296 research outputs found
New Neighbours: Modelling the Growing Population of Gamma-ray Millisecond Pulsars
The Fermi Large Area Telescope, in collaboration with several groups from the
radio community, have had marvellous success at uncovering new gamma-ray
millisecond pulsars (MSPs). In fact, MSPs now make up a sizable fraction of the
total number of known gamma-ray pulsars. The MSP population is characterized by
a variety of pulse profile shapes, peak separations, and radio-to-gamma phase
lags, with some members exhibiting nearly phase-aligned radio and gamma-ray
light curves (LCs). The MSPs' short spin periods underline the importance of
including special relativistic effects in LC calculations, even for emission
originating from near the stellar surface. We present results on modelling and
classification of MSP LCs using standard pulsar model geometries.Comment: 4 pages, 2 figures, proceedings of the ICREA Workshop on The
High-Energy Emission from Pulsars and their Systems (HEEPS), Sant Cugat,
Spai
Cosmic rays and molecular clouds
This paper deals with the cosmic-ray penetration into molecular clouds and
with the related gamma--ray emission. High energy cosmic rays interact with the
dense gas and produce neutral pions which in turn decay into two gamma rays.
This makes molecular clouds potential sources of gamma rays, especially if they
are located in the vicinity of a powerful accelerator that injects cosmic rays
in the interstellar medium. The amplitude and duration in time of the
cosmic--ray overdensity around a given source depend on how quickly cosmic rays
diffuse in the turbulent galactic magnetic field. For these reasons, gamma-ray
observations of molecular clouds can be used both to locate the sources of
cosmic rays and to constrain the properties of cosmic-ray diffusion in the
Galaxy.Comment: To appear in the proceedings of the San Cugat Forum on Astrophysics
2012, 27 pages, 10 figure
IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array
Gamma rays at rest frame energies as high as 90 GeV have been reported from
gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is
considerable hope that a confirmed GRB detection will be possible with the
upcoming Cherenkov Telescope Array (CTA), which will have a larger effective
area and better low-energy sensitivity than current-generation imaging
atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a
detection, we have developed a phenomenological model for GRB emission between
1 GeV and 1 TeV that is motivated by the high-energy GRB detections of
Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower
energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray
Observatory. We show a number of statistics for detected GRBs, and describe how
the detectability of GRBs with CTA could vary based on a number of parameters,
such as the typical observation delay between the burst onset and the start of
ground observations. We also consider the possibility of using GBM on Fermi as
a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM
localization is problematic, the small field-of-view for IACTs can potentially
be overcome by scanning over the GBM error region. Overall, our results
indicate that CTA should be able to detect one GRB every 20 to 30 months with
our baseline instrument model, assuming consistently rapid pursuit of GRB
alerts, and provided that spectral breaks below 100 GeV are not a common
feature of the bright GRB population. With a more optimistic instrument model,
the detection rate can be as high as 1 to 2 GRBs per year.Comment: 28 pages, 24 figures, 4 tables, submitted to Experimental Astronom
Gamma-rays from millisecond pulsars in Globular Clusters
Globular clusters (GCs) with their ages of the order of several billion years
contain many final products of evolution of stars such as: neutron stars, white
dwarfs and probably also black holes. These compact objects can be at present
responsible for the acceleration of particles to relativistic energies.
Therefore, gamma-ray emission is expected from GCs as a result of radiation
processes occurring either in the inner magnetosperes of millisecond pulsars or
in the vicinity of accreting neutron stars and white dwarfs or as a result of
interaction of particles leaving the compact objects with the strong radiation
field within the GC. Recently, GeV gamma-ray emission has been detected from
several GCs by the new satellite observatory Fermi. Also Cherenkov telescopes
reported interesting upper limits at the TeV energies which start to constrain
the content of GCs. We review the results of these gamma-ray observations in
the context of recent scenarios for their origin.Comment: 20 pages, 9 figures, will be published in Astrophysics and Space
Science Series (Springer), eds. N. Rea and D.F. Torre
A review of the scorpion fauna of Saudi Arabia
The scorpions of Saudi Arabia were surveyed in the major regions of Jazan, Al-Medina, Al-Baha, Hail, and Riyadh, in addition to nine provinces surveyed more superficially. Jazan (1,440 specimens) had 10 buthids and two scorpionid species and subspecies; Al-Medina (867) had seven buthid and two scorpionid species and subspecies, one of which, the scorpionid Scorpio maurus (palmatus?), needs further confirmation of identity. The Al-Baha region (2421 specimens) contained five buthids and two scorpionid species and subspecies; Hail (1,921) had eight buthid and two scorpionid species and subspecies - the most common subspecies here was Scorpio maurus kruglovi. Androctonus crassicauda and Leiurus quinquestriatus were only found in Hail and Al-Baha; Androctonus bicolor was newly recorded in Hail and Riyadh. Riyadh (4,164 specimens) had nine buthid, one scorpionid and at least two hemiscorpiid species and subspecies. The Saudi fauna was found to comprise at least 28 species and subspecies of the families Buthidae, Scorpionidae and Hemiscorpiidae.Keywords: Buthidae, Scorpionidae, Diplocentrida
Radio variability of 1st 3-months Fermi blazars at 5 GHz: affected by interstellar scintillation?
