SAX J2239.3+6116 is an X-ray transient which often recurs with a periodicity of 262 d. Because of the Be-star nature of the likely optical counterpart, the periodicity may be identified with the orbital period of the binary. In such a Be X-ray binary system, the transient nature of the X-rays is thought to arise from a combination of episodic mass loss by the Be star, and an eccentric binary orbit. The compact object orbiting the Be star will show enhanced X-ray emission near periastron where it accretes more matter due to an enhanced wind density (e.g., review by White et al. 1995). The 2-10 keV flux history shows fluctuations of a factor of 103. Possibly the true dynamic range is larger. The likely counterpart at an estimated distance of 4.4 kpc implies that the 2-10 keV luminosity ranged between 1 erg s-1 and erg s-1. It seems plausible that SAX J2239.3+6116 was more or less active over the last decades. The peak flux is so low that it can easily be missed. The spectral type of the counterpart, the resulting luminosity and the transient behavior is consistent with the identification of SAX J2239.3+6116 as a Be X-ray binary. This is also confirmed by the value for the X-ray to optical color index (as defined by Van Paradijs & McClintock 1995) where B is the dereddened B magnitude and the 2-10 keV X-ray flux. For and Jy (during quiescence) . If B would be 9.7 (to correct for additional circumstellar extinction) then . These are perfectly normal values for Be X-ray binaries in quiescence and three times lower than for low-mass X-ray binaries.
Usually Be X-ray binaries contain an X-ray pulsar (e.g., review by White et al. 1995). SAX J2239.3+6116 does not appear to contain one with a period shorter than 103 sec, with a fairly sensitive upper limit of 10% on the amplitude. This may be explained by the fact that the pulse period is longer than 103 sec. Corbet (1984) discovered a strong correlation between orbital and pulse period for Be X-ray binaries. If SAX J2239.3+6116 adheres to this correlation, the orbital period of 262 d implies a pulse period near to 103 sec. More dedicated sensitive X-ray observations during future outbursts are obviously needed to study this system in more detail.
© European Southern Observatory (ESO) 2000
Online publication: September 5, 2000