Another intermediate-mass black hole in a starburst galaxy? The luminous X-ray source in NGC 3628 reappears

In a 52 ks long Chandra X-Ray Observatory ACIS-S observation of the nearby starburst galaxy NGC 3628, obtained to study the starburst-driven outflow from this galaxy, we have detected a very luminous (L(X)approximate to1.1 x 10(40) ergs s(-1) in the 0.3-8.0 keV energy band) point source located at least 20 " from the nucleus of the galaxy. No radio, optical, or near-IR counterpart to this source has been found. This is most probably the reappearance of the strongly variable X-ray luminous source discovered by Dahlem, Heckman, & Fabbiano, which faded by a factor of greater than or similar to 27 between 1991 December and 1994 March (at which point it had faded below the detection limit in a ROSAT HRI observation). This source is clearly a member of an enigmatic class of X-ray sources that are considerably more luminous than conventional X-ray binaries but less luminous than active galactic nuclei and which are not found at the dynamical center of the host galaxy. The Chandra spectrum is best fitted by an absorbed power-law model with a photon index of Gamma =1.8 +/-0.2, similar to that seen in Galactic black hole binary candidates in their hard state. Bremsstrahlung models or multicolor disk models (the favored spectral model for objects in this class on the basis of ASCA observations) can provide statistically acceptable fits only if the data at energies E>5 keV are ignored. This is one of the first X-ray spectra of such an object that is unambiguously that of the source alone, free from the spectral contamination by X-ray emission from the rest of the galaxy that affects previous spectral studies of these objects using ASCA.