Discovery of an 11-s X-ray pulsar in the galactic-plane section of the scorpius constellation

Abstract

During a galactic-plane survey with ASCA in 1996 September, we detected a relatively bright, soft source at R.A. = 17h8m46.s6, DEC. = -40°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 1.8 × 1022 cm-2. The observed pulse-phase-averaged flux in the range 0.8-10 keV is 4.3 × 10-11 erg cm-2 s-1, which corresponds to 1.7 × 10-10 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.