1,586 research outputs found
The distance to the Vela pulsar gauged with HST parallax oservations
The distance to the Vela pulsar (PSR B0833-45) has been traditionally assumed
to be 500 pc. Although affected by a significant uncertainty, this value stuck
to both the pulsar and the SNR. In an effort to obtain a model free distance
measurement, we have applied high resolution astrometry to the pulsar V~23.6
optical counterpart. Using a set of five HST/WFPC2 observations, we have
obtained the first optical measurement of the annual parallax of the Vela
pulsar. The parallax turns out to be 3.4 +/- 0.7 mas, implying a distance of
294(-50;+76) pc, i.e. a value significantly lower than previously believed.
This affects the estimate of the pulsar absolute luminosity and of its emission
efficiency at various wavelengths and confirms the exceptionally high value of
the N_e towards the Vela pulsar. Finally, the complete parallax data base
allows for a better measurement of the Vela pulsar proper motion
(mu_alpha(cos(delta))=-37.2 +/- 1.2 mas/yr; mu_delta=28.2 +/- 1.3 mas/yr after
correcting for the peculiar motion of the Sun) which, at the parallax distance,
implies a transverse velocity of ~65 km/s. Moreover, the proper motion position
angle appears specially well aligned with the axis of symmetry of the X-ray
nebula as seen by Chandra. Such an alignment allows to assess the space
velocity of the Vela pulsar to be ~81 km/s.Comment: LaTeX, 21 pages, 5 figures. Accepted for publication in Ap
Evidence for alignment of the rotation and velocity vectors in pulsars
We present strong observational evidence for a relationship between the
direction of a pulsar's motion and its rotation axis. We show carefully
calibrated polarization data for 25 pulsars, 20 of which display linearly
polarized emission from the pulse longitude at closest approach to the magnetic
pole. Such data allow determination of the position angle of the linear
polarisation which in turn reflects the position angle of the rotation axis. Of
these 20 pulsars, 10 show an offset between the velocity vector and the
polarisation position angle which is either less than 10\degr or more than
80\degr, a fraction which is very unlikely by random chance. We believe that
the bimodal nature of the distribution arises from the presence of orthogonal
polarisation modes in the pulsar radio emission. In some cases this orthogonal
ambiguity is resolved by observations at other wavelengths so that we conclude
that the velocity vector and the rotation axis are aligned at birth.
Strengthening the case is the fact that 4 of the 5 pulsars with ages less than
3 Myr show this relationship, including the Vela pulsar. We discuss the
implications of these findings in the context of the Spruit & Phinney
(1998)\nocite{sp98} model of pulsar birth-kicks. We point out that, contrary to
claims in the literature, observations of double neutron star systems do not
rule out aligned kick models and describe a possible observational test
involving the double pulsar system.Comment: MNRAS, In Pres
Pulsar Timing with the Fermi LAT
We present an overview of precise pulsar timing using data from the Large
Area Telescope (LAT) on Fermi. We describe the analysis techniques including a
maximum likelihood method for determining pulse times of arrival from unbinned
photon data. In addition to determining the spindown behavior of the pulsars
and detecting glitches and timing noise, such timing analyses allow the precise
determination of the pulsar position, thus enabling detailed multiwavelength
follow up.Comment: 6 page, 3 figures, to appear in AIP Conference Proceedings of Pulsar
Conference 2010 "Radio Pulsars: a key to unlock the secrets of the Universe",
Sardinia, October 201
Pulsar Spin-Velocity Alignment: Kinematic Ages, Birth Periods and Braking Indices
This paper presents a detailed investigation of the dependence of pulsar
spin-velocity alignment, which has been observed for a sample of 58 pulsars, on
pulsar age. At first, our study considers only pulsar characteristic ages,
resulting in no change in the degree of correlation as a function of age, up to
at least 100 Myr. Subsequently, we consider a more reliable estimate of pulsar
age, the kinematic age, assuming that pulsars are born near the Galactic plane.
We derive kinematic ages for 52 pulsars, based on the measured pulsar proper
motions and positions, by modelling the trajectory of the pulsars in a Galactic
potential. The sample of 52 pulsar kinematic ages constitutes the largest
number of independently estimated pulsar ages to date. Using only the 33 most
reliable kinematic ages from our simulations, we revisit the evolution of spin-
velocity alignment, this time as a function of kinematic age. We find that the
strong correlation seen in young pulsars is completely smeared out for pulsars
with kinematic ages above 10 Myr, a length of time beyond which we expect the
gravitational pull of the Galaxy to have a significant effect on the directions
of pulsar velocities. In the discussion, we investigate the impact of large
distance uncertainties on the reliability of the calculated kinematic ages.
Furthermore, we present a detailed investigation of the implications of our
revised pulsar ages for the braking-index and birth-period distributions.
