31 research outputs found
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
The X-ray Spectrum of the Vela Pulsar Resolved with Chandra
We report the results of the spectral analysis of two observations of the
Vela pulsar with the Chandra X-ray observatory. The spectrum of the pulsar does
not show statistically significant spectral lines in the observed 0.25-8.0 keV
band. Similar to middle-aged pulsars with detected thermal emission, the
spectrum consists of two distinct components. The softer component can be
modeled as a magnetic hydrogen atmosphere spectrum - for the pulsar magnetic
field G and neutron star mass and radius
km, we obtain \tef^\infty =0.68\pm 0.03 MK, erg s, pc (the
effective temperature, bolometric luminosity, and radius are as measured by a
distant observer). The effective temperature is lower than that predicted by
standard neutron star cooling models. A standard blackbody fit gives MK,
erg s ( is the distance in units of 250 pc); the blackbody
temperature corresponds to a radius, km, much
smaller than realistic neutron star radii. The harder component can be modeled
as a power-law spectrum, with parameters depending on the model adopted for the
soft component - , erg s and , erg s for the hydrogen atmosphere and blackbody soft
component, respectively ( is the photon index, is the luminosity
in the 0.2--8 keV band). The extrapolation of the power-law component of the
former fit towards lower energies matches the optical flux at --1.45.Comment: Submitted to ApJ, three figures; color figure 1 can be found at
http://www.xray.mpe.mpg.de/~zavlin/pub_list.htm
The Compact Central Object in Cas A: A Neutron Star with Hot Polar Caps or a Black Hole?
The central pointlike X-ray source of the Cas A supernova remnant was
discovered in the Chandra First Light Observation and found later in the
archival ROSAT and Einstein images. The analysis of these data does not show
statistically significant variability of the source. The power-law fit yields
the photon index 2.6-4.1, and luminosity (2-60)e34 erg/s, for d=3.4 kpc. The
power-law index is higher, and the luminosity lower, than those observed
fromvery young pulsars. One can fit the spectrum equally well with a blackbody
model with T=6-8 MK, R=0.2-0.5 km, L=(1.4-1.9)e33 erg/s. The inferred radii are
too small, and the temperatures too high, for the radiationcould be interpreted
as emitted from the whole surface of a uniformly heated neutron star. Fits with
the neutron star atmosphere models increase the radius and reduce the
temperature, but these parameters are still substantially different from those
expected for a young neutron star. One cannot exclude, however, that the
observed emission originates from hot spots on a cooler neutron star surface.
Because of strong interstellar absorption, the possible low-temperature
component gives a small contribution to the observed spectrum; an upper limit
on the (gravitationally redshifted) surface temperature is < 1.9-2.3 MK.
Amongst several possible interpretations, we favor a model of a strongly
magnetized neutron star with magnetically confined hydrogen or helium polar
caps on a cooler iron surface. Alternatively, the observed radiation may be
interpreted as emitted by a compact object (more likely, a black hole)
accreting from a fossil disk or from a late-type dwarf in a close binary.Comment: 12 pages, 2 figures, submitted to ApJ
Polarization of Thermal X-rays from Isolated Neutron Stars
Since the opacity of a magnetized plasma depends on polarization of
radiation, the radiation emergent from atmospheres of neutron stars with strong
magnetic fields is expected to be strongly polarized. The degree of linear
polarization, typically ~10-30%, depends on photon energy, effective
temperature and magnetic field. The spectrum of polarization is more sensitive
to the magnetic field than the spectrum of intensity. Both the degree of
polarization and the position angle vary with the neutron star rotation period
so that the shape of polarization pulse profiles depends on the orientation of
the rotational and magnetic axes. Moreover, as the polarization is
substantially modified by the general relativistic effects, observations of
polarization of X-ray radiation from isolated neutron stars provide a new
method for evaluating the mass-to-radius ratio of these objects, which is
particularly important for elucidating the properties of the superdense matter
in the neutron star interiors.Comment: 7 figures, to be published in Ap
Detection of Pulsed X-ray Emission from XMM-Newton Observations of PSR J0538+2817
We report on the XMM-Newton observations of the 143 ms pulsar PSR J0538+2817.
We present evidence for the first detections of pulsed X-rays from the source
at a frequency which is consistent with the predicted radio frequency. The
pulse profile is broad and asymmetric, with a pulse fraction of 18 +/- 3%. We
find that the spectrum of the source is well-fit with a blackbody with
T^{infty} = (2.12^{+0.04}_{-0.03}) x 10^6 K and N_{H} = 2.5 x 10^21 cm^{-2}.
