DISCOVERY OF THE 18.7-SECOND ACCRETING X-RAY PULSAR GRO-J1948+32

We have detected an 18.7 s accreting X-ray pulsar in the Cygnus region, using the BATSE large-area detectors on the Compton Gamma Ray Observatory. GRO J1948 + 32 has been localized to within 10 deg(2) using a method we developed for positioning weak pulsed sources with BATSE. During the 33 day outburst, the phase-averaged 20-75 keV pulsed flux rose from 25 mCrab to 50 mCrab over 10 days and then decayed below our detection threshold over nearly 25 days. A photon spectral index of gamma = 2.65 +/- 0.15 (assuming photon flux density dN/dE proportional to E(-gamma)) was measured during a bright interval. The observed modulation of the neutron star's pulse frequency is suggestive of orbital variation over less than one orbit cycle. Assuming a constant spin frequency derivative over the outburst, we can place the following individual 95% confidence limits on each of the pulsar parameters: orbital period 35(d) < P-orb < 70(d); orbital radius 75 lt-sec < a(x) sin i < 300 lt-sec, eccentricity e < 0.25, spin frequency derivative 5 x 10(-13) Hz s(-1) < v < 2.5 x 10(-11) Hz s(-1), X-ray mass function 0.5 M. <f(x)(M)< 5 M.. As the stellar type of the mass-providing companion is still not known for this source, we briefly speculate on the nature of mass transfer in this system.