Okay. Okay.
So, yes, today I'm going to talk to you all good on Sandiford about the spaghetti ification of stars by supermassive black holes,
which I think is basically the most extreme event in the observable universe.
Okay. So what we're going to talk about today, we'll start with a tour of the Galactic Centre.
Okay. Which is an interesting object in itself.
I'll explain what on earth I mean by the phrase spaghetti ification, how we ended up with this ridiculous time.
How we may observe spaghetti fide stars in the observable universe.
And then the rest of the talk about what can we learn from these systems? Okay.
So let's take a step back. We're not going as far back as for George.
This is talk, but a little bit further out in Francesco's talk. Okay, let's think about the galaxy.
Okay. So this is the Milky Way. We live here.
Roughly halfway out. Okay. But we're going to focus instead on a central region.
Okay. And as you can already tell from this slide, the key difference between here and here is the density of the stellar environment.
Okay. If I go outside, I look at the night sky. I do see plenty of stars if I'm not in the centre of the city.
But in the study in the Galactic Centre we get a very different picture.
Okay. And if I went to look at the night sky, it would be truly stunning. The density is orders and orders of magnitude higher.
Okay. So let's actually zoom in on the centre of our own galaxy. Okay.
So this is an image of our own galactic centre by shuttle itself.
So, yes, it's an extremely dense environment. And also that the stars are clustered around the very centre which is inside this box.
So we'll zoom in again. We can start to resolve individual stars.
Now we see again incredible density, some quite bright objects, but we're still clustering right around the centre.
So we zoom in once more. Okay. And we can see the now famous star cluster.
Whether or not these are actually famous depends on how much astrophysics you keep up with.
These won a Nobel Prize a couple of years ago. Okay. And so the star clusters are a group of stars at the very centre of our galaxy.
There's 30 or 40 of them. And this ring here is one light month across.
Okay, so that's the distance. Light can travel in a month.
So for reference for scale, the nearest stellar neighbour to the sun is four light years away.
So this distance is 1/50 of the gap between the star and its nearest neighbour.
And we have 30 or 40 stars. So that's the difference. So we can keep zooming in.
You know, it's what we do in astronomy. Okay. And we can. So this is the Nobel Prize winning work.
You can just watch the Galactic Centre and you can watch the stars evolve through time.
You can follow them around.
Case of particularly interesting is, for example, this yellow star where we see it completes an entire orbits of 20 or so years.
Okay. Now you will see the first obvious thing.
All of these orbits are centred on this white star. Hey, this is a terrible use of a marker for this object.
And if we observe this point of an optical telescope, we see nothing.
There's no emission from the. But there's clearly an object there. Everything is orbiting around this point.
We know Newton's laws of gravity.
So we can basically weigh this object, we can measure the mass, and it comes out at a few million times the mass of the sun.
So it's an incredibly massive object. Okay. Then you can say, okay, well, my yellow star didn't hit anything.
So that gives me an upper bound for the size of this object.
And combining the two measurements, you get a density and you infer that rather than a white star, this is a black hole.
Okay. This is a supermassive black hole. There's no other objects in the universe that can be distance.
We now believe that the centre of all large galaxies is a black hole and is tightly correlated with the properties of the galaxy themselves.
Okay. So as I said, as 2020, Nobel Prize for black holes keep winning Nobel Prizes.
So it's quite nice. So this rather than looking particularly at the black hole, let's look at these orbits themselves.
Okay. Now, one of the very interesting things you see is that they appear to come really quite close to one another.
Okay. Now, this is a two dimensional projection of three dimensional orbits.
Okay. So this is an exaggeration. They're not necessarily quite so close.
But let's think about some of the implications of this dense orbital neighbourhood.
Okay. So now we're going to make graphics from paint.
We have a black hole in the centre and I have a star going on a purple orbit is perfectly happy going around.
It will do that. Whether. Let's introduce a second stop.
Okay. My Blue Star, if I run time forward. Okay.
I can imagine in principle that my two stars can get pretty close to one another.
And if they do, they will interact gravitationally.
