633 research outputs found
Deep XMM-Newton Spectroscopic and Timing Observations of the Isolated Radio Millisecond Pulsar PSR J0030+0451
We present deep XMM-Newton EPIC spectroscopic and timing X-ray observations
of the nearby solitary radio millisecond pulsar, PSR J0030+0451. Its emission
spectrum in the 0.1-10 keV range is found to be remarkably similar to that of
the nearest and best studied millisecond pulsar, PSR J0437-4715, being well
described by a predominantly thermal two-temperature model plus a faint hard
tail evident above ~2 keV. The pulsed emission in the 0.3-2 keV band is
characterized by two broad pulses with pulsed fraction ~60-70%, consistent with
a mostly thermal origin of the X-rays only if the surface polar cap radiation
is from a light-element atmosphere. Modeling of the thermal pulses permits us
to place constraints on the neutron star radius of R>10.7 (95% confidence) and
R>10.4 km (at 99.9% confidence) for M=1.4 M_sun.Comment: 8 pages, 7 figures; accepted for publication in The Astrophysical
Journa
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
Nucleon Superfluidity vs Observations of Cooling Neutron Stars
Cooling simulations of neutron stars (NSs) are performed assuming that
stellar cores consist of neutrons, protons and electrons and using realistic
density profiles of superfluid critical temperatures and
of neutrons and protons. Taking a suitable profile of
with maximum K one can obtain smooth
transition from slow to rapid cooling with increasing stellar mass. Adopting
the same profile one can explain the majority of observations of thermal
emission from isolated middle--aged NSs by cooling of NSs with different masses
either with no neutron superfluidity in the cores or with a weak superfluidity,
K. The required masses range from for (young
and hot) RX J0822-43 and (old and warm) PSR 1055-52 and RX J1856-3754 to
for the (colder) Geminga and Vela pulsars. Observations
constrain the and profiles with respect to the
threshold density of direct Urca process and maximum central density of NSs.Comment: 4 pages, 2 figures, AA Letters, accepte
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
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
Quantum nature of cyclotron harmonics in thermal spectra of neutron stars
Some isolated neutron stars show harmonically spaced absorption features in
their thermal soft X-ray spectra. The interpretation of the features as a
cyclotron line and its harmonics has been suggested, but the usual explanation
of the harmonics as caused by relativistic effects fails because the
relativistic corrections are extremely small in this case. We suggest that the
features correspond to the peaks in the energy dependence of the free-free
opacity in a quantizing magnetic field, known as quantum oscillations. The
peaks arise when the transitions to new Landau levels become allowed with
increasing the photon energy; they are strongly enhanced by the square-root
singularities in the phase-space density of quantum states in the case when the
free (non-quantized) motion is effectively one-dimensional. To explore
observable properties of these quantum oscillations, we calculate models of
hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e.,
electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such
conditions are thought to be typical for the so-called central compact objects
in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that
observable features at the electron cyclotron harmonics form at moderately
large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 -
20. The equivalent widths of the features can reach 100 - 200 eV; they grow
with increasing b_{eff} and are lower for higher harmonics.Comment: 6 pages; shortened, references updated; 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
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