203 research outputs found
Fading of the Transient Anomalous X-ray Pulsar XTE J1810-197
Three observations of the 5.54 s Transient Anomalous X-ray Pulsar XTE
J1810-197 obtained over 6 months with the Newton X-Ray Multi-Mirror Mission
(XMM-Newton) are used to study its spectrum and pulsed light curve as the
source fades from outburst. The decay is consistent with an exponential of time
constant 300 days, but not a power law as predicted in some models of sudden
deep crustal heating events. All spectra are well fitted by a blackbody plus a
steep power law, a problematic model that is commonly fitted to anomalous X-ray
pulsars (AXPs). A two-temperature blackbody fit is also acceptable, and better
motivated physically in view of the faint optical/IR fluxes, the X-ray pulse
shapes that weakly depend on energy in XTE J1810-197, and the inferred emitting
areas that are less than or equal to the surface area of a neutron star. The
fitted temperatures remained the same while the flux declined by 46%, which can
be interpreted as a decrease in area of the emitting regions. The pulsar
continues to spin down, albeit at a reduced rate of (5.1+/-1.6)x10^{-12} s
s^{-1}. The inferred characteristic age Tau_c = P/2Pdot ~17,000 yr, magnetic
field strength B_s ~1.7x10^{14} G, and outburst properties are consistent with
both the outburst and quiescent X-ray luminosities being powered by magnetic
field decay, i.e., XTE J1810-197 is a magnetar.Comment: 10 pages, 5 figures, accepted by Ap.
X-ray Spectrum and Pulsations of the Vela Pulsar
We report the results of the spectral and timing analysis of observations of
the Vela pulsar with the Chandra X-ray Observatory. The spectrum shows no
statistically significant spectral lines in the observed 0.25--8.0 keV band. It
consists of two distinct continuum components. The softer component can be
modeled as either a magnetic hydrogen atmosphere spectrum with kT = 59 +- 3 eV,
R = 15.5 +- 1.5 km, or a standard blackbody with kT = 129 +- 4 eV, R = 2.1 +-
0.2 km (the radii are for a distance of 250 pc). The harder component, modeled
as a power-law spectrum, gives photon indices depending on the model adopted
for the soft component: gamma = 1.5 +- 0.3 for the magnetic atmosphere soft
component, and gamma = 2.7 +- 0.4 for the blackbody soft component. Timing
analysis shows three peaks in the pulse profile, separated by about 0.3 in
phase. Energy-resolved timing provides evidence for pulse profile variation
with energy. The higher energy (E > 1.8 keV) profile shows significantly higher
pulsed fraction.Comment: 4 pages, 2 figures, To appear in "Neutron Stars in Supernova
Remnants" (ASP Conference Proceedings), eds P. O. Slane and B. M. Gaensler
Corrected TYPO
The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Abundance analyses from high-resolution optical spectra are presented for 19
Type II Cepheids in the Galactic field. The sample includes both short-period
(BL Her) and long-period (W Vir) stars. This is the first extensive abundance
analysis of these variables. The C, N, and O abundances with similar spreads
for the BL Her and W Vir show evidence for an atmosphere contaminated with
-process and CN-cycling products. A notable anomaly of the BL Her
stars is an overabundance of Na by a factor of about five relative to their
presumed initial abundances. This overabundance is not seen in the W Vir stars.
