arXiv subjects

Kent S. Wood

Publications and source records attributed to Kent S. Wood.

At least 19 recordsLinked to original sources

arXiv2026

Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models

X-ray pulsars are highly magnetized ($B\sim 10^{12}$ G) neutron stars accreting from a donor star. Their characteristic X-ray emission arises from accreted material decelerated from relativistic velocities near the magnetic poles of the neutron star. As our line of sight onto the magnetic poles changes with the rotation of the neutron star, the X-rays are periodically modulated, resulting in X-ray pulsations. The shape of the pulse profiles depends on the physics of the interaction between the bright X-rays from the magnetic poles with the infalling matter, the location of the magnetic poles on the neutron star with respect to its spin axis, and on the properties of the space-time around the neutron star. In this review we give a pedagogical introduction to the accretion mechanisms operating in the various types of accreting neutron star systems and the observational techniques used to characterize the pulse profiles. We summarize how the pulse profiles depend on X-ray luminosity and energy and discuss the attempts to connect theoretically these observables with the physical accretion mechanisms. We conclude with an outline of future observational needs and further developments for theoretical models of magnetic accretion.

astro-ph.HE↗

arXiv2026

Photon-Count Statistics of Crab X-ray Pulses: Skellam Behavior and Excess Variance in the Main Pulse

The Crab pulsar (PSR B0531+21) provides an unusually rich test bed for statistical studies of high-energy photon-counting data, owing to its extreme brightness and the contrasting behavior of its main pulse (MP) and interpulse (IP) components. Using 78.8 ks of Neutron star Interior Composition Explorer (NICER; Gendreau and Arzoumanian 2017) data-over two million individual X-ray pulses- we construct the single-pulse photon-count distributions of the MP and IP at keV energies. We find that the IP is well described by the Skellam distribution expected for the difference of two Poisson processes, providing a rare, high-statistics empirical demonstration of Skellam behavior in an astrophysical photon-counting context. The MP also shows pulse-by-pulse variability best described by a Skellam framework when compared to Gaussian alternatives, but exhibits a significant excess variance driven by high-count events. When photon counts are summed over successive pulses, this excess averages out and the MP distribution becomes consistent with Skellam expectations, indicating that the enhanced variability does not persist across rotations. We further search for short-lag (memory) correlations between successive X-ray pulses and find no statistically significant lag-1 correlation. Although giant radio pulses occur in the MP phase window, their contribution is insufficient to account for the observed excess variability. Together, these results highlight a clear statistical distinction between the MP and IP and underscore the importance of using statistically appropriate models for high-energy photon-counting analyses. The distributional fits and memory limits reported here provide quantitative constraints on pulsar emission models and illustrate the broader utility of Skellam-based approaches.

astro-ph.HE↗

arXiv2025

NuSTAR broadband X-ray observation of EF Eri following its reawakening into a high accretion state

We present the first NuSTAR X-ray observation of EF Eri, a well-known polar system. The NuSTAR observation was conducted in conjunction with NICER, shortly after EF Eri entered a high accretion state following an unprecedented period of low activity lasting 26 years since 1997. NuSTAR detected hard X-ray emission up to 50 keV with an X-ray flux of $1.2\times10^{-10}$ ergs s$^{-1}$ cm$^{-2}$ ($3\rm{-}50$ keV). Folded X-ray lightcurves exhibit a single peak with $\sim65\%$ spin modulation throughout the $3\rm{-}50$ keV band. We found no evidence of QPO signals at $ν= 0.1\rm{-}100$ Hz with an upper limit on the QPO amplitude below $5\%$ ($90\%$ CL) at $ν\sim 0.5$ Hz where the optical QPO was previously detected. Our 1-D accretion column model, called $\texttt{MCVSPEC}$, was fitted to the NuSTAR spectral data, yielding an accurate WD mass measurement of $M = (0.55\rm{-}0.63) M_\odot$. ${\tt MCVSPEC}$ accounts for radiative cooling by thermal bremsstrahlung and cyclotron emission, X-ray reflection off the WD surface, and a previously constrained range of the accretion column area. The derived WD mass range is in excellent agreement with the previous measurement of $M = (0.55\rm{-}0.65) M_\odot$ in the optical band. This demonstrates a combination of broadband X-ray spectral analysis and the ${\tt MCVSPEC}$ model that can be employed in our ongoing NuSTAR observation campaign of other polars to determine their WD masses accurately.

