A transient event in AD 775 reported by al-Tabari: A bolide – not a nova, supernova, or kilonova

Given that the cause for the strong increase in 14C in AD 774/5 in Japanese and German trees is still a matter of debate (e.g. short Gamma-Ray Burst or solar super-flare), we have searched in Arabic chronicles for reports about unusual transient celestial events. In the {\em History of al-Tabari we found two (almost identical) reports about such an event. The group around caliph al-Mansur observed a transient event while on the way from Baghdad to Mecca on AD 775 Aug 29 – Sep 1 (Julian calendar), most probably during the morning twilight of AD 775 Aug 29. A celestial object kawkab was seen to fall or set inqadda, and its trace atharuhu was seen for at least tens of minutes (up to 70-90 min) during morning twilight. The reports use the Arabic words kawkab and athar(uhu), which were also used in the known Arabic reports about supernovae SN 1006 and 1054, so that one might consider an interpretation as a nova-like event. The kawkab (celestial object) was observed only during the morning twilight at a brightness of probably between about -3 and 0 mag. Such a brightness and time-scale would be expected for optical kilonovae (at 3 to 9 kpc) in the context of short Gamma-Ray Bursts. There are no similar reports from eastern Asia for this time. However, the short reports are fully consistent with a bolide: The word kawkab can be used for meteor, the verb inqadda normally means “falling down”, the word atharuhu can mean “its trace”. We therefore prefer the interpretation as bolide. We discuss in detail how to convert the Muslim calendar date to a date in the Julian calendar using first the calculated Islamic calendar and then considering the time when the crescent new moon could be visible at the given location.