Spectral and Timing Analysis of the Accretion-powered Pulsar 4U 1626–67 Observed with Suzaku and NuSTAR

Abstract

We present an analysis of the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626−67 observed with Suzaku and NuSTAR during a spin-up state. The pulsar, which experienced a torque reversal to spin-up in 2008, has a spin period of ∼7.7 s. Comparing the phase-averaged spectra obtained with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape changed between the two observations: the 3-10 keV flux increased by ∼5% while the 30-60 keV flux decreased significantly by ∼35%. Phase-averaged and phase-resolved spectral analysis shows that the continuum spectrum observed by NuSTAR is well described by an empirical NPEX continuum with an added broad Gaussian emission component around the spectral peak at ∼20 keV. Taken together with the observedṖ value obtained from Fermi/GBM, we conclude that the spectral change between the Suzaku and NuSTAR observations was likely caused by an increase of the accretion rate. We also report the possible detection of asymmetry in the profile of the fundamental cyclotron line. Furthermore, we present a study of the energyresolved pulse profiles using a new relativistic ray tracing code, where we perform a simultaneous fit to the pulse profiles assuming a two-column geometry with a mixed pencil-and fan-beam emission pattern. The resulting pulse profile decompositions enable us to obtain geometrical parameters of accretion columns (inclination, azimuthal and polar angles) and a fiducial set of beam patterns. This information is important to validate the theoretical predictions from radiation transfer in a strong magnetic field.

A Broadband X-Ray View of the Precessing Accretion Disk and Pre-eclipse Dip in the Pulsar Her X-1 with NuSTAR and XMM-Newton

“…This preference may suggest that the structure of magnetized accretion flows are more complex than the current scope of our warped disk model. Future modeling efforts would benefit from considering more complex emission geometries (e.g., Koliopanos & Vasilopoulos 2018;Iwakiri et al 2019), physically motivated accretion column models (e.g., Sokolova-Lapa in prep. ), or the effects of light bending from the neutron star (e.g., Falkner A submitted; Falkner B submitted).…”

PHEMTO: the polarimetric high energy modular telescope observatory

“…Despite this phenomenon being know since 1976, a lively debate is still ongoing on the location of the line-forming region and on the actual geometry of the accretion flow producing the X-ray emission. For instance, it is not clear if cyclotron scattering features are formed at the boundary of the magnetically confined accretion column, or on the neutron star surface [6,82]; and which is the relative contribution and localization of the accretion columns (see e.g., for a recent attempt to characterize the accretion geometry using energy-dependent pulse profiles [49]).…”

“…Analysis of polarization data will benefit from the development of spectral physical models for the accretion column (e.g. [5,25]) combined with the geometrical constraints derived from the study of pulse profiles [49].…”

Common patterns in pulse profiles of high-mass X-ray binaries

“…Such approaches have been used by various authors to try and model the emission geometry for specific sources (e.g. Bulik et al 1995;Leahy 2004;Caballero et al 2011;Sasaki et al 2012;Iwakiri et al 2019), frequently limited to specific observations or energy ranges.…”