Discovery of a Transient X-Ray Source in the Compact Stellar Nucleus of NGC 2403

We report the discovery of an X-ray source coincident with the nuclear star cluster at the dynamical center of the nearby late-type spiral galaxy NGC 2403. The X-ray luminosity of this source varies from below detection levels, ~1035 erg s-1 in the 0.5-8.0 keV band to 7 × 1038 erg s-1 on timescales between observations of <2 months. The X-ray spectrum is well fit by an accretion disk model consisting of multiple blackbody components and corresponding physically to a compact object mass of ≳5 M☉. No pulsations nor aperiodic behavior is evident in its X-ray light curve on the short timescales of the individual observations. The X-ray properties of the source are more similar to those of the nuclear source X-8 in M33, believed to be a low-mass X-ray binary, then to those of the low-luminosity active galactic nucleus (AGN) in NGC 4395. The brightness of the nuclear star cluster, MI ~ -11.8 mag, is typical of clusters in late-type spirals but its effective radius, re ~ 12 pc, is several times larger than average indicating a relatively relaxed cluster and a low probability of a central massive object. Estimating from its observed colors and brightness, the cluster has a mass of ≳106.5 M☉ and an age of ~1.4 Gyr.