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

¹ Université de Toulouse; UPS-OMP; IRAP; Toulouse, France
² CNRS; Institut de Recherche en Astrophysique et Planétologie ; 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France
³ Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125
⁴ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139
⁶ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, DK-2800 Lyngby, Denmark
⁷ Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
⁸ Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
⁹ Istituto di Astrofisica e Planetologia Spaziali, INAF, Via Fosso del Cavaliere 100, Roma I-00133, Italy
¹⁰ CRESST, UMBC, and NASA GSFC, Code 661, Greenbelt, MD 20771, USA
¹¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
¹² Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, 96 049 Bamberg, Germany
¹³ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

^a e-mail: matteo.bachetti@irap.omp.eu

Published online: 8 January 2014

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.