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K. Mukai, A.P. Smale (NASA/GSFC and USRA), C.K. Stahle (NASA/GSFC), E.M. Schlegel (SAO)
MS 1603.6+2600 was discovered as a serendipitous source in an Einstein\/ IPC field; its optical counterpart (UW CrB) is an 111 min, eclipsing binary. The short orbital period immediately establishes UW CrB as a low-mass, compact binary. However, it does not fit well into the known classes of X-ray emitting binaries: the X-ray to optical luminosity ratio is in between typical low-mass X-ray binaries (LMXBs) and typical cataclysmic variables. Moreover, UW CrB has to be unusually X-ray faint for this to be an LMXB within our own galaxy (LX = 1.4 \times 1034 [d/10kpc]2 ergs\,s-1), even more so if we require this high latitude (bII ~47\circ) source to be in or near the Galactic disk.
Here we report on our ASCA observation of UW CrB, carried out in 1997 August. The spectrum has no strong features, and is consistent with a ~4 keV bremsstrahlung. We have examined the light curve, obtained by combining data from all 4 instruments; by folding it on the 111 min period, we find a broad, partial eclipse. Thus the X-ray emitting region is extended, which almost certainly indicates that UW CrB is a high-inclination LMXB in which we only observe the Accretion Disk Corona (ADC). However, it is much fainter than the prototype ADC source, X1822-371 (LX ~ 1036 ergs\,s-1). Moreover, there are few LMXBs known with orbital orbital periods shorter than ~3 hrs. We will consider evolutionary scenarios which might explain both these unusual properties of this system.
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