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 $P$d$V$ 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