Lecture 18

• X-ray instrumentation and calibration (mostly XMM).

• XMM users’ guide: – http://xmm.esac.esa.int/external/xmm_user_ support/documentation/uhb/index.html

NASSP Masters 5003F - Computational Astronomy - 2009

XMM-Newton

• 3 x-ray EPIC telescopes – E uropean P hoton I maging C ameras.

– 2 of these have: • MOS detectors • Reflection Grating Arrays (RGAs) – The 3 rd has • a pn detector • No RGA.

• Mirrors all the same – nested Wolter – f ~ 7 m.

Schematic of the satellite NASSP Masters 5003F - Computational Astronomy - 2009

Attitude

• The orientation of the spacecraft in the sky is called its attitude .

• It isn’t just the direction it points to, attitude specifies the roll angle as well.

– One way to define this is via a pointing vector parallactic angle .

• However, as with any trigonometric system, this has problems at poles.

and a – Better is to define a Cartesian coordinate frame of 3 orthogonal vectors.

• Each vector defined by direction cosines • Attitude is then an attitude matrix

A

.

in the sky frame.

• This is isotropic (no trouble at poles).

• Easy to convert between different reference frames.

– Whatever you do, avoid messing about with Euler angles. Ugh.

NASSP Masters 5003F - Computational Astronomy - 2009

z

Sky frame or basis.

at dec=90 ° Unless you have good reason not to, always construct a Cartesian basis according to the Right-Hand Rule :

y

at RA=6 hr

x



y y



z z



x

screws toward screws toward

z; x;

screws toward

y.

x

at RA=0 “Direction cosines” really just means Cartesian coordinates.

NASSP Masters 5003F - Computational Astronomy - 2009

Boresights

• Ideally, each instrument on a satellite would be perfectly aligned with the spacecraft coordinate axes.

• In real life, there is always some misalignment. This is called the boresight of the instrument (I think it is an old artillery term).

• Define a set of Cartesian axes for each instrument.

• Components of these axes in the spacecraft reference system (basis) form the boresight matrix for that instrument.

NASSP Masters 5003F - Computational Astronomy - 2009

Boresights

• With these matrices, conversion between coordinate systems is easy. Suppose we have an x-ray detection which has a position vector

v

in the instrument frame. If we want to find where in the sky that x-ray came from, we first have to express this vector in the sky frame Cartesian system (new vector =

u

). This is simple:

u



A

 1

B

 1

v

• Then all we need to do is calculate

RA

 atan 2 

u y x

 and

dec

 atan 2

u



z ,

• QED.

u x

2 

u

2

y

 NASSP Masters 5003F - Computational Astronomy - 2009

Attitude and boresights

• NOTE that attitude varies with time as the spacecraft slews from target to target – but there is ALSO attitude jitter within an observation.

– XMM has a star tracker to measure the attitude.

• Attitude samples are available at 10 second intervals.

• So to build up a sky picture from x-ray positions in the instrument frame, one has to change to a new attitude matrix whenever the deviation grows too large.

• Boresights can also change with time, due to flexion of the structure, but this is slow.

– Calibration teams measure them from time to time. NASSP Masters 5003F - Computational Astronomy - 2009

Other coordinate systems:

• The fundamental spatial coord system is the chip coordinate system – in CCD pixels.

• Note that for time and energy as well as in the spatial coordinates, coordinate values are ultimately pixellized or discrete.

– This defines the uncertainty with which they are known (to ±half the pixel width).

– Rebinning can give rise to Moiré effects (somewhat similar to aliasing in the Fourier world).

• ‘Dithering’ can avoid this.

NASSP Masters 5003F - Computational Astronomy - 2009

Moir é example:

Original binned data: bin widths are 2 units.

Re-binned data: bin widths are 3.5 units. Moir é effect causes a dip every 4 th bin (since 4 is the smallest integer

n

such that

n

x3.5 is exactly divisible by 2).