The abundance anomalies running from mild to extreme in W Vir stars but not
seen in the BL Her stars are attributed to dust-gas separation that provides an
atmosphere deficient in elements of high condensation temperature, notably Al,
Ca, Sc, Ti, and -process elements. Such anomalies have previously been seen
among RV Tau stars which represent a long-period extension of the variability
enjoyed by the Type II Cepheids. Comments are offered on how the contrasting
abundance anomalies of BL Her and W Vir stars may be explained in terms of the
stars' evolution from the blue horizontal branch.Comment: 41 pages including 11 figures and 4 tables; Accepted for publication
in Ap
The Compact Central Object in the Supernova Remnant G266.2-1.2
We observed the compact central object CXOU J085201.4--461753 in the
supernova remnant G266.2--1.2 (RX J0852.0--4622) with the Chandra ACIS detector
in timing mode. The spectrum of this object can be described by a blackbody
model with the temperature kT=404 eV and radius of the emitting region R=0.28
km, at a distance of 1 kpc. Power-law and thermal plasma models do not fit the
source spectrum. The spectrum shows a marginally significant feature at 1.68
keV. Search for periodicity yields two candidate periods, about 301 ms and 33
ms, both significant at a 2.1 sigma level; the corresponding pulsed fractions
are 13% and 9%, respectively. We find no evidence for long-term variability of
the source flux, nor do we find extended emission around the central object. We
suggest that CXOU J085201.4--461753 is similar to CXOU J232327.9+584842, the
central source of the supernova remnant Cas A. It could be either a neutron
star with a low or regular magnetic field, slowly accreting from a fossil disk,
or, more likely, an isolated neutron star with a superstrong magnetic field. In
either case, a conservative upper limit on surface temperature of a 10 km
radius neutron star is about 90 eV, which suggests accelerated cooling for a
reasonable age of a few thousand years.Comment: Accepted to ApJ, 13 pages, 1 figur
A Candidate Neutron Star Associated with Galactic Center Supernova Remnant Sagittarius A East
We present imaging and spectral studies of the supernova remnant (SNR)
Sagittarius (Sgr) A East from deep observations with the {\it Chandra X-Ray
Observatory}. The spatially-resolved spectral analysis of Sgr A East reveals
the presence of a two-temperature thermal plasma ( 1 keV and 5 keV)
near the center of the SNR. The central region is dominated by emission from
highly-ionized Fe-rich ejecta. We estimate a conservative upper limit on the
total Fe ejecta mass of the SNR, M 0.27 M. Comparisons
with standard SN nucleosynthesis models suggest that this Fe mass limit is
consistent with a Type II SN explosion for the origin of Sgr A East. On the
other hand, the soft X-ray emission extending toward the north of the SNR can
be described by a single-temperature ( 1.3 keV) thermal plasma with
normal chemical composition. This portion of the SNR is thus X-ray emission
from the heated interstellar medium rather than the metal-rich stellar ejecta.
We point out that a hard pointlike source CXOGC J174545.5285829 (the
so-called ``cannonball'') at the northern edge of the SNR shows unusual X-ray
characteristics among other Galactic center sources. The morphological,
spectral, and temporal characteristics of this source suggest an identification
as a high-velocity neutron star. Based on the suggested Type II origin for the
SNR Sgr A East and the proximity between the two, we propose that CXOGC
J174545.5285829 is a high-velocity neutron star candidate, born from the
core-collapse SN which also created the SNR Sgr A East.Comment: ApJ preprint style 28 pages, 1 color fig (fig1), Accepted by Ap
The importance of the mixed phase in hybrid stars built with the Nambu-Jona-Lasinio model
We investigate the structure of hybrid stars based on two different
constructions: one is based on the Gibbs condition for phase coexistence and
considers the existence of a mixed phase (MP), and the other is based on the
Maxwell construction and no mixed phase is obtained. The hadron phase is
described by the non-linear Walecka model (NLW) and the quark phase by the
Nambu-Jona-Lasinio model (NJL). We conclude that the masses and radii obtained
are model dependent but not significantly different for both constructions.Comment: 8 pages, 7 figures, 3 table
Swift monitoring of the central X-ray source in RCW 103
The X-ray source 1E 161348-5055 lies at the centre of the 2-kyr-old supernova
remnant RCW 103. Owing to its 24-ks modulation, orders-of-magnitude flux
variability over a few months/years, and lack of an obvious optical
counterpart, 1E 161348-5055 defies assignment to any known class of X-ray
sources. Starting from April 2006, Swift observed 1E 161348-5055 with its X-ray
telescope for 2 ks approximately once per month. During the five years covered,
the source has remained in a quiescent state, with an average observed flux of
1.7e-12 erg/cm^2/s (1-10 keV), about 20 times lower than the historical maximum
attained in its 1999-2000 outburst. The long time-span of the Swift data allows
us to obtain an accurate measure of the period of 1E 161348-5055 [P =
24030.42(2) s] and to derive the first upper limit on its period derivative
(|dP/dt| < 1.6e-9 s/s at 3 sigma).Comment: 6 pages, 5 figures, accepted for publication in MNRAS. Figures 2,3
and 5 in reduced qualit
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