Mini-outburst from the black hole candidate MAXI J1348-630 detected at optical frequencies by XB-NEWS

ATel #14016; Authors: M. C. Baglio, D. M. Russell, P. Saikia, D. M. Bramich (NYU Abu Dhabi), F. Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 16 Sep 2020; 10:09 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Maria Cristina Baglio (cristina.baglio@brera.inaf.it)

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The black hole X-ray binary candidate MAXI J1348-630 was discovered on 2019 January 26 (MJD 58509) thanks to the MAXI/GSC satellite (ATel#12425). At the end of April 2019, after a decay phase, the source experienced a first reflare (MJD 58630; ATel #12829), followed by a second re-brightening in September 2019 (#12838, #13188). In February 2020, a further re-brightening was observed, first detected in the optical with our monitoring performed with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes (ATel #13451), and then in the X-rays with MAXI/GSC (ATel #13459) and NICER (ATel #13465). The source then faded again in the optical at the end of March 2020 and went back to its quiescent level (ATel #13710). A further optical re-brightening was detected with LCO in May 2020 (MJD 58973.3; ATel #13710), and the source again reached quiescence on MJD 59007, with magnitudes i'=19.92+/-0.11, r'=20.69+/-0.16, and g'>22.62, comparable to its previous detected quiescent level (see ATe l#13710). Magnitudes have been extracted using the real-time data analysis pipeline, the ''X-ray Binary New Early Warning System (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020, ATel #13451 for details).

Recently, on 2020 September 05 (MJD 59097), a mini-outburst has been detected in the X-rays by MAXI/GSC and reported in ATel #13994. No optical observations were acquired with our monitoring at that time.
On 2020 September 12 (MJD 59104), we detected a re-brightening in the optical, consistent with the X-ray mini-outburst. In particular, we measured the following magnitudes: g'=18.85+/-0.04, r'=17.85+/-0.01, i'=17.19+/-0.01, y=19.91+/-0.03 on MJD 59104. These values are comparable to the peak magnitudes reached in previous re-brightenings of the source.
The last measured magnitudes (September 14; MJD 59106) instead are: g'=18.95+/-0.03, r'=18.00+/-0.01, i'=17.33+/-0.01, implying the flux is decreasing. However, it is not clear if the re-brightening has already reached its peak. Our light curve is linked below. Further multi-wavelength observations are encouraged. Our monitoring with LCO is still ongoing but soon there will be a Sun constraint for a few months.

The LCO observations are part of an on-going monitoring campaign of ~ 50 low-mass X-ray binaries (Lewis et al. 2008) with LCO and the Faulkes Telescopes. This work makes use of observations from the Las Cumbres Observatory (LCO). We acknowledge the support of the NYU Abu Dhabi Research Enhancement Fund under grant RE124.

Optical LCO light curves of MAXI J1348-630