astro-ph.HE↗

arXiv2023

Long-Term Density Trend in the Mesosphere and Lower Thermosphere from Occultations of the Crab Nebula with X-Ray Astronomy Satellites

We present long-term density trends of the Earth's upper atmosphere at altitudes between 71 and 116 km, based on atmospheric occultations of the Crab Nebula observed with X-ray astronomy satellites, ASCA, RXTE, Suzaku, NuSTAR, and Hitomi. The combination of the five satellites provides a time period of 28 yr from 1994 to 2022. To suppress seasonal and latitudinal variations, we concentrate on the data taken in autumn (49< doy <111) and spring (235< doy <297) in the northern hemisphere with latitudes of 0--40 degrees. With this constraint, local times are automatically limited either around noon or midnight. We obtain four sets (two seasons times two local times) of density trends at each altitude layer. We take into account variations due to a linear trend and the 11-yr solar cycle using linear regression techniques. Because we do not see significant differences among the four trends, we combine them to provide a single vertical profile of trend slopes. We find a negative density trend of roughly -5 %/decade at every altitude. This is in reasonable agreement with inferences from settling rate of the upper atmosphere. In the 100--110 km altitude, we found an exceptionally high density decline of about -12 %/decade. This peak may be the first observational evidence for strong cooling due to water vapor and ozone near 110 km, which was first identified in a numerical simulation by Akmaev et al. (2006). Further observations and numerical simulations with suitable input parameters are needed to establish this feature.

physics.space-ph↗

arXiv2021

A Detection of Red Noise in PSR J1824$-$2452A and Projections for PSR B1937+21 using NICER X-ray Timing Data

We have used X-ray data from the Neutron Star Interior Composition Explorer (NICER) to search for long time-scale, correlated variations ("red noise") in the pulse times of arrival from the millisecond pulsars PSR J1824$-$2452A and PSR B1937+21. These data more closely track intrinsic noise because X-rays are unaffected by the radio-frequency dependent propagation effects of the interstellar medium. Our Bayesian search methodology yields strong evidence (natural log Bayes factor of $9.634 \pm 0.016$) for red noise in PSR J1824$-$2452A, but is inconclusive for PSR B1937+21. In the interest of future X-ray missions, we devise and implement a method to simulate longer and higher precision X-ray datasets to determine the timing baseline necessary to detect red noise. We find that the red noise in PSR B1937+21 can be reliably detected in a 5-year mission with a time-of-arrival (TOA) error of 2 microseconds and an observing cadence of 20 observations per month compared to the 5 microsecond TOA error and 11 observations per month that NICER currently achieves in PSR B1937+21. We investigate detecting red noise in PSR B1937+21 with other combinations of observing cadences and TOA errors. We also find that an injected stochastic gravitational wave background (GWB) with an amplitude of $A_{\rm GWB}=2\times10^{-15}$ and spectral index of $γ_{\rm GWB}=13/3$ can be detected in a pulsar with similar TOA precision to PSR B1937+21, but with no additional red noise, in a 10-year mission that observes the pulsar 15 times per month and has an average TOA error of 1 microsecond.

astro-ph.HE↗

arXiv2020

A NICER View of Spectral and Profile Evolution for Three X-ray Emitting Millisecond Pulsars

