597 research outputs found
The Gamma Ray Pulsar Population
We apply a likelihood analysis to pulsar detections, pulsar upper limits, and
diffuse background measurements from the OSSE and EGRET instruments on the
Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray
pulsars and some properties of the gamma-ray pulsar population. We find that
the dependence of luminosity on spin period and dipole magnetic field is much
steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively),
suggesting that different emission mechanisms are responsible for low- and
high-energy gamma-ray emission. Incorporating a spin-down model and assuming a
pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray
background due to unidentified pulsars and find that pulsars may be an
important component of the OSSE diffuse flux, but are most likely not important
at EGRET energies. Using measurements of the diffuse background flux from these
instruments, we are able to place constraints on the braking index, initial
spin period, and magnetic field of the Galactic pulsar population. We are also
able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr).
Our results are based on a large gamma-ray beam, but they do not scale in a
simple way with beam size. With our assumed beam size, the implied gamma-ray
efficiency for the EGRET detections is no more than 20%. We estimate that about
20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use
our model to predict the pulsar population that will be seen by future
gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray
pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
Hard X-ray timing and spectral characteristics of the energetic pulsar PSR J0205+6449 in supernova remnant 3C58
PSR J0205+6449 is a young rotation-powered pulsar in SNR 3C 58. It is one of
only three young (<10,000 year old) pulsars which are so far detected in the
radio and the classical X-ray bands, as well as at hard X-rays above 20 keV and
at high-energy (>100 MeV) -rays. The other two young pulsars are the
Crab and PSR B1509-58. Our aim is to derive the timing and spectral
characteristics of PSR J0205+6449 over the broad X-ray band from ~0.5 to ~270
keV. We used all publicly available RXTE observations of PSR J0205+6449 to
first generate accurate ephemerides over the period September 30, 2000 - March
18, 2006. Next, phase-folding procedures yielded pulse profiles using data from
RXTE PCA and HEXTE, and XMM-Newton EPIC PN. While our timing solutions are
consistent with earlier results, our work shows sharper structures in the PCA
X-ray profile. The X-ray pulse profile consists of two sharp pulses, separated
in phase by 0.488(2), which can be described with 2 asymmetric Lorentzians,
each with the rising wing steeper than the trailing wing, and
full-width-half-maximum 1.41(5) ms and 2.35(22) ms, respectively. We find an
indication for a flux increase by a factor ~2, about 3.5 sigma above the
time-averaged value, for the second, weaker pulse during a two-week interval,
while its pulse shape did not change. The spectrum of the pulsed X-ray emission
is of non-thermal origin, exhibiting a power-law shape with photon index Gamma
= 1.03(2) over the energy band ~0.5 to ~270 keV. In the energy band covered
with the PCA (~3-30 keV) the spectra of the two pulses have the same photon
index, namely, 1.04(3) and 1.10(8), respectively.Comment: 10 pages; 7 figures (2 in color), resubmitted to A&A, including
referee comment
Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197
After spending almost a decade in a radio-quiet state, the Anomalous X-ray
Pulsar XTE J1810-197 turned back on in early December 2018. We have observed
this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of
radio re-activation on 8 December 2018. In this paper, we report on the current
timing properties of XTE J1810-197 and find that the magnitude of the spin
frequency derivative has increased by a factor of 2.6 over our 48-day data set.
We compare our results with the spin-down evolution reported during its
previous active phase in the radio band. We also present total intensity pulse
profiles at five different observing frequencies between 1.5 and 8.4 GHz,
collected with the Lovell and the Effelsberg telescopes. The profile evolution
in our data set is less erratic than what was reported during the previous
active phase, and can be seen varying smoothly between observations. Profiles
observed immediately after the outburst show the presence of at least five
cycles of a very stable ~50-ms periodicity in the main pulse component that
lasts for at least tens of days. This remarkable structure is seen across the
full range of observing frequencies.Comment: 9 pages, 7 figures, updated with additional analysis of the 50-ms
oscillation, accepted for publication in MNRA
An improved solar wind electron-density model for pulsar timing
Variations in the solar wind density introduce variable delays into pulsar
timing observations. Current pulsar timing analysis programs only implement
simple models of the solar wind, which not only limit the timing accuracy, but
can also affect measurements of pulsar rotational, astrometric and orbital
parameters. We describe a new model of the solar wind electron density content
which uses observations from the Wilcox Solar Observatory of the solar magnetic
field. We have implemented this model into the tempo2 pulsar timing package. We
show that this model is more accurate than previous models and that these
corrections are necessary for high precision pulsar timing applications.Comment: Accepted by ApJ, 13 pages, 4 figure
Birth and Evolution of Isolated Radio Pulsars
We investigate the birth and evolution of Galactic isolated radio pulsars. We
begin by estimating their birth space velocity distribution from proper motion
measurements of Brisken et al. (2002, 2003). We find no evidence for
multimodality of the distribution and favor one in which the absolute
one-dimensional velocity components are exponentially distributed and with a
three-dimensional mean velocity of 380^{+40}_{-60} km s^-1. We then proceed
with a Monte Carlo-based population synthesis, modelling the birth properties
of the pulsars, their time evolution, and their detection in the Parkes and
Swinburne Multibeam surveys. We present a population model that appears
generally consistent with the observations. Our results suggest that pulsars
are born in the spiral arms, with a Galactocentric radial distribution that is
well described by the functional form proposed by Yusifov & Kucuk (2004), in
which the pulsar surface density peaks at radius ~3 kpc. The birth spin period
distribution extends to several hundred milliseconds, with no evidence of
multimodality. Models which assume the radio luminosities of pulsars to be
independent of the spin periods and period derivatives are inadequate, as they
lead to the detection of too many old simulated pulsars in our simulations.
