476 research outputs found
On the Interaction Between Cosmic Rays and Dark Matter Molecular Clouds - II. The Age Distribution of Cosmic Ray Electrons
We explore further the proposal in paper I of this series that the
confinement time of cosmic ray nuclei in the Milky Way is determined by their
interaction with dark matter molecular clouds rather than by their escape from
the halo, as is assumed in conventional models of cosmic ray propagation. The
same proposal can be made for cosmic ray electrons. This proposal leads to a
specific age distribution for the electrons which is in agreement with Tang's
(1984) observations of the electron spectrum at high energies but not with
Nishimura et al's (1980) earlier data, which lead to a flatter spectrum.
However, the simplest leaky box and diffusion models disagree with both sets of
data so that our trapping model is supported if Tang's data are correct.Comment: 7 pages, no figures, using MNRAS latex styl
High-energy -ray emission from GRBs
GRBs are nowadays a rather well understood phenomenon in the soft (KeV-MeV)
-ray energy band, while only a few GRBs have been observed at high
photon energies (E_{\gamma} \ut > 1 GeV). It is also widely recognized that
GRBs accelerate protons to relativistic energies and that dense media are often
present nearby the sources. Within this framework we compute in detail the
high-energy -ray flux from the decay of neutral pions produced through
the interaction of accelerate protons with nucleons in the surrounding medium.
We also take into account the local and intergalactic -ray absorption.
The presence of magnetic fields around the GRB sources causes the deflection of
the accelerated protons and so a temporal spread of the produced high-energy
-rays with respect to the signal in the soft -ray band.
Moreover, we analyze the possibility to detect the -ray signal in the
GeV-TeV energy range by the ARGO detector under construction in Tibet.Comment: 9 pages, 7 figures, abstract shortened, to appear in Astronomy and
Astrophysic
Gamma ray astronomy and baryonic dark matter
Recently, Dixon et al. have re-analyzed the EGRET data, finding a
statistically significant diffuse -ray emission from the galactic halo.
We show that this emission can naturally be explained within a
previously-proposed model for baryonic dark matter, in which -rays are
produced through the interaction of high-energy cosmic-ray protons with cold
clouds clumped into dark clusters - these dark clusters supposedly
populate the outer galactic halo and can show up in microlensing observations.
Our estimate for the halo -ray flux turns out to be in remarkably good
agreement with the discovery by Dixon et al. We also address future prospects
to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999
Constraints on Cold H_2 Clouds from Gravitational Microlensing Searches
It has been proposed that the Galaxy might contain a population of cold
clouds in numbers sufficient to account for a substantial fraction of the total
mass of the Galaxy. These clouds would have masses of the order of 10^{-3}
Solar mass and sizes of the order of 10 AU. We consider here the lensing
effects of such clouds on the light from background stars. A semianalytical
formalism for calculation of the magnification event rate produced by such
gaseous lensing is developed, taking into account the spatial distribution of
the dark matter in the Galaxy, the velocity distribution of the lensing clouds
and source stars, and motion of the observer. Event rates are calculated for
the case of gaseous lensing of stars in the Large Magellanic Cloud and results
are directly compared with the results of the search for gravitational
microlensing events undertaken by the MACHO collaboration. The MACHO experiment
strongly constrains the properties of the proposed molecular clouds, but does
not completely rule them out. Future monitoring programs will either detect or
more strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change
Thermal stability of cold clouds in galaxy halos
We consider the thermal properties of cold, dense clouds of molecular
hydrogen and atomic helium. For cloud masses below 10^-1.7 Msun, the internal
pressure is sufficient to permit the existence of particles of solid or liquid
hydrogen at temperatures above the microwave background temperature. Optically
thin thermal continuum emission by these particles can balance cosmic-ray
heating of the cloud, leading to equilibria which are thermally stable even
though the heating rate is independent of cloud temperature. For the Galaxy,
the known heating rate in the disk sets a minimum mass of order 10^-6 Msun
necessary for survival. Clouds of this type may in principle comprise most of
the dark matter in the Galactic halo. However, we caution that the equilibria
do not exist at redshifts z > 1 when the temperature of the microwave
background was substantially larger than its current value; the formation and
survival of such clouds to the present epoch therefore remain open questions.Comment: 5 pp incl 2 figs, LaTeX, emulateapj.sty; ApJ Letters in press.
