The transient X-ray source SAX J2239.3+6116 and its optical counterpart

We have discovered with the Wide Field Cam- eras on board BeppoSAX the weak transient X-ray source SAX J2239.3+6116 whose position coincides with that of 4U 2238+60/3A 2237+608 and is close to that of the fast transient AT 2238+60 and the unidentified EGRET source 3EG 2227+6122. The data suggest that the source exhibits outbursts that last for a few weeks and peak to a flux of 4 10 10 erg cm 2 s 1 (2-10 keV) at maximum. During the peak the X-ray spectrum is hard with a photon index of 1:1 0:1. Follow-up observations with the Narrow-Field In- struments on the same platform revealed a quiescent emission level that is 10 3 times less. Searches through the data archive of the All-Sky Monitor on RXTE result in the recognition of five outbursts in total from this source during 1996-1999, with a regular interval time of 262 days. Optical observations with the KPNO 2.1 m telescope provide a likely optical counterpart. It is a B0 V to B2 III star with broadened emission lines at an ap- proximate distance of 4.4 kpc. The distance implies a 2-10 keV luminosity in the range from10 33 to10 36 erg s 1 . The evidence suggests that SAX J2239.3+6116 is a Be X-ray binary with an orbital period of 262 days.