Fast blue optical transients are a rare class of emerging transients, the source of which is not yet known. A recent study of the most luminous known object in this class, AT2024wpp, reveals clues about their identity.

A New Class of Transients

In 2018, a network of robotic survey telescopes spotted a new transient source toward a galaxy 200 million light-years away. It was bright and getting brighter rapidly, rising in just a few days to become 10–100 times brighter than the brightest supernovae.

That transient, AT2018cow, was the first of now tens of known sources like it that are too blue, too rapidly evolving, and too luminous to be ordinary supernovae. Despite the growing number of luminous fast blue optical transients (FBOTs) discovered, exactly what causes them is still up for debate.

Brightest of the Bright

Enter AT2024wpp: the most luminous known fast blue optical transient to date. In about a month and a half, this record-breaking event radiated away more than 10⁴⁴ Joules — which is roughly equivalent to the Sun’s radiative output over 8 billion years. Nayana A. J. (University of California, Berkeley) and collaborators carried out an extensive multi-wavelength observing campaign to track the behavior of AT2024wpp from 2 to 280 days after it was first discovered, making it only the second FBOT to undergo such detailed study. This research article describes the team’s findings at radio and X-ray wavelengths.

X-ray behavior of AT2024wpp. The top plot demonstrates the hardening of the spectral index (i.e., shifting toward higher energies) around the peak at 50 days. Click to enlarge. [Nayana A. J. et al. 2025]

These observations revealed that AT2024wpp’s X-ray emission remained bright and constant for 7 days before plummeting until day 30. The emission then rocketed up again, peaking at day 50, and shifted toward higher energies. Finally, things settled down to a lower brightness around day 75. In the radio, AT2024wpp’s behavior was complex, showing a never-before-seen meteoric rise at millimeter wavelengths around 17–32 days.

Comparison of the density of the material surrounding AT2024wpp (red stars) and other FBOTs. Click to enlarge. [Nayana A. J. et al. 2025]

Possible Scenarios

Based on these observations, the team favors a scenario involving a central compact object — a black hole or a neutron star — that is rapidly accreting matter from its surroundings. This accretion powers outflows that accelerate to more than 40% the speed of light and send a shock through dense material around the system. Notably, the team found that the density of the surrounding material is similar among FBOTs that have bright radio emission, including AT2024wpp. This suggests that similar processes are responsible for setting up these events.

The team pointed to two scenarios from the literature that are compatible with their data. In the first, a newborn neutron star or black hole collides with a stellar companion, ripping the star apart and accreting its remains. In the second, a Wolf–Rayet star (a massive star that has shed much of its atmosphere and is nearing its eventual end as a supernova) is tidally disrupted by a neutron star or a black hole. Though it’s not possible to narrow the possibilities further, the immense energies involved tilt things in favor of a black hole rather than a neutron star.

Though FBOTs remain mysterious, additional multi-wavelength studies like this one can help to illuminate their origins. And with the Ultraviolet Transient Astronomy Satellite (ULTRASAT) and UltraViolet EXplorer (UVEX) missions slated to launch within the next five years, we can expect many more FBOT discoveries in the near future.

Citation

“The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the X-Rays and Radio,” Nayana A. J. et al 2025 ApJL 993 L6. doi:10.3847/2041-8213/ae0b4d