Scientists from the Purple Mountain Observatory (PMO) of the Chinese Academy of Sciences and the National Institute of Astrophysics of Italy have presented a new method to measure moderately saturated sources of the Ultraviolet Optical Telescope on the Swift satellite (Swift/UVOT). They also identified GRB 220101A as the most energetic UV/optical burst ever detected. The findings were recently published in the journal Nature Astronomy.

Gamma ray bursts (GRBs) are the most powerful explosions in the universe. Their instantaneous radiation occurs mainly in the soft range of gamma rays and lasts for a short period of time (i.e. from milliseconds to maximum hours). The instantaneous emission is followed by an afterglow of X-ray, optical and radio emission that lasts for weeks or even years.

Fast optical emission GRB 080319B broke the world record for ultraviolet/optical brightness in 2008. It was so bright that an observer in a dark place could see it with the naked eye. Optical burst emission from GRB 080319B followed the gamma-ray light curve and hence the activity of the central thruster. But now GRB 220101A has broken the previous record.

On New Year’s Eve 2022, the Swift satellite recorded a new outburst of GRB 220101A. GRB 220101A has a measured redshift of 4.618. At such a high redshift, the optical photons observed were in the ultraviolet range and experienced very significant absorption. Therefore, the intrinsic radiation flux was approximately 100 times higher than the observed value. Just 79 seconds after the explosion, Swift/UVOT conducted a rapid 150-second observation in white-band event mode.

The researchers then performed a high-resolution photometric analysis that revealed the rapid evolution of the flow. Especially during peak times the UVOT telescope was already moderately saturated.

“We proposed a UVOT data processing method based on the point spread function of the telescope and confirmed that it indeed provides reliable flux measurements,” said the study’s corresponding author, Professor FAN Yizhong from PMO. After appropriate corrections for distance and absorption, the UV/optical magnitude of GRB 220101A reached -39.4, making it the only known source today with an absolute magnitude brighter than -39.

“This is also the first case in which a highly energetic ultraviolet/optical flash has been detected using a space telescope,” added the Russian Academy of Sciences professor.

The brightness of GRB 220101A is approximately 400 quadrillion times that of the Sun, breaking the 14-year record of GRB 080319B. It also suggests a new astrophysical process that demonstrates the diversity of physical sources of ultra-bright optical UV flares. The Sino-French Space Variable Object Monitor (SVOM) satellite, scheduled for launch in early 2024, is expected to detect highly energetic UV/optical flares at even higher redshift. Source