hello my name is Andrew Norton I'm in
the school of physical sciences at the
Open University and today I'm going to
give another talk about an area of
astrophysics that I work on so let me
just share my slides so the talk that
I'm going to give is called outbursts
orbits and oscillations it's all about
the time-domain astrophysics the compact
accreting binary stars and I'll explain
just what that means shortly but
basically I'm going to be talking about
white dwarfs and neutron stars and black
holes and many of the images such as the
one I've got on the screen here are by a
very talented space artist called mark
garlic and I would encourage you to take
a look at his website space art co dot
uk'
if you want to see more of this sort of
thing so let's think about how we do
astronomy then and in general terms when
we're trying to study objects out there
in space all we have to go on is the
light that they emit and that we detect
here on earth now when I say light I
might mean any part of the
electromagnetic spectrum and I'll come
on to that shortly but the simplest
thing we tend to think of is images such
as the ones shown here the images I've
got these are all I think these are all
Hubble Space Telescope images so at the
top we've got a supernova remnant and a
so-called planetary nebula and at the
bottom we've got a star cluster and a
star forming region now these extended
objects are all well and good and by
examining the structure we can
understand a great deal about what's
going on in these regions of space from
the distribution of the material its
colors tell us something about the
temperature or the composition and so on
but often what we have to deal with in
astronomy are just a point source of
light a star for most stars they're so
far away we can't resolve them into
anything like an extended disc we just
see a point of light so we have to study
these points of light in a different way
and the ways we can do that are by
looking at the spectrum of the light how
its distributed with wavelength or
frequency and we can look at the time
series of that like how the brightness
if you like there
with time and that's going to be a
particular focus of some of the things
I'm going to talk about in this talk so
I'll start off by talking about compact
objects and what we mean by that and the
modes of accretion how these compact
objects accrete material accumulate
material from a companion star in a
binary system I'll then talk about
various categories of objects novae
bursters I'll talk about dwarf Nobi and
soft transience soft x-ray transient
sorry and I'll talk about things called
intermediate Polar's and x-ray pulsars
now I'll explain all of these as we go
along so first of all then let's think
about these compact objects white dwarfs
neutron stars and black holes these are
the end points in the evolution of stars
so a star like the Sun will end its life
as a white dwarf a star that starts off
with a mass maybe 10 times that of the
Sun when it ends its life it will turn
into a neutron star and the very most
massive stars will end their lives
collapsing to become black holes so when
we see these stellar mass compact
objects out there in the galaxy we see
white dwarfs with masses up to about 1.4
times the mass of the Sun that mass
limit is called the Chandrasekhar limit
a white dwarf any more massive than that
would collapse on itself and in fact
turn into a neutron star the upper limit
for the mass of a neutron star is round
about two and a half solar masses we
don't know precisely what that value is
but it's a limit known as the
Oppenheimer volkov limit and compact
objects more massive than that have no
other way of existing other than as a
black hole now you may wonder why these
masses don't quite mass what I said a
moment ago about the types of stars that
turn into these objects that's because
during their their death throes if you
like stars expel a large amount of mass
so even though a star may have six seven
eight times the mass of the Sun to begin
with as it dies it sheds a lot of that
mass and collapses down to become a
white dwarf less than the mass of the
Sun typically
now when we talk about the radii the
sizes of these compact objects you can
see they really are very small indeed
that's why we call them compact objects
white dwarfs typically have a radius
similar to that of the earth and as
shown in the little graph at the top you
can see that more massive white dwarfs
actually have smaller radii that's a bit
counterintuitive but that's just the way
this material that white dwarfs are made
off known as electron electron
degenerate material the way that it
behaves similarly neutron stars are also
slightly smaller as they get rather more
massive they're supported by something
called neutron degeneracy pressure but
they typically all have radii around
about ten kilometres the radii of these
compact black holes stellar-mass black
holes are also round about ten
kilometres broadly speaking a black hole
with a mass equal to that of the Sun one
solar mass would have a radius of about
three kilometres something three times
as massive would have a radius of about
nine or ten kilometers that's sort of
that sort of size so you can see these
are very small objects indeed compared
to regular stars now in binary stars
where these compact objects are
recruiting from a companion there are
broadly speaking two ways in which they
