6,822 research outputs found
Improving Pulsar Distances by Modelling Interstellar Scattering
We present here a method to study the distribution of electron density
fluctuations in pulsar directions as well as to estimate pulsar distances. The
method, based on a simple two-component model of the scattering medium
discussed by Gwinn et al. (1993), uses scintillation & proper motion data in
addition to the measurements of angular broadening & temporal broadening to
solve for the model parameters, namely, the fractional distance to a discrete
scatterer and the ascociated relative scattering strength. We show how this
method can be used to estimate pulsar distances reliably, when the location of
a discrete scatterer (e.g. an HII region), if any, is known. Considering the
specific example of PSR B0736-40, we illustrate how a simple characterization
of the Gum nebula region (using the data on the Vela pulsar) is possible and
can be used along with the temporal broadening measurements to estimate pulsar
distances.Comment: To be published in MNRAS, 7 pages, 3 figure
Upper Limits on the Pulsed Radio Emission from the Geminga Pulsar at 35 & 327 MHz
We report here our observations at 35 MHz and 327 MHz made in the direction
of the Gamma Ray pulsar Geminga. Based on the observed absence of any
significant pulsed emission from this source above our detection thresholds at
the two frequencies, we obtain useful upper-limits for the average flux to be
75-100 mJy at 35 MHz, and 0.2-0.3 mJy at 327 MHz. We discuss a few possible
reasons for the ``radio-quiet'' nature of this pulsar at frequencies other than
around 100 MHz.Comment: Accepted for publication in Astronomy & Astrophysic
The observational evidence pertinent to possible kick mechanisms in neutron stars
We examine available observations on pulsars for evidence pertaining to
mechanisms proposed to explain the origin of their velocities. We find that
mechanisms predicting a correlation between the rotation axis and the pulsar
velocity are ruled out. Also, that there is no significant correlation between
pulsar magnetic field strengths and velocities. With respect to recent
suggestions postulating asymmetric impulses at birth being solely responsible
for both the spins and velocities of pulsars, single impulses of any duration
and multiple extended duration impulses appear ruled out.Comment: 7 pages. Accepted for publication in Astronomy & Astrophysic
Search for Discrete Refractive Scattering Events
We have searched for discrete refractive scattering events (including effects
due to possible non-multiple diffractive scattering) at meter wavelengths in
the direction of two close by pulsars B0950+08 and B1929+10, where we looked
for spectral signatures associated with the multiple imaging of pulsars due to
scattering in the interstellar medium. We do not find any signatures of such
events in the direction of either source over a spectral periodicity range of
50 KHz to 5 MHz. Our analysis puts strong upper limits on the column density
contrast associated with a range of spatial scales of the interstellar electron
density irregularities along these lines of sight.Comment: Accepted for publication in Astronomy & Astrophysic
Small-Scale Variations of HI Spectra from Interstellar Scintillatio
I suggest that radio-wave scattering by the interstellar plasma, in
combination with subsonic gradients in the Doppler velocity of interstellar HI,
is responsible for the observed small-scale variation in HI absorption spectra
of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can
produce the observed variations. I suggest observational tests to distinguish
between this model and the traditional picture of small-scale opacity
variations from cloudlets.Comment: 24 pages, 2 figures, Latex, uses AASTe
Electroweak Model Independent Tests for SU(3) Symmetry in Hadronic B Decays
We study effects of new physics beyond the Standard Model on SU(3) symmetry
in charmless hadronic two body B decays. It is found that several equalities
for some of the decay amplitudes, such as , , , predicted by SU(3) symmetry in the SM are not affected by new
physics. These relations provide important electroweak model independent tests
for SU(3) symmetry in B decays.Comment: 4 pages, revte
Effects of R-parity violation on direct CP violation in B decays and extraction of
In the standard model, direct CP-violating asymmetries for are roughly 2% based on perturbative calculation. Rescattering effects might
enhance it to at most (20-25)%. We show that lepton-number-violating couplings
in supersymmetric models without R-parity are capable of inducing as large as
100% CP asymmetry in this channel. Such effects drastically modify the allowed
range of the CKM parameter arising from the combinations of the
observed charged and neutral B decays in the modes. With a multichannel
analysis in B decays, one can either discover this exciting new physics, or
significantly improve the existing constraints on it.Comment: Latex, 5 pages; minor changes, to appear in Phys Rev Let
Constraints on the phase and new physics from Decays
Recent results from CLEO on indicate that the phase may
be substantially different from that obtained from other fit to the KM matrix
elements in the Standard Model. We show that extracted using is sensitive to new physics occurring at loop level. It provides
a powerful method to probe new physics in electroweak penguin interactions.
Using effects due to anomalous gauge couplings as an example, we show that
within the allowed ranges for these couplings information about
obtained from can be very different from the Standard
Model prediction.Comment: Revised version with analysis done using new data from CLEO. RevTex,
11 Pages with two figure
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