We present two years of Neutron star Interior Composition Explorer (NICER) X-ray observations of three energetic rotation-powered millisecond pulsars (MSPs): PSRs B1937+21, B1821-24, and J0218+4232. We fit Gaussians and Lorentzians to the pulse profiles for different energy sub-bands of the soft X-ray regime to measure the energy dependence of pulse separation and width. We find that the separation between pulse components of PSR J0218+4232 decreases with increasing energy at $\gt 3σ$ confidence. The 95% upper limit on pulse separation evolution for PSRs B1937+21 and B1821-24 is less than 2 milliperiods per keV. Our phase-resolved spectral results provide updated constraints on the non-thermal X-ray emission of these three pulsars. The photon indices of the modeled X-ray emission spectra for each pulse component of PSR B1937+21 are inconsistent with each other at the 90% confidence level, suggesting different emission origins for each pulse. We find that the PSR B1821-24 and PSR J0218+4232 emission spectra are invariant with phase at the 90% confidence level. We describe the implications of our profile and spectral results in the context of equatorial current sheet emission models for these three MSPs with non-thermal, magnetospheric X-ray emission.

astro-ph.HE↗

arXiv2019

Constraining the Neutron Star Mass-Radius Relation and Dense Matter Equation of State with NICER. I. The Millisecond Pulsar X-ray Data Set

We present the set of deep Neutron Star Interior Composition Explorer (NICER) X-ray timing observations of the nearby rotation-powered millisecond pulsars PSRs J0437-4715, J0030+0451, J1231-1411, and J2124-3358, selected as targets for constraining the mass-radius relation of neutron stars and the dense matter equation of state via modeling of their pulsed thermal X-ray emission. We describe the instrument, observations, and data processing/reduction procedures, as well as the series of investigations conducted to ensure that the properties of the data sets are suitable for parameter estimation analyses to produce reliable constraints on the neutron star mass-radius relation and the dense matter equation of state. We find that the long-term timing and flux behavior and the Fourier-domain properties of the event data do not exhibit any anomalies that could adversely affect the intended measurements. From phase-selected spectroscopy, we find that emission from the individual pulse peaks is well described by a single-temperature hydrogen atmosphere spectrum, with the exception of PSR J0437-4715, for which multiple temperatures are required.

astro-ph.HE↗

arXiv2018

STROBE-X: A probe-class mission for X-ray spectroscopy and timing on timescales from microseconds to years

We describe the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept that will provide an unprecedented view of the X-ray sky, performing timing and spectroscopy over both a broad energy band (0.2-30 keV) and a wide range of timescales from microseconds to years. STROBE-X comprises two narrow-field instruments and a wide field monitor. The soft or low-energy band (0.2-12 keV) is covered by an array of lightweight optics (3-m focal length) that concentrate incident photons onto small solid-state detectors with CCD-level (85-175 eV) energy resolution, 100 ns time resolution, and low background rates. This technology has been fully developed for NICER and will be scaled up to take advantage of the longer focal length of STROBE-X. The higher-energy band (2-30 keV) is covered by large-area, collimated silicon drift detectors that were developed for the European LOFT mission concept. Each instrument will provide an order of magnitude improvement in effective area over its predecessor (NICER in the soft band and RXTE in the hard band). Finally, STROBE-X offers a sensitive wide-field monitor (WFM), both to act as a trigger for pointed observations of X-ray transients and also to provide high duty-cycle, high time-resolution, and high spectral-resolution monitoring of the variable X-ray sky. The WFM will boast approximately 20 times the sensitivity of the RXTE All-Sky Monitor, enabling multi-wavelength and multi-messenger investigations with a large instantaneous field of view. This mission concept will be presented to the 2020 Decadal Survey for consideration.

astro-ph.IM↗

arXiv2017

The NRL Program in X-ray Navigation

This chapter describes the development of X-ray Navigation at the Naval Research Laboratory (NRL) within its astrophysics research programs. The prospects for applications emerged from early discoveries of X-ray source classes and their properties. Starting around 1988 some NRL X-ray astronomy programs included navigation as one of the motivations. The USA experiment (1999) was the first flight payload with an explicit X-ray navigation theme. Subsequently, NRL has continued to work in this area through participation in DARPA and NASA programs. Throughout, the general concept of X-ray navigation (XRNAV) has been broad enough to encompass many different uses of X-ray source observations for attitude determination, position determination, and timekeeping. Pulsar-based X-ray navigation (XNAV) is a special case.

