[Iaude] CBET 5375: T CORONAE BOREALIS = NOVA CrB 1217

[quai at eps.harvard.edu]

                                                  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 at eps.harvard.edu (alternate cbat at iau.org)
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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