AT2023fhn, the LFBOT nicknamed ‘the Finch’: a bizarre explosion in an unexpected place
A very rare, strange burst of extraordinarily bright light in the universe just got even stranger – thanks to the eagle-eye of the NASA/ESA Hubble Space Telescope. The phenomenon, called a Luminous Fast Blue Optical Transient (LFBOT), flashed onto the scene where it wasn’t expected to be found, far away from any host galaxy. Only Hubble could pinpoint its location. The Hubble results suggest astronomers know even less about these objects than previously thought by ruling out some possible theories.
A Hubble Space Telescope image of a Luminous Fast Blue Optical Transient (LFBOT) designated AT2023fhn, indicated by pointers. It shines intensely in blue light and evolves rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim. Only a handful of previous LFBOTs have been discovered since 2018. The surprise is that this latest transient, seen in 2023, lies at a large offset from both the barred spiral galaxy at right and the dwarf galaxy to the upper left. Only Hubble could pinpoint its location. And, the results are leaving astronomers even more confounded because all previous LFBOTs have been found in star-forming regions in the spiral arms of galaxies. It’s not clear what astronomical event would trigger such a blast far outside of a galaxy. Credit: NASA, ESA, STScI, A. Chrimes (Radboud University)
Luminous Fast Blue Optical Transients (LFBOTs) are among the brightest known visible-light events in the universe – going off unexpectedly like camera flashbulbs. Only a handful have been found since the first discovery in 2018. Presently, LFBOTs are detected about once per year.
After its initial detection, the latest LFBOT was observed by multiple telescopes across the electromagnetic spectrum, from X-rays to radio waves. Only Hubble’s exquisitely sharp resolution could pinpoint its location. Designated AT2023fhn and nicknamed ‘the Finch,’ the transitory event showed all the tell-tale characteristics of an LFBOT. It shined intensely in blue light and evolved rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim.
But unlike any other LFBOT seen before, Hubble found that the Finch is located in apparent isolation between two neighbouring galaxies – about 50,000 light-years from a nearby spiral galaxy and about 15,000 light-years from a smaller galaxy – a baffling locale for celestial objects previously thought to exist within host galaxies.
“The Hubble observations were really the crucial thing. They made us realise that this was unusual compared to the other ones like that, because without the Hubble data we would not have known,”
said Ashley Chrimes, lead author of the Hubble paper reporting the discovery in an upcoming issue of the Monthly Notices of the Royal Astronomical Society (MNRAS). He is also a European Space Agency Research Fellow, formerly of Radboud University, Nijmegen in the Netherlands.
This is an artist’s concept of one of the brightest explosions ever seen in space. Called a Luminous Fast Blue Optical Transient (LFBOT), it shines intensely in blue light and evolves rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim. Only a handful of previous LFBOTs have been discovered since 2018. And they all happen inside galaxies where stars are being born. But as this illustration shows, the LFBOT flash discovered in 2023 by Hubble was seen between galaxies. This only compounds the mystery of what these transient events are. Because astronomers don’t know the underlying process behind LFBOTs, the explosion shown here is purely conjecture based on some known transient phenomenon. Credit: NASA, ESA, NSF’s NOIRLab, M. Garlick , M. Zamani
While these awesome explosions have been assumed to be a rare type of supernova (called core-collapse supernovae), the gargantuan stars that turn into supernovae are short-lived by stellar standards. Therefore, the massive progenitor stars to supernovae don’t have time to travel very far from their birthing place – a cluster of newborn stars. All previous LFBOTs have been found in the spiral arms of galaxies where star birth is ongoing.
“The more we learn about LFBOTs, the more they surprise us,” said Chrimes. “We’ve now shown that LFBOTs can occur a long way from the centre of the nearest galaxy, and the location of the Finch is not what we expect for any kind of supernova.”
The Zwicky Transient Facility – an extremely wide-angle ground-based camera that scans the entire northern sky every two days – first alerted astronomers to the Finch on 10 April 2023. Once it was spotted, the researchers triggered a pre-planned program of observations that had been on standby, ready to quickly turn their attention to any potential LFBOT candidates that arose.
Spectroscopic measurements made with the Gemini South telescope in Chile found that the Finch is a scorching 20,000 degrees Celsius. Gemini also helped determine its distance from Earth so its luminosity could be calculated. Together with data from other observatories including the Chandra X-ray Observatory and the Very Large Array radio telescope, these findings confirmed the explosion was indeed an LFBOT.
