19,997 research outputs found
Optically pumped submillimeter-waves and applications
Rapid development of optically pumped lasers has shown the potential to be used as a source for a high resolution spectrometer. In this connection, a compact, stable FIR laser was designed, fabricated, and assembled, integrating both the pump laser and the FIR cavity in the same mechanical structure for its improved (both thermal and mechanical) stability and mobility. Performance of the mixer structure which was designed and constructed for the evaluation of the Schottky diodes was found to be satisfactory. The feasibility of generating tunable sideband for molecular spectroscopy studies was demonstrated
Neutralino Proton Cross Sections in SUGRA and D-BRANE Models
We calculate the spin independent neutralino-proton cross section for
universal SUGRA, non universal SUGRA and D-brane models with R-parity
invariance. The regions of maximum cross section in these models has started to
be probed by the current detectors. The minimum cross section generally is
>~1\times 10^{-(9-10)}pb and hence will be accessible in the future detectors,
barring special regions of parameter space where it can reduce to =~10^{-12}pb.
However, the squarks and gluinos will be heavy (>~1 TeV) in the latter case,
but still accessible at the LHC.Comment: 3 pages, latex, 3 figures, talk at DPF 2000, Columbus, OH, August
9-12, 200
Incidence of HI 21-cm absorption in strong FeII systems at
We present the results from our search for HI 21-cm absorption in a sample of
16 strong FeII systems ((MgII ) \AA\ and
(FeII ) or \AA) at
using the Giant Metrewave Radio Telescope and the Green Bank Telescope. We
report six new HI 21-cm absorption detections from our sample, which have
increased the known number of detections in strong MgII systems at this
redshift range by %. Combining our measurements with those in the
literature, we find that the detection rate of HI 21-cm absorption increases
with , being four times higher in systems with
\AA\ compared to systems with \AA. The (HI)
associated with the HI 21-cm absorbers would be
cm, assuming a spin temperature of K (based on HI 21-cm
absorption measurements of damped Lyman- systems at this redshift
range) and unit covering factor. We find that HI 21-cm absorption arises on an
average in systems with stronger metal absorption. We also find that quasars
with HI 21-cm absorption detected towards them have systematically higher
values than those which do not. Further, by comparing the velocity
widths of HI 21-cm absorption lines detected in absorption- and galaxy-selected
samples, we find that they show an increasing trend (significant at
) with redshift at , which could imply that the absorption
originates from more massive galaxy haloes at high-. Increasing the number
of HI 21-cm absorption detections at these redshifts is important to confirm
various trends noted here with higher statistical significance.Comment: 17 pages, 10 figures, 8 tables, accepted for publication in MNRA
The young cluster NGC 2282 : a multi-wavelength perspective
We present the analysis of the stellar content of NGC~2282, a young cluster
in the Monoceros constellation, using deep optical and IPHAS photometry
along with infrared (IR) data from UKIDSS and -IRAC. Based on the
stellar surface density analysis using nearest neighborhood method, the radius
of the cluster is estimated as 3.15\arcmin. From optical spectroscopic
analysis of 8 bright sources, we have classified three early B-type members in
the cluster, which includes, HD 289120, a previously known B2V type star, a
Herbig Ae/Be star (B0.5 Ve) and a B5 V star. From spectrophotometric analyses,
the distance to the cluster has been estimated as 1.65 kpc. The -band
extinction map is estimated using nearest neighborhood technique, and the mean
extinction within the cluster area is found to be A 3.9 mag. Using
IR colour-colour criteria and H-emission properties, we have
identified a total of 152 candidate young stellar objects (YSOs) in the region,
of which, 75 are classified as Class II, 9 are Class I YSOs. Our YSO catalog
also includes 50 H-emission line sources, identified using slitless
spectroscopy and IPHAS photometry data. Based on the optical and near-IR
colour-magnitude diagram analyses, the cluster age has been estimated to be in
the range of 2 5 Myr, which is in agreement with the estimated age from
disc fraction ( 58\%). Masses of these YSOs are found to be
0.12.0 M. Spatial distribution of the candidate YSOs shows spherical
morphology, more or less similar to the surface density map.Comment: 16 pages, 19 Figure
Effects of P-wave Annihilation on the Angular Power Spectrum of Extragalactic Gamma-rays from Dark Matter Annihilation
We present a formalism for estimating the angular power spectrum of
extragalactic gamma-rays produced by dark matter annihilating with any general
velocity-dependent cross section. The relevant density and velocity
distribution of dark matter is modeled as an ensemble of smooth, universal,
rigid, disjoint, spherical halos with distribution and universal properties
constrained by simulation data. We apply this formalism to theories of dark
matter with p-wave annihilation, for which the relative-velocity-weighted
annihilation cross section is \sigma v=a+bv^2. We determine that this
significantly increases the gamma-ray power if b/a >> 10^6. The effect of
p-wave annihilation on the angular power spectrum is very similar for the
sample of particle physics models we explored, suggesting that the important
effect for a given b/a is largely determined by the cosmic dark matter
distribution. If the dark matter relic from strong p-wave theories is thermally
produced, the intensities of annihilation gamma-rays are strongly p-wave
suppressed, making them difficult to observe. If an angular power spectrum
consistent with a strong p-wave were to be observed, it would likely indicate
non-thermal production of dark matter in the early Universe.Comment: 20 pages, 3 figure
Boost Mass and the Mechanics of Accelerated Black Holes
In this paper we study the concept of the boost mass of a spacetime and
investigate how variations in the boost mass enter into the laws of black hole
mechanics. We define the boost mass as the gravitational charge associated with
an asymptotic boost symmetry, similiar to how the ADM mass is associated with
an asymptotic time translation symmetry. In distinction to the ADM mass, the
boost mass is a relevant concept when the spacetime has stress energy at
infinity, and so the spacetime is not asymptotically flat. We prove a version
of the first law which relates the variation in the boost mass to the change in
the area of the black hole horizon, plus the change in the area of an
acceleration horizon, which is necessarily present with the boost Killing
field, as we discuss. The C-metric and Ernst metric are two known analytical
solutions to Einstein-Maxwell theory describing accelerating black holes which
illustrate these concepts.Comment: 23 pages, A few modifications and clarifications at the referee's
suggestions; References added and correcte
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