Batse observations and orbit determination of the Be/X-ray transient EXO 2030+375

The Be/X-ray binary transient pulsar EXO 2030 + 375 (P-s approximate to 42 s) has been observed with the large-area detectors (LADs) of the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (C GRO). Beginning in 1991 May, 22 outbursts were observed over 4 years. Thirt een outbursts between 1992 February and 1993 August occurred consecutively at intervals of approximate to 46 days, close to the orbital period determi ned by Parmar and colleagues using EXOSAT data. The pulse profiles from the BATSE data are double peaked and show no significant energy or luminosity dependence, unlike the EXOSAT observations of 1985 May-August. An exponenti al model was used to fit the observed hard X-ray energy spectra from the 13 consecutive outbursts. When EXOSAT discovered this pulsar during a giant o utburst in 1985 May, the X-ray luminosity peaked at L-x = 1.0 x 10(38) ergs s(-1) (1-20 keV), assuming a 5 kpc distance to the source. The BATSE outbu rsts are found to be weaker, 0.3 x 10(37) less than or equal to L-x(1-20 ke V) less than or equal to 3.0 x 10(37) ergs s(-1) after extrapolating the ob served flux (20-50 keV) to the EXOSAT energy band. Pulse phases derived fro m the 13 outbursts were fitted to two different models to determine a binar y orbit. The new orbit is used to estimate 95% confidence limits for the me an peak spin frequency change during the outbursts observed with BATSE. Thi s and the mean peak flux are compared to the spin-up rates and fluxes deter mined by EXOSAT from the 1985 giant outburst, where disk accretion was thou ght to have occurred. It is unclear whether these normal outbursts were dri ven by wind or disk accretion.