A supernova-like transient was observed to decline stupendously fast. What could have caused it?

Fast Transients

“Fast transients” are objects whose brightness rises and then falls drastically, usually on the order of weeks. They are not regularly varying objects; they have more in common with supernovae, which brighten once and then fade. However, fast transients change more rapidly than supernovae do, suggesting they have different explosive progenitors.

With the advent of large astronomical surveys, fast transients are spotted more often now than ever before. The “fastest” optical fast transient is kilonova AT2017gfo — the result of the first observed binary neutron star merger. A recent study by Owen McBrien (Queen’s University Belfast) and collaborators discusses a transient that’s right on the heels of AT2017gfo in terms of the speed of its variation: a supernova-like object named SN2018kzr.

Magnetars and Nickel

SN2018kzr was discovered independently by the Zwicky Transient Factory and the Asteroid Terrestrial-impact Last Alert System. Observers were tipped off by its rapid brightening, which took place over hours. SN2018kzr was then observed extensively over the next two weeks by multiple observatories, yielding a wealth of photometric and spectroscopic data. It began declining in brightness the same night it was first detected.

To explain SN2018kzr’s rise and fall, McBrien and collaborators consider mechanisms that have previously been used for fast transients. They start with the reasonable assumption that nickel-56 is involved. The isotope nickel-56 can form in the explosions associated with fast transients and supernovae, and its radioactive decay can contribute greatly to a transient’s brightness. However, SN2018kzr dims too rapidly for it to be explained by nickel-56 alone, and the decay of other radioactive isotopes don’t explain observations either.

One solution is to tweak the progenitor scenario to include a massive remnant: a rotating neutron star with a strong magnetic field, known as a magnetar. A magnetar can contribute to the energy put out by SN2018kzr by slowing its own rotation. When coupled with nickel-56 decay, a magnetar’s spin-down could explain the shape of SN2018kzr’s light curve.

Choose Your Scenario

Assuming that nickel-56 and a magnetar are involved in the progenitor of SN2018kzr, the authors present three possible scenarios: the core-collapse of a helium-rich star, the collapse of a white dwarf that’s accreted too much matter (accretion induced collapse, or AIC), and the merger of a white dwarf and a neutron star.

The first scenario isn’t favored since any remnant it produces wouldn’t rotate fast enough to explain SN2018kzr’s rapid decline. The authors favor the other scenarios, though the AIC model is on shaky ground based on previous studies.

The more fast transients we discover, the better our understanding becomes of how they form. Stay tuned!

Citation

“SN2018kzr: a rapidly declining transient from the destruction of a white dwarf,” Owen R. McBrien et al 2019 ApJL 885 L23. https://doi.org/10.3847/2041-8213/ab4dae