MAXI J1820+070 with ULTRACAM on 2018 Mar 16

SUMMARY: Source is very bright (g'~12.8 observed) as of March 16. It continues to brigthen and is showing dramatic speedy red flaring. Such strong and fast sub-second flares have not been probed before in sources as bright as this (GX 339-4 is several mag fainter at peak, and the flares in V404 Cyg were relatively much weaker). At this brightness, the source may also be observed with small and amateur telescopes. This represents an excellent opportunity to follow a bright black hole outburst rise.

(Top) 200-second segment of simultaneous ULTRACAM lightcurves, sampled at a cycle time of 13.8 ms. The g' band lightcurve has been offset by 0.2 mag to match the r' median. (Bottom) Zoom-in around a rapid sub-second flare with a peak i' amplitude of 1.6 mag and substructure, all within 0.3 s. The stronger red sub-second flaring is apparent in both panels.

Colour-magnitude plot of the ULTRACAM lightcurves, showing the redder-when-brighter trend.