can do that and here's a diagram of one
of these modes of accretion the compact
object is the white object sitting over
here in the centre of this so-called
accretion disk which is a flattened
swirling amount of material that is
transferred from the companion star
swirls around the compact object before
retreating onto it the companion star
itself shown on the left is this object
that's distorted into a sort of pear
shape by the extremely strong gravity of
the compact object and when material on
this companion star reaches this
so-called inner legrangian point the the
point of the pear shape if you like it
can be transferred down onto the compact
object through a stream and an accretion
disk as you see here this mode of
accretion then is known as Roche lobe
overflow
and tends to occur when we when we have
a relatively low mass companion start to
the compact object something that's
comparable to the Sun or perhaps even
less massive than that the other mode of
accretion is shown in this nice image
and this is wind accretion sometimes
called Bondi oil accretion where we have
a giant companion star that has a really
strong stellar wind and as the compact
object perhaps in this case a neutron
star orbits around this this giant star
it can sweep up some of that wind and
again accrete it down onto its surface
now in what follows I'm going to be
talking about different parts of the
electromagnetic spectrum I'll take I'll
principally be talking about visible
that's optical light the light that we
detect with our with our eyes and as you
can see from this diagram of the
electromagnetic spectrum visible light
just occupies a very narrow region a
very small range of wavelengths or
frequencies around about the middle of
this whole spectrum at longer
wavelengths we have the infrared
microwave and radio wave regions but at
shorter wavelengths or higher
frequencies we have the ultraviolet
x-ray and gamma-ray regions and in what
follows I'm going to be talking
particularly about x-ray emission from
these compact interacting binary stars
because in the extreme gravitational
fields the extreme temperature
environments of these systems the
material is hot enough that it emits
x-rays and that's then how we can study
them by looking at the x-rays they emit
which we detect here with x-ray
telescopes which are usually on
satellites in orbit around the Earth
what I'm going to show you now is a
movie of the x-ray sky so this is a
movie that was compiled with a satellite
called the x-ray timing Explorer in the
late 1990s this is a schematic map then
of the sky in galactic coordinates so
the plane of the galaxy runs across the
middle of the image the center of our
galaxy is in the middle and the little
colored circles are different sources of
x-rays x-ray stars the size of the
circle just indicates the brightness in
tres how much how many x-rays we detect
from them and the color is an indication
of the temperature hotter temperature
objects are shown in a bluer color
cooler objects you know in a retic color
and you can see various objects are
identified here in this diagram the
movie will start shortly and at the
bottom of the map is a zoom in on this
very central regions of our galaxy where
many of these objects are much much
closer together and so we've just done a
zoom in there at the bottom so that you
can pick them out so here we go then the
movie is just about to start any moment
now here we go so you see the clock
ticking away in the upper right so it I
think it's four days of real time for
every second of the movie and you can
see that the x-ray sky is a very dynamic
place these x-ray sources are flaring up
fading away getting brighter and fainter
changing temperature maybe as they do so
they remain in the same place of course
because they're relatively fixed within
the galaxy compared to the position of
our solar system that the moving spot on
this map which has just disappeared off
one side and just appeared on the other
side is the position of the Sun in the
sky of course the Sun isn't really
moving it's the earth moving around the
Sun that causes the Sun to appear
projected against a different part of
the sky but you can see then these
various x-ray sources across our galaxy
flaring up and fading away as time
progresses and different ones are
identified many of these x-ray sources
are just identified by some of their
coordinates these numerical names but
others such as the one that's just
popped up there SMC x1 that's an x-ray
source in the small Magellanic Cloud one
of the satellite galaxies associated
with our Milky Way you can see various
of these objects pop up and get labeled
many of them it turns out have neutron
stars as part of their composition some
of them have black holes you can see
that some of these are labeled as
recurrent transients or recurrent Nova
I'll explain just what these are as we
go along in the later parts of this talk
okay I think you've probably got the the
idea of that now so I will move on to my
next slide to give you an idea of the
scale of these so-called x-ray binary
stars the diagram here shows that so at
the top of the diagram is an arrow
showing the distance from the Sun to
Mercury so that's the scale of our solar
system on the inner parts of our solar
system if you like and with the Sun
drawn to scale as well so you can see
that in many of these systems that go by
various names here LMC x 3 ln c x1v 1357