Okay. And they'll be a force between the two objects.
This force will give a kick to my Purple Star, change its momentum, but it will also change its orbit.
Okay. And then if I run the time forward again, I can imagine, at least in principle,
I can random shuffle the occasional star arbitrarily close to the black hole.
Innocent. And then I have a big ball of gas, and I've thrown it at a black hole.
All sorts of colour. Okay. Okay.
So we can measure the density of the Galactic Centre.
We know a little bit about the orbits of individual stars, so you can ask, well, how likely is this?
So my tenuous link to Francesco's talk is not very OC.
So if I sits and I watch the centre of the Milky Way for a whole year, there's a one in 10,000 chance roughly that this happens.
So it's unlikely to happen tomorrow in the Milky Way. But the key is this is per galaxy per year.
So one way of waiting to see one of these is to wait a long time. That's not the most efficient way.
There are many, many millions and billions of galaxies in the observable universe.
Okay.
So while it's extremely unlikely in any one galaxy, it's very likely that today, somewhere in the observable universe, one of the events is going.
And there's a little shout out for Oxford. This calculation was first properly done by John McGowan is in the ferry the Pope.
Okay. So I hope I convince you that in principle I can imagine throwing a star at a black hole.
So let's think about some of the implications of this. So I have a star that's perfectly happy, and then I introduce a black hole.
Well, what happens? Well, we have a force of gravity between the two objects, as is Newton's laws.
Gee, I want them to overall squared. And the leading order effect of a gravitational force is just to put my star on an orbit.
It's very simple. Hey, that's not what we're interested in. What we're interested in is something called the tidal force.
So the tidal forces. The difference in the gravitational force across an object.
You know, we like simplifications in physics, but the sun is not a point particle.
It has some finite size, which I'm going to call bigger.
And that tells us that the blobs of the star on this side of the black hole, well, they're closer to the black hole.
So they'll feel a larger gravitational force. And the points in the middle and the tidal force is simply the difference in the two forces.
So I can calculate that very simply.
I find I write down for each blob which will have mass delta and the force on that object, minus the force on that object.
And the only difference between the two is the difference in the distances to the black hole.
Okay. So we want to understand the properties of the tidal force.
So we'll do a little bit of algebra so I can satirise this.
I have one minus the radius of the star itself, divided by the distance to the black hole.
Now, we're going to assume this is a small number of wise. I'll be inside the black hole and that's less interesting.
Okay. So I have one minus a small number to the minus two.
And if you remember your total expansions from undergraduate, that's the same as one plus 2x1 minus x minus two, and I can simplify.
And so the tidal force has the following properties. If I increase the mass of the black hole, I get a larger tidal force.
Stars of larger radii are easier to distort.
Tidal, okay, because there's a greater distance between the sides.
My star and the force grows as one over r cubed so very far away from the black hole.
I don't have to worry about tidal effects, but if I get closer and closer, this grows very quickly.
Okay. And also importantly, there's a minus sign. Which tells me that relative to the blob in the centre,
the blob on the outside this side wants to go is a greater acceleration towards the black hole.
So wants to move that. Okay. So if I repeat this calculation for the blob on the far side, okay, this is all relative.
Of course, everything wants to go this way, but relative to the blob in the centre, it wants to move further out.
And that's because it's far away. Right.
And if I repeat the calculation for these two points here, I find that they want to go in ones that might be slightly less obvious,
but you can draw a sort of false triangle and you see the radial components between these two blobs will be the same.
But there's this horizontal component. Okay, which is going to cancel.
So from these four points, I can see what happens if I take a bowl of gas and I put it in a black hole.
Okay. It's distorted slightly. Okay. So this is precisely the reason we get to high tides and too low tides a day on the earth.
Okay. But we're not interested in small effects to the moon.
We're interested in taking this to its logical extreme. So I'm going to get an extremely dense object, and I'm going to put it in this store.
Okay. And you can see what happens. I really stretch out my star.
Okay. And if I ramp up the density some more, you know, I stretch it out some more.
And then if I get an unimaginably dense object, you know, I can see what's happening.