The radius determined from the model fit of 1.68 +/- 0.05 km suggests that the
emission is from a heated polar cap. A fit to the spectrum with an atmospheric
model reduces the inferred temperature and hence increases the radius of the
emitting region, however the pulsar distance determined from the fit is then
smaller than the dispersion distance.Comment: 24 pages, 6 figures, 3 tables, accepted for publication in ApJ. Error
in radius calculation corrected, discussion and conclusions remain unchange
Nonideal strongly magnetized plasmas of neutron stars and their electromagnetic radiation
We study the equation of state, polarization and radiation properties for
nonideal, strongly magnetized plasmas which compose outer envelopes of magnetic
neutron stars. Detailed calculations are performed for partially ionized
hydrogen atmospheres and for condensed hydrogen or iron surfaces of these
stars. This is a companion paper to astro-ph/0511803Comment: 7 pages, 3 figures. Invited topical talk at Strongly Coupled Coulomb
Systems (Moscow, June 20-25, 2005); to appear in Journal of Physics
Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar B1821-24
We report here the results of the first Chandra X-Ray Observatory
observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are
detected, of which 12 lie within one core radius of the center. We show that
the apparently extended X-ray core emission seen with the ROSAT HRI is due to
the superposition of multiple discrete sources for which we determine the X-ray
luminosity function down to a limit of about 6xE30 erg/s. For the first time
the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is
measured and found to be best described by a power law with photon index ~ 1.2.
Marginal evidence of an emission line centered at 3.3 keV in the pulsar
spectrum is found, which could be interpreted as cyclotron emission from a
corona above the pulsar's polar cap if the the magnetic field is strongly
different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0
keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest
unidentified sources is presented. Based on the spectral parameters of the
brightest of these sources, we suggest that it is a transiently accreting
neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum
with a hydrogen neutron star atmosphere model yields the effective temperature
T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty =
14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter,
unresolved X-ray emission from the central core of M28. Using the
Chandra-derived positions, we also report on the result of searching archival
Hubble Space Telescope data for possible optical counterparts.Comment: Accepted for publication in ApJ; 22 pages, 8 figures, 5 table
Gamma-Ray Emissions from Pulsars: Spectra of the TEV Fluxes from Outer-Gap Accelerators
We study the gamma-ray emissions from an outer-magnetospheric potential gap
around a rotating neutron star. Migratory electrons and positrons are
accelerated by the electric field in the gap to radiate copious gamma-rays via
curvature process. Some of these gamma-rays materialize as pairs by colliding
with the X-rays in the gap, leading to a pair production cascade. Imposing the
closure condition that a single pair produces one pair in the gap on average,
we explicitly solve the strength of the acceleration field and demonstrate how
the peak energy and the luminosity of the curvature-radiated, GeV photons
depend on the strength of the surface blackbody and the power-law emissions.
Some predictions on the GeV emission from twelve rotation-powered pulsars are
presented. We further demonstrate that the expected pulsed TeV fluxes are
consistent with their observational upper limits. An implication of high-energy
pulse phase width versus pulsar age, spin, and magnetic moment is discussed.Comment: Revised to compute absolute TeV spectra (22 pages, 9 figures
XMM-Newton Observations of PSR B1706-44
We report on the XMM-Newton observations of the young, 102 ms pulsar PSR
B1706-44. We have found that both a blackbody plus power-law and a magnetized
atmospheric model plus power-law provide an excellent fit to the EPIC spectra.
The two scenarios are therefore indistinguishable on a statistical basis,
although we are inclined to prefer the latter on physical grounds. In this
case, assuming a source distance of ~2.3 kpc, the size of the region
responsible for the thermal emission is R~13 km, compatible with the surface of
a neutron star. A comparison of the surface temperature of PSR B1706-44
obtained from this fit with cooling curves favor a medium mass neutron star
with M~1.45 solar masses or M~1.59 solar masses, depending on two different
models of proton superfluidity in the interior. The large collecting area of
XMM-Newton allows us to resolve a substructure in the broad soft X-ray
modulation detected by Chandra, revealing the presence of two separate peaks
with pulsed fractions of 7 +/- 4% and 15 +/- 3%, respectively.Comment: 21 pages, 5 figures, 2 tables, accepted for publication in Ap