A possible phase dependent absorption feature in the transient X-ray pulsar SAX J2103.5+4545

We present an X-ray spectral and timing analysis of two $NuSTAR$ observations of the transient Be X-ray binary SAX J2103.5+4545 during its April 2016 outburst, which was characterized by the highest flux since $NuSTAR$'s launch. These observations provide detailed hard X-ray spectra of this source during its bright precursor flare and subsequent fainter regular outburst for the first time. In this work, we model the phase-averaged spectra for these observations with a negative and positive power law with an exponential cut-off (NPEX) model and compare the pulse profiles at different flux states. We found that the broad-band pulse profile changes from a three peaked pulse in the first observation to a two peaked pulse in the second observation, and that each of the pulse peaks has some energy dependence. We also perform pulse-phase spectroscopy and fit phase-resolved spectra with NPEX to evaluate how spectral parameters change with pulse phase. We find that while the continuum parameters are mostly constant with pulse phase, a weak absorption feature at ~12 keV that might, with further study, be classified as a cyclotron line, does show strong pulse phase dependence.Comment: 10 pages, 7 figures, accepted by ApJ, acknowledgements update

Discovery of Pulsation Dropout and Turn-on during the High State of the Accreting X-Ray Pulsar LMC X-4

Two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the luminous X-ray pulsar LMC X-4 in 2015 October and November captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse "turn-on" in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares, we find pulsations to be weak or nonexistent, with fractional rms amplitudes of less than 0.05. At the onset of the flare, the pulse profiles exhibit a phase shift of 0.25 cycles that could be associated with a change in the emission geometry. This increase in pulse strength occurring well before the flare cannot be explained by the propeller effect, and potentially offers a connection between the magnetic properties of pulsars that accrete close to their Eddington limits and ultra-luminous X-ray pulsars

High-resolution X-ray spectroscopy of supergiant HMXB 4U 1700−37 during the compact object eclipse

We present an analysis of the first observation of the iconic high-mass X-ray binary 4U 1700−37 with the Chandra High-Energy Transmission Gratings (HETGs) during an X-ray eclipse. The goal of the observation was to study the structure/physical conditions in the clumpy stellar wind through high-resolution spectroscopy. We find the following: (a) Emission-line brightness from K-shell transitions, corresponding to near-neutral species, directly correlates with continuum illumination. However, these lines do not greatly diminish during eclipse. This is readily explained if fluorescence K α emission comes from the bulk of the wind. (b) The highly ionized Fe xxv and Fe xxvi Ly α diminish during eclipse. Thus, they must be produced in the vicinity of the compact object where log ξ > 3. (c) To describe the emission-line spectrum, the sum of two self-consistent photoionization models with low ionization (log ξ ∼ −1) and high ionization (log ξ ∼ 2.4) is required. From their emission measures, the clump-to-interclump density ratio can be estimated to be nc/ni ∼ 300. To fit the complex He-like Si xiii profile, the plasma requires a broadening with vbulk ∼ 840 km s−1. Reproducing the observed r ≈ f line fluxes requires the addition of a third collisionally ionized plasma. (d) Emission-line widths appear unresolved at the HETG resolution with the exception of silicon. There is no clear radial segregation between (quasi-)neutral and ionized species, consistent with cold wind clumps interspersed in a hot rarefied interclump medium.This research has been funded under the project ESP2017-85691-P. The research leading to these results has received funding from the European Union’s Horizon 2020 Programme under the Activities for the High-Energy Astrophysiscs Domain (AHEAD) project (grant agreement no. 654215). Victoria Grinberg (VG) was supported through the Margarete von Wrangell fellowship by the ESF and the Ministry of Science, Research and the Arts of Baden-Württemberg. Work at LLNL was performed under the auspices of the U.S. Department of Energy under contract no. DE-AC52-07NA27344 and supported through National Aeronautics and Space Administration (NASA) grants to Lawrence Livermore National Laboratory (LLNL). Lida M. Oskinova acknowledges Deutsches Zentrum für Luft und Raumfahrt (DLR) grant FKZ 50 OR 1508 and partial support by the Russian Government Program of Competitive Growth of Kazan Federal University

