ASCA AND GRO OBSERVATIONS OF GX-301-2

We observed the X-ray binary pulsar GX 301-2 with ASCA (Astro-D) on 19 94 February 13-14 at orbital phase similar to 0.3, when the X-ray emis sion is in a nonflaring state. We also obtained BATSE/GRO observations of the total high-energy flux concurrent with the ASCA observation, a s well as before and after. The BATSE data cover 7.5 orbital periods f rom 1993 September 20 to 1994 July 28. We folded these data over the o rbital period and found that the preperiastron (PP) flare is asymmetri c-it rises more slowly than it falls. We also detected a periodic near -apastron (NA) flare. Both the asymmetry of the PP flare and the exist ence of the NA flare can be explained by an equatorially enhanced stel lar wind or a circumstellar disk around the optical companion WRA 977 that GX 301-2 crosses twice per orbit. We measured the GX 301-2 pulse period to be 675.7 +/- 0.1 s, the lowest value observed to date. This demonstrates that the spin-up that started in 1984 continues and is th e longest lasting secular spin-up of the known pulsars. The secular sp in-up indicates that an accretion disk has formed. The appearance of t he NA flare may be related to the secular spin-up if the circumstellar disk also provides a steady source of angular momentum to the neutron star. The ASCA Gas Imaging Spectrometer (GIS) and Solid-state Imaging Spectrometer (SIS) images indicate the presence of two previously unk nown soft X-ray sources 3' and 8' from GX 301-2. The brighter source a t 8' we identify as a dMe star. Both sources have contributed to prior observations of the low-energy GX 301-2 spectrum. We have combined th e nearly simultaneous spectral data from ASCA and BATSE to determine t he pulse-phase-averaged 4-60 keV spectrum of GX 301-2. It fits a power -law model with low energy absorption, a narrow iron emission line, an iron absorption edge, and a high-energy cutoff. We measured the SIS0/ ASCA iron line energy to be 6.41 +/- 0.02 keV, the energy for neutral iron or with an ionization state no higher than Fe XVII. The iron line equivalent width is 230 +/- 30 eV, with 43 eV < intrinsic width <74 e V (90% confidence). The iron edge is at 7.19 +/- 0.03 keV. The line an d edge energies are consistent with the same iron ionization state if the state is no higher than Fe V.