The LFBOTs could be the result of stars being torn apart by an intermediate-mass black hole (between 100 to 1,000 solar masses). The NASA/ESA/CSA James Webb Space Telescope’s high resolution and infrared sensitivity might eventually be used to find that the Finch exploded inside a globular star cluster in the outer halo of one of the two neighbouring galaxies. A globular star cluster is the most likely place an intermediate-mass black hole could be found.
To explain the unusual location of the Finch, the researchers are considering the alternative possibility that it is the result of a collision of two neutron stars, travelling far outside their host galaxy, that have been spiralling toward each other for billions of years. Such collisions produce a kilonova – an explosion 1,000 times more powerful than a standard supernova. However, one very speculative theory is that if one of the neutron stars is highly magnetised – a magnetar – it could greatly amplify the power of the explosion even further to 100 times the brightness of a normal supernova.
“The discovery poses many more questions than it answers,” said Chrimes. “More work is needed to figure out which of the many possible explanations is the right one.”
Because astronomical transients can pop up anywhere and at any time, and are relatively fleeting in astronomical terms, researchers rely on wide-field surveys that can continuously monitor large areas of the sky to detect them and alert other observatories like Hubble to do follow-up observations.
A larger sample is needed to converge on a better understanding of the phenomenon, say researchers. Upcoming all-sky survey telescopes may be able to detect more, depending on the underlying astrophysics.
A Hubble Space Telescope image of a Luminous Fast Blue Optical Transient (LFBOT) designated AT2023fhn, indicated by pointers. It shines intensely in blue light and evolves rapidly, reaching peak brightness and fading again in a matter of days, unlike supernovae which take weeks or months to dim. Only a handful of previous LFBOTs have been discovered since 2018. The surprise is that this latest transient, seen in 2023, lies at a large offset from both the barred spiral galaxy at right and the dwarf galaxy to the upper left. Only Hubble could pinpoint its location. And, the results are leaving astronomers even more confounded because all previous LFBOTs have been found in star-forming regions in the spiral arms of galaxies. It’s not clear what astronomical event would trigger such a blast far outside of a galaxy. Credit: NASA, ESA, STScI, A. Chrimes (Radboud University)
The Isaac Newton Group of Telescopes (ING) and the WEAVE instrument team present first-light observations with the WEAVE spectrograph. WEAVE is a powerful, next-generation multi-fibre spectrograph on the William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos (La Palma, Canary Islands), now being commissioned on-sky and already generating high-quality data.
First-light observations were carried out with its large integral-field unit (LIFU) fibre bundle, one of WEAVE’s three fibre systems. When using the LIFU, 547 closely packed optical fibres transmit the light in a hexagonal area of the sky to the spectrograph, where it is analysed and recorded.
The LIFU was aimed at NGC 7318a and NGC 7318b, two galaxies at the heart of Stephan’s Quintet, a group of interacting galaxies. It’s been observed with Hubble, Spitzer, Chandra and many other telescopes, and recently by JWST. It’s also famous for its cinematic role in the 1946 Christmas film It’s a Wonderful Life. The group, 280 million light-years from Earth in the constellation Pegasus, is undergoing a major galaxy collision and provides a natural laboratory for the consequences of galaxy collisions on the evolution of galaxies.
Marc Balcells, ING Director, explains: “Our goal was to host a unique instrument that would allow astronomers to carry out cutting-edge astronomical research. It’s been fantastic to receive generous financial support from the national research agencies in the three partner countries, as well as contributions from non-ING countries. We are now happy to show that the LIFU part of WEAVE not only works, but produces high-quality data. The ING telescopes will continue to provide results of high scientific impact for years to come. We expect soon to announce subsequent first-light events for the other observing modes, now undergoing their own final tuning and calibration.”
Gavin Dalton, WEAVE’s Principal Investigator said, “The wealth of complexity revealed in this way by a single detailed observation of this pair of nearby galaxies provides insights into the interpretation of the many millions of spectra that WEAVE will obtain from galaxies in the distant Universe, and provides an excellent illustration of the power and flexibility of the WEAVE facility.”