NASSP Masters 5003F - Computational Astronomy - 2009

MOS

EPIC telescope schematic

(not to scale) R eflection G rating S pectrometer Optic axis Mirror assemblies Optic axis R eflection G rating A rray Filter wheel Mask pn CCDs NASSP Masters 5003F - Computational Astronomy - 2009

Mirror effects: PSF

• No mirror system of finite aperture can produce a perfectly sharp image.

• Rather, each point source is smeared out (convolved) by a Point Spread Function (PSF).

• More usual, high F-number optics produce a PSF which is reasonably independent of off-axis angle; • This isn’t true for x-ray grazing-incidence optics.

– For both XMM and Chandra, the PSF varies markedly with off-axis angle.

NASSP Masters 5003F - Computational Astronomy - 2009

Mirror effects: PSF

NASSP Masters 5003F - Computational Astronomy - 2009

Mirror effects: PSF

• XMM PSF is complicated.

– Asymmetrical core.

– Inner ‘star’.

– Outer wings with shadows from the mirror ‘spider’.

– RGA streak.

• The average radial profile is best described by a King function:

S

King     1 

r A

0 2   –

r

0 , α depend on energy.

NASSP Masters 5003F - Computational Astronomy - 2009

Mirror effects: vignetting.

• The mirror assemblies have a small ‘acceptance angle’ – transmitted flux drops by a factor of 2 to 3 (it’s energy dependent!) from optic axis to outside of field of view (FOV).

• The ratio of transmittance at any position on the detector plane to that at the optic axis is called the vignetting function .

NASSP Masters 5003F - Computational Astronomy - 2009

X-ray interaction with matter

• Can break it into continuum and resonant .

• Both sorts generate ions.

– ‘Continuum’ absorption scales with • Density • 1/

E

.

– Resonant absorption: • electron is kicked out from an inner orbital.

X-ray e Atom + +

K L M

NASSP Masters 5003F - Computational Astronomy - 2009

Absorption

Resonant absorption continued:

• Because it is an inner orbital, doesn’t much matter if atom is in a gas or a solid. The inner orbitals are pretty well insulated from the outside world.

• X-ray must have energy >= the amount needed to just ionize the electron.

– Hence: absorption edges located at energies characteristic of that orbital (labelled eg K or L ) and that element.

X-ray energy NASSP Masters 5003F - Computational Astronomy - 2009

EPIC cameras

• MOS: – “Front-illuminated” – means that the charge detection and movement electronics are on the illuminated surface (same as the retina).

– This means that • pixels can be smaller (1.1”); • the MOS cameras are not very sensitive to soft x-rays (because these are absorbed in the electronics before reaching the detection substrate); • they’re not very sensitive to hard x-rays either (because the substrate is too thin to absorb many).

– 7 chips (each 600x600 pixels square) in a hexagonal array, staggered in height to (very roughly) follow the curved focal surface.

• This causes slight shadowing of the edges of the central chip by the others.

– Readout time is ~2 seconds (full window imaging mode).

NASSP Masters 5003F - Computational Astronomy - 2009

EPIC cameras

• pn: – “Back-illuminated” – the charge detection and movement electronics are on the rear. X-rays strike the detection substrate first.

– This means that • pixels have to be larger (4.1”); • the pn camera is sensitive to x-rays over a much wider bandwidth than MOS.

– 9 ‘chips’, 200x64 rectangles, but all on the same rigid squarish block of silicon.

– Readout time (in normal imaging mode) is ~ 0.07 seconds (much faster than MOS).

NASSP Masters 5003F - Computational Astronomy - 2009

X-rays to events.

• It isn’t as simple as 1 CCD pixel per incident x-ray.

– Each x-ray creates a charge cloud of electrons, with a certain radius.

– The charge cloud can overlap more than 1 pixel.

– Thus patterns of excited pixels which correspond to a single x-ray have to be identified; • then all charge from that set of pixels must be added up  total energy of the x-ray.

