3,478 research outputs found
Arecibo HI Absorption Measurements of Pulsars and the Electron Density at Intermediate Longitudes in the First Galactic Quadrant
We have used the Arecibo telescope to measure the HI absorption spectra of
eight pulsars. We show how kinematic distance measurements depend upon the
values of the galactic constants R_o and Theta_o, and we select our preferred
current values from the literature. We then derive kinematic distances for the
low-latitude pulsars in our sample and electron densities along their lines of
sight. We combine these measurements with all others in the inner galactic
plane visible from Arecibo to study the electron density in this region. The
electron density in the interarm range 48 degrees < l < 70 degrees is [0.017
(-0.007,+0.012) (68% c.l.)] cm^(-3). This is 0.75 (-0.22,+0.49) (68% c.l.) of
the value calculated by the Cordes & Lazio (2002) galactic electron density
model. The model agrees more closely with electron density measurements toward
Arecibo pulsars lying closer to the galactic center, at 30 degrees<l<48
degrees. Our analysis leads to the best current estimate of the distance of the
relativistic binary pulsar B1913+16: d=(9.0 +/- 3) kpc.
We use the high-latitude pulsars to search for small-scale structure in the
interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19
exhibited significant changes in its absorption spectrum over 22 yr, indicating
HI structure on a ~500 AU scale.Comment: Accepted by Astrophysical Journal September 200
Detection of OH absorption against PSR B1849+00
We have searched for OH absorption against seven pulsars using the Arecibo
telescope. In both OH mainlines (at 1665 and 1667 MHz), deep and narrow
absorption features were detected toward PSR B1849+00. In addition, we have
detected several absorption and emission features against B33.6+0.1, a nearby
supernova remnant (SNR). The most interesting result of this study is that a
pencil-sharp absorption sample against the PSR differs greatly from the
large-angle absorption sample observed against the SNR. If both the PSR and the
SNR probe the same molecular cloud then this finding has important implications
for absorption studies of the molecular medium, as it shows that the statistics
of absorbing OH depends on the size of the background source. We also show that
the OH absorption against the PSR most likely originates from a small (<30
arcsec) and dense (>10^5 cm^-3) molecular clump.Comment: 12 pages, 8 figures. Accepted for publication in Ap
Does tiny-scale atomic structure exist in the interstellar medium ?
We report on preliminary results from the recent multi-epoch neutral hydrogen
absorption measurements toward three pulsars, B0823+26, B1133+16 and B2016+28,
using the Arecibo telescope. We do not find significant variations in optical
depth profiles over periods of 0.3 and 9--10 yr, or on spatial scales of 10--20
and 70--85 AU. The large number of non detections of the tiny scale atomic
structure suggests that the AU-sized structure is not ubiquitous in the
interstellar medium and could be quite a rare phenomenon.Comment: Accepted by ApJ Letters, 5 pages, 2 figure
The geometry of the double-pulsar system J0737-3039 from systematic intensity variations
The recent discovery of J0737-3039A & B-two pulsars in a highly relativistic
orbit around one another - offers an unprecedented opportunity to study the
elusive physics of pulsar radio emission. The system contains a rapidly
rotating pulsar with a spin period of 22.7 ms and a slow companion with a spin
period of 2.77 s, hereafter referred to as 'A' and 'B', respectively. A unique
property of the system is that the pulsed radio flux from B increases
systematically by almost two orders-of-magnitude during two short portions of
each orbit. Here, we describe a geometrical model of the system that
simultaneously explains the intensity variations of B and makes definitive and
testable predictions for the future evolution of the emission properties of
both stars. Our model assumes that B's pulsed radio flux increases when
illuminated by emission from A. This model provides constraints on the spin
axis orientation and emission geometry of A and predicts that its pulse profile
will evolve considerably over the next several years due to geodetic precession
until it disappears entirely in 15-20 years
Determination of the geometry of the PSR B1913+16 system by geodetic precession
New observations of the binary pulsar B1913+16 are presented. Since 1978 the
leading component of the pulse profile has weakend dramatically by about 40%.
