Tanabata: The Weaving Princess and the Cow Herder, a happy young couple in the heavens who are separated as punishment for neglecting their duties and only allowed to meet one night each year, is a well-known story in many Asian cultures. Every summer, festivals are held to commemorate this brief but joyous reunion. This Tanabata season, astronomers announced a different type of brief cosmic encounter, even mentioning a “cow.”
The Subaru Telescope’s wide field-of-view and high sensitivity, combined with big data techniques, have enabled researchers to search large areas of the sky for transient phenomena that change rapidly over time. Dr. Ji-a Jiang of Japan’s National Astronomical Observatory (formerly the Kavli Institute for the Physics and Mathematics of the Universe) is leading the “MUltiband Subaru Survey for Early-phase Supernovae” (MUSSES) project, which is looking for fast-evolving transients within one day of their occurrence. In December 2020, the team discovered 20 fast-evolving transients, one of which, MUSSES2020J (AT 2020afay), piqued Jiang’s interest.
The first images of MUSSES2020J were very dim, but it brightened significantly over the course of the observations, indicating that the team caught it as it was happening. Follow-up observations confirmed that this event evolved much faster and was about 50 times brighter than normal supernovae. Astronomers are still puzzled as to what could have caused such an event. Only a few others have been observed, the most well-known being AT 2018cow. This new class of events, according to the team, should be called “fast blue ultraluminous transients” (FBUT).
MUSSES2020J was discovered at a much earlier stage in its evolution than any other FBUT. “We were able to capture this amazing phenomenon for the first time thanks to the high-cadence survey mode and the excellent performance of Subaru/HSC. The early light-curve data should provide some unique insights into the origins of these amazing transients “Jiang explains.
Because of the suddenness and power of FBUTs, astronomers suspect a compact object such as a black hole or neutron star is involved, but exactly what happens and how remains unknown. The team will keep looking for answers, but their brief encounter with a mysterious heavenly phenomenon has made them as happy as the reunited Weaving Princess and Cow Herder.
On July 12, 2022, these findings were published as “MUSSES2020J: The Earliest Discovery of a Fast Blue Ultraluminous Transient at Redshift 1.063″ in The Astrophysical Journal Letters.