10,844 research outputs found
The Small-scale Structure in Interstellar HI: A Resolvable Puzzle
During the past decade or so, measurements of Galactic HI absorption using
VLBI against extra-galactic sources, as well as multi-epoch observatios in
pulsar directions, have detected small-scale transverse variations
corresponding to tens of AU at the distance of the absorbing matter. Hitherto
these measurements have been interpreted as small-scale structure in the HI
distribution with densities n_{HI} ~ 10^4-10^5 per cc, orders of magnitude
greater than those of the parsec-scale structure. Naturally it is difficult to
imagine how such structures could exist in equilibrium with other components of
the ISM.
In this paper we show that structure on all scales contributes to the
differences on neighbouring lines of sight, and that the observed differences
can be accounted for by a natural extension of the distribution of
irregularities in the distribution of HI opacities at larger scales, using a
single power law. This, in our opinion, should put an end to the decades long
puzzle of the so-called small-scale structure in HI and other species in the
Galaxy.Comment: 7 pages including 3 figures, Latex format. Accepted for publication
in MNRAS, 200
Topology and Polarisation of Subbeams Associated With Pulsar B0943+10's `Drifting'-Subpulse Emission: II. Analysis of Gauribidanur 35-MHz Observations
In the previous paper of this series Deshpande & Rankin (2001) reported
results regarding subpulse-drift phenomenon in pulsar B0943+10 at 430 MHz and
111 MHz. This study has led to the identification of a stable system of
subbeams circulating around the magnetic axis of this star. Here, we present a
single-pulse analysis of our observations of this pulsar at 35 MHz. The
fluctuation properties seen at this low frequency, as well as our independent
estimates of the number of subbeams required and their circulation time, agree
remarkably well with the reported behavior at higher frequencies. We use the
`cartographic'-transform mapping technique developed in Paper-I to study the
emission pattern in the polar region of this pulsar. The significance of our
results in the context of radio emission mechanisms is also discussed.Comment: 6 pages, 7 figures (2 color figs), MNRAS, 326 (4), 1249-1254 (2001
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
- …