Discovery of Millihertz X-Ray Oscillations in a Transient Ultraluminous X-Ray Source in M82

We report on the discovery of X-ray quasi-periodic oscillations (QPOs) at frequencies of 3-4 mHz from a transient ultraluminous X-ray source X42.3+59 in M82. The QPOs are strong and broad and appear with weak or absent red noise, and are detected only in Chandra observations when the source is brighter than 10⁴⁰ erg s^-1. The QPO behavior is similar to the type A-I QPOs found in XTE J1550 - 564, which is a subclass of low-frequency QPOs with properties in between types A and B. Therefore, we identify the QPOs in X42.3+59 as of type A or B and rule out the possibility of type C. With this identification, the mass of the black hole in X42.3+59 can be inferred as in the range of 12,000-43,000 solar masses by scaling the QPO frequency to that of the type A/B QPOs in stellar mass black holes. Cool disk emission is detected in one Chandra observation, and the disk inner radius suggests a similar black hole mass range. Black holes of such a high mass are able to produce an energy output in a manner similar to X42.3+59 by accreting from the interstellar medium directly.