XTE J1946+274 = GRO J1944+26: An Enigmatic Be/X‐Ray Binary

XTE J1946+274 = GRO J1944+26 is a 15.8 s Be/X-ray pulsar discoveredsimultaneously in 1998 September with the Burst and Transient Source Experiment(BATSE) on the Compton Gamma Ray Observatory (CGRO) and the All-Sky Monitor(ASM) on the Rossi X-ray Timing Explorer (RXTE). Here we present new resultsfrom BATSE and {\em RXTE} including a pulse timing analysis, spectral analysis,and evidence for an accretion disk. Our pulse timing analysis yielded anorbital period of 169.2 days, a moderate eccentricity of 0.33, and implied amass function of 9.7 M_sun. We observed evidence for an accretion disk, acorrelation between measured spin-up rate and flux, which was fitted to obtaina distance estimate of 9.5 +/- 2.9 kpc. XTE J1946+274 remained active from 1998September - 2001 July, undergoing 13 outbursts that were not locked in orbitalphase. Comparing RXTE PCA observations from the initial bright outburst in 1998and the last pair of outbursts in 2001, we found energy and intensity dependentpulse profile variations in both outbursts and hardening spectra withincreasing intensity during the fainter 2001 outbursts. In 2001 July, opticalHalpha observations indicate a density perturbation appeared in the Be disk asthe X-ray outbursts ceased. We propose that the equatorial plane of the Be staris inclined with respect to the orbital plane in this system and that thisinclination may be a factor in the unusual outburst behavior of the system.Comment: 18 pages, 15 figures, To appear in ApJ v584, Feb 20, 2003 issu