A means to portray the differences between the properties of two objects in our everyday speech is the use of comparative adjectives, and their superlative counterparts have become a staple of astronomical nomenclature, with newly discovered cosmic objects often described as the “largest,” “brightest,” or “most energetic” ever seen. A new discovery of a gargantuan hypernova explosion at a distant galaxy is now set to give new meaning to these comparative expressions by setting new standards to how powerful and energetic these cataclysmic events can be and at the same time pose a new stellar mystery as to the nature and origins of some of the most violent astrophysical phenomena in the Universe.
Often described as being the most powerful cosmic explosions since the Big Bang itself, which set the Universe in motion more than 13.7 billion years ago, hypernovae, otherwise known as “super-luminous supernovae,” represent the extreme end of the scale of stellar evolution and death which is characterised by violently explosive events. At the lower end of this scale are novae, which are caused by the cataclysmic nuclear explosions that occur at the surfaces of dead stellar remnants known as white dwarfs, while the latter accrete material from neighboring companion stars within stellar binary systems. The more well-known supernovae occupy the next step in the ladder and signify the violent death of stars more massive than the Sun. Best known for their brightness, supernova explosions can briefly outshine an entire galaxy at their peak and can be seen across cosmological distances out to the edge of the observable Universe. Yet, at the upper end of explosive stellar prowess, hypernovae can be seen as “supernovae on overdrive.” With energies that can be hundreds or even thousands of times greater than that of a typical supernova explosion, hypervovae are true monsters of the stellar kind—whereas a supernova simply expels all of the outer layers of a star at least eight times more massive than the Sun into space, leaving its exposed hot and dense core behind, hypernovae explosions occur in stars with a mass at least 20 times that of the Sun, while tearing apart the remnant core of the star itself as well. And contrary to the much more studied supernovae whose origins and physical processes are well-understood, the causes behind the much more rare hypernovae explosions remain more or less a mystery to this date.
Adding to this intriguing astrophysical enigma, an international team of astronomers reported earlier this week the discovery of such a colossal hypernova, whose energy output stretches what is physically possible from these violent phenomena to the limit and holds the distinction for the most powerful one of its kind to ever been observed. Named “ASASSN-15lh,” this new addition to the zoo of hyper-giant stellar monsters, which was reported at the journal Science, was first spotted on the night of 14 June 2015 by the All Sky Automated Survey for SuperNovae project, or ASAS-SN for short, which comprises a global network of eight 14-cm telescopes deployed in two pairs of four as part of the Las Cumbres Observatory Global Telescope Network in the U.S. and Chile respectively. Having seen first light in 2013, the Ohio State University-based ASAS-SN project is devoted to the survey of the entire sky which it completes every two to three days, in the search for new supernovae and other transient, variable stellar objects. The modest, $1 million-worth project, which is operated by a team of both professional and amateur astronomers, has already discovered an impressive total of 278 supernovae on both hemispheres to date, as well as other variable and transient stellar events.
Soon after ASASSN-15lh was first spotted, the automated sky survey’s computer software triggered its pre-programmed detection alert to a team of astronomers led by Dr. Subo Dong, a professor at the Kavli Institute for Astronomy and Astrophysics at Peking University in China. Appearing as just an inconspicuous dot of light in the sky, ASASSN-15lh didn’t grab the researchers attention at first. It wasn’t until a spectrum of the newly found object was obtained with the 2.5-meter du Pont telescope in Chile a few days later which indicated that ASASSN-15lh was a supernova and a record-setting one at that. These follow-up observations, coupled with additional ones with the 10-meter Southern African Large Telescope, or SALT, in South Africa as well as NASA’s Swift X-ray telescope, further baffled Dong’s team for they revealed that ASASSN-15lh’s spectrum didn’t match any of the more than 250 supernovae that had been previously detected by the ASAS-SN project. The only one that fit was a super-luminous supernova that had been independently discovered back in 2010. Nevertheless, ASASSN-15lh did seem to exhibit similar characteristics with a certain type of hypernovae called Type I, which lack any hydrogen or helium emission lines in their spectra.
To complicate things further, ASASSN-15lh was also found to lie inside a host galaxy even more massive and bright than the Milky Way. At face value that wouldn’t seem like an odd thing, save for the fact that all of the other super-luminous supernovae that have been discovered to date have been found inside very small and faint galaxies. In addition, the spectral observations with the 10-meter SALT telescope, helped to determine that ASASSN-15lh was located an impressive 3.8 billion light-years away. By observing its light curve in the weeks following its discovery, Dong’s team calculated that ASASSN-15lh had exploded with a jaw-dropping force that was approximately 200 times greater than that of a typical supernova and twice that of previously known hypernovae, making it the most powerful and luminous one ever to have been detected. To put these numbers in perspective, ASASSN-15lh’s energy output at its peak was 570 billion times that of the Sun and some 20 times the output of the entire Milky Way galaxy! “Upon seeing the spectral signatures from SALT and realizing that we had discovered the most powerful supernova yet, I was too excited to sleep the rest of the night,” comments Dong, who got word of the SALT observations during the wee hours on July 1.
Having been established as an absolute record-smasher in regards to stellar explosions, ASASSN-15lh seems to defy almost every known explanation as to the origin of its tremendous power source. One of the leading explanations among astronomers is that behind ASASSN-15lh’s extreme luminosity lies a rapidly-spinning magnetar. The latter are a distinct sub-class of neutron stars, which are the stellar left-overs of powerful supernova explosions. Even though they result from the same cataclysmic events as neutron stars, magnetars are instead surrounded by magnetic fields of such brute force, that is hundreds of trillions of times that of the Earth’s internal magnetic field. As the theory goes, the sheer force of a magnetar stellar core that would happen to rotate thousands of times every second would be enough to power a super-luminous supernova explosion, the likes of which have already been observed in the past. Yet, even that can’t quite explain the origin of ASASSN-15lh, for the latter’s observed energy output would require a magnetar that would be able to convert 100 percent of its rotational energy to visible light. “We have to ask, how is that even possible?” says Dr. Krzysztof Stanek, a professor of astronomy at Ohio State University and member of Dong’s team. “It takes a lot of energy to shine that bright, and that energy has to come from somewhere. If it really is a magnetar, it’s as if nature took everything we know about magnetars and turned it up to 11.”
So, for now astronomers are left with a nagging mystery at their hands. “ASASSN-15lh is the most powerful supernova discovered in human history,” says Dong. “The explosion’s mechanism and power source remain shrouded in mystery because all known theories meet serious challenges in explaining the immense amount of energy ASASSN-15lh has radiated.”
“The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh,” adds Stanek.
To shine more light into the matter, Dong’s team has already been granted observing time on the Hubble Space Telescope later this year, which could help solve some parts of the mystery, like ASASSN-15lh exact whereabouts. Even though initial observations showed that the newly found hypernova seems to be located inside a massive galaxy, it may turn out that it is nested inside a neighboring dimmer one, which would be more consistent with the handful of other hypernovae cases that have been discovered in previous years. Furthermore, if it is determined that ASASSN-15lh lies at the center of its host galaxy, it might turn out that it is related to the antics of a supermassive black hole, the likes of which reside at the centers of most galaxies, than a magnetar.
“ASASSN-15lh may lead to new thinking and new observations of the whole class of superluminous supernova, and we look forward to plenty more of both in the years ahead,” says Dong.
Shinning brightly across the Cosmos, ASASSN-15lh represents just one more compelling target for the next generation of space-based observatories like the James Webb Space Telescope, which is rapidly coming together at NASA’s Goddard Space Flight Center in Greenbelt, Md., for its rendezvous with history in October 2018.