BeppoSAX Serendipitous Discovery of the X-Ray Pulsar SAX J1802.7-2017

We report on the serendipitous discovery of a new X-ray source, SAX J1802.7-2017, ~22' away from the bright X-ray source GX 9+1, during a BeppoSAX observation of the latter source on 2001 September 16-20. SAX J1802.7-2017 remained undetected in the first 50 ks of observation; the source count rate in the following ~300 ks ranged between 0.04 and 0.28 counts s-1, corresponding to an averaged 0.1-10 keV flux of 3.6 × 10-11 ergs cm-2 s-1. We performed a timing analysis and found that SAX J1802.7-2017 has a pulse period of 139.612 s, a projected semimajor axis of ax sin i ~ 70 lt-s, an orbital period of ~4.6 days, and a mass function f(M) ~ 17 ± 5 Msun. The new source is thus an accreting X-ray pulsar in a (possibly eclipsing) high-mass X-ray binary. The source was not detected by previous X-ray astronomy satellites, indicating that it is likely a transient system.