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 $(8.9^{+0.2}_{-0.4}) \times 10^{38}$ erg s$^{-1}$ and the temperature at the inner-disk radius ($kT_{\rm in}$) was $1.12\pm0.04$ keV. At the last detection with Suzaku on 2010 August 4--5, the luminosity decreased to $(2.2^{+0.3}_{-0.8}) \times10^{38}$ erg s$^{-1}$ and $kT_{\rm in}$ was $0.62\pm0.07$ keV. The source was not detected on 2011 January 29, about six months after the first detection, with a luminosity upper limit of $2.4\times10^{38}$ 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 $kT_{\rm in}$ is not reproduced with the standard model of a constant inner radius but is better approximated with a slim-disk model.

中文翻译：

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在 NGC 4945 中发现瞬态 X 射线源朱雀 J1305-4930

我们报告了在赛弗特 2 星系 NGC 4945 核心西南方向偶然发现的瞬态 X 射线源朱雀 J1305$-$4930，$\sim$3 kpc。 在 2005 年至 2011 年对 NGC 4945 的七次朱雀观测中，朱雀J1305$-$4930 在 2010 年 7 月和 8 月被检测到四次。X 射线光谱用多色盘模型比幂律模型更能近似。在2010年7月4--5日首次探测时，其X射线光度为$(8.9^{+0.2}_{-0.4}) \times 10^{38}$ erg s$^{-1}$和内圆盘半径 ($kT_{\rm in}$) 处的温度为 $1.12\pm0.04$ keV。在2010年8月4--5日最后一次探测朱雀时，光度下降到$(2.2^{+0.3}_{-0.8}) \times10^{38}$ erg s$^{-1}$和$ kT_{\rm in}$ 为 $0.62\pm0.07$ keV。2011 年 1 月 29 日，即首次发现后约六个月，未检测到该源，亮度上限为 $2.4\times10^{38}$ erg s$^{-1}$。我们还发现了类似于 Cyg X-1 中报道的吸收特征。假设标准磁盘，我们建议朱雀 J1305$-$4930 由一个质量为 $\sim$10 $M_{\odot}$ 的黑洞组成。圆盘光度和 $kT_{\rm in}$ 之间的关系不是用恒定内半径的标准模型再现的，而是用细圆盘模型更好地近似。