Monitoring a methanol maser flare associated with the massive star-forming region G358.93-0.03

We report the earliest detection of the 19.967-GHz [transition 2₁-3₀E (t = 0)] methanol maser associated with the massive star-forming region G358.93-0.03. The flare was detectable from 2019 January 23 to March 5, for only 44 d. It turned out to be the most powerful 19.967-GHz maser in the Galaxy in the entire history of observations, taking into account the 104-Jy flux from it on the Earth's surface and the distance to the source, 6.75 kpc. The 19.967-GHz maser flared contemporaneously with the first of two flares detected in associated 20.971-GHz methanol masers. We estimated that the ratio of flux densities between these two transitions is F_20.971/F_19.967 = 14 ± 4, increasing to >520 in the second flare. We discuss the differences between the two flares in the 20.971-GHz methanol masers and the consequence thereof.