Okay, so this is the logical extreme. Okay. And this is my stellar spaghetti.
So this is, you know, it's long and thin and yellow. It looks like spaghetti.
This is where this spaghetti ification term came.
So when I was actually preparing these slides, my office meant told me it should be spaghetti because there's only one of them.
But the the important fact is that for the supermassive black holes in the centre of galaxies, this can actually completely destroy the star.
Okay. Now, to see that, take a sort of average effective tidal force.
So add it up over the whole star, you see. Okay. It grows through the black hole mass.
The mass of the star, the red the star. And it goes like one overachieved. Well, what's holding the staff together?
The staff is being held together precisely by gravity. So it's Newton's law again.
GM want them to overall squared, but it's the same mass. It's the mass of a star twice divided by the radius to start.
This is a rough holding the start to get.
Now, if I make my tidal force larger than the force holding the star together, well, then I no longer effectively have a star.
I have a completely unbound set of gas.
I'd like to solve this equation for a special radius called the tidal radius, which this happens.
And then I'm going to do something unpleasant.
I'm going to take the sun and I'm going to throw it at the supermassive black hole in the centre of the galaxy.
And I find, well how big is this number? And it's 70 million kilometres, but it's just a number.
But the important fact is, is that it's bigger than the black hole itself.
So the black hole in the centre is 3 million kilometres. So this complete disruption of the star happens outside of the black hole.
And if this happens outside of a black hole, in principle, I can see it. Okay.
Light from this event will escape. We can observe. Okay.
Now, this is kind of a classic astrophysics. Lots of APR equals two ignored factors of two.
So you might ask, is this really possible?
So an alternative approach to solving this problem is to take a ball of gas in a computer and process a black hole and see what happens.
So that's what Bonura Atoll did in 2015. Okay.
And so the the white again, the black hole is white and you throw a star in and it goes apart.
Okay. At earlier time, you get a sort of nice spaghetti shape, but eventually it's completely torn apart.
Stream stream interactions. It's a bit of a mess, but eventually if we wait long enough, it's going to start to settle down.
Okay. And the reason it settles down is simply conservation of angry moments.
I took a star on some orbits and my favourite and I can't get rid of that angular momentum.
All I can do is redistribute it.
So she ends up with a ring and this is the ring with radius of the circular orbit with the same arguments and as the star I threw in an.
Okay. Now, just to be safe, we should throw another star in on a different orbit.
Okay. And you see. Okay. The early time behaviour is quite sensitively dependent on the star I pick and the orbits I pick.
But after a couple of orbits round, we're going to mangle the star and we will eventually end up with another round.
Okay. Okay. Now, so I said, you know, late time.
So it's important to get a scale of the time here. So from the start of the simulation to this point, here was 22 Earth hours.
Okay, so what a day for the people around the store either.
But, you know, this is this is a relatively rapid. Again.
And the interesting point is, okay, then I have a ring of gas around the black hole.
This will evolve. It could get very hot. And I could potentially emit light that I could observe.
Okay. We've got to remember that these are rare events.
So it's not going to be in our galaxy, it's going to be in another galaxy. And of a galaxy is a very far away.
Okay. So while in principle it's possible to observe these, we need to ask if in practice it's also possible.
Okay. So to answer that question, we have to go, well, how bright is a tidal disruption event?
So we need to answer this question. We start with. How bright is the star?
While a star is effectively an engine for turning mass into light.
Okay, so it has some energy budget. He's got some energy budget and see squid as that's it's an energy this engine has to work with.
And in terms this is a photon of some efficiency and this efficiency is set by the nuclear reactions in the stars.
Cool. And it does this over some time, the entire lifetime of the store.
Okay. So this is a rough and ready approximation of the luminosity of a star.
Okay. I have some total budget and I radiator over my whole lifetime.
Well, if you think about it, a TDI,
so I'm going to start calling them TDs for total disruption events because spaghetti ification is a bit of a mouthful.
So a TD has the same energy budget rather than nuclear reactions.
I'm shredding a star and I'm throwing it into a black hole, but I still have m.c squared to work with.