astro-ph.IM↗

arXiv2017

Characterization of Pulsar Sources for X-ray Navigation

The Station Explorer for X-ray Timing and Navigation Technology (SEXTANT) is a technology demonstration enhancement to the Neutron Star Interior Composition Explorer (NICER) mission, which is scheduled to launch in 2017 and will be hosted as an externally attached payload on the International Space Station (ISS). During NICER's 18-month baseline science mission to understand ultra-dense matter through observations of neutron stars in the soft X-ray band, SEXTANT will, for the first-time, demonstrate real-time, on-board X-ray pulsar navigation. Using NICER/SEXTANT as an example, we describe the factors that determine the measurement errors on pulse times of arrival, including source and background count rates, and pulse profile shapes. We then describe properties of the SEXTANT navigation pulsar catalog and prospects for growing it once NICER launches. Finally, we describe the factors affecting the prediction of pulse arrival times in advance, including variable interstellar propagation effect and red timing noise. Together, all of these factors determine how well a particular realization of an X-ray pulsar-based navigation system will perform.

astro-ph.IM↗

arXiv2016

The NuSTAR X-ray Spectrum of Hercules X-1: A Radiation-Dominated Radiative Shock

We report new spectral modeling of the accreting X-ray pulsar Hercules X- 1. Our radiation-dominated radiative shock model is an implementation of the analytic work of Becker & Wolff on Comptonized accretion flows onto magnetic neutron stars. We obtain a good fit to the spin-phase averaged 4 to 78 keV X-ray spectrum observed by the Nuclear Spectroscopic Telescope Array during a main- on phase of the Her X-1 35-day accretion disk precession period. This model allows us to estimate the accretion rate, the Comptonizing temperature of the radiating plasma, the radius of the magnetic polar cap, and the average scattering opacity parameters in the accretion column. This is in contrast to previous phenomenological models that characterized the shape of the X-ray spectrum but could not determine the physical parameters of the accretion flow. We describe the spectral fitting details and discuss the interpretation of the accretion flow physical parameters.

astro-ph.HE↗

arXiv2016

Standing Shock Instability in Advection-Dominated Accretion Flows

Depending on the values of the energy and angular momentum per unit mass in the gas supplied at large radii, inviscid advection-dominated accretion flows can display velocity profiles with either preshock deceleration or preshock acceleration. Nakayama has shown that these two types of flow configurations are expected to have different stability properties. By employing the Chevalier \& Imamura linearization method and the Nakayama instability boundary conditions, we discover that there are regions of parameter space where disks/shocks with outflows can be stable or unstable. In regions of instability, we find that preshock deceleration is always unstable to the zeroth mode with zero frequency of oscillation, but is always stable to the fundamental mode and overtones. Furthermore, we also find that preshock acceleration is always unstable to the zeroth mode and that the fundamental mode and overtones become increasingly less stable as the shock location moves away from the horizon when the disk half-height expands above $\sim 12$ gravitational radii at the shock radius. In regions of stability, we demonstrate the zeroth mode to be stable for the velocity profiles that exhibit preshock acceleration and deceleration. Moreover, for models that are linearly unstable, our model suggests the possible existence of quasi-periodic oscillations (QPOs) with ratios 2:3 and 3:5. These ratios are believed to occur in stellar and supermassive black hole candidates, for example, in GRS 1915+105 and Sgr A*, respectively. We expect that similar QPO ratios also exist in regions of stable shocks.

astro-ph.HE↗

arXiv2015

The Transient Accereting X-Ray Pulsar XTE J1946+274: Stability of the X-Ray Properties at Low Flux and Updated Orbital Solution

