Miyasaka, Hiromasa12; Bachetti, Matteo4; Harrison, Fiona A.12; Fu¨rst, Felix12; Barret, Didier4; Bellm, Eric C.12; Boggs, Steven E.5; Chakrabarty, Deepto13; Chenevez, Jérôme1; Christensen, Finn Erland1; Craig, William W.5; Grefenstette, Brian W.12; Hailey, Charles J.14; Madsen, Kristin K.12; Natalucci, Lorenzo8; Pottschmidt, Katja9; Stern, Daniel12; Tomsick, John A.5; Walton, Dominic J.12; Wilms, Jo¨rn10; Zhang, William15
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 California Institute of Technology4 Université de Toulouse5 University of California at Berkeley6 Massachusetts Institute of Technology7 Columbia University8 Istituto di Astrofisica e Planetologia Spaziali Via Fosso del Cavaliere9 University of Maryland10 Dr. Karl-Remeis-Sternwarte and Erlangen Center for Astroparticle Physics11 NASA Goddard Space Flight Center12 California Institute of Technology13 Massachusetts Institute of Technology14 Columbia University15 NASA Goddard Space Flight Center
The Nuclear Spectroscopic Telescope Array 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 yr. We performed timing and spectral analysis and measured the X-ray spectrum between 3-79 keV 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 ~ 12.29 s 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 4 s. Such dramatic energy-dependent lags in the pulse profile have never before been reported in high-mass X-ray binary 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 this energy-dependent pulse phase shift. We find the most likely explanation for this effect is a complex beam geometry.