NuSTAR detection of 4s Hard X-ray Lags from the Accreting Pulsar GS 0834-430

• Published in: EPJ Web of conferences Vol. 64; p. 6011
• Main Authors: Bachetti, Matteo, Miyasaka, Hiromasa, Harrison, Fiona, Fürst, Felix, Barret, Didier, Bellm, Eric C., Boggs, Steven E., Chakrabarty, Deepto, Chenevez, Jerome, Christensen, Finn E., Craig, William W., Grefenstette, Brian W., Hailey, Charles J., Madsen, Kristin K., Natalucci, Lorenzo, Pottschmidt, Katja, Stern, Daniel, Tomsick, John A., Walton, Dominic J., Wilms, Jörn, Zhang, William
• Format: Journal Article
• Language: English
• Published: EDP Sciences 2014
• ISSN: 2100-014X; 2100-014X
• DOI: 10.1051/epjconf/20136406011

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.