GRO J1735-27 and GRO J1750-27

T. Koh, D. Chakrabarty, T. A. Prince, and B. Vaughan, California Institute of Technology; and S. N. Zhang, M. Scott, M. H. Finger, and R. B. Wilson, Marshall Space Flight Center, report for the Compton GRO/BATSE team: "Further observations of the recently-discovered, 4.45-s transient hard x-ray pulsar in the Galactic center region (GRO J1735-27; see IAUC 6207) have yielded a refined position, based only on pulsed flux, which indicates that the pulsed and unpulsed emission reported on IAUC 6207 come from different sources. The estimated 90-percent-confidence error box for the pulsed source is approximately a quadrilateral of size 0.7 deg**2, with vertices R.A. = 17h47m, Decl. = -27o.8; 17h50m, -28o.0; 17h52m, -26o.9; 17h49m, -26o.7 (equinox 2000.0). In accordance with the new position, we have designated the pulsed source as GRO J1750-27, the previous designation (GRO J1735-27) referring to the unpulsed source. The pulsed emission from GRO J1750-27, first detected on July 29 UT, continues to be visible as of Aug. 29; its barycentric pulse period on Aug. 29.5 was 4.45311 +/- 0.00003 s, with a period derivative of about -0.11 s/yr. Optical and soft-x-ray observations of the error box are encouraged to constrain further the source position and to identify the accreting companion. The unpulsed source (GRO J1735-27) is not observed in data combined during Aug. 11-21, while pulses from GRO J1750-27 have been detected at a nearly constant level through Aug. 29. The position reported on IAUC 6207 remains valid for the unpulsed source GRO J1735-27."