X-IFU
ATHENA
BACKGROUND
FILES
Doc. no. : IAPS-XIFU-TN-2015-002
Issue
: 0.1
Date
: 30 March 2015
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Title
:
The ATHENA background files for X-IFU
Prepared by
:
S. Lotti
Date
:
Checked by
:
L. Piro, C. Macculi
Date
:
PA agreed by :
Date
:
Authorised by :
Date
:
Distribution:
Athena team
Document Change Record
Issue
Date
0.0
30 March 2015
Changed
Section
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Description of Change
Wrote
30 March 2015
ATHENA
X-IFU
BACKGROUND
FILES
Doc. no. : IAPS-XIFU-TN-2015-002
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Table of contents
Applicable Documents ................................................................................................................. 3
Reference Documents ................................................................................................................. 3
Abbreviations and acronyms ....................................................................................................... 4
1
Background files ................................................................................................................ 5
1.1
Internal Detector Background .............................................................................................. 6
1.2
Cosmic X-Ray Background .................................................................................................. 6
1.3
Total Background .................................................................... Error! Bookmark not defined.
ATHENA
Doc. no. : IAPS-XIFU-TN-2015-002
X-IFU
BACKGROUND
FILES
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Applicable Documents
[AD#]
Doc. Reference
Issue
Title
Reference Documents
[RD#]
Doc. Reference
Issue
Title
[RD1]
S. Lotti et al, A&A Vol 569,
In-orbit background of X-ray microcalorimeters and
September 2014
its effects on observations
[RD2]
[RD3]
S. Lotti et al, NIMA
Vol. 686, pp.
[RD4]
D. Mc Cammon, The
Vol. 576, pp.
Astrophysical Journal
188–203
31-37
Estimate of the impact of background particles on
the X-ray Microcalorimeter Spectrometer on IXO
A high spectral resolution observation of the soft Xray diffuse background with thermal detectors
ATHENA
X-IFU
BACKGROUND
FILES
Abbreviations and acronyms
Item
Meaning
ACD
Anti-Coincidence Detector
X-IFU
X-ray Integral Field Unit
CXB
Cosmic X-ray Background
NXB
Non X-ray Background
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ATHENA
Doc. no. : IAPS-XIFU-TN-2015-002
X-IFU
BACKGROUND
FILES
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Background files
The extracted folder contains several files:

the response matrices (athena_xifu_1469_onaxis_pitch265um_v20150327.rsp and
athena_xifu_1190_onaxis_pitch265um_v20150327.rsp)




the internal background files,
o
int1arcmin2_athena_xifu[…].pha: particle background in cts s -1 keV-1 arcmin-2.
o
int5arcsec_athena_xifu[…].pha: particle background in cts s-1 keV-1.
the CXB and the Galactic foreground background files,
o
diffuse1arcmin2_athena_xifu[…].pha: diffuse background file in cts s-1 keV-1 arcmin-2.
o
diffuse5arcsec_athena_xifu[…].pha: diffuse background file in cts s -1 keV-1.
two total background files, that are simply the sum of the internal and diffuse backgrounds,
o
TOT_1arcmin2_athena_xifu[…].pha: total background file in cts s-1 keV-1 arcmin-2.
o
TOT_5arcsec_athena_xifu[…].pha: total background file in cts s -1 keV-1.
The diffuse background model, reported also in
Table 1: Diffuse_model.xcm
The background files for point and extended sources are given in two different folders. For extended sources
the background files are normalized to 1 arcmin 2, while for point sources they assume an extraction radius of
5”. All the background files are given in rates.
The folder contains also the calibration files (ARF and RMF).
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ATHENA
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1.1 Internal Detector Background
The internal background has been calculated with a Geant4 simulation and is consistent with a constant
value of 0.05 cts cm-2 s-1 in the 0.2-10 keV. It is induced by charged particles (cosmic rays and solar
protons, see [RD1] and [RD2]). For the purpose of XSPEC simulations the distributed files are based on a
constant internal background.
1.2 Cosmic X-Ray Background and Galactic foreground
The diffuse X-ray background consists of a soft thermal spectrum at low energies (< 1 keV), associated with
the galactic plane and halo hot plasma, and of a hard power-law spectrum at higher energies produced by
the integrated emission of many faint hard spectra of unresolved extragalactic point sources.
It is modeled inside XSPEC as the sum of a thermal emission apec (emission spectrum from collisionallyionized diffuse gas that includes line emissions from several elements) and a powerlaw (a simple photon
powerlaw that dominates at higher energies) models, multiplied by a wabs model for the photoelectric
absorption at low energies, with the addition of an unabsorbed thermal apec component, as reported by
[RD3].
The model parameters are reported in
Table 1 (normalizations refer
to 1 arcmin2) and assume that 80% of
the extragalactic component can be resolved.
Table 1
Model parameters: apec+wabs*(apec+powerlaw)
Component
Parameter
Unit
Value
apec
kT
keV
9.90E-02
apec
Abundance
1
apec
Redshift
0.0
apec
norm
wabs
nH
1022
1.8E-02
apec
kT
keV
0.225
apec
Abundance
1
apec
Redshift
0.0
apec
norm
7.3E-7
powerlaw
PhoIndex
1.52
powerlaw
Norm
2.0E-7
1.7E-06
ATHENA
X-IFU
BACKGROUND
FILES
Doc. no. : IAPS-XIFU-TN-2015-002
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1.3 Variations
If the user wants to vary the extraction area/response matrix/focal lenght of the telescope the procedure to
modify the background files is the following 1:
o
Different extraction areas: In this case the internal and diffuse backgrounds scale the same way,
so the user can just modify the total background files.
o
Open a terminal and type
fv TOT_1arcmin2_athena_xifu[…].pha
o
Press the “all” button to open a new window with the file columns
o
From the menu Tools --> Calculator
o
In the new window set RATE=RATE*NEA (where NEA is the new extraction area in arcmin 2)
o
Press Calculate
o
Close
o
From the menu File --> Save
And the new file will be scaled to the new extraction area
o
New response matrix: in this case the diffuse background has to be recalculated from the model,
and added to the particle one.
o
Open XSPEC and type
o
@Diffuse_model.xcm
o
fakeit none
o
the software will ask you (leave default for anything else):

