12,484 research outputs found
General relativistic treatment of LISA optical links
LISA is a joint space mission of the NASA and the ESA for detecting low
frequency gravitational waves in the band Hz. In order to attain
the requisite sensitivity for LISA, the laser frequency noise must be
suppressed below the other secondary noises such as the optical path noise,
acceleration noise etc. This is achieved by combining time-delayed data for
which precise knowledge of time-delays is required. The gravitational field,
mainly that of the Sun and the motion of LISA affect the time-delays and the
optical links. Further, the effect of the gravitational field of the Earth on
the orbits of spacecraft is included. This leads to additional flexing over and
above that of the Sun. We have written a numerical code which computes the
optical links, that is, the time-delays with great accuracy
metres - more than what is required for time delay interferometry (TDI) - for
most of the orbit and with sufficient accuracy within metres for an
integrated time window of about six days, when one of the arms tends to be
tangent to the orbit. Our analysis of the optical links is fully general
relativistic and the numerical code takes into account effects such as the
Sagnac, Shapiro delay, etc.. We show that with the deemed parameters in the
design of LISA, there are symmetries inherent in the configuration of LISA and
in the physics, which may be used effectively to suppress the residual laser
noise in the modified first generation TDI. We demonstrate our results for some
important TDI variables
Optimising the directional sensitivity of LISA
It was shown in a previous work that the data combinations canceling laser
frequency noise constitute a module - the module of syzygies. The cancellation
of laser frequency noise is crucial for obtaining the requisite sensitivity for
LISA. In this work we show how the sensitivity of LISA can be optimised for a
monochromatic source - a compact binary - whose direction is known, by using
appropriate data combinations in the module. A stationary source in the
barycentric frame appears to move in the LISA frame and our strategy consists
of "coherently tracking" the source by appropriately "switching" the data
combinations so that they remain optimal at all times. Assuming that the
polarisation of the source is not known, we average the signal over the
polarisations. We find that the best statistic is the `network' statistic, in
which case LISA can be construed of as two independent detectors. We compare
our results with the Michelson combination, which has been used for obtaining
the standard sensitivity curve for LISA, and with the observable obtained by
optimally switching the three Michelson combinations. We find that for sources
lying in the ecliptic plane the improvement in SNR increases from 34% at low
frequencies to nearly 90% at around 20 mHz. Finally we present the
signal-to-noise ratios for some known binaries in our galaxy. We also show
that, if at low frequencies SNRs of both polarisations can be measured, the
inclination angle of the plane of the orbit of the binary can be estimated.Comment: 16 pages, 8 figures, submitted to Phys Rev
Schwinger Mechanism for Gluon Pair Production in the Presence of Arbitrary Time Dependent Chromo-Electric Field
We study Schwinger mechanism for gluon pair production in the presence of
arbitrary time-dependent chromo-electric background field with
arbitrary color index =1,2,...8 in SU(3) by directly evaluating the path
integral. We obtain an exact expression for the probability of non-perturbative
gluon pair production per unit time per unit volume and per unit transverse
momentum from arbitrary . We show that the
tadpole (or single gluon) effective action does not contribute to the
non-perturbative gluon pair production rate . We find
that the exact result for non-perturbative gluon pair production is independent
of all the time derivatives where
and has the same functional dependence on two casimir invariants
and as the constant
chromo-electric field result with the replacement: . This
result may be relevant to study the production of a non-perturbative
quark-gluon plasma at RHIC and LHC.Comment: 13 pages latex, Published in European Physical Journal
Radial Flow from Electromagnetic Probes and Signal of Quark Gluon Plasma
A first attempt has been made to extract the evolution of radial flow from
the analysis of the experimental data on electromagnetic probes experimentally
measured at SPS and RHIC energies. The spectra of photons and dileptons
measured by WA98 and NA60 collaborations respectively at CERN-SPS and the
photon spectra obtained by PHENIX collaboration at BNL-RHIC have been used to
constrain the theoretical models, rendering the outcome of the analysis largely
model independent. We argue that the variation of the radial velocity with
invariant mass is indicative of a phase transition from initially produced
partons to hadrons at SPS and RHIC energies.Comment: One LaTeX and 9 eps files, to appear in Phys. Rev.
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