Observing the transient pulsations of SMC X-1 with NuSTAR

We report on NuSTAR observations of transient pulsations in the neutron star X-ray binary SMC X-1. The transition from non-pulsing to pulsing states was not accompanied by a large change in flux. Instead, both pulsing and non-pulsing states were observed in a single observation during the low-flux super-orbital state. During the high-state, we measure a pulse period of $P = 0.70117(9)\,\mathrm{s}$ at $T_{ref} = 56145\,\mathrm{MJD}$. Spectral analysis during non-pulsing and pulsing states reveals that the observations can be consistently modeled by an absorbed power law with a phenomenological cutoff resembling a Fermi-Dirac distribution, or by a partially obscured cutoff power law. The shapes of the underlying continua show little variability between epochs, while the covering fraction and column density vary between super-orbital states. The strength of pulsations also varies, leading us to infer that the absence and reemergence of pulsations are related to changing obscuration, such as by a warped accretion disk. SMC X-1 is accreting near or above its Eddington limit, reaching an unabsorbed X-ray luminosity of $L_{\rm X}({\rm 2-10~keV}) \approx 5 \times 10^{38}\, {\rm erg}\, {\rm s}^{-1}$. This suggests that SMC X-1 may be a useful local analog to ultraluminous X-ray pulsars (ULXPs), which likewise exhibit strong variability in their pulsed fractions, as well as flux variability on similar timescales. In particular, the gradual pulse turn-on which has been observed in M82 X-2 is similar to the behavior we observe in SMC X-1. Thus we propose that pulse fraction variability of ULXPs may also be due to variable obscuration.Comment: Accepted for publication in the Astrophysical Journal; 13 pages, 4 figure

Discovery of Pulsation Dropout and Turn-on during the High State of the Accreting X-Ray Pulsar LMC X-4

Two Nuclear Spectroscopic Telescope Array (NuSTAR) observations of the luminous X-ray pulsar LMC X-4 in 2015 October and November captured several bright accretion flares from this source, which has a long history of stable pulse and superorbital behavior. We present a timing analysis of these data in which we detect a rapid pulse "turn-on" in association with the accretion flares, during which the source reaches super-Eddington luminosities. Pulsations, which are normally seen from this source, are found to only occur for approximately one hour before and during the bright flares. Beyond one hour before and after the flares, we find pulsations to be weak or nonexistent, with fractional rms amplitudes of less than 0.05. At the onset of the flare, the pulse profiles exhibit a phase shift of 0.25 cycles that could be associated with a change in the emission geometry. This increase in pulse strength occurring well before the flare cannot be explained by the propeller effect, and potentially offers a connection between the magnetic properties of pulsars that accrete close to their Eddington limits and ultra-luminous X-ray pulsars

Physical Models for Accreting Pulsars at High Luminosity

A new window for better understanding the accretion onto strongly magnetized neutron stars in X-ray binaries is opening. In these systems the accreted material follows the magnetic field lines as it approaches the neutron star, forming accretion columns above the magnetic poles. The plasma falls toward the neutron star surface at near-relativistic speeds, losing energy by emitting X-rays. The X-ray spectral continua are commonly described using phenomenological models, i.e., power laws with different types of curved cut-offs at higher energies. Here we consider high luminosity pulsars. In these systems the mass transfer rate is high enough that the accreting plasma is thought to be decelerated in a radiation-dominated radiative shock in the accretion columns. While the theory of the emission from such shocks had already been developed by 2007, a model for direct comparison with X-ray continuum spectra in xspec or isis has only recently become available. Here we analyze the broadband X-ray spectra of the accreting pulsars Centaurus X-3 and 4U1626-67 obtained withNuSTAR. We present results from traditional empirical modeling as well as successfully apply the radiation-dominated radiative shock model. We also fit the energy-dependent pulse profiles of 4U 1626-67 using a new relativistic light bending model

NuSTAR Detection of hard x-ray phase lags from the accreting pulsar GS 0834-430

The Nuclear Spectroscopic Telescope Array hard X-ray telescope observed the transient Be/X-ray binary GS 0834-430 during its 2012 outburst-the first active state of this system observed in the past 19 yr. We performed timing and spectral analysis and measured the X-ray spectrum between 3-79 keV with high statistical significance. We find the phase-averaged spectrum to be consistent with that observed in many other magnetized, accreting pulsars. We fail to detect cyclotron resonance scattering features that would allow us to constrain the pulsar's magnetic field in either phase-averaged or phase-resolved spectra. Timing analysis shows a clearly detected pulse period of similar to 12.29 s in all energy bands. The pulse profiles show a strong, energy-dependent hard phase lag of up to 0.3 cycles in phase, or about 4 s. Such dramatic energy-dependent lags in the pulse profile have never before been reported in high-mass X-ray binary pulsars. Previously reported lags have been significantly smaller in phase and restricted to low energies (E < 10 keV). We investigate the possible mechanisms that might produce this energy-dependent pulse phase shift. We find the most likely explanation for this effect is a complex beam geometry