We present a timing and spectral analysis of the X-ray pulsar XTE J1946+274 observed with Suzaku during an outburst decline in 2010 October and compare with previous results. XTE J1946+274 is a transient X-ray binary consisting of a Be-type star and a neutron star with a 15.75 s pulse period in a 172 d orbit with 2-3 outbursts per orbit during phases of activity. We improve the orbital solution using data from multiple instruments. The X-ray spectrum can be described by an absorbed Fermi-Dirac cutoff power law model along with a narrow Fe K line at 6.4 keV and a weak Cyclotron Resonance Scattering Feature (CRSF) at ~35 keV. The Suzaku data are consistent with the previously observed continuum flux versus iron line flux correlation expected from fluorescence emission along the line of sight. However, the observed iron line flux is slightly higher, indicating the possibility of a higher iron abundance or the presence of non-uniform material. We argue that the source most likely has only been observed in the subcritical (non-radiation dominated) state since its pulse profile is stable over all observed luminosities and the energy of the CRSF is approximately the same at the highest (~$5 \times 10^{37} $erg s$^{-1}$) and lowest (~$5 \times 10^{36} $erg s$^{-1}$) observed 3-60 keV luminosities.

astro-ph.HE↗

arXiv2015

Looking into the Theory of Pulsar Accretion: Cen X-3 and XTE J1946+274

This is an overview of pulsar accretion modeling. The physics of pulsar accretion, i.e., the process of plasma flow onto the neutron star surface, can be constrained from the spectral properties of the X-ray source. We discuss a new implementation of the physical continuum model developed by Becker and Wolff (2007, ApJ 654, 435). The model incorporates Comptonized blackbody, bremsstrahlung, and cyclotron emission. We discuss preliminary results of applying the new tool to the test cases of Suzaku data of Cen X-3 and XTE J1946+274. Cen X-3 is a persistent accreting pulsar with an O-star companion observed during a bright period. XTE J1946+274 is a transient accreting pulsar with a Be companion observed during a dim period. Both sources show spectra that are well described with an empirical Fermi Dirac cutoff power law model. We extend the spectral analysis by making the first steps towards a physical description of Cen X-3 and XTE J1946+274.

astro-ph.HE↗

arXiv2014

Discovery of PSR J1227-4853: A transition from a low-mass X-ray binary to a redback millisecond pulsar

XSS J12270-4859 is an X-ray binary associated with the Fermi LAT gamma-ray source 1FGL J1227.9-4852. In 2012 December, this source underwent a transition where the X-ray and optical luminosity dropped and the spectral signatures of an accretion disc disappeared. We report the discovery of a 1.69 millisecond pulsar (MSP), PSR J1227-4853, at a dispersion measure of 43.4 pc cm$^{-3}$ associated with this source, using the GMRT at 607 MHz. This demonstrates that, post-transition, the system hosts an active radio MSP. This is the third system after PSR J1023+0038 and PSR J1824-2452I showing evidence of state switching between radio MSP and low-mass X-ray binary (LMXB) states. We report timing observations of PSR J1227-4853 with the GMRT and Parkes, which give a precise determination of the rotational and orbital parameters of the system. The companion mass measurement of 0.17 to 0.46 M$_{sun}$ suggests that this is a redback system. PSR J1227-4853 is eclipsed for about 40% of its orbit at 607 MHz; with additional short-duration eclipses at all orbital phases. We also find that the pulsar is very energetic, with a spin-down luminosity of ~ 10$^{35}$ erg s$^{-1}$. We report simultaneous imaging and timing observations with the GMRT, which suggests that eclipses are caused by absorption, rather than dispersion smearing or scattering.

astro-ph.HE↗

arXiv2011

The Advanced X-ray Timing Array (AXTAR): A US MIDEX Mission Concept

AXTAR is a NASA MIDEX mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument is a collimated, thick Si pixel detector with 2-50 keV coverage and over 3 square meters effective area. For timing observations of accreting neutron stars and black holes, AXTAR provides at least a factor of five improvement in sensitivity over the RXTE PCA. AXTAR also carries a sensitive sky monitor that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and design choices that face a next generation timing mission. We then describe the technical concept for AXTAR and summarize a preliminary mission design study at the NASA/MSFC Advanced Concepts Office.

astro-ph.IM↗

arXiv2010

AXTAR: Mission Design Concept

The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument, the Large Area Timing Array (LATA) is a collimated instrument with 2-50 keV coverage and over 3 square meters effective area. The LATA is made up of an array of supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present results from a preliminary mission design study.

astro-ph.IM↗

arXiv2009

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.

astro-ph↗