Discovery of a transient X-ray source Suzaku J1305-4930 in NGC 4945. (arXiv:2007.04465v1 [astro-ph.HE])

Discovery of a transient X-ray source Suzaku J1305-4930 in NGC 4945. (arXiv:2007.04465v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Ide_S/0/1/0/all/0/1">Shuntaro Ide</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Hayashida_K/0/1/0/all/0/1">Kiyoshi Hayashida</a> (1 and 2 and 3), <a href="http://arxiv.org/find/astro-ph/1/au:+Noda_H/0/1/0/all/0/1">Hirofumi Noda</a> (1 and 2), <a href="http://arxiv.org/find/astro-ph/1/au:+Kurubi_H/0/1/0/all/0/1">Hiroyuki Kurubi</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Yoneyama_T/0/1/0/all/0/1">Tomokage Yoneyama</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Matsumoto_H/0/1/0/all/0/1">Hironori Matsumoto</a> (1 and 2) ((1) Dept. of Earth and Space Science, Osaka Univ., (2) Project Research Center, Osaka Univ., (3) ISAS/JAXA)

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 $1.12pm0.04$ 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
$0.62pm0.07$ keV. The source was not detected on 2011 January 29, about six
months after the first detection, with a luminosity upper limit of
$2.4times10^{38}$ 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.

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 $1.12pm0.04$ 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
$0.62pm0.07$ keV. The source was not detected on 2011 January 29, about six
months after the first detection, with a luminosity upper limit of
$2.4times10^{38}$ 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.

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