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Hypernova SN 2015L is the Brightest and Most Powerful Ever Seen
Posted by Guy Pirro on 5/23/2017 5:30 PM

Images showing the host galaxy before the explosion of SN 2015L on the left and after on the right. SN 2015L is the brightest and most powerful explosion ever seen. Located about three billion light years away, the source appears tremendously bright for anything so far away -- roughly 200 times brighter than an average Supernova and temporarily 20 times brighter than all of the stars in our Milky Way Galaxy combined. Light emitted by SN 2015L at this rate in all directions at once would make it the most powerful explosion yet recorded. No known stellar object was thought to create an explosion this powerful, although pushing the theoretical limits for the spin-down of a highly-magnetized neutron star -- a magnetar -- gets close. (Image Credit: The Dark Energy Survey, B. Shappee, Las Cumbres Observatory, and the ASASSN team)


Records are made to be broken, as the expression goes, but rarely are records left so thoroughly in the dust. In 2015, stunned astronomers witnessed a cosmic explosion about 200 times more powerful than a typical Supernova event. And Supernovae already rank amongst the mightiest outbursts in the universe. This one was more than twice as luminous as the previous record-holding supernova.

At its peak intensity, SN 2015L (originally designated ASASSN-15lh) exhibited 570 billion times the luminosity of the Sun. If that statistic does not impress, consider that this luminosity level is approximately 20 times the entire output of the 100 billion stars in our Milky Way galaxy.

The record breaking blast is thought to be an outstanding example of a Superluminous Supernova (or Hypernova), a recently discovered and supremely rare variety of explosion unleashed by certain stars when they die. Scientists are frankly at a loss to explain what sorts of stars and stellar scenarios might be responsible for these types of extreme events.

SN 2015L is one of the closest Superluminous Supernovae ever observed, at around 3.8 billion light years away. Given its uncanny brightness and closeness, SN 2015L might offer key clues in unlocking the secrets of this baffling class of celestial detonation.

SN2015L "is the most powerful supernova discovered in human history," said Subo Dong, an astronomer and Research Professor at the Kavli Institute for Astronomy and Astrophysics (KIAA) at Peking University in China. "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."

SN 2015L was first glimpsed in June 2015 by twin telescopes with 14 centimeter diameter lenses in Cerro Tololo, Chile while conducting the "All Sky Automated Survey for SuperNovae" (ASASSN), an international collaboration headquartered at The Ohio State University. (Hence ASASSN-15lh's designation). These two tiny telescopes sweep the skies to detect suddenly appearing objects like SN2015L that are intrinsically very bright, but are too far away for human observers to notice.

"ASASSN is the first astronomical project in history to frequently scan the entire optical sky for optical transients," said Krzysztof Stanek, professor of astronomy at the Ohio State University and the co-Principal Investigator of ASASSN. "Every time in science we open up a new discovery in space, exciting findings should follow. The trick is not to miss them."

Dong and colleagues immediately put out word about the sighting of SN 2015L so that as much data as possible could be gathered by astronomers wordwide. Multiple and far larger ground-based telescopes across the globe, as well as NASA's Swift satellite, have since taken part in an intense observing campaign that continues to this day.

In just the first four months after it exploded, so much energy was beamed out of SN 2015L, that it would take our Sun in its current state more than 90 billion years to equal its emissions. By examining this bright, slowly fading afterglow, astronomers have gleaned a few basic clues about the origin of SN 2015L.

Using the 2.5 meter du Pont telescope in Chile, Dong's colleagues Ben Shappee and Nidia Morrell at the Carnegie Observatories in the United States took the first spectrum of SN 2015L to identify the signatures of chemical elements scattered by the explosion. This spectrum puzzled the team members, for it did not resemble any of the spectra from the 200 or so Supernovae the project had discovered to date.

Inspired by suggestions from Jose Prieto at Universidad Diego Portales in Chile, Dong realized that SN 2015L might in fact be a Superluminous Supernova. Dong found a close spectral match for SN 2015L in a 2010 Superluminous Supernova, and if they were indeed of a similar kind, then SN 2015L's distance could be confirmable with additional observations.

Nearly ten days passed as three other telescopes, stymied by bad weather and instrument mishaps, attempted to gather these necessary spectra. Finally, Dong's colleague Saurabh Jha at Rutgers University in New Jersey was able to use the 10 meter Southern African Large Telescope (SALT) to secure the observations of elemental signatures verifying SN 2015L's distance and extreme potency.

"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," said Dong, who had received word of the SALT results at 2:00 AM in Beijing on July 1, 2015.

The ongoing observations have further revealed that SN 2015L bears certain features consistent with "Hydrogen-poor" (Type I) Superluminous Supernovae, which are one of the two main types of these epic explosions so named for lacking signatures of the chemical element hydrogen in their spectra. SN 2015L has likewise shown a rate of temperature decrease and radius expansion similar to some previously discovered Type I Superluminous Supernova.

Yet in other ways, besides its brute power, SN 2015L stands apart. It is way hotter, and not just brighter, than its Supernova kin. The galaxy it calls home is also without precedent. Type I Superluminous Supernova seen to date have all burst forth in dim galaxies that are smaller in size and churn out stars much faster than the Milky Way.

Noticing the pattern, astronomers hoped this specific sort of galactic environment had something to do with Superluminous Supernovae, either in the creation of the exotic stars that spawn them or in setting these stars off. Exceptionally, however, SN 2015L's galaxy appears even bigger and brighter than the Milky Way. On the other hand, SN 2015L might in fact reside in an as yet unseen, small, faint neighboring galaxy of its presumed, larger galactic home.

To clear up where exactly SN 2015L is located, as well as numerous other mysteries, the research team was granted valuable time on the Hubble Space Telescope. With Hubble, Dong and colleagues will obtain the most detailed views yet of the aftermath of SN 2015L's stunning explosion. Important insights into the true wellspring of its power should then come to light.

One of the best hypotheses is that Superluminous Supernovae's stupendous energy comes from highly magnetized, rapidly spinning neutron stars called magnetars, which are the leftover, hyper-compressed cores of massive, exploded stars. But SN 2015L is so potent that this compelling magnetar scenario just falls short of the required energies. Instead, SN 2015L-like supernovae might be triggered by the demise of incredibly massive stars that go beyond the top tier of masses most astronomers would speculate are even attainable.

"The honest answer is at this point that we do not know what could be the power source for ASASSN-15lh," said Dong. "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."

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