XB-NEWS detects a new outburst from MAXI J1348-630

ATel #13710; Maria Cristina Baglio, David M. Russell, D. M. Bramich, Payaswini Saikia, Saarah F. Pirbhoy (NYU Abu Dhabi), Fraser Lewis (Faulkes Telescope Project & Astrophysics Research Institute, LJMU)
on 6 May 2020; 05:32 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 candidate MAXI J1348-630 was discovered by MAXI/GSC on 2019 January 26 (ATel #12425). The discovery outburst lasted until the end of April 2019, when the source entered a quiescent phase (ATel #12441, #12447, #12456, #12469, #12470, #12471, #12477, #12491, #12497). Later on, two reflares were observed (in May and September 2019, respectively; ATel #12829, #12838, #13188), and at the beginning of February 2020 the source started a new re-brightening that was first detected at optical frequencies by our monitoring performed with the Las Cumbres Observatory (LCO) 2-m and 1-m robotic telescopes (ATel #13451), and was then confirmed by MAXI/GSC (ATel #13459). According to NICER observations, MAXI J1348 was in the hard state at the start of this last re-brightening (ATel #13465).

We have been closely monitoring the source since the start of its first outburst in January 2019 with LCO. The analysis of the LCO data are performed with a new real-time data analysis pipeline, the ''X-ray Binary New Early Warning System (XB-NEWS; see Russell et al. 2019 and Pirbhoy et al. 2020, ATel #13451 for details).

After the detection of the last re-brightening, the source faded again at optical frequencies, going back to the quiescent level that was reached before the start of the re-brightening. In particular, the source magnitude faded from a peak of i'=16.83+/-0.01, g'=18.44+/-0.01 on MJD 58882.0 (February 03) to its faintest magnitude of i'=20.04+/-0.08 on MJD 58935.7 (March 27).

Very recently, a new brightening was observed by LCO, with magnitudes going from i'=19.93+/-0.03 on MJD 58961.5 (April 22) to i'=16.98+/-0.01 on MJD 58973.3 (May 4). The most recent magnitudes (MJD 58973.7, May 4) show that the brightening is still ongoing, with i'=16.91+/-0.01, r'=17.47+/-0.01, g'=18.46+/-0.02. The brightening has probably not reached its peak yet.

We noticed that a slight X-ray brightening is observed in both the MAXI and Swift/BAT monitoring at the same time as the optical one. However, more observations are necessary in order to confirm this. The optical monitoring of the source with LCO is still ongoing. Further multiwavelength observations are therefore encouraged in order to confirm the brightening and determine the accretion state of the source.

The LCO observations are part of an on-going monitoring campaign of ~ 40 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