Transcript Lecture 1

Lecture 22
• XMM instrumentation and calibration.
– Mirror effects
• PSF
• Vignetting
– EPIC cameras
• EPIC background
• Event lists and selection
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XMM-Newton
• 3 x-ray EPIC
telescopes
– European Photon
Imaging Cameras.
– 2 of these have:
• MOS detectors
• Reflection Grating
Arrays (RGAs)
– The 3rd has
• a pn detector
• No RGA.
• Mirrors all the same
– nested Wolter
– f ~ 7 m.
Schematic of the satellite
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EPIC telescope schematic
(not to scale)
Reflection Grating
Spectrometer
MOS
Optic axis
Reflection
Grating Array
Mask
Mirror assemblies
Filter wheel
Optic axis
CCDs
pn
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.
– PSF caused by scattering, not diffraction
– For both XMM and Chandra, the PSF varies
markedly with off-axis angle.
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Mirror effects: PSF
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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:
SKing r  
A
1  r r  
2 
0
– r0, 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
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).
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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.
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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.
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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 xrays absorbed during this time.
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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.
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OOTEs continued
Bright pn OOTEs
Faint MOS OOTEs
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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.
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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 fullwindow 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.
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100x100 MOS small window mode example:
Shift and discard rows 1 to 250 (quick)
Shift and read rows 251 to 350 (slow)
Shift and discard pixels 1 to 250 (quick)
Integrate
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
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Flux calibration
• It is difficult to find x-ray sources which
stay constant over time.
• One can calibrate on the ground, but the
ride to orbit shakes things up.
• The Crab nebula (remnant of a supernova
in AD 1054) is the best compromise.
– The nebula itself is pretty stable but there is a
pulsar (with x-ray pulses) in the centre.
– Crab is also very bright – saturates modern,
sensitive detectors like XMM.
• ‘Crab units’ commonly used as a measure
of x-ray flux.
NASSP Masters 5003F - Computational Astronomy - 2009
Software packages
• It seems common practice in the HE community to
develop a new package for each new mission.
– EG
• ROSAT: EXSAS
• XMM: SAS
• Chandra: ciao
– Not ideal either from software engineering principles, nor
efficient use of resources.
– Packages tend to be non-portable, non-generic
• There’s also small incentive to document things properly!
– Ciao is the best in my view. The USA tends to do
software better than Europe – they spend more money
on it for one thing. (But big is not always beautiful...)
– SAS: some good generic FITS stuff there – but difficult to
get at.
• I’d like to do something about that!
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Background
• Background from instrumental noise
– Worse at low energies & higher chip temperatures.
• X-ray background
– Cosmic
– Fluorescence
• Si of course, but also Al and Cu from support structure.
• Particle background
– Hard, penetrating – “cosmic rays”.
• Fairly constant in time;
• Fairly isotropic.
– Soft protons (~100 eV).
• Flaring time behaviour.
• Funnelled by the mirrors.
• These weren’t suspected before launch! A major headache,
because too strong a flare can damage the CCDs.
NASSP Masters 5003F - Computational Astronomy - 2009
Background examples
Mostly from flares.
Note the background in the masked areas.
dec
MOS
pn
Cu fluorescence.
pn
(Masking here is done via software.)
RA
Instrumental noise at low energy.
pn
(Masking here is done via software.)
NASSP Masters 5003F - Computational Astronomy - 2009
Background – what to do with it
• Significance of background depends on
what you want to do.
– Spectra: obviously one needs to know the
spectrum of the background as well as
possible.
– Images, in particular source detection and flux
measurement: spatial properties of the
background are important.
• Cosmic ray, x-ray and flares all have different
spatial behaviour – so working out the proportions
is important.
– Time series:
• Soft proton flares dominate the problem.
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Other mainly spatial problems with EPICs:
Optical loading from a bright visible-light
source (filters minimize this)
Single-reflection arcs from far-field
sources
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Event lists
• In high-energy astronomy, we deal not
with voltages or brightnesses (essentially
floating-point quantities) but with lists of
events – 1 event per photon.
• Each event comes with the following data:
– Its pixel position on the CCD.
– If necessary, the number of the CCD.
– Its frame number.
– Its energy. (XMM: the column is called PI.)
• Maybe also: a quality flag, event pattern, etc.
• In XMM output the events are stored in a
table in a FITS file.
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Event selection
• The aim is to separate ‘interesting’ events
from ‘boring’ events – eg divide the events
into those which probably come from a
source and those which don’t.
E
Define a selection
volume
t
All events
Good
Bad
•Limits in defining volume shapes.
•Problems integrating over overlapping volumes.
•FITS format for storing selections: Data SubSpace (DSS)
r
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Diagnostic plots:
Cu fluorescence
line
Photon energy
Al fluorescence
line
PN
‘Soft proton’ bursts
It’s helpful to plot 2 of the event
coordinates – here energy vs time.
Time
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Diagnostic plots:
MOS 1
‘Gatti’ events
Al fluorescence
line
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Gatti process – a kind of dithering.
Histogram
of events with
voltage V.
ADC levels are analog
- thus not evenly spaced.
Distorted digitized
histogram.
Undistorted
histogram.
V
V
-
+
V
t
ADC
=
t
NASSP Masters 5003F - Computational Astronomy - 2009