Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+27.pdf

a r X i v : a s t r o - p h / 0 1 1 2 4 6 8 v 1 2 0 D e c 2 0 0 1 DRAFT VERSION FEBRUARY 1, 2008 Preprint typeset using LATEX style emulateapj v. 14/09/00 DISCOVERY OF A CYCLOTRON RESONANCE SCATTERING FEATURE IN THE X-RAY SPECTRUM OF XTE J1946+274 W.A. HEINDL, W. COBURN, D.E. GRUBER, R.E. ROTHSCHILD, Center for Astrophysics and Space Sciences, Code 0424, University of California, San Diego, La Jolla, CA 92093 I. KREYKENBOHM, J. WILMS, R. STAUBERT Institut für Astronomie und Astrophysik – Astronomie, University of Tübingen, Waldh?user Strasse 64, D-72076 Tübingen, Germany wheindl@ucsd.edu Draft version February 1, 2008 ABSTRACT Observations of the transient accreting pulsar XTE J1946+274 made with the Rossi X-ray Timing Explorer during the course of the 1998 September–November outburst, reveal a cyclotron resonance scattering feature (or “cyclotron line”) in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2+0.5 ?0.7 keV, which implies a magnetic field strength of 3.1(1+z)×1012 G, where z is the gravitational redshift of the scattering region. The optical depth, τ = 0.33+0.07 ?0.06, and width, σ = 3.37 +0.92 ?0.75 keV, are typical of known cyclotron lines in other pulsars. This discovery makes XTE J1946+274 one of thirteen pulsars with securely detected cyclotron lines resulting in direct magnetic field measurements. Subject headings: stars: individual (XTE J1946+274) — stars: neutron — stars: magnetic fields — X-rays: binaries — X-rays: stars 1. INTRODUCTION The transient accreting X-ray pulsar XTE J1946+274 was discovered during an outburst in 1998 September with the All Sky Monitor (ASM) on board the Rossi X-ray Timing Explorer (RXTE) (Smith Takeshima 1998). At the same time, 15.8 second pulsations were observed with the CGRO/BATSE, with the pulsating source designated GRO J1944+26 (Wilson et al. 1998). The best X-ray position (BeppoSAX/MECS: 1 ′ radius, 90% confidence, Campana et al. 1998) falls within the Ariel V error region for t