Abstract
Significant sinusoidal modulations with a best fit period of 22 45 hour have been discovered for an X-ray source CXO J140336 0 541925 in M101 from the M101 ultra-deep Chandra survey and an earlier observation The phase has been preserved remarkably over a timeline of 5 years The source had an average luminosity of about 1e38 erg sec but reached down to zero at the sine valleys in ten observations suggesting a nearly edge-on viewing geometry This period requires a B5V secondary and a 1 6 Msol primary a B8V secondary and a 4 Msol black hole primary or a B8V secondary and a 600 Msol black hole primary if the secondary is filling its Roche lobe Alternatively the binary could be a low mass black hole binary in its outburst phase with a less massive secondary not filling its Roche lobe e g a G5V secondary half filling its Roche lobe and a 1 2 Msol neutron star or a 40 Msol black hole primary Future telescopes such as JWST and GMT will be able to monitor the secondary and the accretion disk to determine the binary masses beyond doubt