– Each recognized pattern is called an event .

– What XMM calls patterns , Chandra calls grades .

NASSP Masters 5003F - Computational Astronomy - 2009

Example MOS patterns

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X-rays to events.

• Complications: – X-rays are not the only things which can cause ionization in the chips: can also have cosmic rays.

• However, these tend, on average, to produce elongated electron clouds.

– These patterns are easy to filter out.

– What can’t be avoided however is a slight loss of detection capability – where a cosmic ray has struck, an x ray can’t also be detected (for that frame). See later discussion of exposure.

– Dead or ‘hot’ CCD pixels.

– Chip edges.

NASSP Masters 5003F - Computational Astronomy - 2009

Out Of Time Events (OOTEs)

• As said last lecture, CCDs (at least in imaging mode) are operated in a cyclic fashion.

• Each cycle (called a frame ) is composed of an integration interval followed by a readout interval.

• But! X-ray cameras don’t have shutters! So even during the readout part of the frame, as the rows are being shunted towards the base of the CCD, x-rays are being absorbed.

– This results in a vertical smearing of all the x rays absorbed during this time.

NASSP Masters 5003F - Computational Astronomy - 2009

OOTEs continued

• The MOS chips use a more complicated readout strategy: – Each chip has in fact twice as many pixels as ‘advertised’.

– The extra pixels (which can be made much smaller, since they don’t have to detect x-rays, just hold charge) are located behind an x-ray absorbing shield.

– The readout phase is divided into 2 parts: • a quick phase during which all the exposed rows are shunted into this frame store ; • a slow phase during which the frame store is read out to the ADC.

• Result: MOS have far fewer OOTEs.

NASSP Masters 5003F - Computational Astronomy - 2009

OOTEs continued

Bright pn OOTEs Faint MOS OOTEs NASSP Masters 5003F - Computational Astronomy - 2009

Pileup

• Earlier it was said that, in order to preserve the relation between charge size and x-ray energy, the frame time had to be short enough for the probability of 2 x-rays landing on the same pixel, same frame to be small.

• It does happen, however... and obviously the brighter the source, the more likely it is.

• The phenomenon is known as pileup .

NASSP Masters 5003F - Computational Astronomy - 2009

Pileup

• Because of patterns, interaction between 2 events is difficult to calculate (but has been done however).

• Broadly speaking, 2 piled-up photons look like a single photon of the sum of their energies.

– This mucks up the spectrum of the source.

• Many piled-up events generate ‘cosmic ray-like’ patterns and are thus discarded.

– MOS diagonal doubles are a good diagnostic.

• Heavy pileup leads to the event energy being greater than the accepted cutoff – these events are then also discarded.

– The result is that ‘holes’ are seen at the centres of very bright sources.

NASSP Masters 5003F - Computational Astronomy - 2009

Other modes of operating the CCDs

• So far what has been described is full window imaging mode .

• But there are at least 2 other modes: – Small-window imaging mode.

• If we’re prepared to sacrifice some imaging area, we can have a shorter frame time.

– A way to image very bright sources while avoiding pileup.

• See timing diagram next slide...

– Timing mode. In this mode, the CCD is read out continually  much finer time resolution. This only works where the x-ray flux is dominated by a single bright source.

• The pn has an additional ‘burst mode’ which can give time resolution down to 7 μs.

NASSP Masters 5003F - Computational Astronomy - 2009

100x100 MOS small window mode example:

Integrate Shift and discard rows 1 to 250 (quick) Shift and read rows 251 to 350 (slow) Shift and discard pixels 1 to 250 (quick) Shift to ADC pixels 251 to 350 (slow) Shift and discard pixels 351 to 600 (quick) Shift and discard rows 351 to 600 (quick) NASSP Masters 5003F - Computational Astronomy - 2009

Windowed imaging examples

NASSP Masters 5003F - Computational Astronomy - 2009