nassp masters 5003f - computational astronomy - 2009 lecture 19 epic background event lists and...

NASSP Masters 5003F - Computational Astronomy - 2009

Lecture 19

• EPIC background

• Event lists and selection

• The RGA

• Calibration quantities

• Exposure calculations


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.


Background examplesNote the background in the masked areas.

Mostly from flares.

Cu fluorescence. Instrumental noise at low energy.

(Masking here is done via software.) (Masking here is done via software.)

MOSpn

pn pn

dec

RA


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.


Other mainly spatial problems with EPICs:

Optical loading from a bright visible-lightsource (filters minimize this)

Single-reflection arcs from far-fieldsources


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.


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.

All events

Good Bad

r

E

t

Define a selectionvolume

•Limits in defining volume shapes.•Problems integrating over overlapping volumes.•FITS format for storing selections: Data SubSpace (DSS)


Diagnostic plots:It’s helpful to plot 2 of the eventcoordinates – here energy vs time.

PN

Cu fluorescenceline

Al fluorescenceline

Time

Pho

ton

ener

gy

‘So

ft p

roto

n’

bu

rsts


Diagnostic plots:MOS 1

Al fluorescenceline

‘Gatti’ events


V

Gatti process – a kind of dithering.

Histogramof events withvoltage V.

ADC levels are analog- thus not evenly spaced.

Distorted digitizedhistogram.

+

V

t ADC

-

Undistortedhistogram.

V

t

=


The Reflection Grating Spectrometer (RGA)

• Each MOS has one.

• They divert about ½ the x-rays.

• Diffraction grating array of 9 CCDs.

• Pixel position in the dispersion direction is a function of x-ray energy.– But not a linear function (I think there is a

cosine term in it).

• Energy resolution is much sharper than via amount of charge the photons generate.

• Spectral orders overlap –– but the 2nd order has even finer resolution.


RGA –plot showing the event pixel locations:


The ‘banana plot’


An example RGS spectrum:

Spectral resolution:about 2 eV


An example EPIC spectrum:

Spectral resolution:about 100 eV


Charge redistribution• Photons of a single, narrow energy give rise to

broadened charge redistribution spectrum.– Partly because of Poisson (quantum) statistical variation;– Partly because of smearing out during the transfer of charges

from row to row during readout.• The relation between true spectrum S and measured

spectrum S':

• R is called the redistribution matrix (RM).• As the chips degrade with age (due mostly to particle

impacts), the RM changes and has to be recalibrated.• The philosophy with x-ray spectra is not to subtract

background or deconvolve RM, but to begin with a model, and add background and RM-convolve this before comparing it with the measured spectrum.– See the program XSPEC.

ESEEREdES ,


MOS RM cross-section at 800 eVEnergy of the x-rays


Evolution of the energy dispersion

Black: pnRed and Green: the MOS chips

MOS temperatures werelowered here.

1.5

keV

6.0

keV

nassp masters 5003f - computational astronomy - 2009 lecture 19 epic background event lists and...

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