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Swift detects an outburst of the recurrent supersoft X-ray transient in NGC 300

ATEL # 1560; A.K.H. Kong (NTHU, Taiwan), R. Di Stefano (CfA)
on 5 Jun 2008; 17:40 UT
Password Certification: Albert Kong (

Subjects: X-ray, Binaries, Black Holes, Transients

We report Swift XRT observations of the supersoft X-ray transient (XMMU J005510.7-373855) in NGC 300 (Kong & Di Stefano 2003). The source was first detected as a supersoft X-ray source (kT ~ 50 eV) in 1992 with ROSAT and disappeared in subsequent X-ray observations. The source was active again in 2000 December and 2001 January and was detected with XMM-Newton with luminosities of 1e38 - 1e39 erg/s. However, the source fell below the detection limit of Chandra HRC-I on 2006 June 1 with a 99% upper limit of 3.5e37 erg/s (0.2-2 keV). NGC 300 was observed with Swift on 2008 May 20 and June 4 for 6 ks and 6.9 ks, respectively. The supersoft X-ray transient was clearly detected with XRT in both observations and all the source photons are from below 0.7 keV. The 0.2-2 keV unabsorbed luminosity is about 2e38 erg/s (d=2Mpc), assuming a blackbody spectrum with kT=60 eV and N_H=1e21 cm^-2 (Kong & Di Stefano 2003).

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