Blazars from the first-three-months Fermi-AGN list were observed with the
Urumqi 25m radio telescope at 5GHz in IDV (Intra-Day Variability) mode and
inter-month observation mode. A significant correlation between the flux
density at 5GHz and the gamma-ray intensity for the Fermi-LAT detected blazars
is seen. There is a higher IDV detection rate in Fermi detected blazars than
those reported for other samples. Stronger variability appears at lower
Galactic latitudes; IDV appears to be stronger in weaker sources, indicating
that the variability is affected by interstellar scintillation.Comment: 4 pages, 4 figures, in proceedings of 'Multiwavelength Variability of
Blazars' in Guangzhou Uni. of China, 22-24, Sep. 2010, to appear in JA
Radio Band Observations of Blazar Variability
The properties of blazar variability in the radio band are studied using the
unique combination of temporal resolution from single dish monitoring and
spatial resolution from VLBA imaging; such measurements, now available in all
four Stokes parameters, together with theoretical simulations, identify the
origin of radio band variability and probe the characteristics of the radio jet
where the broadband blazar emission originates. Outbursts in total flux density
and linear polarization in the optical-to-radio bands are attributed to shocks
propagating within the jet spine, in part based on limited modeling invoking
transverse shocks; new radiative transfer simulations allowing for shocks at
arbitrary angle to the flow direction confirm this picture by reproducing the
observed centimeter-band variations observed more generally, and are of current
interest since these shocks may play a role in the gamma-ray flaring detected
by Fermi. Recent UMRAO multifrequency Stokes V studies of bright blazars
identify the spectral variability properties of circular polarization for the
first time and demonstrate that polarity flips are relatively common.
All-Stokes data are consistent with the production of circular polarization by
linear-to-circular mode conversion in a region that is at least partially
self-absorbed. Detailed analysis of single-epoch, multifrequency, all-Stokes
VLBA observations of 3C 279 support this physical picture and are best
explained by emission from an electron-proton plasma.Comment: 6 pages, 5 figures, uses, jaa.sty. Invited talk presented at the
conference Multifrequency Variability of Blazars, Guangzhou, China, September
22-24, 2010. To appear in J. Astrophys. Ast
Goldstone Fermion Dark Matter
We propose that the fermionic superpartner of a weak-scale Goldstone boson
can be a natural WIMP candidate. The p-wave annihilation of this `Goldstone
fermion' into pairs of Goldstone bosons automatically generates the correct
relic abundance, whereas the XENON100 direct detection bounds are evaded due to
suppressed couplings to the Standard Model. Further, it is able to avoid
indirect detection constraints because the relevant s-wave annihilations are
small. The interactions of the Goldstone supermultiplet can induce non-standard
Higgs decays and novel collider phenomenology.Comment: 25 pages, 6 figures. References added, minor typos corrected.
Submitted to JHE
Cosmic Ray Anomalies from the MSSM?
The recent positron excess in cosmic rays (CR) observed by the PAMELA
satellite may be a signal for dark matter (DM) annihilation. When these
measurements are combined with those from FERMI on the total () flux
and from PAMELA itself on the ratio, these and other results are
difficult to reconcile with traditional models of DM, including the
conventional mSUGRA version of Supersymmetry even if boosts as large as
are allowed. In this paper, we combine the results of a previously
obtained scan over a more general 19-parameter subspace of the MSSM with a
corresponding scan over astrophysical parameters that describe the propagation
of CR. We then ascertain whether or not a good fit to this CR data can be
obtained with relatively small boost factors while simultaneously satisfying
the additional constraints arising from gamma ray data. We find that a specific
subclass of MSSM models where the LSP is mostly pure bino and annihilates
almost exclusively into pairs comes very close to satisfying these
requirements. The lightest in this set of models is found to be
relatively close in mass to the LSP and is in some cases the nLSP. These models
lead to a significant improvement in the overall fit to the data by an amount
dof in comparison to the best fit without Supersymmetry
while employing boosts . The implications of these models for future
experiments are discussed.Comment: 57 pages, 31 figures, references adde
Flavor conversion of cosmic neutrinos from hidden jets
High energy cosmic neutrino fluxes can be produced inside relativistic jets
under the envelopes of collapsing stars. In the energy range E ~ (0.3 - 1e5)
GeV, flavor conversion of these neutrinos is modified by various matter effects
inside the star and the Earth. We present a comprehensive (both analytic and
numerical) description of the flavor conversion of these neutrinos which
includes: (i) oscillations inside jets, (ii) flavor-to-mass state transitions
in an envelope, (iii) loss of coherence on the way to observer, and (iv)
oscillations of the mass states inside the Earth. We show that conversion has
several new features which are not realized in other objects, in particular
interference effects ("L- and H- wiggles") induced by the adiabaticity
violation. The neutrino-neutrino scattering inside jet and inelastic neutrino
interactions in the envelope may produce some additional features at E > 1e4
GeV. We study dependence of the probabilities and flavor ratios in the
matter-affected region on angles theta13 and theta23, on the CP-phase delta, as
well as on the initial flavor content and density profile of the star. We show
that measurements of the energy dependence of the flavor ratios will, in
principle, allow to determine independently the neutrino and astrophysical
parameters.Comment: 56 pages, 19 figures. Minor changes. Accepted by JHEP
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