887 research outputs found
X-Ray Eclipse Timing in the LMXB EXO0748-676
Orbital period changes are an important diagnostic for understanding low mass
X-ray binary (LMXB) accretion-induced angular momentum exchange and overall
system evolution. We present our most recent results for the eclipse timing of
the LMXB EXO0748-676. Since its discovery in 1985 it has apparently undergone
three distinct orbital period "epochs", each characterized by a different
orbital period than the previous epoch. We outline the orbital period behavior
for EXO0748-676 over the past 18 years and discuss the implications of this
behavior in light of current theoretical ideas for LMXB evolution.Comment: 4 Pages, 3 Figures, Submitted to the X-Ray Timing 2003: Rossi and
Beyond conference, November 200
Comptonization and the Spectra of Accretion-Powered X-Ray Pulsars
Accretion-powered X-ray pulsars are among the most luminous X-ray sources in
the Galaxy. However, despite decades of theoretical and observational work
since their discovery, no satisfactory model for the formation of the observed
X-ray spectra has emerged. In this paper, we report on a self-consistent
calculation of the spectrum emerging from a pulsar accretion column that
includes an explicit treatment of the bulk and thermal Comptonization occurring
in the radiation-dominated shocks that form in the accretion flows. Using a
rigorous eigenfunction expansion method, we obtain a closed-form expression for
the Green's function describing the upscattering of monochromatic radiation
injected into the column. The Green's function is convolved with
bremsstrahlung, cyclotron, and blackbody source terms to calculate the emergent
photon spectrum. We show that energization of photons in the shock naturally
produces an X-ray spectrum with a relatively flat continuum and a high-energy
exponential cutoff. Finally, we demonstrate that our model yields good
agreement with the spectra of the bright pulsar Her X-1 and the low luminosity
pulsar X Per.Comment: 6 Pages, 2 Figures, To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors" (Cefalu, Sicily, June 2006).
Eds. L. Burderi et al. (New York: AIP
Spectral Formation in X-Ray Pulsar Accretion Columns
We present the first self-consistent model for the dynamics and the radiative
transfer occurring in bright X-ray pulsar accretion columns, with a special
focus on the role of the shock in energizing the emerging X-rays. The pressure
inside the accretion column of a luminous X-ray pulsar is dominated by the
photons, and consequently the equations describing the coupled
radiative-dynamical structure must be solved simultaneously. Spectral formation
in these sources is therefore a complex, nonlinear phenomenon. We obtain the
analytical solution for the Green's function describing the upscattering of
monochromatic radiation injected into the column from the thermal mound located
near the base of the flow. The Green's function is convolved with a Planck
distribution to model the X-ray spectrum resulting from the reprocessing of
blackbody photons produced in the thermal mound. These photons diffuse through
the infalling gas and eventually escape out the walls of the column, forming
the observed X-ray spectrum. We show that the resulting column-integrated,
phase-averaged spectrum has a power-law shape at high energies and a blackbody
shape at low energies, in agreement with the observational data for many X-ray
pulsars.Comment: Accepted for publication in ApJ Letters. Several typos noticed during
the proof review were correcte
Possible Magnetic Activity in the Low Mass X-ray Binary EXO 0748-676
We report evidence of magnetic activity associated with the secondary star in
the EXO 0748-676 low mass X-ray binary system. An analysis of a sequence of
five consecutive X-ray eclipses observed during December 2003 with the RXTE
satellite brings out a feature occurring during ingress we interpret as the
X-ray photoelectric absorption shadow, as seen by an observer at Earth, of a
plasma structure suspended above the surface of the secondary star. The light
curve feature consists of an initial drop in count rate to near zero (the
absorption shadow) with a very short rebound to a significant fraction of the
pre-ingress count rate and then a final plunge to totality over a total time
scale of ~25 s. The ingress feature persists for at least 5 consecutive orbital
periods (a total of ~19 hr), and possibly up to 5 days in our data. Our data
also show significant post-egress dipping during this eclipse sequence, unusual
for this source, indicating possible secondary star mass ejection during this
episode.Comment: Accepted by Astrophysical Journal Letters; 11 Pages including 3
figures and 1 tabl
Prospective Prediction of Juvenile Homicide/Attempted Homicide among Early-Onset Juvenile Offenders
