384 research outputs found
Variations in the spin period of the radio-quiet pulsar 1E 1207.4-5209
The X-ray source 1E 1207.4-5209 is a compact central object in the
G296.5+10.0 supernova remnant. Its spin period of 424 ms, discovered with the
Chandra X-ray Observatory, suggests that it is a neutron star. The X-ray
spectrum of this radio-quiet pulsar shows at least two absorption lines, first
spectral features discovered in radiation from an isolated neutron star. Here
we report the results of timing analysis of Chandra and XMM-Newton observations
of this source showing a non-monotonous behavior of its period. We discuss
three hypotheses which may explain the observational result. The first one
assumes that 1E 1207.$-5209 is a glitching pulsar, with frequency jumps of
\Delta f > 5 \muHz occurring every 1-2 years. The second hypothesis explains
the deviations from a steady spin-down as due to accretion, with accretion rate
varying from \sim 10^{13} to >10^{16} g s^{-1}, from a disk possibly formed
from ejecta produced in the supernova explosion. Finally, the period variations
could be explained assuming that the pulsar is in a wide binary system with a
long period, P_orb \sim 0.2-6 yr, and a low-mass companion, M_2 < 0.3 M_\odot.Comment: 20 pages, 5 figures, accepted for publications in ApJ. 2004 ApJ, in
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The pulsar wind nebula of the Geminga pulsar
The superb spatial resolution of Chandra has allowed us to detect a 20''-long
tail behind the Geminga pulsar, with a hard spectrum (photon index 1.0+/-0.2)
and a luminosity (1.3+/-0.2) 10^{29} ergs/s in the 0.5 - 8 keV band, for an
assumed distance of 200 pc. The tail could be either a pulsar jet, confined by
a toroidal magnetic field of about 100 microGauss, or it can be associated with
the shocked relativistic wind behind the supersonically moving pulsar confined
by the ram pressure of the oncoming interstellar medium. We also detected an
arc-like structure 5'' - 7'' ahead of the pulsar, extended perpendicular to the
tail, with a factor of 3 lower luminosity. We see a 3-sigma enhancement in the
Chandra image apparently connecting the arc with the southern outer tail that
has been possibly detected with XMM-Newton. The observed structures imply that
the Geminga's pulsar wind is intrinsically anisotropic.Comment: Revised version: data analysis described in more detail, Figure 2
replaced; 6 pages, 2 color figures; accepted for publication in ApJ (v.643,
2006 June 1
Thermal Radiation from Neutron Stars: Chandra Results
The outstanding capabilities of the Chandra X-ray observatory have greatly
increased our potential to observe and analyze thermal radiation from the
surfaces of neutron stars (NSs). Such observations allow one to measure the
surface temperatures and confront them with the predictions of the NS cooling
models. Detection of gravitationally redshifted spectral lines can yield the NS
mass-to-radius ratio. In rare cases when the distance is known, one can measure
the NS radius, which is particularly important to constrain the equation of
state of the superdense matter in the NS interiors. Finally, one can infer the
chemical composition of the NS surface layers, which provides information about
formation of NSs and their interaction with the environment. We overview the
recent Chandra results on the thermal radiation from various types of NSs --
active pulsars, young radio-quiet neutron stars in supernova remnants, old
radio-silent ``dim'' neutron stars -- and discuss their implications.Comment: URL changed for Figures 1, 12 and 18:
ftp://ftp.xray.mpe.mpg.de/people/zavli
X-ray emission from PSR J1809-1917 and its pulsar wind nebula, possibly associated with the TeV gamma-ray source HESS J1809-193
We detected X-ray emission from the 50-kyr-old pulsar J1809-1917 and resolved
its pulsar wind nebula (PWN) with Chandra. The pulsar spectrum fits PL+BB model
with the photon index of 1.2 and the BB temperature of 2 MK for n_{H}=0.7\times
10^{22} cm^{-2}. The luminosities are(4\pm 1)\times 10^{31} ergs s^{-1} for the
PL component (in the 0.5-8 keV band) and ~1\times 10^{32} ergs s^{-1} for the
BB component (bolometric) at a plausible distance of 3.5 kpc. The bright inner
PWN component of a 3''\times12'' size is elongated in the north-south
direction, with the pulsar close to its south end. This component is immersed
in a larger (20''\times40''), similarly elongated outer PWN component of lower
surface brightness. The elongated shape of the compact PWN can be explained by
the ram pressure confinement of the pulsar wind due to the supersonic motion of
the pulsar. The PWN spectrum fits a PL model with photon index of 1.4\pm0.1 and
0.5-8 keV luminosity of 4\times10^{32} ergs s^{-1}. The compact PWN appears to
be inside a large-scale (~4'\times4') emission more extended to the south of
the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the
extended X-ray emission ahead of the moving pulsar, one has to invoke strong
intrinsic anisotropy of the pulsar wind or assume that this emission comes from
a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN
are located within the extent of the unidentified TeV source HESS J1809-193
whose brightest part is offset by ~8' to the south of the pulsar, i.e. in the
same direction as the large-scale X-ray emission. Although the association
between J1809-1917 and HESS J1809-193 is plausible, an alternative source of
relativistic electrons powering HESS J1809-193 might be the serendipitously
discovered X-ray source CXOU J180940.7-192544.Comment: 13 pages, 10 figures and 3 tables, submitted to ApJ. Version with the
high-resolution figures is available at
http://www.astro.psu.edu/users/green/J1809/ms_astroph.pd
The jets of the Vela pulsar
Chandra observations of the Vela pulsar-wind nebula (PWN) have revealed a jet
in the direction of the pulsar's proper motion, and a counter-jet in the
opposite direction, embedded in diffuse nebular emission. The jet consists of a
bright, 8''-long inner jet, between the pulsar and the outer arc, and a dim,
curved outer jet that extends up to 100'' in approximately the same direction.
From the analysis of thirteen Chandra observations spread over about 2.5
years we found that this outer jet shows particularly strong variability,
changing its shape and brightness. We observed bright blobs in the outer jet
moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on
time-scales of days to weeks. The spectrum of the outer jet fits a power-law
model with a photon index of 1.3\pm0.1. The X-ray emission of the outer jet can
be interpreted as synchrotron radiation of ultrarelativistic
electrons/positrons. This interpretation allows one to estimate the magnetic
field, ~100 microGauss, maximum energy of X-ray emitting electrons, ~2\times
10^{14} eV, and energy injection rate, ~8\times 10^{33} erg/s, for the outer
jet. In the summed PWN image we see a dim, 2'-long outer counter-jet, which
also shows a power-law spectrum with photon ined of 1.2-1.5. Southwest of the
jet/counter-jet an extended region of diffuse emission is seen. Relativistic
particles responsible for this radiation are apparently supplied by the outer
jet.Comment: 4 pages, including 1 figure, accepted for publication in New
Astronomy Reviews; proceedings of the conference "The Physics of Relativistic
Jets in the CHANDRA and XMM Era", 23-27 September 2002, Bologna. The full
resolution versions of the images shown in the fugure are avaliable at
http://www.astro.psu.edu/users/green/vela_jet_proc/vela_jet_proc.htm
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