signe and so on the companion star the
the donor star in the binary is rather
bigger than the Sun in some of the
others like this one down the bottom v
fide 1:8 Persei for instance the donor
star is rather smaller than the Sun but
nonetheless that the scale of the system
the distance between the the accretion
disk and the donor star is very compact
they would all fit in between the Sun
and Mercury in our solar system and some
of these smaller ones would even fit
sort of in between the earth and the
moon perhaps they're very compact very
close binaries and so they tend to have
orbital periods the two stars orbiting
around each other which are measured in
days or even hours in some cases so
they're very compact systems and that's
important to remember so there's a huge
range of these different types of
compact binary stars and I've attempted
to describe them here in this taxonomy
chart now don't worry about the details
of this I'm going to look at some of the
specific objects as we go through but
broadly speaking the ones in red are
systems where we have a white dwarf the
systems in yellow are ones we have a
neutron where we have a neutral
star and the systems in blue are the
ones where we have a black hole that's
creating material so if I start then
with one of the red ones one like the
white dwarf systems and this is type of
system known as Novi so no they look
something like this they have a system
where a small low mass red dwarf star is
distorted into this sort of pear shape
and is its material from it is being
drawn down onto a white dwarf which sits
down here at the center of this
flattened accretion disk so nova
outbursts then look something like this
this is the light curve of nova
sagittario 2012 so this is an object
that underwent an outburst in 2012 star
in the constellation of Sagittarius and
what we're plotting here is a graph
showing just the brightness of the star
as a function of time and as you can see
this graph spans about a week or so a
couple of weeks and it rises in
brightness fairly rapidly and then fades
away rather slowly okay when we see
these novae mostly we we see them occur
once and we never see them recur again
it's thought that some of them probably
do recur maybe most of them do recur but
on very long timescales maybe thousands
or even tens of thousands of years
between the successive Nova outbursts
but they increase in brightness by a
factor of maybe a thousand or 10,000
before fading away gradually over time
if I now jump to another part of this
taxonomic chart in yellow here one of
these neutron star systems called
bursters x-ray bursts so these are
rather similar again a little image here
showing what these may look like the
difference here is that we've got
accretion from a companion star down
onto a neutron star and what we see here
when we look at the x-rays from that
system shown in the top graph is a
series of x-ray bursts
these are rapid bursts of x-rays that
last just a few seconds and repeat every
few hours as you can see this graph
spend spans a few hours
over the course of a day the 29th of
January 2003 looking at this particular
x-ray bursts are called for you 1323
minus 62 and if we zoom in on just one
of those x-ray bursts happens to be from
a different system but the principle is
the same now in the lower graph you can
see we're spreading out the light from
that burst over maybe 200 seconds or so
you can see that they have a very
characteristic shape again a steep rise
and then this shallow decay so the
bursts themselves last just a few
minutes and recur every few hours it
turns out that both novi in the case of
accreting white dwarfs and x-ray bursts
in the case of accreting neutron stars
have the same cause and it's this shown
here it's a thermonuclear explosion on
the surface of the white dwarf or
neutron star what happens is material
from the companion star builds up on the
surface of the compact star until it
reaches a critical mass when
thermonuclear runaway can occur it goes
bang blows material off and it starts
accreting again the difference is in the
white dwarfs that recurs every few
thousand years and the outburst is
mostly seen as an optical of visible
light flash whereas in the case of
neutron stars the bursts recurs every
few hours and we see it in an x-ray
situation we see x-rays from the system
instead but the principle behind both
types is the same one of the useful
things with x-ray bursts is that they
allow us to measure the size of neutron
stars now I've put a couple of equations
here but don't worry about that I've not
got many equations in this talk this is
just to illustrate that if we see an
x-ray burst from a neutron star and we
measure the flux of light the amount of
light we measure in say watts per square
meter from that burst and we measure the
temperature of the spectrum of the
x-rays then we can combine those
quantities to actually work out the
radius of the neutron star shown in the
third equation there the radius of the
neutron star just depends on the
distance to the system which we can get
by other means
the
flux that we measure from the burst a
constant Sigma called the
stefan-boltzmann constant and the
temperature of the spectrum that we
measure and from measurements and
calculations like this we can then
measure the radius of neutron stars to
be this number of order 10 kilometers
that I mentioned earlier back to the
taxonomy chart again if we look at
