Discovery of a transient X-ray source Suzaku J1305-4930 in NGC 4945

Abstract

We report the serendipitous discovery of a transient X-ray source, Suzaku J1305-4930, ∼3 kpc southwest of the nucleus of the Seyfert 2 galaxy NGC 4945. Among the seven Suzaku observations of NGC 4945 from 2005 to 2011, Suzaku J1305-4930 was detected four times in July and August in 2010. The X-ray spectra are better approximated with a multi-color disk model than a power-law model. At the first detection on 2010 July 4-5, its X-ray luminosity was $(8.9^{+0.2}_{-0.4}) \times 10^{38}\:$ erg s^-1 and the temperature at the inner-disk radius (kT_in) was 1.12 ± 0.04 keV. At the last detection with Suzaku on 2010 August 4-5, the luminosity decreased to $(2.2^{+0.3}_{-0.8}) \times 10^{38}\:$ erg s^-1 and kT_in was 0.62 ± 0.07 keV. The source was not detected on 2011 January 29, about six months after the first detection, with a luminosity upper limit of 2.4 × 10³⁸ erg s^-1. We also find an absorption feature which is similar to that reported in Cyg X-1. Assuming the standard disk, we suggest that Suzaku J1305-4930 consists of a black hole with a mass of ∼10 $M_\odot$ . The relation between the disk luminosity and kT_in is not reproduced with the standard model of a constant inner radius but is better approximated with a slim-disk model.