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 starcluster 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, ~1e35erg/s in the 0.5-8.0 keV band, to 7e38 erg/s on timescales between observationsof less than 2 months. The X-ray spectrum is well-fit by an accretion diskmodel consisting of multiple blackbody components and corresponding physicallyto a compact object mass of greater than approximately 5 solar masses. Nopulsations nor aperiodic behavior is evident in its X-ray light curve on theshort timescales of the individual observations. The X-ray properties of thesource are more similar to those of the nuclear source X-8 in M33, believed tobe a low-mass X-ray binary, then to those of the low-luminosity active galacticnucleus in NGC 4395. The brightness of the nuclear star cluster, M_I ~ -11.8mag, is typical of clusters in late-type spirals but its effective radius, r_e\~ 12 pc, is several times larger than average indicating a relatively relaxedcluster and a low probability of a central massive object. The cluster has amass 3e6 solar masses and an age of 1.4 Gyr estimating from its observed colorsand brightness.Comment: 7 pages, submitted to Ap