For the first time, a decrease in component separation is observed, consistent
with expectations of geodetic precession. Assuming the correctness of general
relativity and a circular hollow-cone like beam, a fully consistent model for
the system geometry is developed. The misalignment angle between pulsar spin
and orbital momentum is determined giving direct evidence for an asymmetric
kick during the second supernova explosion. It is argued that the orbital
inclination angle is 132\fdg8 (rather than 47\fdg2). A prediction of this
model is that PSR B1913+16 will not be observable anymore after the year 2025.Comment: 16 pages, incl. 5 figures, accepted for publication in Ap
The Radial Extent and Warp of the Ionized Galactic Disk. I. A VLBA Survey of Extragalactic Sources Toward the Anticenter
We report multifrequency Very Long Baseline Array observations of twelve
active galactic nuclei seen toward the Galactic anticenter. All of the sources
are at |b| < 10 degrees and seven have |b| < 0.5 degrees. Our VLBA observations
can detect an enhancement in the angular broadening of these sources due to an
extended H II disk, if the orientation of the H II disk in the outer Galaxy is
similar to that of the H I disk. Such an extended H II disk is suggested by the
C IV absorption in a quasar's spectrum, the appearance of H I disks of nearby
spiral galaxies, and models of Ly-alpha cloud absorbers and the Galactic
fountain. We detect eleven of the twelve sources at one or more frequencies;
nine of the sources are compact and suitable for an angular broadening
analysis. A preliminary analysis of the observed angular diameters suggests
that the H II disk does not display considerable warping or flaring and does
not extend to large Galactocentric distances (R >~ 100 kpc). A companion paper
(Lazio & Cordes 1997) combines these observations with those in the literature
and presents a more comprehensive analysis.Comment: 19 pages, LaTeX2e with AASTeX macro aaspp4, accepted for publication
in ApJS, Vol. 115, 1998 April; Figures 1, 3, and 4 included, for figures of
individual sources see
http://astrosun.tn.cornell.edu/students/lazio/Anticenter/anticenterI.htm
PSR J1829+2456: a relativistic binary pulsar
We report the discovery of a new binary pulsar, PSR J1829+2456, found during
a mid-latitude drift-scan survey with the Arecibo telescope. Our initial timing
observations show the 41-ms pulsar to be in a 28-hr, slightly eccentric, binary
orbit. The advance of periastron, omegadot = 0.28 +/- 0.01 deg/yr is derived
from our timing observations spanning 200 days. Assuming that the advance of
periastron is purely relativistic and a reasonable range of neutron star masses
for PSR J1829+2456 we constrain the companion mass to be between 1.22 Msun and
1.38 Msun, making it likely to be another neutron star. We also place a firm
upper limit on the pulsar mass of 1.38 Msun. The expected coalescence time due
to gravitational-wave emission is long (~60 Gyr) and this system will not
significantly impact upon calculations of merger rates that are relevant to
upcoming instruments such as LIGO.Comment: Accepted MNRAS, 5 pages, 3 figure
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Faraday Rotation Measure Synthesis
We extend the rotation measure work of Burn (1966) to the cases of limited
sampling of lambda squared space and non-constant emission spectra. We
introduce the rotation measure transfer function (RMTF), which is an excellent
predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation
measure synthesis can be implemented very efficiently on modern computers.
Because the analysis is easily applied to wide fields, one can conduct very
fast RM surveys of weak spatially extended sources. Difficult situations, for
example multiple sources along the line of sight, are easily detected and
transparently handled. Under certain conditions, it is even possible to recover
the emission as a function of Faraday depth within a single cloud of ionized
gas. Rotation measure synthesis has already been successful in discovering
widespread, weak, polarized emission associated with the Perseus cluster (De
Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as
good as traditional linear fits to polarization angle versus lambda squared
plots. However, when the situation is more complex or very weak polarized
emission at high rotation measures is expected, it is the only viable option.Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected
typo
- …