Details have been released of the most powerful supernova ever recorded. Astronomers hope the study of ASASSN-15lh will advance our understanding of super-luminous supernovae (SLSNs), astonishing explosions that manage to exceed even regular supernovae in the quantity of energy released in a single enormous burst.
ASASSN-15lh is a freak even among supernovae, twice as bright as the previous record held. It released in four months about ten times as much energy as the Sun will emit in its lifetime. This has left astronomers scratching their heads about what processes could cause such an event. Explanations for previous over the top supernovae don't predict anything this big.
Type Ia supernovae, formed when white dwarfs gain material from a neighboring star, have a fairly consistent intrinsic brightness, making them exceptionally useful for measuring the scale of the universe. Supernovae that occur when the cores of giant stars collapse are more varied in their brightness, but still tend to release similar amounts of energy within a factor of ten. Fewer than one supernovae in a thousand stands out by being a hundred times brighter, and scientists are keen to understand these exceptions. We have seen so few of these, however, and at such great distances, that we know little about them.
The All Sky Automated Survey for Supernovae team (ASASSN) scans the sky with a network of 14-centimeter (5.5-inch) telescopes seeking supernovae of any type. Despite being a size more commonly used by amateurs than professionals, the telescopes are sensitive enough to pick up normal supernovae within 350 million light-years.
“On June 14, we spotted a newly occurring explosion in a galaxy of an unknown distance," Carnegie Observatory's Dr. Benjamin Shappee said in a statement. "Subsequent observations – including those made at our Las Campanas Observatory by Nidia Morrell and Ian Thompson – allowed the team to confirm the existence of the supernova ASASSN-15lh."
Large telescopes measured the distance to the galaxy in which it occurred as 3.8 billion light-years, allowing astronomers to calculate its true power.
Despite its enormous distance, ASASSN-15lh is one of the closest SLSNs yet observed, enhancing our prospects of observing it well enough to explain it.
A false color image of ASASSN-15lh's galaxy pre-explosion and some foreground stars, and one taken as the supernovae was fading. Credit:Benjamin Shappee
Besides its brightness, and unusual absence of hydrogen and helium spectra, ASASSN-15lh has other exceptional features. Most super-luminous supernova occur in dwarf galaxies that are not very luminous, but are actively forming abundant new stars. ASASSN-15lh's suspected galaxy is brighter than the Milky Way and shows little sign of star formation.
Total radiated energy over a four month period is estimated at 1.1x1045 Joules, and at its peak it outshone the entire Milky Way 20 to 50 times.
The favored explanation for SLSNs has been that they are powered by neutron stars with exceptionally powerful magnetic fields, known as magnetars, and these fields' spin feeds extra energy into the explosion.
However, in Science, Shappee and 28 other authors argue that ASASSN-15lh throws this theory into doubt. "The astounding amount of energy released by this supernova strains the magnetar-formation theory," Shappee said. The rate of decline in brightness was also too rapid for magnetar explosion models.
"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," said first author Professor Subo Dong of Peking University in a statement.
Time has been booked on the Hubble telescope this year to see what more we can learn.
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