Okay, so, yeah, once again, welcome.

So this will be the last sort of introductory talk for this session on the implementation

of the AthenaXI-FU, which will go along similar lines as the talk of Max yesterday.

In fact, I'll just pick up where he left off on this slide.

So you've already seen Athena yesterday.

And Max mostly talked about the AthenaWFI, the imaging instrument, which is a high count

rate detector with moderate spectral resolution, so 170 electron volts, with a large field

of view of 40 by 40 arc minutes, and also this high count rate chip.

The other instrument which I will talk about more now is the XI-FU, which is a micro-color

emitter and can perform high-resolution imaging, so high spectral resolution imaging observations,

which it does by using a so-called technique called micro-colorimetry.

The general idea of the XI-FU is that it lives in this big cryostat here and operates, in

fact, in the focal plane, this array of so-called, in this case, photon absorbers.

So they are on a rectangular grid that has sort of a border of a hexagon.

And this circle here indicates the field of view of five arc minutes diameter.

Each of these pixels in the XI-FU are linked to so-called transition edge sensors.

The final number is being consolidated, right now it's assumed about 3,000.

And each of these individual detectors have a size of about five arc seconds, and are

all each their own spectrometer, meaning each of them can take 2.5 EV resolution spectrum

and spectra at 7K EV.

And the way this works is that each of these pixels is a transition edge sensor, which

is effectively a superconductor that is operated near its transition temperature.

So you have this transition edge sensor, which is linked to a thermal bath, as well as an

absorber, which is a photon absorber, usually some sort of high Z material that is good

at absorbing X-rays.

And what happens when a photon is absorbed in this absorber is that it heats up.

The absorber then is suddenly coupled to the transition edge sensor, which makes it so

that its temperature increases as a photon impacts.

Now the transition edge sensor is being operated at a point where its resistance jumps from

superconducting, so zero resistance, to some normal resistance very rapidly.

This is this transition you see here.

And what happens when a photon impacts is that the temperature goes up, meaning the

resistance goes up very dramatically.

It is somewhat like a resistive thermometer, but much more sensitive.

What then happens is that the temperature of the TES goes up, meaning the resistance

goes up.

And if you apply a constant voltage, you can measure a current decrease.

And the temperature and the current will then eventually relax back to their starting state,

producing these characteristic pulses in the TES output, whose height and shape can be

used to reconstruct the energy of the photon that has been absorbed.

If you want to look at this physical system from a, you know, solve it as a differential

equation system, the standard way to do this is to have two differential equations of power,

of temperature and current.

Where effectively the thermal equation takes into account the power taken from the bath,

and then the joule power of current flowing through the TES, and the photon power.

And the electrical equation effectively takes into account the TES resistance as a function

of temperature and current, as well as, for instance, effects like the bias voltage and

load resistors.

What then happens is when you solve, for instance, this equation and apply multiple or simulate

multiple pulses, is that you will see a timeline somewhat like this.