There's going to be a different efficiency and efficiency associated with the accretion process and there's going to be a different time.
So to get a rough and ready idea for how bright a TD will be, you take the luminosity of a star,
you divide these two equations, assume the efficiencies are roughly the same,
and you get an amplification of the luminosity the star by a factor of ratio of
the time the star lives four divided by the time in which the two can happen.
And this is a big number. This is about 10 billion. The sun will live for 10 billion years.
And when all said and done, it takes roughly a year to shred the star and throw it into the black.
Now, this is a very big number in the case of the luminosity of the entire Milky Way galaxy is roughly 10 billion solar luminosity.
So what we're talking about here is taking all of the light from this galaxy and putting it as a single point source right at the centre.
Okay. Now, if one of these events ever did happen in the Milky Way.
Would be truly phenomenal. Okay, so from our perspective, halfway out, this object would have 30 times the brightness of the full moon.
Can you be able to see it in the day? People probably lose their minds.
It would be it would be fantastic for funding, if nothing else. Okay.
And so. Okay, now we understand that, you know, in principle, these can happen and we can observe them at cosmological distances.
Okay. So in practice, how do we go about doing this rare event?
Well, we sit, we watch the whole sky and we wait.
Okay. It's it's actually it's a very effective technique, actually.
So there's whole groups of astronomers out there, and they take a picture of the whole sky each night and then they compare on any two months.
Okay. Now, the group with the best appreciation of branding is the assassin collaboration.
Okay, so this is the All-sky Automated Survey for Supernova.
Okay. It shows astronomers love for forcing acronyms. Okay.
And so what assassin do is they have a whole bunch of telescopes dotted around the earth and they take
an image of the whole sky and they know precisely where everything was yesterday and how bright it was.
And if in today's image, suddenly something's twice as bright, that means that a tidal disruption event may have happened.
Okay. That principally interested in finding supernova, which is another way of blowing up the star.
And so. But occasionally, for basically every one supernova they find, they're going to find one of these tidal disruption event.
And they let everyone know. Okay.
So these objects were first theorised in the late seventies, early eighties, but it was wasn't until the 20 tens when we started to get solid data.
Okay. There's now about 30 of these objects. Okay.
So we can start to do population studies.
There's going to be hopefully tens of thousands by the end of the 2030s because we've got some new telescope service coming online.
Okay. I'm going to tell you about two of them today. I'm going to tell you about Assassin 14 and I an assassin 15 now.
Okay. So the naming convention is discovered by assassin.
We always named things after ourselves in astronomy. And it was found in 2014.
That's the 14 an assassin. Start with 14 A and they count up.
So Ali, was the 321st objects found by assassin in 2014.
Okay. So this was a hugely exciting event. It was actually very nearby in cosmological terms.
So everyone turned the telescopes and followed up.
Okay. And what do we do when we get brightness versus time?
Okay, that's all we're going to get in astrophysics. That's all we ever have to work with.
So some groups pointed in actually telescope. This was actually done over in Oxford astrophysics.
So so that's a nice sort of connection.
I mean, the telescopes in space, but the people we were using it were sort of a groups appointed, say a U.V. telescope, optical telescope.
So so we have a huge amount of data. Okay.
There's also Assassin 15 LH So if I'd given this talk yesterday, I would have said this was the brightest one of these ever.
And then another one blew up yesterday, too, which is even brighter.
But. But Assassin 50 LH went off in 2015. It was phenomenally bright.
So all sorts of groups followed up. So we have infrared data, we have optical data.
We have easy data. We're not struggling for data. The problem is, if we run our simulation, we can probe this sort of time scale, the powerful gap.
Okay, so if we're really lucky, we may be able to explain one data point.
Okay. So the problem is, how do we go from this ring?
Okay. Which we know is roughly the endpoint of our simulation. How do we go from this ring to this data?
Okay. So that's the question. And that's what I spent my Ph.D. doing, is trying to work out what's this popular?
Okay. So to understand these systems, we need to do fluid dynamics in general relativity.
Okay, we have a ring of gas. Okay.