Finally, we discuss the predictions of various SN-kick mechanisms and their
compatibility with our results.Comment: 24 pages, 19 figures, MNRAS accepte
Dissecting cosmic-ray electron-positron data with Occam's Razor: the role of known Pulsars
We argue that both the positron fraction measured by PAMELA and the peculiar
spectral features reported in the total electron-positron (e+e-) flux measured
by ATIC have a very natural explanation in electron-positron pairs produced by
nearby pulsars. While this possibility was pointed out a long time ago, the
greatly improved quality of current data potentially allow to reverse-engineer
the problem: given the regions of pulsar parameter space favored by PAMELA and
by ATIC, are there known pulsars that explain the data with reasonable
assumptions on the injected e+e- pairs? In the context of simple benchmark
models for estimating the e+e- output, we consider all known pulsars, as listed
in the most complete available catalogue. We find that it is unlikely that a
single pulsar be responsible for both the PAMELA e+ fraction anomaly and for
the ATIC excess, although two single sources are in principle enough to explain
both experimental results. The PAMELA excess e+ likely come from a set of
mature pulsars (age ~ 10^6 yr), with a distance of 0.8-1 kpc, or from a single,
younger and closer source like Geminga. The ATIC data require a larger (and
less plausible) energy output, and favor an origin associated to powerful, more
distant (1-2 kpc) and younger (age ~ 10^5$ yr) pulsars. We list several
candidate pulsars that can individually or coherently contribute to explain the
PAMELA and ATIC data. Although generally suppressed, we find that the
contribution of pulsars more distant than 1-2 kpc could contribute for the ATIC
excess. Finally, we stress the multi-faceted and decisive role that Fermi-LAT
will play in the very near future by (1) providing an exquisite measurement of
the e+e- flux, (2) unveiling the existence of as yet undetected pulsars, and
(3) searching for anisotropies in the arrival direction of high-energy e+e-.Comment: revised version, references and new figures added, changes in the
discussion and figure
HST/WFPC2 observations of the LMC pulsar PSR B0540-69
The study of the younger, and brighter, pulsars is important to understand
the optical emission properties of isolated neutron stars. PSRB0540-69, the
second brightest (V~22) optical pulsar, is obviously a very interesting target
for these investigations. The aim of this work is threefold: constraining the
pulsar proper motion and its velocity on the plane of the sky through optical
astrometry, obtaining a more precise characterisation of the pulsar optical
spectral energy distribution (SED) through a consistent set of multi-band,
high-resolution, imaging photometry observations, measuring the pulsar optical
phase-averaged linear polarisation, for which only a preliminary and uncertain
measurement was obtained so far from ground-based observations. We performed
high-resolution observations of PSRB0540-69 with the WFPC2 aboard the HST, in
both direct imaging and polarimetry modes. From multi-epoch astrometry we set a
3sigma upper limit of 1 mas/yr on the pulsar proper motion, implying a
transverse velocity <250 km/s at the 50 kpc LMC distance. Moreover, we
determined the pulsar absolute position with an unprecedented accuracy of 70
mas. From multi-band photometry we characterised the pulsar power-law spectrum
and we derived the most accurate measurement of the spectral index
(0.70+/-0.07) which indicates a spectral turnover between the optical and X-ray
bands. Finally, from polarimetry we obtained a new measurement of the pulsar
phase-averaged polarisation degree (16+/-4%),consistent with magnetosphere
models depending on the actual intrinsic polarisation degree and depolarisation
factor, and we found that the polarisation vector (22+/-12deg position angle)
is possibly aligned with the semi-major axis of the pulsar-wind nebula and with
the apparent proper motion direction of its bright emission knot.Comment: 14 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
A new nearby pulsar wind nebula overlapping the RX J0852.0-4622 supernova remnant
Energetic pulsars can be embedded in a nebula of relativistic leptons which
is powered by the dissipation of the rotational energy of the pulsar. The
object PSR J0855-4644 is an energetic and fast-spinning pulsar (Edot =
1.1x10^36 erg/s, P=65 ms) discovered near the South-East rim of the supernova
remnant (SNR) RX J0852.0-4622 (aka Vela Jr) by the Parkes multibeam survey. The
position of the pulsar is in spatial coincidence with an enhancement in X-rays
and TeV gamma-rays, which could be due to its putative pulsar wind nebula
(PWN).
The purpose of this study is to search for diffuse non-thermal X-ray emission
around PSR J0855-4644 to test for the presence of a PWN and to estimate the
distance to the pulsar. An X-ray observation was carried out with the
XMM-Newton satellite to constrain the properties of the pulsar and its nebula.
The absorption column density derived in X-rays from the pulsar and from
different regions of the rim of the SNR was compared with the absorption
derived from the atomic (HI) and molecular (12CO) gas distribution along the
corresponding lines of sight to estimate the distance of the pulsar and of the
SNR.
The observation has revealed the X-ray counterpart of the pulsar together
with surrounding extended emission thus confirming the existence of a PWN. The
comparison of column densities provided an upper limit to the distance of the
pulsar PSR J0855-4644 and the SNR RX J0852.0-4622 (d<900 pc). Although both
objects are at compatible distances, we rule out that the pulsar and the SNR
are associated. With this revised distance, PSR J0855-4644 is the second most
energetic pulsar, after the Vela pulsar, within a radius of 1 kpc and could
therefore contribute to the local cosmic-ray e-/e+ spectrum.Comment: 10 pages, 9 Figures. Accepted for publication in A&
Cut-off Characterisation of Energy Spectra of Bright Fermi Sources: Current instrument limits and future possibilities
In this paper some of the brightest GeV sources observed by the Fermi-LAT
were analysed, focusing on their spectral cut-off region. The sources chosen
for this investigation were the brightest blazar flares of 3C~454.3 and 3C~279
and the Vela pulsar with a reanalysis with the latest Fermi-LAT software. For
the study of the spectral cut-off we first explored the Vela pulsar spectrum,
whose statistics in the time interval of the 3FGL catalog allowed strong
constraints to be obtained on the parameters. We subsequently performed a new
analysis of the flaring blazar SEDs. For these sources we obtained constraints
on the cut-off parameters under the assumption that their underlying spectral
distribution is described by a power-law with a stretched exponential cut-off.
We then highlighted the significant potential improvements on such constraints
by observations with next generation ground based Cherenkov telescopes,
represented in our study by the Cherenkov Telescope Array (CTA). Adopting
currently available simulations for this future observatory, we demonstrate the
considerable improvement in cut-off constraints achievable by observations with
this new instrument when compared with that achievable by satellite
observations.Comment: total number of pages 24, including 6 pages of references. Accepted
by Astroparticle Physic
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