GRO J1735-27

R. B. Wilson, S. N. Zhang, M. Scott, and B. A. Harmon, Marshall Space Flight Center, NASA; and T. Koh, D. Chakrabarty, B. Vaughan, and T. A. Prince, California Institute of Technology, report for the Compton Observatory BATSE team: "Pulsations from a previously- uncatalogued source were detected on July 29 in the galactic-center region by BATSE. The observed period history shows a clear orbital signature. The maximum pulsed intensity was observed on Aug. 13, with a measured barycentric pulse period of 4.45565 +/- 0.000025 s; the pulses continue to be observed as of Aug. 16, at energies from 20 to at least 70 keV. Using occultation imaging, we observed a source of about 100 mCrab in the energy range 20-50 keV not present during July 2-14. A preliminary location (90-percent confidence) is bounded by a quadrilateral with vertices (equinox 2000.0) at R.A. = 17h36m, Decl. = -26o.9; 17h37m, -28o.0; 17h33m, -27o.3; 17h32m, -26o.1. Positions estimated independently from the pulsed and DC flux are consistent at a 68-percent confidence level. If the detected emissions are from the same source, the pulsed fraction is about 30 percent. The energy spectrum of the pulsed component on Aug. 5 was well fitted by a power-law model F(E) = C[E/(30 keV)]E(-alpha), with C = (2.7 +/- 1.0) x 10E-4 photon cmE-2 sE-1 keVE-1, and alpha = 3.5 +/- 1.0."