Ancient Chinese, Korean, and Vietnamese observers left us records of celestial sightings, the so‐called “guest stars” dated up to ∼2500 years ago. Their identification with modern observable targets could open interesting insights into the long‐term behavior of astronomical objects, as shown by the successful identification of eight galactic supernovae. Here, we evaluate the possibility to identify ancient classical novae with presently known cataclysmic variables (CVs). For this purpose, we have developed a method which reconsiders in detail positions and sizes of ancient asterisms, in order to define areas on the sky that should be used for a search of modern counterparts. These areas range from a few to several 100 square degrees, depending on the details given in ancient texts; they should replace the single coordinate values given by previous authors. Any appropriate target (CVs, X‐ray binaries etc.) within these areas can be considered as a valid candidate for identification with the corresponding ancient event. Based on the original descriptions of several 100 old events, we selected those without movement and without a tail (to exclude comets) and which was not only visible within a certain hour (to exclude meteors). This way, we present a shortlist of 24 most promising events which could refer to classical nova eruptions. Our method is checked by applying it to the known SN identifications, leading to a margin of error between 0 and 4.5 degrees, meaning that some SN remnants lay exactly inside the areas given by the historical reports while in some other cases they are laying at considerable distances.
Historical star magnitudes from catalogs by Ptolemy (137 AD), al‐Sūfī (964), and Tycho Brahe (1602/27) are converted to the Johnson V‐mag scale and compared to modern‐day values from the HIPPARCOS catalog. The deviations (or “errors”) are tested for dependencies on three different observational influences. The relation between historical and modern magnitudes is found to be linear in all three catalogs as it had previously been shown for the Almagest data by Hearnshaw, J. B., 1999, New A Rev., 43, 403. A slight dependency on the color index (B‐V) is shown throughout the data sets and al‐Sūfī's, as well as Brahe's data also give fainter values for stars of lower culmination height (indicating extinction). In all three catalogs, a star's estimated magnitude is influenced by the brightness of its immediate surroundings. After correction for the three effects, the remaining variance within the magnitude errors can be considered approximate accuracy of the pre‐telescopic magnitude estimates. The final converted and corrected magnitudes are available via the Vizier catalog access tool (Ochsenbein, F., Bauer, P., & Marcout, J., 2000, A&AS, 143, 23).
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