1,426 research outputs found
An empirical Bayesian analysis applied to the globular cluster pulsar population
We describe an empirical Bayesian approach to determine the most likely size
of an astronomical population of sources of which only a small subset are
observed above some limiting flux density threshold. The method is most
naturally applied to astronomical source populations at a common distance
(e.g.,stellar populations in globular clusters), and can be applied even to
populations where a survey detects no objects. The model allows for the
inclusion of physical parameters of the stellar population and the detection
process. As an example, we apply this method to the current sample of radio
pulsars in Galactic globular clusters. Using the sample of flux density limits
on pulsar surveys in 94 globular clusters published by Boyles et al., we
examine a large number of population models with different dependencies. We
find that models which include the globular cluster two-body encounter rate,
, are strongly favoured over models in which this is not a factor. The
optimal model is one in which the mean number of pulsars is proportional to
. This model agrees well with earlier work by Hui et al.
and provides strong support to the idea that the two-body encounter rate
directly impacts the number of neutron stars in a cluster. Our model predicts
that the total number of potentially observable globular cluster pulsars in the
Boyles et al. sample is 1070, where the uncertainties signify
the 95% confidence interval. Scaling this result to all Galactic globular
clusters, and to account for radio pulsar beaming, we estimate the total
population to be 2280.Comment: 8 pages, 6 figures, 3 tables, corrected a few minor formatting errors
which have also been submitted as an erratum to MNRA
Why the distance of PSR J0218+4232 does not challenge pulsar emission theories
Recent VLBI measurements of the astrometric parameters of the millisecond
pulsar J0218+4232 by Du et al. have suggested this pulsar is as distant as 6.3
kpc. At such a large distance, the large {\gamma}-ray flux observed from this
pulsar would make it the most luminous {\gamma}-ray pulsar known. This
luminosity would exceed what can be explained by the outer gap and slot-gap
pulsar emission models, potentially placing important and otherwise elusive
constraints on the pulsar emission mechanism. We show that the VLBI parallax
measurement is dominated by the Lutz-Kelker bias. When this bias is corrected
for, the most likely distance for this pulsar is 3.15(+0.85/-0.60) kpc. This
revised distance places the luminosity of PSR J0218+4232 into a range where it
does not challenge any of the standard theories of the pulsar emission
mechanism.Comment: 3 pages, 2 figures, 1 table. Accepted for publication in MNRA
Isolated pulsar spin evolution on the P-Pdot Diagram
We look at two contrasting spin-down models for isolated radio pulsars and,
accounting for selection effects, synthesize observable populations. While our
goal is to reproduce all of the observable characteristics, in this paper we
pay particular attention to the form of the spin period vs. period derivative
(P-Pdot) diagram and its dependence on various pulsar properties. We analyse
the initial spin period, the braking index, the magnetic field, various beaming
models, as well as the pulsar's luminosity. In addition to considering the
standard magnetic dipole model for pulsar spin-down, we also consider the
recent hybrid model proposed by Contopoulos & Spitkovsky. The magnetic dipole
model, however, does a better job of reproducing the observed pulsar
population. We conclude that random alignment angles and period dependent
luminosity distributions are essential to reproduce the observed P-Pdot
diagram. We also consider the time decay of alignment angles, and attempt to
reconcile various models currently being studied. We conclude that, in order to
account for recent evidence for the alignment found by Weltevrede & Johnston,
the braking torque on a neutron star should not depend strongly on the
inclination. Our simulation code is publically available and includes a
web-based interface to examine the results and make predictions for yields of
current and future surveys.Comment: 9 pages, 4 figure
Lutz-Kelker bias in pulsar parallax measurements
Lutz & Kelker showed that parallax measurements are systematically
overestimated because they do not properly account for the larger volume of
space that is sampled at smaller parallax values. We apply their analysis to
neutron stars, incorporating the bias introduced by the intrinsic radio
luminosity function and a realistic Galactic population model for neutron
stars. We estimate the bias for all published neutron star parallax
measurements and find that measurements with less than ~95% certainty, are
likely to be significantly biased. Through inspection of historic parallax
measurements, we confirm the described effects in optical and radio
measurements, as well as in distance estimates based on interstellar dispersion
measures. The potential impact on future tests of relativistic gravity through
pulsar timing and on X-ray--based estimates of neutron star radii is briefly
discussed.Comment: 9 pages, 3 tables, 1 figure. Accepted for publication in MNRA
Gravitational wave background from rotating neutron stars
The background of gravitational waves produced by the ensemble of rotating
neutron stars (which includes pulsars, magnetars and gravitars) is
investigated. A formula for \Omega(f) (commonly used to quantify the
background) is derived, properly taking into account the time evolution of the
systems since their formation until the present day. Moreover, the formula
allows one to distinguish the different parts of the background: the
unresolvable (which forms a stochastic background) and the resolvable. Several
estimations of the background are obtained, for different assumptions on the
parameters that characterize neutron stars and their population. In particular,
different initial spin period distributions lead to very different results. For
one of the models, with slow initial spins, the detection of the background can
be rejected. However, other models do predict the detection of the background
by the future ground-based gravitational wave detector ET. A robust upper limit
for the background of rotating neutron stars is obtained; it does not exceed
the detection threshold of two cross-correlated Advanced LIGO interferometers.