It's a fluid. It's going to move. It's dynamics.
And there's a black hole in the middle. We're playing with general relativity.
Now, this is slightly tricky because I was told to produce some slides for a second year undergrad audience.
This is a third year course, and I don't think they taught this in anything other than the maths departments or quite recently.
But this is the Saturday morning of theoretical physics. You know, we're going to do it anyway.
Okay. We're physicists. We give physicists. Okay. So what we're going to do is we're going to conserve mass.
Okay. The only way mass can change my system is if it ends up in the black hole.
Well, conserve angular momentum. Exactly the same principle.
We'll conserve energy and then we'll see what we get. Okay, so so that's that's the process.
Okay. So so you've probably heard in popular media the film Interstellar perhaps about accretion disks.
Okay. So these are the sorts of systems we describing here. So imagine you have a fluid, okay?
And it's a roughly two dimensional structure. It has radial behaviour and it's some useful behaviour but it's very thin.
So my flow has some high take, it has some density rho, it's moving radially.
We've lost you and the distance off in the black hole.
Okay, now mass conservation tells me if I draw a purple box in the flow, I cannot create or destroy mass in that box.
Okay. So if I wait some time in the mass, in my box changes.
That tells me mass is either left the box or entered the box. It's a very simple principle.
In the language of general relativity, we have a partial differential equation.
Okay. So drove it. T is how does my massive my box change over time.
And the second time tells me how does mass flow in and out of my box? If I add them up, I, of course, have to get zero.
Okay. So there's an additional factor, which I'm going to call the interstellar term.
Okay. So if anyone's seen the film Interstellar, you know, the an astronaut leaves Earth, goes to a black hole,
spends a few hours there in his frame, and then comes home where 40 years have passed and his daughter hates him.
Okay. Now, so we have to be very careful in general relativity when we say something like day to normal physics problems.
But it is fine. We can all agree. Maybe not in cosmology, but we can all agree what we mean.
But we're interested in the rate of change of this disk with respect to time on earth.
That's what we measure. That's our data. Okay. But from the fluids perspective, that can be a very different time.
Okay. So this tells me the connection between the two clocks in the system.
Okay. And it's in a textbook. You can look at it. Okay.
So this is the this is the procedure. So then we move on to energy and then some conservation.
Okay. So this fluid is is orbiting in a ring. It's moving very quickly.
It has an event. And again, I cannot create or destroy and condense them in my system.
So I wrote all the ways in which all the ways in which angular momentum can change.
And this one's a little bit more intimidating. But again, we can understand each of the terms.
So the terms in the square bucket, square brackets tell me I have a box.
I am still thinking about my post box.
If the mass in this box changes and that mass had angular momentum, then the angry momentum of the box has changed.
Okay. So kind of trivial to. The second term describes.
Okay, well, what if I had two boxes next to each other and fluid leaves this box and moves into this box.
But this box has a different angle and then some profile.
If you imagine moving from one circular orbit onto a second circular orbit, there's orbits have different time movements.
So this term, in effect describes the fluid moving between these two states.
Okay. Now, this third term is a little bit more complicated, and my supervisor will hate me for using the word friction here.
Okay. You may ask, well, why? Why does the fluid want to go into the black hole at all?
The earth is on an orbit circle, but it's basically a ring.
It doesn't charge headfirst at the sun. Okay. So why would my fluid do any different?
Well, the difference between the fluid and the earth is that there are neighbouring elements in the
flow and they can rub off against each other and there's friction between layers in the flow.
We in reality this is turbulence, it's so much more complicated process, but for our purposes you can model it as a friction or a viscosity.
Fluid elements rub past each other. Energy is dissipated and angular momentum is moved around.
Okay. This actually drive secretion accretion. Without this, there is no accretion.
And this is a final term which tells me, okay, imagine I emit light out of the top of my desk while light travels carries momentum as well as energy.
And these are very bright objects. So that can appreciably change the flow.
Okay. So energy conservation, we're nearly there.
Okay, so this tells me, okay, I have fluid, it's in a box, I move it around.
I can't just do that or create and change energy.
