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Discovery of a Transient X-ray Source in the Compact Stellar Nucleus of NGC 2403.pdf

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

a r X i v : a s t r o - p h / 0 7 0 2 7 4 4 v 1 2 8 F e b 2 0 0 7 Submitted to Astrophysical Journal Preprint typeset using LATEX style emulateapj v. 04/03/99 DISCOVERY OF A TRANSIENT X-RAY SOURCE IN THE COMPACT STELLAR NUCLEUS OF NGC 2403 Mihoko Yukita1, Douglas A. Swartz2, Roberto Soria3, and Allyn F. Tennant4 Submitted to Astrophysical Journal ABSTRACT 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 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. The cluster has a mass ～10 6.5 M⊙ and an age of ～1.4 Gyr estimating from its observed colors and brightness. Subject headings: galaxies: individual (NGC 2403) — galaxies: nuclei — galaxies: star clusters — galaxies: evolution — X-rays: galaxies — X-rays: binaries 1. INTRODUCTION Tight correlations have been measured between the masses of central supermassive black holes in early-type galaxies and their bulge mass (Magorrian et al. 1998; Ha?ring Rix 2004), luminosity (Kormendy Richstone 1995), central velocity dispersion (