5,586 research outputs found
Identifying Dark Matter Burners in the Galactic center
If the supermassive black hole (SMBH) at the center of our Galaxy grew
adiabatically, then a dense "spike" of dark matter is expected to have formed
around it. Assuming that dark matter is composed primarily of weakly
interacting massive particles (WIMPs), a star orbiting close enough to the SMBH
can capture WIMPs at an extremely high rate. The stellar luminosity due to
annihilation of captured WIMPs in the stellar core may be comparable to or even
exceed the luminosity of the star due to thermonuclear burning. The model thus
predicts the existence of unusual stars, i.e. "WIMP burners", in the vicinity
of an adiabatically grown SMBH. We find that the most efficient WIMP burners
are stars with degenerate electron cores, e.g. white dwarfs (WD) or degenerate
cores with envelopes. If found, such stars would provide evidence for the
existence of particle dark matter and could possibly be used to establish its
density profile. In our previous paper we computed the luminosity from WIMP
burning for a range of dark matter spike density profiles, degenerate core
masses, and distances from the SMBH. Here we compare our results with the
observed stars closest to the Galactic center and find that they could be
consistent with WIMP burners in the form of degenerate cores with envelopes. We
also cross-check the WIMP burner hypothesis with the EGRET observed flux of
gamma-rays from the Galactic center, which imposes a constraint on the dark
matter spike density profile and annihilation cross-section. We find that the
EGRET data is consistent with the WIMP burner hypothesis. New high precision
measurements by GLAST will confirm or set stringent limits on a dark matter
spike at the Galactic center, which will in turn support or set stringent
limits on the existence of WIMP burners at the Galactic center.Comment: 2 pages, 2 figures; to appear in the Proc. of the First Int. GLAST
Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan,
AIP Conf. Pro
Time Domain Studies of X-ray Shot Noise in Cygnus X-1
We investigate the variability of Cygnus X-1 in the context of shot moise
models, and employ a peak detection algorithm to select individual shots. For a
long observation of the low, hard state, the distribution of time intervals
between shots is found to be consistent with a purely random process, contrary
to previous claims in the literature. The detected shots are fit to several
model templates and found to have a broad range of shapes. The fitted shots
have a distribution of timescales from below 10 milliseconds to above 1 second.
The coherence of the cross spectrum of light curves of these data in different
energy bands is also studied. The observed high coherence implies that the
transfer function between low and high energy variability is uniform. The
uniformity of the tranfer function implies that the observed distribution of
shot widths cannot have been acquired through Compton scattering. Our results
in combination with other results in the literature suggest that shot
luminosities are correlated with one another. We discuss how our experimental
methodology relates to non-linear models of variability.Comment: Accepted for publication in Astrophysical Journal on July 16, 200
Malmquist Indices of Pre and Post-Deregulation Productivity, Efficiency and Technological Change in the Singaporean Banking Sector
By the end of the 1990s, the Singaporean government had recognised the need to open up its banking sector so as to remain competitive in the global economy. The Monetary Authority of Singapore thus began deregulation of the banking sector in 1999 to strengthening the competitiveness of local banks relative to their foreign competition through mergers. This paper employs a nonparametric Malmquist productivity index to provide measure of productivity, technological change and efficiency gains over the period 1995-2005. The findings reveal some total factor productivity growth associated with deregulation and scale efficiency improvement largely from mergers amongst the local banks.Efficiency, productivity; deregulation; Malmquist indices; banking
Looking for a light Higgs boson in the overlooked channel
The final state obtained when a Higgs boson decays to a photon and a Z boson
has been mostly overlooked in current searches for a light Higgs boson.
However, when the Z boson decays leptonically, all final state particles in
this channel can be measured, allowing for accurate reconstructions of the
Higgs mass and angular correlations. We determine the sensitivity of the Large
Hadron Collider (LHC) running at center of masses energies of 8 and 14 TeV to
Standard Model (SM) Higgs bosons with masses in the 120 - 130 GeV range. For
the 8 TeV LHC, sensitivity to several times the the SM cross section times
branching ratio may be obtained with 20 inverse femtobarns of integrated
luminosity, while for the 14 TeV LHC, the SM rate is probed with about 100
inverse femtobarns of integrated luminosity.Comment: 4 pages, 4 figures. Improves on version 1 in that 8 and 14 TeV LHC
running is considered, the case of a 125 GeV Higgs is treated specifically,
and the effect of an additional jet in the final state has been taken into
account in studying experimental sensitivit
Snatch trajectory of elite level girevoy (Kettlebell) sport athletes and its implications to strength and conditioning coaching
Girevoy sport (GS) has developed only recently in the West, resulting in a paucity of English scientific literature available. The aim was to document kettlebell trajectory of GS athletes performing the kettlebell snatch. Four elite GS athletes (age = 29-47 years, body mass = 68.3-108.1 kg, height 1.72-1.89 m) completed one set of 16 repetitions with a 32.1 kg kettlebell. Trajectory was captured with the VICON motion analysis system (250 Hz) and analysed with VICON Nexus (1.7.1). The kettlebell followed a ‘C’ shape trajectory in the sagittal plane. Mean peak velocity in the upwards phase was 4.03 ± 0.20 m s –1, compared to 3.70 ± 0.30 m s–1 during the downwards phase, and mean radial error across the sagittal and frontal planes was 0.022 ± 0.006 m. Low error in the movement suggests consistent trajectory is important to reduce extraneous movement and improve efficiency. While the kettlebell snatch and swing both require large anterior-posterior motion, the snatch requires the kettlebell to be held stationary overhead. Therefore, a different coaching application is required to that of a barbell snatch
Enhancing the Critical Current of a Superconducting Film in a Wide Range of Magnetic Fields with a Conformal Array of Nanoscale Holes
The maximum current (critical current) a type-II superconductor can transmit
without energy loss is limited by the motion of the quantized magnetic flux
penetrating into a superconductor. Introducing nanoscale holes into a
superconducting film has been long pursued as a promising way to increase the
critical current. So far the critical current enhancement was found to be
mostly limited to low magnetic fields. Here we experimentally investigate the
critical currents of superconducting films with a conformal array of nanoscale
holes that have non-uniform density while preserving the local ordering. We
find that the conformal array of nanoscle holes provides a more significant
critical current enhancement at high magnetic fields. The better performance
can be attributed to its arching effect that not only gives rise to the
gradient in hole-density for pinning vortices with a wide range of densities
but also prevent vortex channeling occurring in samples with a regular lattice
of holes.Comment: 5 pages, 3 figure
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