So energy conservation tells me that all of the energy change in my box is dumped into heat, which is then radiated out the top of my box.
Okay, that's where the light. And this tells me the temperature of my desk.
Okay. So then you just do everything at once.
You confirm you conserve all free quantities and you combine and you get a set of equations which are right here.
Okay. And it's a diffusion equation. So what you find is that my ring is going to diffuse out and I'm going to end up with a disk.
Okay. It looks pretty complicated, but the key point is if I write down a ring of material and put a black hole in the middle,
all I have to do is solve this equation, and I can do that on a computer.
And this is sort of the algorithm for solving these problems. Okay.
I evolve the density forward. I use energy conservation to work out the temperature.
And from the temperature, I remember Planck's law of radiation.
And so I can get the brightness as a function of frequency. So every last bit of my disk has some temperature, it's got some brightness.
I add it all up and I can compare to my telescope. So this is a procedure.
Okay. So let's see what happens. Okay. This is time is zero.
There's a black hole over on this side, which I've added thing.
So any fluid that passes this black line will go since the black hole it's got.
This is my ring at an initial time. Okay.
And we're just going to press go and solve the equation and see what happens.
Okay. You have to watch quite closely early times because it moves quickly. Okay.
So what happens is that my fluid rushes in, okay?
The the friction in effects moves material around, and it wants to go towards the black hole.
So the fluid rushes in and my temperature goes up. Okay.
But then I lose material into the black hole, so the density drops, I have less energy available and my temperature starts to come down.
So this besides quite easy to understand. Now you might wonder what's going on here.
Okay. Well, a lot of my food appears to be running away from the black hole.
What's going on here? Okay, so that's angular momentum conservation.
Okay. I can't just move material in. We have this picture of an accretion flow is just material diving towards a black hole.
But actually, to conserve total momentum, I need to move a lot of my fluid outwards.
Okay. All right. So this, you know, you might think this is the end.
Okay. Can we compare to the telescope now? Unfortunately there's this extra effect switch morphing start to go wrong when
there's a black hole in the sensor and we return to the film Interstellar.
So if you've seen this film Interstellar or basically any black hole in pop culture,
you'll probably see this strange, glowing, three dimensional UFO type shape surrounding it.
Okay. This doesn't look anything like my two dimensional flat desk that I've been describing.
So so what's what's the difference here? Okay. And the difference is an effect called gravitational lensing.
So this is a purely optical effect. The physical system is a black hole in a 2D structure.
But if I look at the system, you know, I'm effectively measuring photons and where they've come from and I observe a warped structure.
Okay. So this is, you know, the bending of light raised by a by a gravitational field is one of the classical tests of general relativity.
Okay. And the you know, the stars this famous Eddington expedition moved ever so slightly.
Now, if you throw a light ray, it's a black hole. You can get much more substantial deviations in the flow.
Okay. So we call this the pub orbit.
Okay. So you've got a photon on its way home. It stops off for one, and then it's instead.
Okay, so this pub orbits like they're highly, highly warped things.
Okay. Now, just as a note of caution, as any undergrads will tell you, pub orbits are very dangerous.
And that's because they're chaotic. Now, if I start arbitrarily close to a pub of it, I stop off for one.
I can actually end up at any point in the universe. Right.
Okay. So you got to be very careful here. Okay.
And so this is warping. It's this it's just the bending of light rays.
And so my 2D structure, which is physically remains physically two D is observed to be warped.
Okay. And I get this classic UFO shape. And the UFO shape comes because.
Oh, my laser pointer seems to go. The photons behind the black hole.
Emitted a needle bent over the top effectively.
And so I can actually see them when in a naive Newtonian gravity, they'll be absorbed by the black hole.
And so I can see light that appears to come from behind the black hole.
And that's the warp. Okay. Really the one last effect.
Okay. And for this last effect, I want you to imagine that I have a lime green accretion disk.
Okay. It's not a particularly realistic physical model, but it highlights an important point,
and that is that the fluid in this system is rushing around at nearly the speed of light.