Scott Trager, WEAVE’s Project Scientist: “These observations show the power of WEAVE to unravel the complex phenomena involved in the evolution of galaxies throughout the history of the Universe. The more than 500 members of the WEAVE Science Team and the members of the wider ING community will certainly make great discoveries with WEAVE’s exciting new capabilities.”
The William Herschel Telescope with WEAVE. The WEAVE positioner is housed in the 1.8-metre black box above the top-end ring. Optical fibres run along the telescope structure to the enclosure on the left, which houses the WEAVE spectrograph. Credit: Sebastian Kramer
WEAVE First Light: the JWST image with the WEAVE LIFU pointing at Stephan’s Quintet for the first-light observation. The LIFU gathers light from 547 points on the sky for analysis by the WEAVE spectrograph (each circle indicates an optical fibre 2.6 arcseconds in diameter). The observation provides physical information from each separate region of each galaxy as well as their immediate environment, covering 120,000 light years from top to bottom. Credits: NASA, ESA, CSA, STScI (background image); This composite has made use of “Aladin sky atlas” developed at CDS, Strasbourg Observatory, France
About the First-Light Observations
The WEAVE LIFU measures separate spectra for 547 different regions of the two galaxies and their vicinity, recording the colours of their light from the ultraviolet to the near-infrared. These spectra reveal the motions of stars and gas, the chemical composition of the stars, the temperatures and densities of the gas clouds, and more. From these data, astronomers can learn how galaxy collisions transform the galaxies in the group.
WEAVE First Light: Blue, green and red colours, according to velocities derived from the WEAVE spectra, are overlaid on a composite image of Stephan’s Quintet, which features galaxy star light (CFHT telescope), and X-ray emission of hot gas (blue vertical band, Chandra X-ray observatory). The velocities indicate that the centre-left galaxy (NGC 7318b, blue), is a late intruder, entering the group from behind at 800 km/s (nearly 3,000,000 km/h) through the centre of Stephan’s Quintet. This high-speed collision creates havoc in NGC 7318b. Hydrogen gas clouds, the fuel for the formation of new stars, are being stripped off the galaxy. This will likely cause formation of new stars in the galaxy to severely slow down. The WEAVE spectra will help to figure out the fate of the stripped gas as it evolves in the space between the group galaxies. Credits: X-ray (blue): NASA/CXC/CfA/E. O’Sullivan, optical (brown): Canada-France-Hawaii-Telescope/Coelum, WEAVE’s LIFU: WEAVE Team
WEAVE, A Next Generation Spectrograph
WEAVE is a multi-mode, multi-fibre spectrograph installed on the WHT at the Observatorio Roque de los Muchachos (ORM), La Palma in the Canary Islands. WEAVE was built by a consortium of European astronomical institutions, led by the UK Science and Technology Facilities Council to become the next-generation spectroscopic facility for the WHT.
WEAVE uses optical fibres to gather light from celestial sources and transmits it to a two-arm spectrograph. The spectrograph separates the light into its different wavelengths, or colours, and records them on large-format CCD light detectors. The raw data are transferred over the internet to computers in Cambridge and Tenerife, and the science-ready products are stored in an archive on La Palma for scientific use. The resulting spectra contain the fingerprint of physical and chemical properties of stars, galaxies, and interstellar and intergalactic gas, which astronomers use to test their theories about the Universe.
WEAVE’s versatility is one of its biggest strengths. While the LIFU mode hosts 547 fibres closely-packed to image extended areas of the sky, in the MOS mode up to 960 individual fibres can be separately positioned using two robots to gather light from many hundreds of stars, galaxies or quasars. In the mIFU mode, the fibres are organised into 20 units, each consisting of 37 fibres, that are used to study small extended targets such as nebulae and distant galaxies.
WEAVE provides velocities along the line of sight, through the Doppler effect. Depending on the scientific objective, astronomers choose among two spectral resolving powers: at low resolution the spectra distinguish velocity differences of approximately 5 km/s and, at high resolution, 1.2 km/s. Even at its low resolving power, WEAVE records the line-of-sight velocities of stars with accuracies similar to those of the transverse velocities measured by ESA’s Gaia satellite.
Who Will Use WEAVE?
In the coming five years the ING will assign 70% of the time available on the WHT to eight major surveys with WEAVE, selected out of those proposed by the astronomical communities of the partner countries. Together, these surveys will require spectra of several million stars and galaxies, a goal now obtainable thanks to WEAVE’s ability to observe almost 1000 objects at a time.
