Highly-magnetic star could have boosted brightness of super-luminous supernovae

Rapidly spinning neutron stars called magnetars could explain two extremely unusual stellar explosions

An artist's impression of a high-magnetic star known as a magnetar. Image Credit: ESA

An artist’s impression of a highly-magnetic star known as a magnetar. Image Credit: ESA

Calculations by scientists have found highly magnetised, rapidly spinning neutron stars called magnetars could explain the energy source behind two extremely unusual stellar explosions.

Stellar explosions known as supernovae usually shine a billion times brighter than the Sun. Super-luminous supernovae (SLSNe) are a relatively new and rare class of stellar explosions, 10 to 100 times brighter than normal supernovae. But the energy source of their super-luminosity, and explosion mechanisms are a mystery and remain controversial amongst scientists.

A group of researchers led by Melina Bersten, an Instituto de Astrofisica de La Plata researcher and affiliate member of Kavli IPMU, and including Kavli IPMU Principal Investigator Ken’ichi Nomoto, tested a model that suggests that the energy to power the luminosity of two recently discovered SLSNe, SN 2011kl and ASASSN-15lh, is mainly due to the rotational energy lost by a newly born magnetar.

“These supernovae can be found in very distant universe, thus possibly informing us the properties of the first stars of the universe,” says Nomoto.

Super-luminous supernovae such as ASASSN-15lh (pictured)

Super-luminous supernovae such as ASASSN-15lh (pictured) usually shine 10 to 100 times brighter than standard supernovae. Image Credit: NASA

Interestingly, both explosions were found to be extreme cases of SLSNe. First, SN 2011kl was discovered in 2011 and is the first supernovae to have an ultra long gamma-ray burst that lasted several hours, whereas typical long-duration gamma-ray bursts fade in a matter of minutes. The second, ASASSN-15lh, was discovered in 2015 and is possibly the most luminous and powerful explosion ever seen, more than 500 times brighter than normal supernovae. For more than a month its luminosity was 20 times brighter than the whole Milky Way galaxy.

The team performed numerical calculations to explore the magnetar hypothesis, and found both SLSNe could be understood in the framework of magnetar-powered supernovae. In particular, for ASASSN-15lh, they were able to find a magnetar source with physically allowed properties of magnetic field strength and rotation period. The solution avoided the prohibited realm of neutron star spins that would cause the object to breakup due to centrifugal forces.

“These two extreme super-luminous supernovae put to the test our knowledge of stellar explosions,” says Bersten.

To confirm the team’s calculations, further observations would need to be carried out when the material ejected by the supernova is expected to become thin. The most powerful telescopes, including the NASA/ESA Hubble Space Telescope, will be required for this purpose. If correct, these observations will allow scientists to probe the inner part of an exploding object, and provide new insight on its origin, and evolution of stars in the universe.

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