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Type I bursts in 3D

Periodic Reporting for period 2 - Burst3D (Type I bursts in 3D)

Okres sprawozdawczy: 2018-10-01 do 2019-09-30

Dr Cavecchi runs computer simulations of the Type I Bursts that
take place on neutron stars.

The neutron stars are fascinating objects that result from the
explosion of massive stars, the so called supernovae. What is left of
the core of the original star collapses to form a new object, the
neutron star, which encloses the mass of almost two suns within 10 Km;
the approximate size of a small city.

Due to their high mass in such a small volume, the density of matter
near the centre of the neutron stars exceed even the density found in
the nuclei of atoms on Earth. The physics governing this state of
matter is not well understood and is the subject of theoretical and
experimental work.

The neutron stars offer a great opportunity to study the behaviour of
this matter, but the centres of the stars cannot be observed directly
and we have to reply on other proxies. The Type I Bursts are
thermonuclear explosion on the surface of neutron stars that strip
matter from a companion star. They produce extremely bright X-ray
flashes that makes them ideal to observe the neutron stars.

Dr Cavecchi runs magnetohydrodynamical simulations of these
explosions, trying to understand how the nuclear flame propagates on
the surface of the star. With precise models of the flame propagation,
it is possible to construct synthetic images of the bursts that, once
compared to the observed ones, can inform us about the properties of
the star where they burn. In this way, we can shed light on the
mysterious behaviour of matter in the centre of the neutron stars.

During this action, Dr. Cavecchi showed how to reconcile the theory of
nuclear burning with the observed frequency of ignition of Type I Bursts.
He also showed how the subsequent flame propagates. Contrary to our
previous picture, the flame does not develop with an orderly front: the latter
breaks down into small vortices which make the flame proceed up to 10 times
faster. These vortices may also hold the key to explain the lightcurves during
the cooling phase of the bursts.
During his Marie Sklodowska Curie action, Dr Cavecchi
went to Princeton and to Southampton to work on his 3D simulations
of the flame. The simulations showed a complex behaviour of the flame,
which develops in an unstable fashion forming intricated structures of
cyclones and anticyclones. His results change the picture that was previously
had of the flame propagation: showing that the flame front is bound to fragment
into many vortices. He has also worked on explaining the relation between
the burst explosion frequency and the accretion rate and spin of the star,
showing how to reconcile the observed behaviour and the theoretical
predictions. His results have been published on peer-reviewed journals and
presented at international conferences, scientific schools and public outreach events.

Meanwhile, he has also contributed to theoretical work on the
modelling of oscillations in the fluid burning during the bursts and
to observational papers discussing the cooling of heated neutron stars.
These are the first 3D simulations of flame propagation during the
type I bursts. They show for the first time the development of
instabilities at the flame front.

The way the flame propagates influences the emission pattern. The
latter is what is needed to extract information about the core of the
neutron stars from the observation of their X-ray flashes. This will
help the progress in understanding the physics of matter at the high
densities of neutron star cores.

This Marie Sklodowska Curie action provided for the first time
fundamental knowledge about the 3D development of the flame from which
unambiguously infer the emission pattern.
The vortices in the burning fluid after the front is disrupted
Instability developing at the flame front