Rxte and Bepposax Observations of the Transient X-ray Pulsar Xte J1859+083

We present observations of the 9.8 s X-ray pulsar XTEJ1859+083 made with the ASM and PCA on board RXTE, and the WFC on board BeppoSAX. The ASM data cover a 12 year time interval and show that an extended outburst occurred between approximately MJD 50,250 and 50,460 (1996 June 16 to 1997 January 12). The ASM data excluding this outburst interval suggest a possible modulation with a period of 60.65 ± 0.08 days. Eighteen sets of PCA observations were obtained over an approximately one month interval in 1999. The flux variability measured with the PCA appears consistent with the possible period found with the ASM. The PCA measurements of the pulse period showed it to decrease non-monotonically and then to increase significantly. Doppler shifts due to orbital motion rather than accretion torques appear to be better able to explain the pulse period changes. Observations with the WFC during the extended outburst give a position which is consistent with a previously determined PCA error box, but which has a significantly smaller error. The transient nature of XTE J1859+083 and the length of its pulse period are consistent with it being a Be/neutron star binary. The possible 60.65 day orbital period would be of the expected length for a Be star system with a 9.8 s pulse period. Subject headings: stars: individual (XTE J1859+083) — stars: neutron — Xrays: binaries University of Maryland, Baltimore County; [email protected] CRESST/Mail Code 662, NASA Goddard Space Flight Center, Greenbelt, MD 20771 SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands [email protected]; Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA 02139 Mail Code 660.1, NASA Goddard Space Flight Center, Greenbelt, MD 20771