If gravitars exist and constitute more than a few percent of the neutron star
population, then they produce an unresolvable background that could be detected
by ET. Under the most reasonable assumptions on the parameters characterizing a
neutron star, the background is too faint. Previous papers have suggested
neutron star models in which large magnetic fields (like the ones that
characterize magnetars) induce big deformations in the star, which produce a
stronger emission of gravitational radiation. Considering the most optimistic
(in terms of the detection of gravitational waves) of these models, an upper
limit for the background produced by magnetars is obtained; it could be
detected by ET, but not by BBO or DECIGO.Comment: 25 pages, 15 figure
Low-Mass X-Ray Binaries, Millisecond Radio Pulsars, and the Cosmic Star Formation Rate
We report on the implications of the peak in the cosmic star-formation rate
(SFR) at redshift z ~ 1.5 for the resulting population of low-mass X-ray
binaries(LMXB) and for that of their descendants, the millisecond radio pulsars
(MRP). Since the evolutionary timescales of LMXBs, their progenitors, and their
descendants are thought be significant fractions of the time-interval between
the SFR peak and the present epoch, there is a lag in the turn-on of the LMXB
population, with the peak activity occurring at z ~ 0.5 - 1.0. The peak in the
MRP population is delayed further, occurring at z < 0.5. We show that the
discrepancy between the birthrate of LMXBs and MRPs, found under the assumption
of a stead-state SFR, can be resolved for the population as a whole when the
effects of a time-variable SFR are included. A discrepancy may persist for
LMXBs with short orbital periods, although a detailed population synthesis will
be required to confirm this. Further, since the integrated X-ray luminosity
distribution of normal galaxies is dominated by X-ray binaries, it should show
strong luminosity evolution with redshift. In addition to an enhancement near
the peak (z ~ 1.5) of the SFR due to the prompt turn-on of the relatively
short-lived massive X-ray binaries and young supernova remnants, we predict a
second enhancement by a factor ~10 at a redshift between ~ 0.5 and ~ 1 due to
the delayed turn-on of the LMXB population. Deep X-ray observations of galaxies
out to z ~ 1 by AXAF will be able to observe this enhancement, and, by
determining its shape as a function of redshift, will provide an important new
method for constraining evolutionary models of X-ray binaries.Comment: 13 pages, including 1 figure. Accepted for publication in ApJ Letter
The Arecibo 430-MHz Intermediate Galactic Latitude Survey: Discovery of Nine Radio Pulsars
We have used the Arecibo Radio Telescope to search for millisecond pulsars in
two intermediate Galactic latitude regions (7 deg < | b | < 20 deg) accessible
to this telescope. For these latitudes the useful millisecond pulsar search
volume achieved by Arecibo's 430-MHz beam is predicted to be maximal. Searching
a total of 130 square degrees, we have discovered nine new pulsars and detected
four previously known objects. We compare the results of this survey with those
of other 430-MHz surveys carried out at Arecibo and of an intermediate latitude
survey made at Parkes that included part of our search area; the latter
independently found two of the nine pulsars we have discovered.
