Hubble finds luminous fast blue optical transient far from any host galaxy

A rare and puzzling burst of exceptionally bright light in the universe has become even more enigmatic, thanks to the keen eye of the NASA/ESA Hubble Space Telescope. This phenomenon, known as a Luminous Fast Blue Optical Transient (LFBOT), made a surprising appearance far from any host galaxy, and only Hubble’s precision could pinpoint its location. The findings from Hubble’s observations have raised questions about our understanding of these objects, challenging some previously held theories.

Luminous Fast Blue Optical Transients (LFBOTs) are some of the most dazzling visible-light events in the cosmos, erupting suddenly and shining intensely like camera flashbulbs. Since their initial discovery in 2018, only a few of these events have been identified, occurring at a rate of approximately one per year.

Following its initial detection, the most recent LFBOT, dubbed AT2023fhn and affectionately called “the Finch,” was observed by various telescopes across the electromagnetic spectrum, from X-rays to radio waves. Yet, it was Hubble’s unparalleled resolution that provided the crucial details. The Finch exhibited all the classic characteristics of an LFBOT, radiating in vibrant blue light and undergoing rapid changes, from reaching peak brightness to fading within days, in stark contrast to the gradual dimming of supernovae over weeks or months.

However, what sets the Finch apart is its unconventional location. Hubble revealed that it resides in apparent solitude between two neighboring galaxies, situated roughly 50,000 light-years from a nearby spiral galaxy and approximately 15,000 light-years from a smaller galaxy. This peculiar placement challenges the previous belief that celestial objects of this nature exist exclusively within host galaxies.

Ashley Chrimes, the lead author of the Hubble paper detailing this discovery and a European Space Agency Research Fellow, formerly affiliated with Radboud University in the Netherlands, emphasized the significance of Hubble’s observations in unveiling this extraordinary find. Without Hubble’s data, the uniqueness of the Finch would have remained undiscovered.

This groundbreaking study is set to be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, and the paper is currently accessible on the arXiv preprint server.

Hubble image of a Luminous Fast Blue Optical Transient (LFBOT) This Hubble photograph shows three galaxies against the velvet-black backdrop of space. The largest is the white and blue spiral-shaped galaxy at the image center. Two smaller galaxies are whitish patches toward the left. A curious white spot near the top of the image is marked out with two perpendicular orange lines. It is the glow from some unknown object that exploded, but isn’t associated with any of the galaxies.] Credit: NASA, ESA, STScI, A. Chrimes (Radboud University)

While these incredible bursts of light have long been associated with a rare type of supernova known as core-collapse supernovae, the massive stars that give rise to these supernovae have relatively short lifespans in the cosmic sense. As a result, these massive progenitor stars were thought to stay close to their birthplaces, typically within clusters of newly formed stars. Previous Luminous Fast Blue Optical Transients (LFBOTs) had all been located in the spiral arms of galaxies where ongoing star formation occurs.

Ashley Chrimes remarked, “The more we delve into LFBOTs, the more they defy our expectations. We’ve now demonstrated that LFBOTs can occur in locations far from the central regions of the nearest galaxy, and the Finch’s location doesn’t fit the profile of any known type of supernova.”

The initial alert about the Finch came from the Zwicky Transient Facility, a wide-angle ground-based camera scanning the entire northern sky every two days. Astronomers promptly initiated a pre-planned observation program when they spotted it, ready to investigate any potential LFBOT candidates.

Spectroscopic measurements taken with the Gemini South telescope in Chile revealed that the Finch blazed at a scorching 20,000 degrees Celsius. Gemini also helped calculate its distance from Earth, enabling the determination of its luminosity. Data from various observatories, including the Chandra X-ray Observatory and the Very Large Array radio telescope, collectively confirmed the explosion as a genuine LFBOT.

One intriguing theory suggests that LFBOTs may result from stars being torn apart by intermediate-mass black holes, those with masses between 100 to 1,000 times that of our Sun. The NASA/ESA/CSA James Webb Space Telescope, with its high resolution and infrared sensitivity, might eventually unveil that the Finch exploded within a globular star cluster situated in the outer halo of one of the neighboring galaxies. Such clusters are prime locations to search for intermediate-mass black holes.

This Hubble photograph shows three galaxies against the black backdrop of space. The largest is the white and blue spiral-shaped galaxy at the image center. Two smaller galaxies are whitish patches toward the left. A curious white spot near the top of the image is marked out with two perpendicular orange lines. The image is labeled with the name of the white spot, coordinates and a scale marker. Credit: NASA, ESA, STScI, A. Chrimes (Radboud University)

To account for the Finch’s unusual location, researchers are also contemplating an alternative scenario where it stems from the collision of two neutron stars, spiraling together over billions of years and venturing far from their home galaxy. These collisions give rise to kilonovae, explosions 1,000 times more potent than standard supernovae. A speculative notion even proposes that if one of these neutron stars is a highly magnetized object known as a magnetar, it could significantly amplify the explosion’s power, making it 100 times brighter than a typical supernova.

“The discovery raises more questions than answers,” Chrimes noted. “Further investigation is necessary to pinpoint the correct explanation from the many possibilities.”

Given that astronomical transients can appear anywhere and at any time, with fleeting lifetimes on cosmic scales, researchers rely on wide-field surveys capable of continuously monitoring vast sections of the sky to detect these events and signal other observatories, such as Hubble, for follow-up observations.

Researchers emphasize the need for a more extensive dataset to arrive at a deeper understanding of this phenomenon. Future all-sky survey telescopes may play a pivotal role in detecting additional LFBOTs, dependent on the underlying astrophysical processes.

Source: NASA

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