Discovery of the 18.7-Second Accreting Pulsar GRO J1948+32

Abstract

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 11948 + 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 γ = 2.65 ± 0.15 (assuming photon flux density d N/ dE α E^(-γ) 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 35d < P_(orb) < 70d; orbital radius 75 It-sec < α_x sin i < 300 It-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.