ASASSN-15lh is the brightest supernova discovered so far

The galaxy that hosts ASASSN-15lh before its explosion taken by the Dark Energy Camera (DECam) [Left], and the supernova by the Las Cumbres Observatory Global Telescope Network (LCOGT) 1-meter telescope network [Right] (Image courtesy The Dark Energy Survey, B. Shappee and the ASAS-SN team))
The galaxy that hosts ASASSN-15lh before its explosion taken by the Dark Energy Camera (DECam) [Left], and the supernova by the Las Cumbres Observatory Global Telescope Network (LCOGT) 1-meter telescope network [Right] (Image courtesy The Dark Energy Survey, B. Shappee and the ASAS-SN team))
An article published in the journal “Science” describes the discovery of the supernova ASASSN-15lh, the brightest discovered so far. A team of astronomers led by Subo Dong, of the Kavli Institute for Astronomy and Astrophysics at Peking University, China, studied this explosion that is extraordinary even by the standards of these events: it’s more than twice as bright as the one that held the record, about 200 times brighter than the average supernova, 570 billion times brighter than the Sun, and 20 times brighter than all the stars in the Milky Way put together.

When a star with a certain mass reaches the end of its normal life the result is a supernova but some of them are called superluminous or hypernovae because they are much brighter than the common ones. This is the case of ASASSN-15lh, discovered by the automated network of telescopes scattered across the world in an international collaboration called ASAS-SN (All Sky Automated Survey for SuperNovae).

The news of the discovery of the hypernova ASASSN-15lh came on June 14, 2015 and at that point various telescopes started observing this supernova. Its estimated distance is about 3.8 billion light years from Earth but if its distance was the same as Sirius, 8.6 light years, now it would look to as almost brilliant as the Sun.

Finding an explanation for the really out of the ordinary brightness of ASASSN-15lh is difficult and Subo Dong admits that we don’t know what source of energy might power it. The scientist speculated that the star that exploded was particularly massive, of a very rare type with a mass hundreds of times the Sun’s. The problem is that in that case the spectral analysis should show the presence of the nickel created in its core.

Todd Thompson, a professor of astronomy at Ohio State University, proposed the hypothesis that the supernova generated a magnetar, a neutron star with an incredibly powerful magnetic field that spins more than a thousand times per second. This seems to go beyond the limits possible for a magnetar but those are extreme objects so that hypothesis seems the most plausible.

ASASSN-15lh is certainly a very interesting object from the scientific point of view so it will be studied again with various telescopes, including the Hubble Space Telescope. Phenomena of this type are short-term so the scientists involved in this research will try to make the largest number of observations with different instruments to solve this mystery.

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