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Burning on Accreting Compact Objects

Periodic Reporting for period 1 - BACCO (Burning on Accreting Compact Objects)

Reporting period: 2017-04-01 to 2019-03-31

"Neutron stars (or ""pulsars"") contain the densest stable form of matter
that we can see in the Universe. In this MSCA project, BACCO, we have
studied how thermonuclear burning proceeds on their surface, at
different accretion rates. Our new approach compares thermonuclear
flashes on the surface of accreting neutron stars and white dwarfs,
from a theoretical as well as observational view point.

We have also tackled in the course of this project one of the most
fundamental and long-lasting questions in the history of neutron star
research: what is the maximum mass they can reach? This is important
because it tells us how particles interact in their cores, in a
physical state of matter that is not accessible from laboratories on
We compiled a catalog of recurrence times of recurrent novae (RNe),
in order to compare bursts from neutron stars at the lowest mass
accretion rates with recurrent novae from accreting white dwarfs. This
allows us to compare quantitatively the recurrence times of
thermonuclear explosions on neutron stars and white dwarfs.

We have quantified the dependence of helium ignition on neutron star
mass. We built and analized helium ignition models for neutron star
masses of 1.2 1.4 1.8 and 2.2 Solar masses, over a wide range of
mass accretion rates. The results show a subtle dependence of
recurrence time on neutron star mass, not sufficient to explain the
shortest recurrence times (substantially below one hour).

We incorporated to BACCO the search and characterization of new
millisecond pulsars in compact binary systems.

We developed a new method to measure pulsar masses using optical
observations. We combined optical spectroscopy with detailed modelling
of an extremely irradiated companion to a millisecond pulsar, using
the largest optical telescope on Earth.
"Dynamical modelling of the new redback pulsar candidate 3FGL
J0212.1+5320: we successfully reproduced the clearly asymmetric
optical light curves, and found evidence for a 1.85 Solar-mass pulsar
(undetected as of yet).

Discovery of the optical & X-ray counterparts and the orbital period
of PSR J1306-40: we showed that the X-ray and optical flux is
modulated with the same 1.09716 days period, establishing this as a
new compact binary millisecond pulsar.

Discovery of a 2.3 Solar-mass neutron star: we have found one of the
most massive neutron stars known to date in the ""redback"" millisecond
pulsar PSR J2215+5135, using a new method to measure the velocity of
both sides of the companion star.

These results have been presented in international scientific
conferences, and to the broader public by means of press releases,
public talks and radio interviews.