Supernovae astrophysics from Middle Age documents

The supernova explosion of 1054 AD, which originated the Crab Nebula and Pulsar, is probably the astronomical event which has been most deeply studied by means of historical sources. However, many mysteries and inconsistencies, both among the different sources and between what is deduced by the historical records and the present day astronomical data, are demanding extraordinary efforts by theoretical astrophysicists in order to put all the data in a meaningful framework. An accurate analysis of the historical sources, like the one we are presenting here, may contribute to solve some of these problems. Galactic Supernovae are rare events and their testimonies are extremely important for astrophysics. To date, seven astronomical events documented by historical texts are believed to have been galactic supernovae (see Tab.1). Information gathered from the historical sources concerning all of these events have been used in some way in astrophysical studies, though it is not always easy to extract quantitative data from ancient measurements. Before the Cepheid distance scale was extended by HST to include the host galaxies of many SNe of Type Ia with good photometry, historical Type Ia SNe had been used to compute the Hubble constant by combining their brightness at maximum with the modern distances to their remnant in order to estimate the peak absolute magnitude. For instance, Schaefer (1996), from a careful reconstruction of the Type Ia SN1572 light curve, obtained a peak magnitude of V=-4.53±0.18, corresponding to an absolute magnitude of Vo=-18.64±0.31, deriving Ho=66±12 km s −1 Mpc: this is an astonishing precise result, considering that it was obtained by using data gathered by naked eye more than 400 years ago! However, it may be risky to use historical astronomical data for astrophysics if they have not been carefully checked from a historical point of view. The most indicative case is the one of SN185, which was used for calibrating the brightness of Type Ia SNe and hence for deriving the Hubble constant. But, as it has been shown by Schaefer (1995), most likely this event was not a supernova, but a transit of comet P/Swift-Tuttle – indeed the derived value of Ho was the unrealistic one of Ho≃ 150 km s −1 Mpc. The importance of a careful historical analysis of the historical sources before using them for astrophysical purposes is further illustrated by the case of SN1054. The Song Empire sources were the first to be suggested as witnesses of the birth of the Crab Nebula by Hubble (1928) and Mayall (1939). It was found that they report a date when the Emperor was notified by the astronomer Yang Weide about the 1054 ”guest star” appearance (4 July), the length of the period in which this star was visible in † Present address: CESR, Toulouse, France