NuSTAR detection of hard X-ray phase lags from the accreting pulsar GS 0834-430

TL;DR

This paper reports the first detection of significant energy-dependent hard X-ray phase lags in the pulsar GS 0834-430, revealing complex beam geometry in a high-mass X-ray binary during its 2012 outburst.

Contribution

It presents the first observation of large, energy-dependent phase lags in a HMXB pulsar, expanding understanding of pulsar emission mechanisms.

Findings

Detected a ~12.29s pulse period across energies

Observed energy-dependent phase lag up to 0.3 cycles (~4s)

No cyclotron features detected in spectra

Abstract

The NuSTAR 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 years. We performed timing and spectral analysis, and measured the X-ray spectrum between 3-79keV 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 ~12.29s 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 4s. Such dramatic energy-dependent lags in the pulse profile have never before been reported in high-mass X-ray binary (HMXB) pulsars. Previously reported lags have been significantly smaller in phase and restricted to low-energies (E<10keV). We investigate the possible mechanisms that might produce this energy-dependent pulse phase shift. We find the most likely explanation for this effect to be a complex beam geometry.