SVOM - Indico

Report
Influence of the earth on the background of the SVOM instruments
Study performed by Donghua Zhao during her one year stay in France
Ms. Donghua is a IHEP PhD student. Her stay was founded by the CSC-FCPPL PhD grants
programs.
She arrived in Saclay 2010, December the 1rst, and she leaved 2011, November the 31.
She plans to finish her PhD In 2012 June
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
The SVOM mission is devoted to the study of Gamma-ray burst
It is a french chinese collaboration. The main laboratories involved are:
NAOC beijing, IHEP beijing, XIOPM Xian, CEA-Irfu, IRAP Toulouse
Launch date 2017
The scientific payload aboard SVOM
GRM
VT
ECLAIRs
MXT
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
SVOM Pointing Strategy
Sun
Night side
Sun
Night side
SVOM orbit (i ~ 30°)
Sun
Night side
A pointing close to the antisolar in the night part of the orbit is required in order to
permit the fast follow-up of localized GRBs with large telescope on Earth
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
SVOM Pointing Strategy
At each orbit (every 90 minutes) the Earth enters inside the instruments FOV
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
X and gamma ray background issues in a low earth orbit
Cosmic protons
Cosmic X-ray background
The presence of the Earth in the FOV introduces:
- An occultation of the diffused X-ray background
- A reflection of the X-ray background on the Earth
- An albedo component du to cosmic proton interactions in the earth atmosphere
In this work we focus on the assessment of the albedo component
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
Albedo gamma : computation of the emissivity 1/4
For 1 cm2 of earth atmosphere :
dN
C
=
dE E / 44keV 5 + E / 44keV 1.4

ph / cm
3μ 1+ μ  1.47  0.0178   / 2.8 +  / 2.8
C=
2
5π


Rcut
1+ 
0.25
0.4 
1.3 1+ 2.5 
where,
μ = cosθ
Rcut = 59.4 
1.5 1
0.4

,
/ s / sr / keV
ph / cm
2

/ s / sr


cos4 λm
1+ 1+ cos λ sinθsinξ  
1/ 2 2
3

2
GV 
m
Sazonov et al. 2007
Albedo gamma : computation of the emissivity 1/3
For a given atmosphere cell of 1 cm2, the emissivity depends on:
1 the solar modulation at the epoch of observation
2 the zenithal angle θ of the cell viewed from the spacecraft
3 the rigidity cut off at the cell position
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
Albedo gamma : computation of the emissivity 2/4
The spacecraft is moving, it intercepts different atmosphere spherical caps
At a given satellite position, one consider a spherical cap
We divide the considered cap in elementary cells
For each cell we compute Rcut
At the end one integrate the emissivity over the spherical cap area.
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
Albedo gamma : computation of the emissivity 4/4
Emissivity computed along the SVOM orbit (Inclination 30°)
Taking into account the 2015 magnetic field model, the worse position on the SVOM orbit is:
Alpha=287°E delta=30°N
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier
X and gamma ray Background computation versus the position of
the earth relative to the instrument field of view
Using the previous results we performed Geant4 simulations
Hypotheses position on the orbit Alpha=287°E delta=30°N (worse case)
Cosmic gamma
Reflected cosmic gamma
Albedo gamma
Conclusions
• Donghua Zhao did a very serious work during
her stay in Saclay
• Thanks to her work, the collaboration is now
in a position to compute the X and Gamma ray
background and its variation at any time and
at each point of the SVOM orbit.
• A paper summarizing this work has been
submitted to NIM last month (Donghua Zhao et al.)
5th France China Particle Physics Laboratory (FCPPL) Workshop - 2012 March – Orsay
B. Cordier

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