344 research outputs found
X-ray emission from the old pulsar B0950+08
We present the timing and spectral analyses of theXMM-newton data on the
17-Myr-old, nearby radio pulsar B0950+08. This observation revealed pulsations
of the X-ray flux of the pulsar at its radio period. The pulse shape and pulsed
fraction are apparently different at lower and higher energies of the observed
0.2-10 keV energy range, which suggests that the radiation cannot be explained
by a single emission mechanism. The X-ray spectrum of the pulsar can be fitted
with a power-law model with a photon index about 1.75 and an (isotropic)
luminosity about 9.8e29 erg/s in the 0.2-10 keV. Better fits are obtained with
two-component, power-law plus thermal, models with index of 1.30 and 9.7e29
erg/s for the power-law component that presumably originates from the pulsar's
magnetosphere. The thermal component, dominating at E>0.7 keV, can be
interpreted as radiation from heated polar caps on the neutron star surface
covered with a hydrogen atmosphere. The inferred effective temperature, radius,
and bolometric luminosity of the polar caps are about 1 MK, 250 m, and 3e29
erg/s. Optical through X-ray nonthermal spectrum of the pulsar can be described
as a single power-law with index 1.3-1.4 for the two-component X-ray fit. The
ratio of the nonthermal X-ray (1-10 keV) luminosity to the nonthermal optical
(4000-9000 \AA) luminosity is within the range of 1e2-1e3 observed for younger
pulsars, which suggests that the magnetospheric X-ray and optical emissions are
powered by the same mechanism in all pulsars. An upper limit on the temperature
of the bulk of the neutron star surface, inferred from the optical and X-ray
data, is about 0.15 MK. We also analyze X-ray observations of several other old
pulsars, B2224+65, J2043+2740, B0628-28, B1813-36, B1929+10, and B0823+26.Comment: To be published in ApJ. Nonthermal optical and X-ray luminosities of
seven radio pulsars are updated and presented in a new Table. Figure 6
showing the ratios of the luminosities vs. spin-down energy is also update
Mass-to-Radius Ratio for the Millisecond Pulsar J0437-4715
Properties of X-ray radiation emitted from the polar caps of a radio pulsar
depend not only on the cap temperature, size, and position, but also on the
surface chemical composition, magnetic field, and neutron star's mass and
radius. Fitting the spectra and the light curves with neutron star atmosphere
models enables one to infer these parameters. As an example, we present here
results obtained from the analysis of the pulsed X-ray radiation of a nearby
millisecond pulsar J0437-4715. In particular, we show that stringent
constraints on the mass-to-radius ratio can be obtained if orientations of the
magnetic and rotation axes are known, e.g., from the radio polarization data.Comment: 2 figures, aasms4.sty; accepted for publication in ApJLetter
XMM observations of three middle-aged pulsars
X-ray observations of middle-aged pulsars allow one to study nonthermal
radiation from pulsar magnetospheres and thermal radiation from neutron star
(NS) surfaces. In particular, from the analysis of thermal radiation one can
infer the surface temperatures and radii of NSs, which is important for
investigating evolution of these objects and constraining the equation of state
of the superdense matter in the NS interiors. Here we present results of XMM
observations of three middle-aged pulsars, J0538+2817, B0656+14 and J0633+1746
(Geminga), and briefly discuss mechanisms of their X-ray emission.Comment: 6 pages, 8 figures; to be published in Memorie della Societa'
Astronomica Italiana, the Proceedings of the EPIC Consortium (held on Oct
14-16, 2003 in Palermo
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
pres
XMM-Newton observations of four millisecond pulsars
I present an analysis of the XMM-Newton observations of four millisecond
pulsars, J0437-4715, J2124-3358, J1024-0719, and J0034-0534. The new data
provide strong evidence of thermal emission in the X-ray flux detected from the
first three objects. This thermal component is best interpreted as radiation
from pulsar polar caps covered with a nonmagnetic hydrogen atmosphere. A
nonthermal power-law component, dominating at energies E>3 keV, can also be
present in the detected X-ray emission. For PSR J0437-4715, the timing analysis
reveals that the shape and pulsed fraction of the pulsar light curves are
energy dependent. This, together with the results obtained from the
phase-resolved spectroscopy, supports the two-component (thermal plus
nonthermal) interpretation of the pulsar's X-ray radiation. Highly significant
pulsations have been found in the X-ray flux of PSRs J2124-3358 and J1024-0719.
For PSR J0034-0534, a possible X-ray counterpart of the radio pulsar has been
suggested. The inferred properties of the detected thermal emission are
compared with predictions of radio pulsar models.Comment: 33 pages, 13 figures (of them 4 are color); to be published in Ap
GEMINGA'S SOFT X-RAY EMISSION AND THE STRUCTURE OF ITS SURFACE
We present a model to explain the decrease in the amplitude of the pulse
profile with increasing energy observed in Geminga's soft X-ray surface thermal
emission. We assume the presence of plates surrounded by a surface with very
distinct physical properties: these two regions emit spectra of very distinct
shapes which present a crossover, the warm plates emitting a softer spectrum
than the colder surrounding surface. The strongly pulsed emission from the
plates dominates at low energy while the surroundings emission dominates at
high energy, producing naturally a strong decrease in the pulsed fraction. In
our illustrative example the plates are assumed to be magnetized while the rest
of the surface is field free.
This plate structure may be seen as a schematic representation of a
continuous but very nonuniform distribution of the surface magnetic field or as
a quasi realistic structure induced by past tectonic activity on Geminga.Comment: 10 pages, AASTeX latex, + 3 figures (compressed 7 uuencoded).
Submitted to Ap. J. Let
X-ray Pulsations from the Central Source in Puppis A
There are several supernova remnants which contain unresolved X-ray sources
close to their centers, presumably radio-quiet neutron stars. To prove that
these objects are indeed neutron stars, to understand the origin of their X-ray
radiation, and to explain why they are radio-quiet, one should know their
periods and period derivatives. We searched for pulsations of the X-ray flux
from the radio-quiet neutron star candidate RX J0822-4300 near the center of
the Puppis A supernova remnant observed with the ROSAT PSPC and HRI. A standard
timing analysis of the separate PSPC and HRI data sets does not allow one to
detect the periodicity unequivocally. However, a thorough analysis of the two
observations separated by 4.56 yr enabled us to find a statistically
significant period ms and its derivative s s. The corresponding characteristic parameters of
the neutron star, age kyr, magnetic field G, and rotational energy loss erg
s, are typical for young radio pulsars. Since the X-ray radiation has a
thermal-like spectrum, its pulsations may be due to a nonuniform temperature
distribution over the neutron star surface caused by anisotropy of the heat
conduction in the strongly magnetized crust.Comment: 9 pages, 2 postscript figures, to appear in ApJ Letters; an
acknowledgment is adde
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
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