1.4 GHz = 21 cm. Masers use the same transition AFAIK. In fact, natural H-masers have been found: https://www.cfa.harvard.edu/news/hydrogen-masers-space https

1.4 GHz = 21 cm. Masers use the same transition AFAIK. In fact, natural H-masers have been found: https://www.cfa.harvard.edu/news/hydrogen-masers-space https://www.cfa.harvard.edu/news/hydrogen-masers-space