the name of the new response matrix

fake data file name (i.e., yournewdifffile.fak, WARNING: do NOT use dashes “-” in the
filename or mathpha will interpret them as minus signs, use underscores “_” instead)

Exposure time, correction norm, bkg exposure time: set an high exposure time to make
sure every bin of the response matrix gets enough counts (i.e., 1e8) and leave the
other fields blank
o
o
Exit XSPEC
Now XSPEC has created the new background file in counts, you need to convert it in rate, so:
o
mathpha yournewdifffile.fak
o
when asked “Units algebraic expression to be performed in” set R
o
when asked “O/p PHA filename” set yournewdifffile.pha
o
when asked “Exposure time flag/value” set CALC
o
when asked “Areascal flag/value” set 1
and your file in counts will be converted in a rate file.
o
Now you have to add yournewdifffile.pha to the internal background:
o
mathpha
o
when asked “Expression to be evaluated” set yournewdifffile.pha +
int1arcmin2_athena_xifu[…].pha
o
1
when asked “Units algebraic expression to be performed in” set R
note that this is not the only way to do this, if you feel confident with your own procedure you should probably stick to it.
ATHENA
X-IFU
BACKGROUND
FILES
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o
when asked “O/p PHA filename” set yournewtotfile.pha
o
when asked “Exposure time flag/value” set CALC
o
when asked “Areascal flag/value” set 1
and you will have your new total background file yournewtotfile.pha
o
New focal lenght: the particle background for sky unit observed goes with the ratio of the focal length
squared, so the internal background file has to be scaled accordingly:
o
Open a terminal and type
fv int1arcmin2_athena_xifu[…].pha
o
Press the “all” button to open a new window with the file columns
o
From the menu Tools --> Calculator
o
In the new window set RATE=RATE*FR (where 𝐹𝑅 = (
o
Press Calculate
o
Close
o
From the menu File --> Save
𝑛𝑒𝑤𝑓𝑜𝑐𝑎𝑙 2
𝑜𝑙𝑑𝑓𝑜𝑐𝑎𝑙
) )
And the new internal particle background file will be scaled according to the new focal length.
o
Now you have to add the new internal background to the diffuse one:
o
mathpha
o
when asked “Expression to be evaluated” set int1arcmin2_athena_xifu[…].pha +
diffuse1arcmin2_athena_xifu[…].pha
o
when asked “Units algebraic expression to be performed in” set R
o
when asked “O/p PHA filename” set yournewtotfile.pha
o
when asked “Exposure time flag/value” set CALC
o
when asked “Areascal flag/value” set 1
and you will have your new total background file yournewtotfile.pha