When you imagine a zombie, you may picture a fictional, rotting human, dragging their
feet and moaning in some state of decomposition. But what if I told you that zombies exist,
in space? What if I told you that when a star dies, it doesn't just disappear, but it begins
a whole new life as a compact object? And what if I told you that these compact objects
are so extreme, and pack so much mass into tiny areas, that they bend time and space
itself? This is what my PhD research is focused on understanding; a specific type of compact
object called a neutron star. A neutron star is a dense remnant of a dead star, but is
about the size of Melbourne, but holds about one and a half times the mass of our Sun.
Now this is pretty hard to imagine, so to put this into perspective: if you were to
go to a neutron star, and grab a handful of material, and bring it back to Earth, on Earth
it would weigh a whopping five billion tonnes! Just that one handful. Now that's about five
Mount Everests. So you can see that neutron stars operate at the extremes of physics;
from the insane densities reached inside of them, to their strong gravitational fields,
to their strong magnetic fields, neutron stars are positively bursting with physics and represent
conditions that we just can't get to on Earth. So I like to think of neutron stars as "space
laboratories" for our scientists. But, there's a problem, as there always is in science:
neutron stars don't emit light from nuclear burning, like a normal star, and they're very
small objects, and space is really big! In fact, we've only found about 2,000 neutron
stars in total, when there should be about a billion in our galaxy alone. So how do we
find them? Well, one of the ways, is we look for their interaction with things around them.
For example, if the neutron star was born in a binary orbit, with a normal star. This
brings me to accreting neutron stars. So these are neutron stars in close, binary orbits,
and because the neutron star has such a strong gravitational field and it's in a very close
orbit with a normal star, it can actually pull material, so like gas, from its companion
star in towards the neutron star. And this gas will spiral in and form a structure called
an accretion disk. And as the gas is spiralling in through the accretion disk, it's actually
heated up, and it emits light and energy. And this is exactly what I'm searching for,
when I'm looking for these space zombies. The light they emit is actually so energetic
that it's primarily emitted in the X-ray portion of the electromagnetic spectrum. But we're
not done yet, these things are even cooler! They can create huge explosions on their surface
that emit more energy than the Sun does in ten thousand years in less than a minute.
These explosions are called thermonuclear X-ray bursts and happen when their accreted
fuel, so the fuel that the neutron star has consumed, builds up on its surface and ignites
in an unstable thermonuclear runaway. Hundreds of nuclear reactions happen in these explosions,
and they're super-interesting to study from a nuclear physics perspective, because a lot
of these nuclear reactions are really difficult to replicate from it. So I bet that you're
all as excited as I am about these star-consuming space zombies, aka "accreting neutron stars."
This brings me to what my PhD research is actually about; the primary goal of my research
is to further understand accreting neutron stars by studying: how their strong gravitational
field affects their environments, the thermonuclear explosions that can occur on their surfaces
and the geometry and structure of the accretion disk and accretion flow. To achieve these
goals, I have used a combination of observations and models. For observations, I've used more
than ten telescopes around the world, and this here is a map of some of those locations
of those telescopes, including some telescopes in space. And the reason we use space telescopes
is because X-rays actually don't pass through the Earth atmosphere, which is lucky for us,
so it means we're not constantly being irradiated by all this high-energy radiation, but it
kind of sucks for someone like me, because it means it's really difficult for us to get
observations of X-rays from things outside of our solar system. On the modelling side
of things, I have developed my own models, as well as using models that are part of larger
efforts. So, to the science! The first exciting thing I discovered, and you actually may have
seen this in the news kind of recently, is that I coordinated observations of the rise
to outburst, so this means the beginning of the accretion process onto a neutron star,
for the first time, in multiple wavelengths. So this means that we were using more than
seven telescopes to watch this thing switching on. Now these things are really hard to catch
when they're in the process of beginning accretion, so with a combination of luck and coordination,
we were able to watch the whole process from where there was no accretion to the beginnings
of the activity, to when the accretion actually happened. On this graph here, shows the light
curve we obtained - so the light curve means that the light that was coming from this accreting
neutron star during this process - and the different coloured lines indicate different
telescopes, which means different wavelengths of light that we were able to obtain. Now
these observations are super important in understanding the accretion disk structure
and the accretion flow to the neutron star. On the modelling side of things, I developed
an accretion model that models the accretion geometry and how the accreted fuel may spread
over the neutron star if it is channelled to the poles based on the magnetic field.
And this means because the neutron star has such a strong magnetic field, the magnetic
field can actually channel some of the accreted fuel to the poles of the neutron star, and
you end up with a "hot spot," where all of the fuel is hitting the one spot on the neutron
star. And we think this could affect the internal structure, going down into the neutron star,
and that's exactly what I modelled. And you can see this rainbow graph here shows an output
of this model where there is a hot spot on the surface and goes deeper into the neutron
star, and you can see that there is a temperature gradient going downwards. I also was able
to develop a simple relation for total energy produced in X-ray bursts depending on the
fuel composition. And in the process I actually discovered that we had been over-estimating
the amount of neutrinos produced in an X-ray burst in all of our models, and this is super-important
for us to get right, because obviously, in order to correctly model the total energy
in an X-ray burst, we need to know how much energy they release as neutrinos. And finally,
I published some public software that predicts the accreting neutron star parameters that
we can't observe by matching observations and X-ray bursts with models. This software
is called BEANS, and that stands for Bayesian Estimation of Accreting Neutron Star parameters.
I was very proud of that acronym! Now this is not all of the research I have done during
my PhD, but it's just a few highlights. So, to summarise, my research has helped us gain
an understanding of how an outburst is triggered, to how the accretion process is triggered
onto the neutron star, as well as providing constraints on the accretion disk structure
and flow, using both observations and models. I've also developed a simple relation for
X-ray burst energy released based on fuel composition and this can be used by both observers
and modellers to further understand accreting neutron star systems. I also published some
public software that can predict unobservable neutron star parameters, mainly mass and radius,
and that understanding the neutron star mass and radius means that this can help us understand
the equation of state of dense matter, so that means how matter behaves at the densities
reached inside a neutron star, because this is not something that we currently know. My
thesis will include six scientific journal articles - I've already published five and
I've got one in the works - and generally this work contributes to our understanding
of dense matter, strong gravitational fields, thermonuclear reactions, and the population
of accreting neutron stars as a whole. So, the next time you look up at the night sky
and see all the beautiful, bright stars, maybe you'll think about the things you can't see,
like the space zombies, lurking out there, consuming stars and, in the process, helping
us understand the universe we live in.