2020ApJ...890L..22C

We report the first detection of isotopic methanol (¹³CH₃OH) maser emission in interstellar space. The emission was detected toward the high-mass young stellar object G358.93-0.03 during monitoring of a flare in the 6.7 GHz methanol (CH₃OH) maser emission in this source. We find that the spectral and spatial distribution of the ¹³CH₃OH masers differs from the CH₃OH masers imaged at the same epoch, contrary to expectations from similarity of their pumping. This conclusively demonstrates that isotopic methanol masers are bright under different physical conditions and suggests that they can provide additional, complementary information to the CH₃OH masers from the same source. We detect a rapid decay of the ¹³CH₃OH maser lines suggesting that they are transient phenomena (masing for only a few months), likely associated with rapid changes in radiation field due to an accretion burst induced by massive disk fragmentation. Changes in the line flux density are faster than required to achieve equilibrium in the energy level populations, indicating that the pumping of these masers is likely variable.