Wednesday, February 15, 2023

Spherical symmetry in the kilonova AT2017gfo/GW170817

 Nature (via arXiv):

The mergers of neutron stars expel a heavy-element enriched fireball which can be observed as a kilonova. The kilonova's geometry is a key diagnostic of the merger and is dictated by the properties of ultra-dense matter and the energetics of the collapse to a black hole. 

We conclude that energy injection by radioactive decay is insufficient to make the ejecta spherical. A magnetar wind or jet from the black hole disk could inject enough energy to induce a more spherical distribution in the overall ejecta, however an additional process seems necessary to make the element distribution uniform.

Kilonovae are one of few objects that allow a relatively direct asymmetry measurement, via the expanding photospheres method. Normally, this method uses information about the expanding photosphere to fix the distance; however, since we can measure the explosion time very precisely from the gravitational wave, and we know the distance fairly accurately by measuring the distance to the host galaxy, we can use this method to work backwards to measure a transverse velocity of the ejecta. Comparing the two velocities gives a direct measurement of the asymmetry of the expansion.

For every epoch we find a line shape that is consistent with a completely spherical expansion to within a few percent. These line-shape constraints are independent of the EPM measurements and verify the spherical nature of the kilonova at early epochs. 

This is pretty interesting. Current GRMHD simulations show that asymmetry is usually favored in explosions like these. Spherical symmetry is of course possible, but it is not a generic attribute, and we would not expect to see it very frequently. Understanding what kind of mechanism could generate spherical symmetry in an explosion that is typically asymmetric is an important problem.

Energy may also be injected in an anisotropic fashion as a relativistic wind from the remnant neutron star or black hole by tapping the rotational energy of the system. Within the first few seconds of the explosion a polar outflow could be launched and produce a rapidly-expanding balloon of high-Ye material with low opacity, dominated by elements like Sr. This polar outflow, outpacing the equatorial ejecta, would expand sideways, covering the low-Ye material and providing a near-spherical kilonova.


The colossal explosion resulting from a merger between two neutron stars has an unexpectedly perfect shape...The spherical explosion they actually found suggests that our understanding of neutron star mergers is lacking.

"You have two super-compact stars that orbit each other 100 times a second before collapsing. Our intuition, and all previous models, say that the explosion cloud created by the collision must have a flattened and rather asymmetrical shape," Sneppen says.

"The most likely way to make the explosion spherical is if a huge amount of energy blows out from the center of the explosion and smooths out a shape that would otherwise be asymmetrical. So the spherical shape tells us that there is probably a lot of energy in the core of the collision, which was unforeseen."

However, this theory does not explain another aspect of the researchers' discovery. According to the previous models, while all elements produced are heavier than iron, the extremely heavy elements, such as gold or uranium, should be created in different places in the kilonova than the lighter elements such as strontium or krypton, and they should be expelled in different directions. The researchers, on the other hand, detect only the lighter elements, and they are distributed evenly in space.

They therefore believe that the enigmatic elementary particles, neutrinos, about which much is still unknown, also play a key role in the phenomenon.


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