Abstract
The NuSTAR hard X-ray telescope observed the transient Be/X-ray binary GS 0834-430 during its 2012 outburst. The source is detected between 3 - 79 keV with high statistical significance, and we were able to perform very accurate spectral and timing analysis. The phase-averaged spectrum is consistent with that observed in many other magnetized accreting pulsars. We fail to detect cyclotron resonance scattering features in either phase-averaged nor phase-resolved spectra that would allow us to constrain the pulsar's magnetic field. We detect a pulse period of ~ 12:29 s in all energy bands. The pulse profile can be modeled with a double Gaussian and shows a strong and smooth hard lag of up to 0.3 cycles in phase, or about 4s between the pulse at ~ 3 and ≳ 30 keV. This is the first report of such a strong lag in high-mass X-ray binary (HMXB) 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 such lags. We find the most likely explanation for this effect to be a complex beam geometry.
Highlights
We find no evidence of cyclotron resonance scattering features (CRSF), often observed in accreting pulsars and yielding an approximate measure of the magnetic field in these sources [see 7, for a review]
A CRSF can produce a similar effect as the one we observe, producing an apparent delay in the spectral emission due to the different contributions of the feature in the various phases of the pulse profile [e.g. 14], but we do not find evidence for CRSFs in the spectrum in the relevant bands
It is possible that the effect is produced by the different emission components coming from the different regions of the accretion column
Summary
At the peak of its latest outburst [4], on UT 2012 July 11, the source was observed by NuSTAR [5] for about 31ks. We could perform very high quality timing and spectral analysis up to 79 keV. The shift is about 0.3 cycles in phase, or 4 s in time, between 3 and 30 keV.
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