Hubble Telescope Witnessed Intergalactic Flash of Light

Last April, the Zwicky Transient Facility in California was scanning the sky, when it picked up a fleeting astronomical phenomenon — an event that burst into view and then disappeared within a relatively short timeframe. Most such transients are supernovae, though others can occur, too.

Owing to transients’ short shelf lives, astronomers wasted no time in following up on the finding. The initial discovery triggered a prepared sequence of observations; within weeks, they were measuring the event’s visible spectrum with the Gemini South telescope in Chile, its radio waves with the Very Large Array in New Mexico, and its X-rays with the Chandra telescope in space. By May, the Hubble Space Telescope was trained on the target, too.

Out of that data, a team in the Netherlands, the UK, Denmark, and South Africa pieced together a picture of the event, posted on the arXiv astronomy preprint server. It turned out to be a member of a rare subclass of transients known as luminous fast blue optical transients (luminous FBOTs). Officially designated AT2023fhn, it was dubbed “the Finch,” continuing the trend of nicknaming FBOTs after animals. (Others include “the Koala,” “the Camel,” and “the Cow.”)

Luminous FBOTs are intensely bright — up to 100 times brighter than typical supernovae — and, like other FBOTs, maintain a persistent blue color throughout the entirety of their evolution. Even among transients, FBOTs are unusually short-lived, rising rapidly to high peak luminosities before dimming in a matter of days. Unlike other transients, their precise nature remains a mystery.

As a result, FBOTs have generated considerable intrigue, and a slew of scenarios have been proposed to account for their common features. The tendency of most FBOTs to occur in star-forming galaxies, for instance, has led to the suggestion that they are an end result of massive-star evolution, perhaps even a special type of supernova. Exceptional circumstances, such as a tidal disruption event (TDE), in which a star is ripped apart by a black hole, could also be at play.

The Finch differs from previous FBOTs in that it lies far from the two nearest candidate host galaxies. It’s 50,000 light-years from a large spiral galaxy and 15,000 light-years from the spiral’s satellite, a much smaller dwarf galaxy. Star-forming regions are typically found in galaxies, so if the Finch is indeed a supernova, how did it stray so far from home before exploding?

As study lead Ashley Chrimes (ESA and Radboud University, The Netherlands), puts it, “The more we learn about LFBOTs, the more they surprise us. We’ve now shown that LFBOTs can occur a long way from their nearest galaxy, and the location of the Finch is not what we expect for a supernova.”

The astronomers established with a high degree of confidence that the Finch is aligned with the two neighboring galaxies, as opposed to being caught in the same field of view by chance. They also calculated that, unlikely though it may be, it is theoretically possible for a massive progenitor star originating in one of the galaxies and travelling at a sufficiently high velocity to have reached the position in question before exploding as a supernova.

They add, however, that there could also be an unseen globular cluster in the vicinity that the observations failed to capture. It might have hosted the Finch, as it offered the opportunity for a star to disrupt near an intermediate-mass black hole. This scenario would admittedly also be unusual, given that only a tiny fraction of globular clusters are located as far out from galactic centers as the Finch is.

However, a TDE is consistent with the authors’ observations and “should be taken seriously,” according to Brian Metzger (Columbia University and the Flatiron Institute), who was not involved in the study. He proposes, though, that a stellar-mass black hole or a neutron star might be a more likely source of disruption than an intermediate-mass black hole, as these objects sometimes coexist with old stars in binaries. The explosive merger of such a binary could generate an FBOT.

Cosimo Inserra (Cardiff University), another researcher unaffiliated with the study, finds all the researchers’ suggestions plausible, but he favors the scenario with the high-velocity massive star, since such stars have already been observed. He adds that more confirmed FBOT events and more multiwavelength data following their evolution will help get to the heart of the matter. This jibes with the authors’ calls for enhanced modeling of the Finch’s spectra and light curves as well as later imaging to search for any stellar survivors.  

Data gathering proves a real challenge, given the brevity of the transients, but is set to receive a boost with a new telescope in Chile: The Vera Rubin Observatory is due to begin operations in late 2024. As part of its “Legacy Survey of Space and Time,” it will map the southern sky at regular intervals, revealing variability over time and filling in some of the blanks in our knowledge. Its wide field and sensitive detectors should be capable of picking up many more transients, including FBOTs.

“There is still a long way to go,” Inserra observes, “but we are on the right path.”