ionized medium, but it can also be excited by
impacts of low-energy cosmic rays. This paper aims to provide signatures
allowing identification of radiation from low-energy cosmic rays in X-ray
spectra showing the 6.4 keV line. We study in detail the production of
nonthermal line and continuum X-rays by interaction of accelerated electrons
and ions with a neutral ambient gas. Corresponding models are then applied to
XMM-Newton observations of the X-ray emission emanating from the Arches cluster
region near the Galactic center. Bright 6.4 keV line structures are observed
around the Arches cluster. This emission is very likely produced by cosmic
rays. We find that it can result from the bombardment of molecular gas by
energetic ions, but probably not by accelerated electrons. Using a model of
X-ray production by cosmic-ray ions, we obtain a best-fit metallicity of the
ambient medium of 1.7 plus-minus 0.2 times the solar metallicity. A large flux
of low-energy cosmic ray ions could be produced in the ongoing supersonic
collision between the star cluster and an adjacent molecular cloud. We find
that a particle acceleration efficiency in the resulting shock system of a few
percent would give enough power in the cosmic rays to explain the luminosity of
the nonthermal X-ray emission. Depending on the unknown shape of the kinetic
energy distribution of the fast ions above 1 GeV per nucleon, the Arches
cluster region may be a source of high-energy gamma-rays detectable with the
Fermi Gamma-ray Space Telescope. At present, the X-ray emission prominent in
the 6.4 keV Fe line emanating from the Arches cluster region probably offers
the best available signature for a source of low-energy hadronic cosmic rays in
the Galaxy.

3D Simulations of the Thermal X-ray Emission from Young Supernova Remnants Including Efficient Particle Acceleration. (arXiv:1210.0085v1 [astro-ph.HE])

3D Simulations of the Thermal X-ray Emission from Young Supernova Remnants Including Efficient Particle Acceleration. (arXiv:1210.0085v1 [astro-ph.HE]):
Supernova remnants (SNRs) are believed to be the major contributors to
Galactic cosmic rays. The detection of non-thermal emission from SNRs
demonstrates the presence of energetic particles, but direct signatures of
protons and other ions remain elusive. If these particles receive a sizeable
fraction of the explosion energy, the morphological and spectral evolution of
the SNR must be modified. To assess this, we run 3D hydrodynamic simulations of
a remnant coupled with a non-linear acceleration model. We obtain the
time-dependent evolution of the shocked structure, impacted by the
Rayleigh-Taylor hydrodynamic instabilities at the contact discontinuity and by
the back-reaction of particles at the forward shock. We then compute the
progressive temperature equilibration and non-equilibrium ionization state of
the plasma, and its thermal emission in each cell. This allows us to produce
the first realistic synthetic maps of the projected X-ray emission from the
SNR. Plasma conditions (temperature, ionization age) can vary widely over the
projected surface of the SNR, especially between the ejecta and the ambient
medium owing to their different composition. This demonstrates the need for
spatially-resolved spectroscopy. We find that the integrated emission is
reduced with particle back-reaction, with the effect being more significant for
the highest photon energies. Therefore different energy bands, corresponding to
different emitting elements, probe different levels of the impact of particle
acceleration. Our work provides a framework for the interpretation of SNR
observations with current X-ray missions (Chandra, XMM-Newton, Suzaku) and with
upcoming X-ray missions (such as Astro-H).

Jets and gamma-ray emission from isolated accreting black holes. (arXiv:1209.0293v1 [astro-ph.HE])

Jets and gamma-ray emission from isolated accreting black holes. (arXiv:1209.0293v1 [astro-ph.HE]):
The large number of isolated black holes (IBHs) in the Galaxy, estimated to
be 10^8, implies a very high density of 10^-4 pc^-3 and an average distance
between IBHs of 10 pc. Our study shows that the magnetic flux, accumulated on
the horizon of an IBH because of accretion of interstellar matter, allows the
Blandford-Znajeck mechanism to be activated. Thus, electron-positron jets can
be launched. We have performed 2D numerical modelling which allowed the jet
power to be estimated. Their inferred properties make such jets a feasible
electron accelerator which, in molecular clouds, allows electron energy to be
boosted up to 1 PeV. For the conditions expected in molecular clouds the
radiative cooling time should be comparable to the escape time. Thus these
sources can contribute both to the population of unidentified point-like
sources and to the local cosmic ray (CR) electron spectrum. The impact of the
generated electron CRs depends on the diffusion rate inside molecular clouds
(MCs). If the diffusion regime in a MC is similar to Galactic diffusion, the
produced electrons should rapidly escape the cloud and contribute to the
Galactic CR population at very high energies >100 TeV. However, due to the
modest jet luminosity (at the level of 10^35 erg s^-1) and low filling factor
of MC, these sources cannot make a significant contribution to the spectrum of
cosmic ray electrons at lower energies. On the other hand, if the diffusion
within MCs operates at a rate close to the Bohm limit, the CR electrons
escaping from the source should be confined in the cloud, significantly
contributing to the local density of CRs. The IC emission of these
locally-generated CRs may explain the variety of gamma ray spectra detected
from nearby MCs.

