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, $\sim$3 kpc southwest of the nucleus of the Seyfert 2 galaxy NGC 4945. Among the seven Suzaku observations of NGC 4945 from 2005 to 2011, Suzaku J1305$-$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 $(8.9_{-0.4}^{+0.2})\times 10^{38}$ erg s${}^{-1}$ and the temperature at the inner-disk radius ($k{T}_{\mathrm{in}}$) was $1.12\pm 0.04$ keV. At the last detection with Suzaku on 2010 August 4--5, the luminosity decreased to $(2.2_{-0.8}^{+0.3})\times 10^{38}$ erg s${}^{-1}$ and $k{T}_{\mathrm{in}}$ was $0.62\pm 0.07$ keV. The source was not detected on 2011 January 29, about six months after the first detection, with a luminosity upper limit of $2.4\times 10^{38}$ 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 $\sim$10 $M_{\odot }$. The relation between the disk luminosity and $k{T}_{\mathrm{in}}$ is not reproduced with the standard model of a constant inner radius but is better approximated with a slim-disk model.

17 pages, 10 figures

References in corpus (4)

• Modelling the behaviour of accretion flows in X-ray binaries
• Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state
• X-Ray Sources in the Star Forming Galaxies NGC 4194 and NGC 7541
• Long-term Spectral Variability of the Ultra-luminous X-ray source Holmberg IX X--1