While homicide perpetrated by juveniles is a relatively rare occurrence, between 2010 and 2014, approximately 7%–8% of all murders involved a juvenile offender. Unfortunately, few studies have prospectively examined the predictors of homicide offending, with none examining first-time murder among a sample of adjudicated male and female youth. The current study employed data on 5908 juvenile offenders (70% male, 45% Black) first arrested at the age of 12 or younger to prospectively examine predictors of an arrest for homicide/attempted homicide by the age of 18. Among these early-onset offenders, males, Black youth, those living in households with family members with a history of mental illness, those engaging in self-mutilation, and those with elevated levels of anger/aggression (all measured by age 13) were more likely to be arrested for homicide/attempted homicide by age 18. These findings add to the scant scientific literature on the predictors of homicide, and illustrate potential avenues for intervention
A Strong X-Ray Burst from the Low Mass X-Ray Binary EXO0748-676
We have observed an unusually strong X-ray burst as a part of our regular
eclipse timing observations of the low mass binary system EXO0748-676. The
burst peak flux was 5.2x10^-8 ergs cm^-2 s^-1, approximately five times the
normal peak X-ray burst flux observed from this source by RXTE. Spectral fits
to the data strongly suggest that photospheric radius expansion occurred during
the burst. In this Letter we examine the properties of this X-ray burst, which
is the first example of a radius expansion burst from EXO0748-676 observed by
RXTE. We find no evidence for coherent burst oscillations. Assuming that the
peak burst luminosity is the Eddington luminosity for a 1.4 solar mass neutron
star we derive a distance to EXO0748-676 of 7.7 kpc for a helium-dominated
burst photosphere and 5.9 kpc for a hydrogen-dominated burst photosphere.Comment: 15 pages including 2 figures and 1 table. Accepted for publication in
the Astrophysical Journa
A Generalized Analytical Model For Thermal And Bulk Comptonization In Accretion-Powered X-Ray Pulsars
We develop a new theoretical model describing the formation of the radiation
spectrum in accretion-powered X-ray pulsars as a result of bulk and thermal
Comptonization of photons in the accretion column. The new model extends the
previous model developed by the authors in four ways: (1) we utilize a conical
rather than cylindrical geometry; (2) the radiation components emitted from the
column wall and the column top are computed separately; (3) the model allows
for a non-zero impact velocity at the stellar surface; and (4) the velocity
profile of the gas merges with Newtonian free-fall far from the star. We show
that these extensions allow the new model to simulate sources over a wide range
of accretion rates. The model is based on a rigorous mathematical approach in
which we obtain an exact series solution for the Green's function describing
the reprocessing of monochromatic seed photons. Emergent spectra are then
computed by convolving the Green's function with bremsstrahlung, cyclotron, and
blackbody photon sources. The range of the new model is demonstrated via
applications to the high-luminosity source Her X-1, and the low-luminosity
source X Per. The new model suggests that the observed increase in spectral
hardness associated with increasing luminosity in Her X-1 may be due to a
decrease in the surface impact velocity, which increases the d work done
on the radiation field by the gas
Eclipse Timings of the Transient Low Mass X-ray Binary EXO0748-676. IV. The Rossi X-Ray Timing Explorer Eclipses
We report our complete database of X-ray eclipse timings of the low mass
X-ray binary EXO0748-676 observed by the Rossi X-Ray Timing Explorer (RXTE)
satellite. As of this writing we have accumulated 443 full X-ray eclipses, 392
of which have been observed with the Proportional Counter Array on RXTE. These
include both observations where an eclipse was specifically targeted and those
eclipses found in the RXTE data archive. Eclipse cycle count has been
maintained since the discovery of the EXO0748-676 system in February 1985. We
describe our observing and analysis techniques for each eclipse and describe
improvements we have made since the last compilation by Wolff et al. (2002).
The principal result of this paper is the database containing the timing
results from a seven-parameter fit to the X-ray light curve for each observed
eclipse along with the associated errors in the fitted parameters. Based on the
standard O-C analysis, EXO0748-676 has undergone four distinct orbital period
epochs since its discovery. In addition, EXO0748-676 shows small-scale events
in the O-C curve that are likely due to short-lived changes in the secondary
star.Comment: Accepted for publication in The Astrophysical Journal Supplement
Series, 5 figures. Analysis revised. Tables 1 & 3 update
- …