DISCOVERY OF AN 11-S X-RAY PULSAR IN THE GALACTIC-PLANE SECTION OF THE SCORPIUS CONSTELLATION

During a galactic-plane survey with ASCA in 1996 September, we detecte d a relatively bright, soft source at R.A. = 17(h)8(m)46.(s)6, DEC. = -40 degrees 9'27'' (J2000), and discovered an 11-s X-ray pulsation fro m the source. This source has been identified with the ROSAT source 1R XS J170849.0-400910. From a timing analysis of the source, we obtained a barycentric pulse period of P = 10.99759+/-0.00005 s with a broad s inusoidal shape of a pulse fraction of similar to 30%. The energy spec trum in the 0.8-10 keV region is very soft, and can be fitted by a pow er-law model with a photon index of 3.5 and an absorption column densi ty of 1.8 x 10(22) cm(-2). The observed pulse-phase-averaged flux in t he range 0.8-10 keV is 4.3 x 10(-11) erg cm(-2) s(-1), which correspon ds to 1.7 x 10(-10) erg cm(-2) s(-1) after correcting for soft X-ray a bsorption. During an observation interval of about 14 hr, neither a si gnificant change in the pulsation period, nor a significant variation in the phase-averaged flux was detected. From these X-ray properties, we suggest that this newly discovered X-ray pulsar might be a member o f a small subgroup of ''anomalous'' X-ray pulsars with a period close to 6-9 s.