50 research outputs found
Hardness-Intensity Correlations in Magnetar Afterglows
We explore the hardness-intensity correlations observed in several AXPs and
SGRs within the framework of a thermally emitting magnetar model. Using our
detailed atmosphere models and taking into account reprocessing of the surface
emission by the magnetosphere, we show that the hardness of the surface spectra
increases with increasing temperature and hence the changes in the effective
temperatures of the outer layers of the star alone can account for the observed
correlations. We conclude that the slow release of the heat deposited in the
deep crust during a magnetar burst naturally accounts for the spectral changes
during the afterglow. The correlations are further enhanced by changes in the
structures of the magnetic currents during or following a burst. However, the
additional hardening produced by scattering of the surface photons off the
magnetospheric charges saturates at moderate values of the scattering optical
depth.Comment: Submitted to the Astrophysical Journal Letter
The mass and the radius of the neutron star in the transient low mass X-ray binary SAX J1748.9−2021
We use time resolved spectroscopy of thermonuclear X-ray bursts observed from SAX J1748.9−2021 to infer the mass and the radius of the neutron star in the binary. Four X-ray bursts observed from the source with RXTE enable us to measure the angular size and the Eddington limit on the neutron star surface. Combined with a distance measurement to the globular cluster NGC 6440, in which SAX J1748.9−2021 resides, we obtain two solutions for the neutron star radius and mass, R = 8.18±1.62 km and M = 1.78±0.3 M_sun or R = 10.93±2.09 km and M = 1.33 ± 0.33 M_sun
The Mass and Radius of the Neutron Star in the Bulge Low-Mass X-ray Binary KS 1731-260
Measurements of neutron star masses and radii are instrumental for
determining the equation of state of their interiors, understanding the
dividing line between neutron stars and black holes, and for obtaining accurate
statistics of source populations in the Galaxy. We report here on the
measurement of the mass and radius of the neutron star in the low-mass X-ray
binary KS 1731-260. The analysis of the spectroscopic data on multiple
thermonuclear bursts yields well-constrained values for the apparent angular
area and the Eddington flux of the source, both of which depend in a distinct
way on the mass and radius of the neutron star. The binary KS 1731-260 is in
the direction of the Galactic bulge, allowing a distance estimate based on the
density of stars in that direction. Making use of the Han & Gould model, we
determine the probability distribution over the distance to the source, which
is peaked at 8 kpc. Combining these measurements, we place a strong upper bound
on the radius of the neutron star, R <= 12 km, while confining its mass to M <=
1.8 M_sun.Comment: submitted to Ap
Astrophysical Measurement of the Equation of State of Neutron Star Matter
We present the first astrophysical measurement of the pressure of cold matter
above nuclear saturation density, based on recently determined masses and radii
of three neutron stars. The pressure at higher densities are below the
predictions of equations of state that account only for nucleonic degrees of
freedom, and thus present a challenge to the microscopic theory of neutron star
matter.Comment: replaced with the published versio
Mapping the Surface of the Magnetar 1E 1048.1-5937 in Outburst and Quiescence Through Phase Resolved X-ray Spectroscopy
We model the pulse profiles and the phase resolved spectra of the anomalous
X-ray pulsar 1E 1048.1-5937 obtained with XMM-Newton to map its surface
temperature distribution during an active and a quiescent epoch. We develop and
apply a model that takes into account the relevant physical and geometrical
effects on the neutron star surface, magnetosphere, and spacetime. Using this
model, we determine the observables at infinity as a function of pulse phase
for different numbers and sizes of hot spots on the surface. We show that the
pulse profiles extracted from both observations can be modeled with a single
hot spot and an antipodal cool component. The size of the hot spot changes from
in 2007, 3 months after the onset of a dramatic flux
increase, to during the quiescent observation in 2011,
when the pulsed fraction returned to the pre-outburst 65\% level. For
the 2007 observation, we also find that a model consisting of a single 0.4 keV
hot spot with a magnetic field strength of G accounts for
the spectra obtained at three different pulse phases but under predicts the
flux at the pulse minimum, where the contribution to the emission from the
cooler component is non-negligible. The inferred temperature of the spot stays
approximately constant between different pulse phases, in agreement with a
uniform temperature, single hot spot model. These results suggest that the
emitting area grows significantly during outbursts but returns to its
persistent and significantly smaller size within a few year timescale.Comment: Accepted for publication in The Astrophysical Journa
Probing X-ray Absorption and Optical Extinction in the Interstellar Medium Using Chandra Observations of Supernova Remnants
We present a comprehensive study of interstellar X-ray extinction using the
extensive Chandra supernova remnant archive and use our results to refine the
empirical relation between the hydrogen column density and optical extinction.
In our analysis, we make use of the large, uniform data sample to assess
various systematic uncertainties in the measurement of the interstellar X-ray
absorption. Specifically, we address systematic uncertainties that originate
from (i) the emission models used to fit supernova remnant spectra, (ii) the
spatial variations within individual remnants, (iii) the physical conditions of
the remnant such as composition, temperature, and non-equilibrium regions, and
(iv) the model used for the absorption of X-rays in the interstellar medium.
Using a Bayesian framework to quantify these systematic uncertainties, and
combining the resulting hydrogen column density measurements with the
measurements of optical extinction toward the same remnants, we find the
empirical relation NH = (2.87+/-0.12) x 10^21 AV cm^(-2), which is
significantly higher than the previous measurements
X-ray Perspective of the Twisted Magnetospheres of Magnetars
Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are
recognized as the most promising magnetar candidates, as indicated by their
energetic bursts and rapid spin-downs. It is expected that the strong magnetic
field leaves distinctive imprints on the emergent radiation both by affecting
the radiative processes in atmospheres of magnetars and by scattering in the
upper magnetospheres. We construct a self-consistent physical model that
incorporates emission from the magnetar surface and its reprocessing in the
three-dimensional (3D) twisted magnetosphere using a Monte Carlo technique. The
synthetic spectra are characterized by four parameters: surface temperature kT,
surface magnetic field strength , magnetospheric twist angle ,
and the normalized electron velocity . We also create a tabular model
(STEMS3D) and apply it to a large sample of XMM-Newton spectra of magnetars.
The model successfully fits nearly all spectra, and the obtained magnetic field
for 7 out of the 11 sources are consistent with the values inferred from the
spin-down rates. We conclude that the continuum-fitting by our model is a
robust method to measure the magnetic field strength and magnetospheric
configuration of AXPs and SGRs. Investigating the multiple observations of
variable sources, we also study the mechanism of their spectral evolution. Our
results suggest that the magnetospheres in these sources are highly twisted
(), and the behavior of magnetospheric twisting and untwisting
is revealed in the 2002 outburst of 1E 2259+586.Comment: 20 pages, 12 figures, 9 tables, published in Ap