Okay, so let's imagine it's orbiting counterclockwise, which means that this blob of fluid is moving directly at an incredibly high velocity,
and this blob of fluid is moving away from it at an incredibly high velocity.
Now if an ambulance charges towards you in the street, the side at the frequency of the siren goes up.
And then as it charges away from you, it goes down. And that's called the Doppler shift.
Okay. Now, what doesn't happen with an ambulance is the blue flashing light doesn't go ultraviolet and that infrared as it travels away.
And that's simply because ambulances don't travel close to the speed of light.
Okay. Whereas, you know, they travel roughly close to the speed of sound.
And that's why sound waves are changed.
But if you can show that for the fluid on the very inner edge, the circular orbit can be half the speed of light.
Okay. So we're going to get not just warping but strong shifting in the radiation.
Okay. And I bother to do an actually accurate lime green accretion flow as observed from Earth.
Okay. And you see that on this side, which is rushing towards you.
The Doppler shift is so strong that actually ends up ultraviolet and you won't be able to see it, but it might give you cancer.
This side over here would end up blue, perpendicular to my line of sight.
There is no relative velocity of the source and I recover my lime green.
Hello. And on this side, it's rushing away from me. And I get red and then infrared.
Okay. So they actually deliberately kept this out of the film, Interstellar.
They calculated it properly. You know, kit form was on the film.
They calculated it properly, but then it looks rubbish. Like you just see a tiny corner of the mission.
Okay. These are boosted as well. So it's much brighter.
And black holes look boring. So? So they took it out. Okay, so now we're ready to go.
Okay. We have a ring of material. We press go.
We fold the temperature through in time. Every last bit of my disk has a temperature.
It emits some light. And then we we understand that we have to take the image of this flow with a special sort of camera,
which takes into effect the warping in the shifting.
And what that allows us to do is calculate the brightness as a function of frequency at a given snapshot.
And this is great because this is exactly what we compared to our telescope.
So she pointed an X-ray telescope. What you can really see is here a few points, a TV telescope.
What you really see is this little band here. Okay. So we see what happens.
Okay. So we see that as we press go, the fluid rushes in and it gets to high temperatures.
That means that my spectra, my blue curve moves up to higher energies because hotter objects emit more higher energy photons.
Okay. But as the schools, even though it it cools only a little bit, the change as observed by an X-ray telescope will be quite strong.
And that's because it's in the V tail. So it's exponentially sensitive to the peak of the distribution.
Okay. So we have a clear theoretical prediction. X-ray light curves of these events should decay away rapidly.
Over in the U.S., we get a very different story. Sure, it goes up again to early times, but then it just sort of sits there.
It sits there for two years. This will run up to a thousand days and basically nothing happens.
Okay. And we have two competing effects here. My whole system is cooling.
I'm losing material and I'm cooling. If I take a black body and I call it, I make it less bright.
So that's what's going on up here. But I'm also spreading. I'm conserving increments.
And so my area is getting bigger. Now, if I take a black body of constant temperature and I make this area bigger, it's brightness goes up.
And so these two effects completely cancel out in the evening. So we have another theoretical prediction.
Okay. So does it work? Well, I probably won't be giving this talk if it didn't work.
So sweet. And presto. And compared to the X-ray data collected over in this department and we see, okay,
the blue curve is the theoretical prediction and the orange is the observed data.
There are various gaps when the thing went behind the sun and you can say, okay, we're clearly modelling that the average trend of the data will.
The real world is is messy. It's not a nice little two dimensional simple model occurs, but we're clearly modelling the trend.
Okay. So what about the other data? This may be a disappointing slide.
Okay. So a day, 200 onwards. Okay, we're doing a very good job.
Fantastic. But what on earth is going on here?
Okay, now, these are strange astronomy units where bigger numbers are less bright and this is a factor of 100.
So effectively, we're a factor of 102 early times. So what's going on?
Well, we learned from this that this early time admission is not coming from the disk.
Actually, there's no way you can force a disk model to do this sort of behaviour.
It doesn't want to change it. You've. It's the wrong part of the spectrum.