Significant revisions, results qualitatively unchange
Gravitational wave scintillation by a stellar cluster
The diffraction effects on gravitational waves propagating through a stellar
cluster are analyzed in the relevant approximation of Fresnel diffraction
limit. We find that a gravitational wave scintillation effect - similar to the
radio source scintillation effect - comes out naturally, implying that the
gravitational wave intensity changes in a characteristic way as the observer
moves.Comment: 9 pages, in press in IJMP
Gravitational waves from hyperbolic encounters
The emission of gravitational waves from a system of massive objects
interacting on hyperbolic orbits is studied in the quadrupole approximation.
Analytic expressions are derived for the gravitational radiation luminosity,
the total energy output and the gravitational radiation amplitude. An
estimation of the expected number of events towards different targets (i.e.
globular clusters and the center of the Galaxy) is also given. In particular,
for a dense stellar cluster at the galactic center, a rate up to one event per
year is obtained.Comment: 6 pages, 2 figure
A Search for Stellar Obscuration Events due to Dark Clouds
The recent detections of a large population of faint submillimetre sources,
an excess halo gamma-ray background, and the extreme scattering events observed
for extragalactic radio sources have been explained as being due to baryonic
dark matter in the form of small, dark, gas clouds. In this paper we present
the results of a search for the transient stellar obscurations such clouds are
expected to cause. We examine the Macho project light curves of 48 x 10^6 stars
toward the Galactic bulge, LMC and SMC for the presence of dark cloud
extinction events. We find no evidence for the existence of a population of
dark gas clouds with Av > 0.2 and masses between ~ 10^-4 and 10^-2 M_solar in
the Galactic disk or halo. However, it is possible that such dark cloud
populations could exist if they are clustered in regions away from the observed
lines of sight.Comment: 13 pages, 9 figures, submitted to Ap
Optical, near-IR and -ray observations of SN 2015J and its host galaxy
SN 2015J was discovered on April 27th 2015 and is classified as a type IIn
supernova. At first, it appeared to be an orphan SN candidate, i.e. without any
clear identification of its host galaxy. Here, we present the analysis of the
observations carried out {by the VLT 8-m class telescope with the FORS2 camera
in the R band and the Magellan telescope (6.5 m) equipped with the IMACS
Short-Camera (V and I filters) and the FourStar camera (Ks filter)}. We show
that SN 2015J resides in what appears to be a very compact galaxy establishing
a relation between the SN event and its natural host. We also present and
discuss archival and new -ray data centred on SN 2015J. At the time of the
supernova explosion, Swift/XRT observations were made and a weak X-ray source
was detected at the location of SN 2015J. Almost one year later, the same
source was unambiguously identified during serendipitous observations by
Swift/XRT and -Newton, clearly showing an enhancement of the 0.3-10 keV
band flux by a factor with respect to the initial state. Swift/XRT
observations show that the source is still active in the -rays at a level of
counts s. The unabsorbed X-ray luminosity derived from the
{\it XMM}-Newton slew and SWIFT observations, erg
s, places SN 2015J among the brightest young supernovae in X-rays.Comment: The Astrophysical Journal, Volume 850, Number
HP Cet and Swift J0820.6-2805: two candidate intermediate polars observed by XMM-Newton
We report on the XMM-Newton observation of HP Cet and Swift J0820.6-2805, two
X-ray photon sources that are candidates to be members of the intermediate
polar class of cataclysmic variables. If the historical optical light curve of
HP Cet shows a periodic feature at minutes, a clear identification
of such a signature in the high energy band (apart for a variability on a time
scale of minutes as detected by the ROSAT satellite) is lacking. By
using XMM-Newton archive data, we clearly identify a feature (at
minutes) which is marginally consistent with one of the binary system orbital
periods reported in the literature. We also found a signature of a periodic
features on the time scale of minutes. In the case of Swift
J0820.6-2805, the intermediate polar nature was previously unclear and the
orbital and the white dwarf spin periods were unknown. Here, the 0.3-10 keV
data undoubtedly reveal an orbital period and a white dwarf spin of minutes and minutes, respectively. The spectral analysis
showed that both HP Cet and Swift J0820.6-280 are members of the under-luminous
IP subclass since their luminosity in the keV band is estimated to be
erg s and erg
s, respectively.Comment: Accepted for publication on MNRAS, main journal, 2020. 9 Pages. 9
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