At least six of our discoveries are isolated pulsars with ages between 5 and
300 Myr; one of these, PSR J1819+1305, exhibits very marked and periodic
nulling. We have also found a recycled pulsar, PSR J2016+1948. With a
rotational period of 65 ms, this is a member of a binary system with a 635-day
orbital period. We discuss some of the the properties of this system in detail,
and indicate its potential to provide a test of the Strong Equivalence
Principle. This pulsar and PSR J0407+16, a similar system now being timed at
Arecibo, are by far the best systems known for such a test.Comment: Accepted for publication in ApJ Referee format: 22 pages, 7 figure
X-ray Timing of PSR J1852+0040 in Kesteven 79: Evidence of Neutron Stars Weakly Magnetized at Birth
The 105-ms X-ray pulsar J1852+0040 is the central compact object (CCO) in SNR
Kes 79. We report a sensitive upper limit on its radio flux density of 12 uJy
at 2 GHz using the NRAO GBT. Timing using XMM and Chandra over a 2.4 yr span
reveals no significant change in its spin period. The 2 sigma upper limit on
the period derivative leads, in the dipole spin-down formalism, to an energy
loss rate E-dot < 7e33 ergs/s, surface magnetic field strength B_p < 1.5e11 G,
and characteristic age tau_c = P/2P-dot > 8 Myr. This tau_c exceeds the age of
the SNR by 3 orders of magnitude, implying that the pulsar was born spinning at
its current period. However, the X-ray luminosity of PSR J1852+0040, L(bol) ~
3e33(d/7.1 kpc)^2 ergs/s is a large fraction of E-dot, which challenges the
rotation-powered assumption. Instead, its high blackbody temperature,
0.46+/-0.04 keV, small blackbody radius ~ 0.8 km, and large pulsed fraction, ~
80%, may be evidence of accretion onto a polar cap, possibly from a fallback
disk made of supernova debris. If B_p < 1e10 G, an accretion disk can penetrate
the light cylinder and interact with the magnetosphere while resulting torques
on the neutron star remain within the observed limits. A weak B-field is also
inferred in another CCO, the 424-ms pulsar 1E 1207.4-5209, from its steady spin
and soft X-ray absorption lines. We propose this origin of radio-quiet CCOs:
the B-field, derived from a turbulent dynamo, is weaker if the NS is formed
spinning slowly, which enables it to accrete SN debris. Accretion excludes
neutron stars born with both B_p 0.1 s from radio pulsar
surveys, where B_p
40 Myr) or recycled pulsars. Finally, such a CCO, if born in SN 1987A, could
explain the non-detection of a pulsar there.Comment: 8 pages, 3 figures, to appear in The Astrophysical Journa
New limits on the population of normal and millisecond pulsars in the Large and Small Magellanic Clouds
We model the potentially observable populations of normal and millisecond
radio pulsars in the Large and Small Magellanic Clouds (LMC and SMC) where the
known population currently stands at 19 normal radio pulsars. Taking into
account the detection thresholds of previous surveys, and assuming optimal
period and luminosity distributions based on studies of Galactic pulsars, we
estimate there are (1.79 +/- 0.20) x 10^4 and (1.09 +/- 0.16) x 10^4 normal
pulsars in the LMC and SMC respectively. When we attempt to correct for beaming
effects, and the fraction of high-velocity pulsars which escape the clouds, we
estimate birth rates in both the LMC and SMC to be comparable and in the range
0.5--1 pulsar per century. Although higher than estimates for the rate of
core-collapse supernovae in the clouds, these pulsar birth rates are consistent
with historical supernova observations in the past 300 yr. A substantial
population of active radio pulsars (of order a few hundred thousand) have
escaped the LMC and SMC and populate the local intergalactic medium. For the
millisecond pulsar (MSP) population, the lack of any detections from current
surveys leads to respective upper limits (at the 95% confidence level) of
15,000 for the LMC and 23,000 for the SMC. Several MSPs could be detected by a
currently ongoing survey of the SMC with improved time and frequency resolution
using the Parkes multibeam system. Giant-pulse emitting neutron stars could
also be seen by this survey.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
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