So what does it. It depends which of my colleagues you ask effectively as this is.
This is one of the big open questions. Okay. It's a real shame because it's really easy to observe, but we don't really have a good model for it.
There are lots of plausible theories. I'm going to tell you about my personal favourite and to go back to the one I actually believe.
It's true. It's not just my favourite. If we press go on a simulation, my spaghetti goes once round the black hole.
Absolutely fine. But then on the second time on the flight, call, the front of the spaghetti smashes into the back of the spaghetti.
Okay. This is a extremely high velocity collision of a gas.
So he's probably going to shock.
I tell you, there's going to be a collision is going to heat my gas and it's going to give off a whole bunch of light.
And the reason we we like this particular interpretation is it has a set of very nice properties.
Okay. It's going to be very bright. But but it's also going to decay away very quickly, because eventually I end up with my ring, okay.
Within 22 hours or a couple of days. And then I no longer have these intersections between the ends of my spaghetti.
Okay. So it should go away very quickly. Okay.
So if we just add in by hand the sort of exponentially decaying components.
We see that the sum of the two, we only basically need one extra component that's decaying away very quickly,
and then we can have a nice description of the entire entire evolution.
Okay. So this is really cool.
Okay, this is really cool because they have a model which reproduces the data, but not just that we have to put physical parameters into the model.
Okay, we start with a black hole and a ring of material so we can actually use these effectively as scales of the system.
We can weigh the black hole in the middle. Okay.
And so for assessing 14 alloy, we find that the mass of the black hole has to be very similar to the one at the centre of our own galaxy.
Around 2 million solar masses. And I know that I had to put about 5% of a solar mass into my ring in the storm.
Okay. So this could be a powerful technique to learn more about these systems.
So does it work? So.
So does it work on more sources so we can go to the formerly record holder, 15 Al Hage And this is a real difficult test case for the theory.
Okay. We've got nine different bands at different parts of the spectrum, and we've only got four free parameters in the model.
Okay. So we press go and you can see, okay, again,
we find this rapidly decaying for air component at early times and then it looks like it's all going to go wrong.
But then the disk shown by these purple dashed lines and each figure starts to
kick in and the disk component nicely explains each of the bumps and wiggles,
which we see in every single component. Okay.
So it's there's pronounced rises in luminosity up in the easy and just a gentle kink in the light curve in the infrared.
But every single light curve shows some deviation from the early time behaviour at roughly 100 days.
And this is when, this is when the disk component is kicking in. And then beyond that we nicely describe within the turbulent fluctuations all the.
And The reason assassin 15 LH was such a bright object is that the black hole at the centre was a real beast.
Okay, this had a billion solar masses. Okay.
So it's very difficult to imagine. And it was about 0.1 Solomon.
Okay. So this is really cool because what we've developed is effectively a set of weighing scales of black holes in the universe.
We sit around, we wait for a black for one of these events to happen.
We follow up with multi wavelength telescopes, we fit in the data and we can just measure the mass of the black hole.
We can look at the galaxy and we can see how big the galaxy and we can start to contribute to these these plots that astronomers love to make,
which is actually hence why we're in this case.
Why is the mass of the black hole an x is the mass accounts like?
And one of the problems of doing this in conventional methods is that there's
a real systematic bias and there's a systematic bias towards big black holes.
Big black holes are effectively easier to see because they have a larger effect on their surroundings.
Okay, so we have one down here. So the Milky Way black hole is here.
Okay, so and there's lots more small galaxies.
So N30 two just happens to be nearby so we can resolve this problem.
Okay. But there's this big gap which is on physical. It's a systematic bias and we can now fill that in.
Okay, so we take all these TDs. I said there's 30 of them so far, and each of these Red Stars is one of these systems and we see that,
okay, we can fill in the gap and this is the blue points are just these points.
And so this is very exciting. Okay. So thank you very much.
So in today's talk, I told you this specification occurs due to the extreme tidal forces near to black holes.
Stellar spaghetti ification produces emission observable at cosmological distances.
And understanding these events allows us to weigh supermassive black holes.
Thank you very much.