Electronic Telegram No. 5375 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network T CORONAE BOREALIS = NOVA CORONAE BOREALIS 1217 B. E. Schaefer, Louisiana State University, Baton Rouge, reports his discovery of an apparent long-lost nova eruption of the recurrent nova T CrB at visual magnitude somewhat brighter than +2, based on the eyewitness report of Burchard, Abbot of the Ursberg Abbey (near Augsberg, Germany), in the autumn of the year 1217. This record appears in Latin in the Ursperger Chronicle, a medieval monastic annal written in the year 1225, and the nova was one of the two most important events of the year. Burchard writes "In the autumn season of the same year [1217 AD], in the early evening, a wonderful sign ["signum mirabile"] was seen in a certain star ["stella"] in the west. This star was located a little west of south, in what astrologers call Ariadne's Crown [Corona Borealis]. As we ourselves observed, it was originally a faint star that for a time shone with great light, and then returned to its original faintness", and "This was seen for many days that autumn." The transient was reported for dates after the autumnal equinox, while CrB gets too low in the evening sky for seeing a moderately bright transient by the end of October. The observed duration is poorly defined but must be longer than a week or so and shorter than the month of autumn visibility. For discovery with the unaided eye by lay sky watchers, the peak visual magnitude was likely +2 or brighter. The connection from Burchard's transient to T CrB comes as the only possible source of a stellar transient of magnitude V < 2 and lasting 1-4 weeks in Corona Borealis. Only three classes of celestial transients can get to V = 2 or brighter: (1) Burchard explicitly labels the transient as "stella", a point source, rather than any of the various medieval terms for comet as used in Germany. Further Burchard calls the star as "signum mirabile", a very positive omen, which is contrary to the universal useage throughout medieval Germany and Europe with comets as amongst the worst and most-evil omens. So the star was not a comet. (2) The star could not have been a supernova because the progenitor would be roughly 100 kpc away and far outside any possible host galaxy. Further, any supernova would not have been seen to fade to invisibility in the four weeks of autumnal visibility. Further, an 800-year- old supernova at that distance should produce a remnant that is bright at radio and x-ray wavelengths, and such is not seen anywhere in the area around CrB. (3) If Burchard's star is a nova peaking at V < 2, for nova peak absolute V magnitudes of -7.45 +/- 1.3, the required distance is roughly 800 pc or closer, with T CrB itself at a distance of 914 +/- 23 pc. As one of the nearest nova systems, such a nearby cataclysmic variable would be prominent in quiescence, discovered many decades ago with variability, color/spectrum, and x-ray surveys. The only such nova system in the sky area around CrB is T CrB itself. It now appears that T CrB has had visible classical-nova eruptions peaking in 1217.8, 1787.9 (cf. CBET 5374), 1866.4, 1946.1, and likely soon in 2024.4 +/- 0.3 yr. With the average recurrence time of 78.8 +/- 0.4 years from the last four eruptions, there were seven eruption intervals going back to 1217.8. For seven recurrence times from 1217.8 to 1787.9, the average recurrence time is 81.4 years. Full details are given in Schaefer (2023, JHA 54, 436-455; see also URL https://arxiv.org/abs/2308.13668). NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 March 28 (CBET 5375) Daniel W. E. Green