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We report the discovery of X-ray quasi-periodic oscillations (QPOs) atfrequencies of 3-4 mHz from a transient ultraluminous X-ray source (ULX)X42.3+59 in M82. The QPOs are strong and broad and appear with weak or absentred noise, and are detected only in Chandra observations when the source isbrighter than 10^40 ergs/s. The QPO behavior is similar to the type A-I QPOsfound in XTE J1550-564, which is a subclass of low frequency QPOs withproperties in between type A and B. Therefore, we identify the QPOs in X42.3+59as of type A or B, and rule out the possibility of type C. With thisidentification, the mass of the black hole in X42.3+59 can be inferred as inthe range of 12,000-43,000 solar masses by scaling the QPO frequency to that ofthe type A/B QPOs in stellar mass black holes. Cool disk emission is detectedin one Chandra observation, and the disk inner radius suggests a similar blackhole mass range. Black holes of such a high mass are able to produce an energyoutput in a manner similar to X42.3+59 by accreting from the interstellarmedium directly.

Discovery of mHz X-ray Oscillations in a Transient Ultraluminous X-ray Source in M82