X-ray detection of the transient and candidate recurrent nova SSS 1 in NGC 300

ATel #16827; Marina Orio, University of Wisconsin, Madison, USA and INAF-Padova, Italy, and Kim Page, School of Physics and Astronomy, The University of Leicester, UK
on 24 Sep 2024; 06:57 UT
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Credential Certification: Marina Orio (orio@astro.wisc.edu)

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We announce a new serendipitous detection of the luminous, recurrent supersoft X-ray source (SSS) XMMU J005510.7-373855 with the Swift X-Ray Telescope (XRT) in the NGC 300 galaxy, between 2024-06-07 and 2024-08-29. The source was detected in 13 summed short exposures (including three in 2024 September) with average count rate 0.0013±0.002 counts/s. In each individual exposure lasting for 4 to 5 ks, the source was detected at the 2 σ or 3 σ level. We notice that there is no detection of XMMU J005510.7-373855 yet in four short exposures lasting between 1.5 and 3.5 ks done in September, but it is too early to conclude that the SSS has faded again. All counts were always at energy lower than 1 keV, as in the previous recorded outbursts, and although the statistics are poor, a blackbody model fits the spectrum with a 65 eV temperature and a column density 4 x 10²⁰ cm^-2, which is also consistent with previous outbursts. The count rate reported here is about the same the Swift XRT measured in a 2016 outburst, which was confirmed in that occasion also with deep XMM-Newton observations (see Carpano et al. 2019, MNRAS, 490, 4804 and references therein). Swift has very often monitored NGC 300 since the last outburst of XMMU J005510.7-373855 in 2016, and there has never been a new detection of this source before 2024 June, not in longer (7-8 ks) exposures, neither in summed exposures obtained within 10 days. Swift monitoring of NGC 300, with approximately 5 ks every 10 days, is ongoing with a primary goal of following another, unrelated X-ray source. XMMU J005510.7-373855 is known as a transient, luminous SSS in NGC 300 and it has been observed in outburst with ROSAT, Chandra, Swift and XMM-Newton in 1992, 2000, 2008, and 2016, suggesting a recurrence period of 8 years (see Carpano et al. 2019, MNRAS, 490, 4804). The source has also displayed a periodic fluctuation of the X-ray flux with a (possibly orbital) ~4.7 hours period and it is likely to be a luminous recurrent nova. However, since its unabsorbed luminosity is borderline high for a nova (~4 x 10³⁸ erg/s), it may also be a more exotic type of transient, especially a black hole transient cannot be ruled out (see discussion by Carpano et al. 2019). Carpano et al. (2019) did not find any quiescent counterpart more luminous than 25th magnitude in Hubble Space Telescope images; however the source has never been observed in outburst, so optical photometry and spectroscopy is necessary and urgent.