another of the white dwarf creatures
these are things called dwarf novi now
they were named originally because they
they look like just dimmer versions of
novi they occur in the same sort of
systems where we've got a chrétien via a
disc onto a white dwarf but they recur
much more frequently here's a light
curve of a dwarf Nova called SS signe
over a year or so from june 1992 to
september 1993 and you can see that this
object underwent what seven bursts plus
another rather odd that period of
activity over here but seven bursts in
this period of a year and a bit the
increase in brightness is not as
significant as with a nova hence the
name dwarf Nova only increases by
brightness by maybe a factor of a
hundred or so the outbursts themselves
last for weeks and recur every few
months now there's a similar type of
object it turns out on the other side of
this taxonomic diagram in terms of black
hole and neutron star systems that we
call x-ray transients in fact they're
usually called soft x-ray transients
because the x-ray emission from them is
quite low energy so these then are due
to accretion via disk onto a neutron
star or black hole and the outbursts
look like this here's the light curve in
x-rays and a system called a zero 620
minus zero this is a plot of the x-ray
brightness against time and you can see
that in this case the outburst lasts
four months and they actually recur
every few years or even decades it turns
out that these then are the x-ray
equivalent the neutron star equivalent
to the black hole equivalent of what
happens in dwarf Nova and it's due to
what's called a thermal viscous
instability
in the accretion disk we can
characterize the accretion disk by two
numbers the mass transfer rate the rate
at which material flows through the disk
and the density of the disk and if we
plot those quantities on the graph we
find that the physically allowed values
lie on this green line this sort of
s-curve that we see here the upper and
lower branches of this curve are both
stable but the middle branch is an
unstable regime and that's key to this
this process so what happens is the
system the accreting white dwarf or
neutron star or black hole with its
accretion disk around it can find itself
in a situation where the mass transfer
is somewhere in the region that would
put it on this unstable branch of the S
curve it can't remain there so what
happens well what happens is that matter
accumulates in the disk it heats up as
it does so so material flows into the
disk from the companion star it can't
flow out of the disk quick enough so it
builds up in the disk the disk therefore
gets hotter it reaches a point where the
disk can't move on to the unstable
branch of this S curve so instead it
heats up rapidly the hydrogen in the
disk becomes ionized and the system goes
into outburst when it's on this then hot
branch of the S curve material can then
flow out of the disk at a greater rate
mass is passed down onto the compact
object the disk cools down it reaches a
point where it cools rapidly the
hydrogen D ionisers recombines and the
system falls into quiescence so you can
see that there's a cycle here going
around this s curve on the graph of mass
transfer rate against density and that's
how the system's cycle quasi period
quasi periodically between a cool
quiescent state and a hot outburst state
and that's what explains both dwarf novi
when the accretor is a white dwarf and
soft x-ray transients when the accretor
is a neutron star or black hole
so in these four types of system that
I've talked about then we have two
different types of complex objects and
two different types of outburst
mechanism when the compact object is a
white dwarf and call these things
cataclysmic variables and when the
compact object is a neutron star or a
black hole we call them
x-ray binaries and the four types of
systems that I've talked about shown
here then dwarf Nova classical Nova soft
x-ray transient and x-ray burst ur have
this characteristic duration for the
different outbursts days and weeks in
the terms of the duration of dwarf Nova
tens of days for the duration of
classical novi fairly similar months for
the duration of soft x-ray transients
but seconds or minutes for the durations
of x-ray bursts --is if it's so the
duration we look at the recurrence
interval well with dwarf novi they recur
every few months the classical Nova they
maybe recur every 10,000 years or so
soft x-ray transients recur every few
decades x-ray bursters recur every few
hours there's one point I didn't mention
earlier of course I said x-ray binaries
have either a neutron star or a black
hole but x-ray bursts ha's are not seen
in the black hole systems and the reason
for that is simply that black holes
don't have a physical surface on which
the material can accumulate in order to
undergo the thermonuclear runaway so if
we see an x-ray burst we know it must be
a neutron star compact object and not a
black hole
conversely the soft x-ray transients can
occur in either type of system you
Transtar or black hole because that
outburst is occurring in the disk which
is independent of the compact object in
the center of it now back to my taxonomy
diagram again I'm going to talk about a
couple more types of objects over on the
right hand side with the red systems the
white dwarf systems I'm going to talk
about some systems called intermediate
