The Advanced Telescope for High ENergy Astrophysics
The X-IFU background
S. Lotti, C. Macculi, L. Piro, T. Mineo, S. Molendi, M. D’Andrea
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
Overview
• The Athena mission
• Importance of background in x-ray observations
• Particle background components
• Monte Carlo simulations for background estimates
• High energy particles induced background
• Low energy particles induced background
• Conclusions
The ATHENA Background Working Group
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The Advanced Telescope for High ENergy Astrophysics
The Athena Observatory
L2 orbit Ariane V
Mass < 5100 kg
Power 2500 W
5 year mission
Silicon Pore Optics:
2 m2 at 1 keV
5 arcsec HEW
Focal length: 12 m
Sensitivity: 3 10-17 erg cm-2 s-1
X-ray Integral Field Unit:
DE: 2.5 eV
Field of View: 5 arcmin
Operating temp: 50 mk
Barret et al., 2013 arXiv:1308.6784
1. How does ordinary matter assemble into
the large scale structures we see today?
•
The formation and evolution of clusters
and groups of galaxies
•
The chemical evolution of hot baryons
•
The Warm-Hot intergalactic medium
(WHIM)
Wide Field Imager:
DE: 125 eV
Field of View: 40 arcmin
High countrate capability
Rau et al. 2013 arXiv1307.1709
2. How do black holes grow and shape the Universe?
• Cosmic feedback:
the origin of black hole winds
• Cosmic feedback:
black hole and galaxy co-evolution
• Black hole growth in the early Universe
The ATHENA Background Working Group
The first stars, the first BH,
the first metals
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The Advanced Telescope for High ENergy Astrophysics
Background: why it matters
Particles component
ns nBsi Ad + BBdQWAs Diffuse component
Fmin =Fmin =
QAs QAs tDE
tDE
• Observation of diffuse/faint/distant objects
WHIM
Cluster outskirts
The ATHENA Background Working Group
Distant AGNs
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The Advanced Telescope for High ENergy Astrophysics
Particle background
Induced by 2 populations of charged particles able to
reach the focal plane, depositing part or all of their energy
ns
Fmin =
QAs
Bi Ad + BdQWAs
tDE
• High energy (>100 MeV)
- Solar protons
- Cosmic Rays
– Cross the spacecraft
– Generate secondaries
• Low energy (<100s keV)
– Concentrated by the optics
– Unstable and flaring
There are no experimental background data in L2 for X-ray detectors:
we proceed with MC simulations
L2 environment characterization
Accurate mass model
Physics model validation
Tuning simulation settings
Data analysis
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
High energy particles background
Requirement: unrejected background
< 5 x 10-3 p/cm2/s/keV
In the 2-10 keV band:
• Background level expected without CryoAC:
0.56 p cm-2 s-1 keV-1
• Inserting the CryoAC:
1.7 x 10-2 p cm-2 s-1 keV-1
• Inserting the kapton shield:
1.1 x 10-2 p cm-2 s-1 keV-1
• Inserting a bilayer (kapton + Bi):
7.5 x 10-3 p cm-2 s-1 keV-1
• Side CryoAC:
3 x 10-3 p cm-2 s-1 keV-1
<Bkg>~ 8E-3 (2-10 keV)
Req.
<Bkg>~ 3E-3 (2-10 keV)
The ATHENA Background Working Group
6
The Advanced Telescope for High ENergy Astrophysics
Background origin and composition
250 mm
Kapton
250 mm Kapton
10 mm W
250 mm Kapton
20 mm Bi
10 mm W
250 mm Kapton
250 mm SiC
10 mm W
250 mm Kapton
300 mm Si3N4
10 mm W
10 mm W
300 mm Si3N4
250 mm Kapton
1.3 mm Si3N4
250 mm Kapton
20 mm Bi
250 mm Kapton
1 mm SiC
Total [x10-3]
cts/cm2/s/keV
10
7.6
8.8
8.4
8.1
7.8
7.4
8
7.3
gamma [x10-3]
cts/cm2/s/keV
2
1.7
1.7
1.4
1.3
1
1.2
1.9
1.4
E.P.
W:
8.4 keV
9.6 keV
Bi:
10.8 keV
No
No
Si:
1.72 keV
No
Bi:
10.8 keV
E.P.
Bkg
2-12 keV
Shield
Lines?
Might be a problem for AGN at z~2-3
The ATHENA Background Working Group
5.7 keV
The Advanced Telescope for High ENergy Astrophysics
Low energy particles
Magnetic diverter required transmission efficiency
L2 low energy
environment
analysis
Geant4
simulations
Ray-tracing
simulations
Geant4
simulations
We want this flux < 0.1 x BGCR = 5 x 10-3 p cm-2 s-1
The ATHENA Background Working Group
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The Advanced Telescope for High ENergy Astrophysics
External soft protons flux
•
The low energy L2 environment is curre
ntly poorly known, complex, and highly
dynamical
•
In the solar wind:
•
F∝E-1.5
•
Fluxes @80 keV ~ 10.5 p cm-2 s-1 sr-1
keV-1
• From GEOTAIL/EPIC beyond
150 Re (Athena ~230):
• F∝E-3.3
• Fluxes @80 keV ~ 10.5 p cm-2 s-1 sr-1 keV-1
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
Mirrors focalization efficiency
Two independent estimates of the mirrors focalization efficiency:
• Ray-tracing simulations using the Firsov model on ATHENA optics
(T. Mineo):
I inc 
ndet
N inc
Aopt ( )
N det

Adet
ndet
( ) Aopt N det
3
3

 0.5  10 
   4 ~ 6.28  10 sr
4 Adet N inc
I inc
•
assume that the protons will behave like 1 keV photons (S. Molendi):
•
Firsov and Remizovich models implemented in Geant4  new sets of
simulations
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
Energy loss: X-IFU filters
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
Expected fluxes at focal plane
Putting everything together the expected flux for X-IFU: ~0.145 p cm-2 s-1
We want this flux < 0.1 x BGCR = 5 x 10-3 p cm-2 s-1  we need to reject 96.5% of the
incoming flux
From the cumulative this means we have to block up to ~75 keV with 100% efficiency
The ATHENA Background Working Group
The Advanced Telescope for High ENergy Astrophysics
Magnetic diverter
•
We performed an exercise sharing the results
with ESA:
ARF
OPEN
ELECTRODE
– Worst case: 75 keV proton entering the MD from
the edge of the optics to the opposite side of the
MD
NORMAL
ELECTRODE
– All the variables involved will depend on the MD
location and size
•
The required B ranges from 0.095 T to 0.38 T
(WFI) and from B=0.125 T to B=0.83 T (X-IFU
dewar entrance)
•
Further improvements:
– XMM proton response matrix constructed with
success, under construction for Athena
– AREMBES WP2 is providing directional proton
fluxes for the different magnetotail regions
The ATHENA Background Working Group
RMF: Input energy 40 keV
The Advanced Telescope for High ENergy Astrophysics
Conclusions
• Geant4 simulations allowed us to predict the X-IFU instrument
background when experimental date were not available
• High energy particles: simulations allowed study the expected
background level, origin and composition
• The background level was too high wrt the scientific
requirement, so we investigated several solutions:
• Insertion and optimization of an anticoincidence detector
• Insertion and optimization of a secondary electron shield
• More exotic solutions (lateral CryoAC, electron filter…)
• Low energy particles: simulations showed an expected
background level way above the requirement
• Insertion of a magnetic diverter to reduce their flux
• MD dimensioning according to the particles expected features (i.e.,
energy spectrum)
The ATHENA Background Working Group