400 research outputs found
Can Dark Matter Annihilation Dominate the Extragalactic Gamma-Ray Background?
Annihilating dark matter (DM) has been discussed as a possible source of
gamma-rays from the galactic center (GC) and contributing to the extragalactic
gamma-ray background (EGB). Assuming universality of the density profile of DM
halos, we show that it is quite unlikely that DM annihilation is a main
constituent of EGB, without exceeding the observed gamma-ray flux from the GC.
This argument becomes stronger when we include enhancement of the density
profiles by supermassive black holes or baryon cooling. The presence of
substructure may loosen the constraint, but only if a very large cross section
as well as the rather flat profile are realized.Comment: 4 pages, 2 figures, accepted by PR
Dark Matter Searche with GLAST
The Gamma-Ray Large Area Space Telescope (GLAST), scheduled to be launched in
fall 2007, is the next generation satellite for high-energy gamma-ray
astronomy. The Large Area Telescope (LAT),
GLAST main instrument, with a wide field of view (> 2 sr), a large effective
area (> 8000 cm^2 at 1 GeV) and 20 MeV - 300 GeV energy range, will provide
excellent high energy gamma-ray observations for Dark Matter searches. In this
paper we examine the potential of the LAT to detect gamma-rays coming from
WIMPS annihilation in the context of supersymmetry. As an example, two search
regions are investigated: the galactic center and the galactic satellites.Comment: Comment: 6 pages, 3 figures, 36th COSPAR proceeding accepted for
publication in "Advances in Space Research
Is the PAMELA Positron Excess Winos?
Recently the PAMELA satellite-based experiment reported an excess of galactic
positrons that could be a signal of annihilating dark matter. The PAMELA data
may admit an interpretation as a signal from a wino-like LSP of mass about 200
GeV, normalized to the local relic density, and annihilating mainly into
W-bosons. This possibility requires the current conventional estimate for the
energy loss rate of positrons be too large by roughly a factor of five. Data
from anti-protons and gamma rays also provide tension with this interpretation,
but there are significant astrophysical uncertainties associated with their
propagation. It is not unreasonable to take this well-motivated candidate
seriously, at present, in part because it can be tested in several ways soon.
The forthcoming PAMELA data on higher energy positrons and the FGST (formerly
GLAST) data, should provide important clues as to whether this scenario is
correct. If correct, the wino interpretation implies a cosmological history in
which the dark matter does not originate in thermal equilibrium.Comment: 7 pages, 4 figue
The final COS-B database: In-flight calibration of instrumental parameters
A method for the determination of temporal variation of sensitivity is designed to find a set of parameters which lead to maximum consistency between the intensities derived from different observation periods. This method is briefly described and the resulting sensitivity and background variations presented
Mobility as a Feature (MaaF): Why and how ride sharing platforms have evolved into super apps
With Mobility as a Feature (MaaF), transportation scholars propose an extension of the Mobility as a Service (MaaS) concept. Leveraging the ongoing trend of platformization, MaaF intends to integrate mobility with unrelated services such as food delivery, grocery delivery, financial services, or e-commerce and shopping. In this research, I show that some ride sharing platforms basically already provide MaaF functionality. Uber, Grab, Didi Chuxing, Bolt, and others have transformed into super apps, offering a wide range of services beyond their core ride sharing business. The findings of this study offer valuable insights into which ride sharing platforms are actively pursuing the super app strategy and shed light on the motivations driving their expansion into diversified service offerings. While the phenomenon of super apps is already established in the global South, it is now gaining interest in Europe and the US. Indeed, the present study argues that super apps are poised to shape the future of mobility and service provision, representing a significant shift in how people access mobility and other services. It therefore calls for additional research on this topic to better understand the implications of MaaF and super apps in the transportation sector
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*
A point-like source of ~TeV gamma-rays has recently been seen towards the
Galactic center by HESS and other air Cerenkov telescopes. In recent work
(Ballantyne et al. 2007), we demonstrated that these gamma-rays can be
attributed to high-energy protons that (i) are accelerated close to the event
horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and
(iii) finally interact to produce gamma-rays. The same hadronic collision
processes will necessarily lead to the creation of electrons and positrons.
Here we calculate the synchrotron emissivity of these secondary leptons in the
same magnetic field configuration through which the initiating protons have
been propagated in our model. We compare this emission with the observed ~GHz
radio spectrum of the inner few pc region which we have assembled from archival
data and new measurements we have made with the Australia Telescope Compact
Array. We find that our model predicts secondary synchrotron emission with a
steep slope consistent with the observations but with an overall normalization
that is too large by a factor of ~ 2. If we further constrain our theoretical
gamma-ray curve to obey the implicit EGRET upper limit on emission from this
region we predict radio emission that is consistent with observations, i.e.,
the hadronic model of gamma ray emission can, simultaneously and without
fine-tuning, also explain essentially all the diffuse radio emission detected
from the inner few pc of the Galaxy.Comment: 11 pages, 2 figures. Two references missing from published version
added and acknowledgements extende
Evidence for a Galactic gamma ray halo
We present quantitative statistical evidence for a -ray emission halo
surrounding the Galaxy. Maps of the emission are derived. EGRET data were
analyzed in a wavelet-based non-parametric hypothesis testing framework, using
a model of expected diffuse (Galactic + isotropic) emission as a null
hypothesis. The results show a statistically significant large scale halo
surrounding the center of the Milky Way as seen from Earth. The halo flux at
high latitudes is somewhat smaller than the isotropic gamma-ray flux at the
same energy, though of the same order (O(10^(-7)--10^(-6)) ph/cm^2/s/sr above 1
GeV).Comment: Final version accepted for publication in New Astronomy. Some
additional results/discussion included, along with entirely revised figures.
19 pages, 15 figures, AASTeX. Better quality figs (PS and JPEG) are available
at http://tigre.ucr.edu/halo/paper.htm
Search for periodicities near 59 s in the COS-B gamma-ray data of 2CG195+04 (Geminga)
The COS-B data relating to five observations in the general direction of Geminga, spanning 6.7 years, were searched for pulsation near 59 s. The SAS-2 indication is not confirmed. An indication of a 59 s pulsation in the gamma ray emission from 2CG195+04 (Geminga) was reported. Early analysis of COS-B data supported the result while later improved statistics did not confirm it. Subsequently, detection of a 59 s pulsation in the emission from the direction of Geminga at ultra high gamma and X-rays was reported. Geminga was identified with the X-ray source 1E0630+128. The final COS-B data on Geminga which was observed five times for a total of 214 days are reported
- …