XMM-Newton evidence of shocked ISM in SN 1006: indications of hadronic acceleration. (arXiv:1208.5966v1 [astro-ph.HE])

XMM-Newton evidence of shocked ISM in SN 1006: indications of hadronic acceleration. (arXiv:1208.5966v1 [astro-ph.HE]):
Shock fronts in young supernova remnants are the best candidates for being
sites of cosmic ray acceleration up to a few PeV, though conclusive
experimental evidence is still lacking. Hadron acceleration is expected to
increase the shock compression ratio, providing higher postshock densities, but
X-ray emission from shocked ambient medium has not firmly been detected yet in
remnants where particle acceleration is at work. We exploited the deep
observations of the XMM-Newton Large Program on SN 1006 to verify this
prediction. We performed spatially resolved spectral analysis of a set of
regions covering the southeastern rim of SN 1006. We studied the spatial
distribution of the thermodynamic properties of the ambient medium and
carefully verified the robustness of the result with respect to the analysis
method. We detected the contribution of the shocked ambient medium. We also
found that the postshock density of the interstellar medium significantly
increases in regions where particle acceleration is efficient. Under the
assumption of uniform preshock density, we found that the shock compression
ratio reaches a value of ~6 in regions near the nonthermal limbs. Our results
support the predictions of shock modification theory and indicate that effects
of acceleration of cosmic ray hadrons on the postshock plasma can be observed
in supernova remnants.

Simulating the Toothbrush: Evidence for a triple merger of galaxy clusters. (arXiv:1206.6118v2 [astro-ph.CO] UPDATED)

Simulating the Toothbrush: Evidence for a triple merger of galaxy clusters. (arXiv:1206.6118v2 [astro-ph.CO] UPDATED):
The newly discovered galaxy cluster 1RXS J0603.3+4214 hosts a 1.9 Mpc long,
bright radio relic with a peculiar linear morphology. Using hydrodynamical
+N-body AMR simulations of the merger between three initially hydrostatic
clusters in an idealised setup, we are able to reconstruct the morphology of
the radio relic. Based on our simulation, we can constrain the merger geometry,
predict lensing mass measurements and X-ray observations. Comparing such models
to X-ray, redshift and lensing data will validate the geometry of this complex
merger which helps to constrain the parameters for shock acceleration of
electrons that produces the radio relic.

Discovery of radio halos and double-relics in distant MACS galaxy clusters: clues to the efficiency of particle acceleration. (arXiv:1206.6102v1 [astro-ph.CO])

Discovery of radio halos and double-relics in distant MACS galaxy clusters: clues to the efficiency of particle acceleration. (arXiv:1206.6102v1 [astro-ph.CO]):
We have performed 323 MHz observations with the Giant Metrewave Radio
Telescope of the most promising candidates selected from the MACS catalog. The
aim of the work is to extend our knowledge of the radio halo and relic
populations to z>0.3, the epoch in which massive clusters formed. In
MACSJ1149.5+2223 and MACSJ1752.1+4440, we discovered two double-relic systems
with a radio halo, and in MACSJ0553.4-3342 we found a radio halo. Archival Very
Large Array observations and Westerbork Synthesis Radio Telescope observations
have been used to study the polarization and spectral index properties. The
radio halo in MACSJ1149.5+2223 has the steepest spectrum ever found so far in
these objects (alpha > 2). The double relics in MACSJ1149.5+2223 are peculiar
in their position that is misaligned with the main merger axis. The relics are
polarized up to 30% and 40% in MACSJ1149.5+2223 and MACSJ1752.040+44,
respectively. In both cases, the magnetic field is roughly aligned with the
relics' main axes. The spectra in the relics in MACSJ1752.040+44 steepen
towards the cluster centre, in agreement with model expectations. X-ray data on
MACSJ0553.4-3342 suggests that this cluster is undergoing a major merger, with
the merger axis close to the plane of the sky. The cores of the disrupted
clusters have just passed each other, but no radio relic is detected in this
system. If turbulence is responsible for the radio emission, we argue that it
must develop before the core passage. A comparison of double relic plus halo
system with cosmological simulations allows a simultaneous estimate of the
acceleration efficiencies at shocks (to produce relics) and of turbulence (to
produce the halo).