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 detected a relatively bright, soft source at R.A.=17() h8() m46. s6, DEC.=-40(deg) 9'27'' (J2000), and discovered an 11-s X-ray pulsation from the source. This source has been identified with the ROSAT source 1RXS 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 sinusoidal shape of a pulse fraction of ~ 30%. The energy spectrum in the 0.8--10 keV region is very soft, and can be fitted by a power-law model with a photon index of 3.5 and an absorption column density of 1.8times 10(22) cm(-2) . The observed pulse-phase-averaged flux in the range 0.8--10 keV is 4.3*E(-11) erg cm(-2) s(-1) , which corresponds to 1.7*E(-10) erg cm(-2) s(-1) after correcting for soft X-ray absorption. During an observation interval of about 14 hr, neither a significant 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 of a small subgroup of ``anomalous'' X-ray pulsars with a period close to 6--9 s.