analysis of nustar bkg analysis of nustar bkg fabio gastaldello & silvano molendi...
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Analysis of NuSTAR Bkg Analysis of NuSTAR Bkg
Fabio Gastaldello Fabio Gastaldello
&&
Silvano Molendi Silvano Molendi
(IASF-Milano/INAF)(IASF-Milano/INAF)1
Data Data
Blank fields from COSMOS and ECDFS NuSTAR surveys
• 64 COSMOS fields• 16 ECDFS fields
• COSMOS fields short DE passages, few ks Sun angle 130+ deg
• ECDFS 15-17 ks DE Sun angle 70+ deg
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Data Data
Blank fields from COSMOS and ECDFS NuSTAR surveys- Sky ELV > 3
- Bright Earth BE < -5- Dark Earth (BE > 5)&&(ELV <-5)
Energy band 1.8keV<E< 3.5keV
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Spectra Spectra Blank fields from COSMOS and ECDFS NuSTAR surveys
30+ COSMOS fields16 ECDFS fields
COSMOS fields short DE observation few ksECDFS longer 15-17 ks
- Sky ELV > 3- Sky and nosunshine - Sky and sunshine
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What could it be? What could it be?
• Compton Scattering of solar light from elements in the satellite affording a direct view of the detectors
• Aperture stops by construction have direct line of sight to the detectors.
• Back-scattering off aperture stops one of the major contributors, if sun angle is larger than 90
Reflected Solar X-rays
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Scattering elements Scattering elements
Other scattering elements also present
Sun angle ~70deg
ECDFS observations feature significantly weaker contamination.Many observations are effectively contamination-free.Plausible explanation aperture stops provide most significant contribution for Sun Angle > 90,For smaller angles other elements provide some scattering surface.Needs to be verified through analysis of more fields.
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ImplicationsImplications• Presence of contamination in an observation
can be easily assesed by comparing sunshine with no-sunshine spectra
• Most of the time contamination, if present, is restricted to E<6-7 keV
• In a few instances can extend up to 15 keV.
• In these rare cases filtering against sunshine will remove contaminating component
• More complex strategies can be implemented to minimize data loss.
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Dark Earth Dark Earth is not so dark! is not so dark!
During «quiescent» periods DE spectra feature a component which is consistent with being a rescaled version of the aperture background seen during SKY observations.
Scaling factor is ~ 2
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Dark Earth Dark Earth is not so dark! is not so dark!
During «quiescent» periods DE spectra feature a component which is consistent with being a rescaled version of the aperture background seen during SKY observations.
Scaling factor is ~ 2Most likely explanation is Compton
backscattering of the CXB.
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Exploring BKG Exploring BKG phenomenology phenomenology cr[4-20keV] full FOV unit A
In ECDFS fields In COSMOS fields mean stdev mean stdev DEARTH 8.21e-2 0.41e-2 1.05e-1 0.54e-1Sky 2.00e-1 0.10e-1 2.15e-1 0.51e-1Ratio 0.410 0.025 0.489 0.220
When Solar contamination is modest ratio of Sky/DEarth cr is well constrained 6%
In COSMOS ratio is far less constrained, however can be reduced to ~6% by excluding sunshine data
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