The ING will also make 30% of the time available for projects selected competitively from those proposed by astronomers in the ING partner countries. These projects will leverage WEAVE’s versatility to provide fast responses to immediate questions. There are also channels for programmes that jointly exploit WEAVE and the diverse capabilities of the telescopes in the Canarian Observatories such as the 10.4-metre Gran Telescopio Canarias.
Eight Surveys with WEAVE
Over 500 astronomers from across Europe have organized eight major surveys using WEAVE, covering studies of stellar evolution, Milky Way science, galaxy evolution and cosmology. In synergy with the European Space Agency’s Gaia satellite, the MOS mode of WEAVE will be used to obtain spectra of several million stars in the disk and the halo of our host galaxy, enabling Milky Way archaeology. Galaxies near and far, some detected by the LOFAR radio telescope will be studied to learn the history of their growth. And quasars will be used as beacons to map the spatial distribution and interaction of gas and galaxies when the Universe was only around 20% of its current age.
WEAVE Funding and Construction
ING initiated plans to build WEAVE after extensive consultation with the ING user community about what was needed for the future. There was a broad consensus that a world-class wide-field multi-object spectrograph was required, to exploit from the ground the huge surveys being undertaken by powerful telescopes such as as ESA’s Gaia, thereby helping to address the main astrophysical challenges foreseen for the next decade or so.
WEAVE First Light: WEAVE spectrograph in the laboratory. Credit: NOVA
In 2016, the countries of the ING partnership (the UK, Spain and the Netherlands), joined by France and Italy, signed an agreement to design and build WEAVE, with each country contributing major components as listed below, and with the ING providing auxiliary systems and overall project management.
The instrument construction team is led by Gavin Dalton from Oxford University as Principal Investigator, Scott Trager from Groningen University as Project Scientist, Don Abrams from ING as Project Manager, and Chris Benn from ING as Instrument Scientist. The main components of WEAVE are:
Fibre–positioner, developed by the University of Oxford and RAL Space in the UK, with support from the IAC.
Prime-focus corrector, designed by ING and SENER, provided by the Instituto de Astrofísica de Canarias (IAC) in Spain and manufactured by SENER. Support from Konkoly Observatory (HU). Lenses were polished by KiwiStar in New Zealand, funded from STFC, NOVA, INAF and ING, and mounted at SENER Aeroespacial (Spain) by SENER and ING.
Spectrograph, built by NOVA in the Netherlands with optical design by RAL Space in the UK, optics manufactured at INAOE (MX) and support from INAF (IT) and the IAC (ES).
Field Rotator, provided by the Instituto de Astrofísica de Canarias (IAC) in Spain and manufactured by IDOM (Spain).
Optical fibres, provided by the Observatoire de Paris in France, manufactured in France, Canada and USA.
LIFU, built by NOVA (NL).
CCD detectors system, provided by Liverpool John Moores University in the UK.
Data processing, analysis and archiving, led by the University of Cambridge (UK) with support from the IAC (ES) and INAF (IT).
Observation control system, built by the ING.
WEAVE’s construction has been funded by the Science and Technology Facilities Council (STFC, UK), the Netherlands Research School for Astronomy (NOVA, NL), the Dutch Science Foundation (NWO, NL), the Isaac Newton Group of Telescope (ING, UK/NL/ES), the Astrophysical Institute of the Canaries (IAC, ES), the Ministry of Economy and Competitiveness (MINECO, ES), the Ministry of Science and Innovation (MCI), the European Regional Development Fund (ERDF), the National Institute for Astrophysics (INAF, IT), the French National Centre for Scientific Research (CNRS, FR), Paris Observatory – University of Paris Science and Letters (FR), Besançon Observatory (FR), Region île de France (FR), Region Franche-Comté (FR), Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE, MX), National Council for Science and Technology (CONACYT, MX), Lund Observatory (SE), Uppsala University (SE), the Leibniz Institute for Astrophysics (AIP,DE), Max-Planck Institute for Astronomy (MPIA, DE), University of Pennsylvania (US), and Konkoly Observatory (HU).
Related publications
Shoko Jin et al., 2022, “The wide-field, multiplexed, spectroscopic facility WEAVE: Survey design, overview, and simulated implementation”, MNRAS, accepted for publication. Paper.
Press release from the Isaac Newton Group of Telescopes – ING
