The Outbursts and Orbit of the Accreting Pulsar GS 1843−02 = 2S 1845−024

We present observations of a series of 10 outbursts of pulsed hard X-ray fluxfrom the transient 10.6 mHz accreting pulsar GS 1843-02, using the Burst andTransient Source Experiment on the Compton Gamma Ray Observatory. Theseoutbursts occurred regularly every 242 days, coincident with the ephemeris ofthe periodic transient GRO J1849-03 (Zhang et al. 1996), which has recentlybeen identified with the SAS 3 source 2S 1845-024 (Soffitta et al. 1998). Ourpulsed detection provides the first clear identification of GS 1843-02 with 2S1845-024. We present a pulse timing analysis which shows that the 2S 1845-024outbursts occur near the periastron passage of the neutron star's highlyeccentric (e = 0.88+-0.01) 242.18+-0.01 day period binary orbit about a highmass (M > 7 solar masses) companion. The orbit and transient outburst patternstrongly suggest the pulsar is in a binary system with a Be star. Ourobservations show a long-term spin-up trend, with most of the spin-up occurringduring the outbursts. From the measured spin-up rates and inferred luminositieswe conclude that an accretion disk is present during the outbursts.Comment: Accepted for publication in Astrophysical Journa