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 observed Pdot 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 energy-resolved 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.

19 pages, 14 figures, Accepted for publication in ApJ on May 5, 2019

References in corpus (15)

• Thermal and Bulk Comptonization in Accretion-Powered X-Ray Pulsars
• NuSTAR Observations of the Bullet Cluster: Constraints on Inverse Compton Emission
• A model for cyclotron resonance scattering features
• IACHEC Cross-Calibration of Chandra, NuSTAR, Swift, Suzaku, and XMM-Newton with 3C 273 and PKS 2155-304
• Pulse phase resolved analysis of the HMXB Cen X-3 over two binary orbits
• Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+53
• On the dependence of the X-ray continuum variations with luminosity in accreting X-ray pulsars
• NuSTAR Detection Of A Cyclotron Line In The Supergiant Fast X-ray Transient IGR J17544-2619
• Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling
• Detection of Cyclotron Resonance Scattering Feature in High Mass X-ray Binary Pulsar SMC X-2
• INTEGRAL and Swift observations of EXO 2030+375 during a giant outburst
• NuSTAR Discovery of a Cyclotron Line in the Be/X-ray Binary RX J0520.5-6932 During Outburst
• Formation of phase lags at the cyclotron energies in the pulse profiles of magnetized, accreting neutron stars
• Looking at A 0535+26 at low luminosities with NuSTAR
• A broadband self-consistent modelling of the X-ray spectrum of 4U 1626--67

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• Observations of 4U 1626-67 with the Imaging X-ray Polarimetry Explorer
• 4U 1626-67 Returns to Spin-Down: Timing features toe the line
• A broad-band X-ray view of the precessing accretion disk and pre-eclipse dip in the pulsar Her X-1 with NuSTAR and XMM-Newton
• Beam pattern evolution of accreting X-ray pulsar 1A 0535+262 during its 2020 giant outburst
• Disk vs wind accretion in X-ray pulsar GX 301-2
• Chandra/LETGS Studies of the Collisional Plasma in 4U 1626-67