Dithered radio luminosities proportional to the square root of the spin-down
luminosity accommodate the observations well and provide a natural mechanism
for the pulsars to dim uniformly as they approach the death line, avoiding an
observed pile-up on the latter. There is no evidence for significant torque
decay (due to magnetic field decay or otherwise) over the lifetime of the
pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate
and total number of pulsars in the Galaxy.Comment: 27 pages, including 15 figures, accepted by Ap
Optical Pulse-Phased Photopolarimetry of PSR B0656+14
We have observed the optical pulse profile of PSR B0656+14 in 10 phase bins
at a high signal-to-noise ratio, and have measured the linear polarization
profile over 30% of the pulsar period with some significance. The pulse profile
is double-peaked, with a bridge of emission between the two peaks, similar to
gamma-ray profiles observed in other pulsars. There is no detectable unpulsed
flux, to a 1-sigma limit of 16% of the pulse-averaged flux. The emission in the
bridge is highly (~ 100%) polarized, with a position angle sweep in excellent
agreement with the prediction of the Rotating Vector Model as determined from
radio polarization observations. We are able to account for the gross features
of the optical light curve (i.e., the phase separation of the peaks) using both
polar cap and outer gap models. Using the polar cap model, we are also able to
estimate the height of the optical emission regions.Comment: 27 pages, 11 figures, accepted by ApJ (scheduled v597 n2, November
10, 2003
Locating the intense interstellar scattering towards the inner Galaxy
We use VLBA+VLA observations to measure the sizes of the scatter-broadened
images of 6 of the most heavily scattered known pulsars: 3 within the Galactic
Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the
measured sizes with temporal pulse broadening data from the literature and
using the thin-screen approximation, we locate the scattering medium along the
line of sight to these 6 pulsars. At least two scattering screens are needed to
explain the observations of the GC sample. We show that the screen inferred by
previous observations of SGR J1745-2900 and Sgr A*, which must be located far
from the GC, falls off in strength on scales < 0.2 degree. A second scattering
component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC
produces most or all of the temporal broadening observed in the other GC
pulsars. Outside the GC, the scattering locations for all three pulsars are ~2
kpc from Earth, consistent with the distance of the Carina-Sagittarius or
Scutum spiral arm. For each object the 3D scattering origin coincides with a
known HII region (and in one case also a supernova remnant), suggesting that
such objects preferentially cause the intense interstellar scattering seen
towards the Galactic plane. We show that the HII regions should contribute >
25% of the total dispersion measure (DM) towards these pulsars, and calculate
reduced DM distances. Those distances for other pulsars lying behind HII
regions may be similarly overestimated.Comment: 16 pages, 10 figures, MNRAS, in pres
Pulsar Constraints on Neutron Star Structure and Equation of State
With the aim of constraining the structural properties of neutron stars and
the equation of state of dense matter, we study sudden spin-ups, glitches,
occurring in the Vela pulsar and in six other pulsars. We present evidence that
glitches represent a self-regulating instability for which the star prepares
over a waiting time. The angular momentum requirements of glitches in Vela
indicate that at least 1.4% of the star's moment of inertia drives these
events. If glitches originate in the liquid of the inner crust, Vela's
`radiation radius' must exceed ~12 km for a mass of 1.4 solar masses.
Observational tests of whether other neutron stars obey this constraint will be
possible in the near future.Comment: 5 pages, including figures. To appear in Physical Review Letter
Double Neutron Star Systems and Natal Neutron Star Kicks
We study the four double neutron star systems found in the Galactic disk in
terms of the orbital characteristics of their immediate progenitors and the
natal kicks imparted to neutron stars. Analysis of the effect of the second
supernova explosion on the orbital dynamics, combined with recent results from
simulations of rapid accretion onto neutron stars lead us to conclude that the
observed systems could not have been formed had the explosion been symmetric.
Their formation becomes possible if kicks are imparted to the radio-pulsar
companions at birth. We identify the constraints imposed on the immediate
progenitors of the observed double neutron stars and calculate the ranges
within which their binary characteristics (orbital separations and masses of
the exploding stars) are restricted. We also study the dependence of these
limits on the magnitude of the kick velocity and the time elapsed since the
second explosion. For each of the double neutron stars, we derive a minimum
kick magnitude required for their formation, and for the two systems in close
orbits we find it to exceed 200km/s. Lower limits are also set to the
center-of-mass velocities of double neutron stars, and we find them to be
consistent with the current proper motion observations.Comment: 25 pages, 6 figs (9 parts), 4 tables, AASTeX, Accepted in Ap
New Pulsars from an Arecibo Drift Scan Search
We report the discovery of pulsars J0030+0451, J0711+0931, and J1313+0931
that were found in a search of 470 square degrees at 430 MHz using the 305m
Arecibo telescope. The search has an estimated sensitivity for long period, low
dispersion measure, low zenith angle, and high Galactic latitude pulsars of ~1
mJy, comparable to previous Arecibo searches. Spin and astrometric parameters
for the three pulsars are presented along with polarimetry at 430 MHz. PSR
J0030+0451, a nearby pulsar with a period of 4.8 ms, belongs to the less common
category of isolated millisecond pulsars. We have measured significant
polarization in PSR J0030+0451 over more than 50% of the period, and use these
data for a detailed discussion of its magnetospheric geometry. Scintillation
observations of PSR J0030+0451 provide an estimate of the plasma turbulence
level along the line of sight through the local interstellar medium.Comment: 21 pages, 4 figures, Accepted for Publication in Ap
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