Polar's now these are systems where the
white dwarf has a strong magnetic field
and as such it's able to disrupt the
inner regions of the accretion disk as
shown here in this cartoon you can see
that the accretion disk is truncated at
some radius where material latches onto
the field
of the white dwarf and he said channel
down to the magnetic poles of the white
dwarf which is a little blue object you
can see they're spinning at the center
of the accretion disk so in these
systems when we look at the x-rays
coming from close in to the white dwarf
itself and here's the x-ray light curve
of a system called a Opium the light
curve spans about 16 hours we can see a
clear pulsation in the x-rays every 15
minutes or so and that's the rate at
which the white dwarf is spinning but we
can also see a longer modulation every
few hours and that represents the
orbital period at which the two objects
the white dwarf from the companion star
are orbiting around each other so we see
if you like two clocks in the system
here the spinning white dwarf and the
orbiting binary if I move now to the
other side of a taxonomy diagram and
look at some of these things called
x-ray pulsars these are systems with
again a a neutron star in them but these
are now magnetic neutron stars very
magnetic neutron stars we see here what
are called x-ray pulsars and there's a
little image there showing what one of
these may look like from the surface of
a / handy nearby asteroid that we're
standing on what happens here is that
the magnetic field of the neutron star
the x-ray pulsar is able to funnel again
material from maybe the wind of its
companion star down onto the magnetic
poles of the neutron star and as it
spins on its axis so this beam of x-rays
passes by our line of sight so here's a
particular x-ray pulse are called Veiler
x1 consists of a neutron star and a
giant star the x-ray light curve is
shown there on the left and we see a
regular pulsation going on if we extract
that pulsation if you'd like fold the
light curve over on itself every 283
seconds in this case which is the rate
at which the neutron star is spinning we
see the x-ray pulsations there going up
and down this first one beam from the
pole of the neutron star and then the
other beam from the other pole sweeps
across our line of sight in this system
the orbital period is about nine days so
that's the time it takes the two stars
to orbit around each other
one thing we can do is measure the speed
at which that x-ray pulsar is orbiting
its companion style and we do that using
the Doppler effect as this x-ray pulsar
is moving away from us so the pulses
needs 283 second pulses arrive more
slowly as the neutron star moves away
from us as it's coming towards us in the
orbit the 283 second pulses arrive more
quickly so if we measure that time delay
advanced and retarded as the neutron
star orbits we can plot that delay as a
curve and we see that it varies over the
nine-day orbit of the system the
amplitude of that graph then indicates
how fast the neutron star is moving
around its orbit and we can measure that
now it's not just that the neutron star
is orbiting around the giant style the
two stars are orbiting around their
common center of mass and we can measure
the speed of the giant star by looking
at its spectrum here's a segment of the
spectrum of that giant star in Veiler x1
and you can see in this graph at the
bottom the various absorption lines in
that part of the spectrum ranging from
the blue end to the green part of the
visible spectrum and what we can do is
measure the precise wavelengths of those
spectral lines because as the giant star
is moving around in its orbit so the
Doppler shift comes into play again as
the star is moving away from us so those
spectral lines get shifted to longer
wavelengths and as the giant star is
moving towards us so those spectral
lines get shifted to shorter wavelengths
we can convert those shifts into the
speed at which the giant star is moving
and again plot that on a graph as shown
here so the giant star over the 9-day or
which is alternately moving towards us
and away from us as the two stars orbit
around their common center of mass and
essentially what we can do is then
measure the speed of motion of the
neutron star from the time delay of the
pulsation the speed of motion of the
companion star from the Doppler shifts
which spectral lines we measure the
orbital period in this case the system
also eclipses that is the neutron star
passes behind the giant star for about a
tenth of each orbit and we can use that
to work out the
angle at which reviewing the system we
put all that information together we can
actually weigh the two stars and in this
case it turns out the neutron star has a
mass and about twice that of the Sun and
the giant star has a mass of around
about thirty times that of the Sun so
that's a rapid Whistlestop tour then of
these compact accreting binary stars I
think I've hopefully shown you that they
undergo these outbursts we can measure
their oscillations and their orbits and
they allow us to study both extreme
gravitational fields extreme
temperatures and extreme magnetic fields
in these systems and they allow us to
measure amongst other things the masses
of compact objects and the sizes of
compact objects - I'll just finish the
slides there I hope you've enjoyed
listening to this talk and I hope you
watch some more of the lectures on our
website -
