MS 1603.6+2600: an Accretion Disc Corona source?

Abstract

We have observed the eclipsing low-mass X-ray binary MS1603.6+2600 with Chandra for 7 ksec. The X-ray spectrum is well fit with a single absorbed powerlaw with an index of ~2. We find a clear sinusoidal modulation in the X-ray lightcurve with a period of 1.7+-0.2 hours, consistent with the period of 1.85 hours found before. However, no (partial) eclipses were found. We argue that if the X-ray flare observed in earlier X-ray observations was a type I X-ray burst then the source can only be an Accretion Disc Corona source at a distance between ~11-24 kpc (implying a height above the Galactic disk of ~8-17 kpc). It has also been proposed in the literature that MS1603.76+2600 is a dipper at ~75 kpc. We argue that in this dipper scenario the observed optical properties of MS1603.6+2600 are difficult to reconcile with the optical properties one would expect on the basis of comparisons with other high inclination low-mass X-ray binaries, unless the X-ray flare was not a type I X-ray burst. In that case the source can be a nearby soft X-ray transient accreting at a quiescent rate as was proposed by Hakala et al.(1998) or a high inclination source at ~15-20 kpc.