supernova designator

Supernova designators for confirmed supernovae have an IAU standard: the prefix SN followed by a year its electromagnetic radiation reached Earth, and if needed, a suffix distinguishing the year's confirmed supernovae. For many historical supernovae, the year is sufficient, e.g., SN 1604. More recently many are confirmed for each year, and a suffix is used to indicate the order of their discovery:

• for discoveries 1-26, using upper case letters A to Z (e.g., SN 1987A).
• for 27-52, using lower case letter combinations: aa to az.
• for 53-78, similarly using ba to bz.
• for 79 to 702 (the latter is 26+26²), similarly using ca to cz, etc., up to zz.
• if two letters are not enough, three letters are used: aaa, aab, etc.
• similarly for four letters, then five letters, etc.

Example: SN 1998bw, sometimes written sn1998bw or in context, abbreviated to 1998bw. Methods of identifying transients that could be supernovae have improved, with thousands of supernovae now detected each year and the Rubin Observatory promises to increase the discovery rate. The sequence of letter suffixes formerly approximated the confirmation-order of the transient as a supernova, but now (since 2016) the year and suffix are assigned to transients on discovery (without waiting for confirmation as a supernova) using a default prefix of AT, for astronomical transient. If and when confirmed as a supernova, the prefix SN (with exactly the same year/discovery indicator) is also considered valid, though the AT version of the designator may still be frequently used, e.g., AT2018cow. Thus, some of the suffixes only apply to transients that are not considered supernovae. The IAU's Transient Name Server (TNS) is a clearinghouse for such naming, taking reports of transients, assigning AT- and SN-based designations, and providing online access to the names.

Such transients often are first given a survey-based designator for the survey that discovered them. In sifting through data for transients, such surveys must track "candidate-transients" that have even less confirmation, often using formats for the survey-specific order of discovery that are analogous but recognizably-different. One convention used by such surveys is to list the years as two-digits instead of four, e.g., ATLAS18qqn, an ATLAS survey discovery; in this case, the suffix is survey-specific and not the same as that used with AT or SN: for example, ATLAS18qqn was the ATLAS designator for AT2018cow.