Possible ∼0.4 h X-ray quasi-periodicity from an ultrasoft active galactic nucleus J. R. Song, X. W. Shu, L. M. Sun, Y. Q. Xue, C. Jin, W. J. Zhang, N. Jiang, L. M. Dou and T. G. Wang A&A, 644 (2020) L9 Published online: 14 December 2020 DOI: 10.1051/0004-6361/202039410