3,031 research outputs found
GLAST and Dark Matter Substructure in the Milky Way
We discuss the possibility of GLAST detecting gamma-rays from the
annihilation of neutralino dark matter in the Galactic halo. We have used "Via
Lactea", currently the highest resolution simulation of Galactic cold dark
matter substructure, to quantify the contribution of subhalos to the
annihilation signal. We present a simulated allsky map of the expected
gamma-ray counts from dark matter annihilation, assuming standard values of
particle mass and cross section. In this case GLAST should be able to detect
the Galactic center and several individual subhalos.Comment: 4 pages, 2 figures, to be published in "The Proceedings of the First
International GLAST Symposium", February 5-8, 2007, Stanford University, AIP,
Eds. S. Ritz, P.F. Michelson, and C. Meega
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
The Use of Weighting in Periodicity Searches in All-Sky Monitor Data: Applications to the GLAST LAT
The light curves produced by all-sky monitors, such as the Rossi X-ray Timing
Explorer All-Sky Monitor and the Swift Burst Alert Telescope (BAT), generally
have non-uniform error bars. In searching for periodic modulation in this type
of data using power spectra it can be important to use appropriate weighting of
data points to achieve the best sensitivity. It was recently demonstrated that
for Swift BAT data a simple weighting scheme can actually sometimes reduce the
sensitivity of the power spectrum depending on source brightness. Instead, a
modified weighting scheme, based on the Cochran semi-weighted mean, gives
improved results independent of source brightness. We investigate the benefits
of weighting power spectra in period searches using simulated GLAST LAT
observations of gamma-ray binaries.Comment: 2 pages. To appear in the Proceedings of the First International
GLAST Symposium, Febuary 5-8, 2007, Stanford University, AIP, Eds. S. Ritz,
P. F. Michelson, and C. Meega
Trans-Relativistic Supernovae, Circumstellar Gamma-Ray Bursts, and Supernova 1998bw
Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct
evidence that supernovae are the progenitors of some GRBs. However, this burst
was unusually dim, smooth and soft compared to other bursts with known
afterglows. Whether it should be considered a prototype for cosmological GRBs
depends largely on whether the supernova explosion and burst were asymmetrical
or can be modeled as spherical. We address this question by treating the
acceleration of the supernova shock in the outermost layers of the stellar
envelope, the transition to relativistic flow, and the subsequent expansion
(and further acceleration) of the ejecta into the surrounding medium. We find
that GRB 980425 could plausibly have been produced by a collision between the
relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model
requires no significant asymmetry. This event therefore belongs to a dim
subclass of GRBs and is not a prototype for jet-like cosmological GRBs.Comment: 5 pages, 2 figures, to appear in Gamma 2001, eds. S. Ritz, N.
Gehrels, and C. Shrade
The Origin of Cosmic Rays and the Diffuse Galactic Gamma-Ray Emission
Cosmic-ray interactions with interstellar gas and photons produce diffuse
gamma-ray emission. In this talk we will review the current understanding of
this diffuse emission and its relationship to the problem of the origin of
cosmic rays. We will discuss the open issues and what progress might be
possible with GLAST, which is planned for launch in 2006.Comment: 10 pages, 5 figures. Invited talk at the "Gamma 2001" Symp., April
4-6, 2001, Baltimore, MD. AIP Conf. Proc. v.587, eds. S. Ritz, N. Gehrels, &
C. R. Shrader, in pres
Discovery of a Pulsar Candidate Associated with the TeV Gamma-ray Source HESS J1813-178
We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio
supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We
resolve the X-ray emission from the co-located ASCA source into a compact
object surrounded by structured diffuse emission that fills the interior of the
radio shell. The morphology of the diffuse emission strongly resembles that of
a pulsar wind nebula. The spectrum of the compact source is well-characterized
by a power-law with index approx. 1.3, typical of young and energetic
rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray
absorption, the 2-10 keV X-ray luminosity of the putative pulsar and nebula is
L(PSR) = 3.2E33 erg/s and L(PWN) = 1.4E34 erg/s, respectively. Both the flux
ratio of L(PWN)/L(PSR) = 4.3 and the total luminosity of this system imply a
pulsar spin-down power greater then 1E37 erg/s, on a par with the top ten most
energetic young pulsars in the Galaxy. We associate the putative pulsar with
the radio remnant and the TeV source and discuss the origin of the gamma-ray
emission.Comment: 3 Pages, 3 Figures. Latex, aipproc.cls. To be published in "The
Proceedings of the First International GLAST Symposium" (Held Febuary 5-8,
2007, Stanford Univ., USA), AIP, Eds. S. Ritz, P.F. Michelson, and C. Meegan.
Updated coordinate
An AGN Identification for 3EG J2006-2321
We present a multiwavelength analysis of the high-energy gamma-ray source 3EG
J2006-2321. The flux of this source above 100 MeV is shown to be variable on
time scales of days and months. Optical observations and careful examination of
archived radio data indicate that its radio counterpart is PMN J2005-2310, a
flat-spectrum radio quasar with a 5-GHz flux density of 260 mJy. Study of the
optical counterpart indicates a redshift of 0.833 and variable linear
polarization. No X-ray source has been detected near the position of PMN
J2005-2310, but an X-ray upper limit is derived from ROSAT data. This upper
limit provides for a spectral energy distribution with global characteristics
similar to those of known gamma-ray blazars. Taken together, these data
indicate that 3EG J2006-2321, listed as unidentified in the 3rd EGRET Catalog,
is a member of the blazar class of AGN. The 5-GHz radio flux density of this
blazar is the lowest of the 68 EGRET-detected AGN. The fact that EGRET has
detected such a source has implications for unidentified EGRET sources,
particularly those at high latitudes (), many of which may be
blazars.Comment: 22 pages, 6 figures. To appear in ApJ v569 n1, 10 April 200
Gamma-Ray Pulsar Studies with GLAST
Some pulsars have their maximum observable energy output in the gamma-ray
band, offering the possibility of using these high-energy photons as probes of
the particle acceleration and interaction processes in pulsar magnetospheres.
After an extended hiatus between satellite missions, the recently-launched
AGILE mission and the upcoming Gamma-ray Large Area Space Telescope (GLAST)
Large Area Telescope (LAT) will allow gamma-ray tests of the theoretical models
developed based on past discoveries. With its greatly improved sensitivity,
better angular resolution, and larger energy reach than older instruments,
GLAST LAT should detect dozens to hundreds of new gamma-ray pulsars and measure
luminosities, light curves, and phase-resolved spectra with unprecedented
resolution. It will also have the potential to find radio-quiet pulsars like
Geminga, using blind search techniques. Cooperation with radio and X-ray pulsar
astronomers is an important aspect of the LAT team's planning for pulsar
studies.Comment: 8 pages, 5 figures. To appear in the proceedings of "40 Years of
Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
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