1. Analysis of NuSTAR Bkg Analysis of NuSTAR Bkg Fabio Gastaldello & Silvano Molendi (IASF-Milano/INAF) 1

2. Goal Provide description of recent findings following 2 week stay at Caltech 2

3. 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 3

4. 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 4

5. Data Blank fields from COSMOS and ECDFS NuSTAR surveys 5

6. Data -Sunshine -No sunshine 6

7. Sun Rise clearly associated to sunshine Modulation over longer timescales 7

8. Spectra Blank fields from COSMOS and ECDFS NuSTAR surveys 30+ COSMOS fields 16ECDFS fields COSMOS fields short DE observation few ks ECDFS longer 15-17 ks -Sky ELV > 3 -Sky and nosunshine -Sky and sunshine 8

9. 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 9

10. 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. 10

11. Implications 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. 11

12. Dark Earth Spectra 12

13. Dark Earth Spectra 13

14. Dark Earth 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 14

15. Dark Earth 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 Most likely explanation is Compton backscattering of the CXB. 15

16. Exploring BKG 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-1 Sky 2.00e-1 0.10e-1 2.15e-1 0.51e-1 Ratio 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 16

17. Future Work Reduction of more fields to verfiy dependence on sun angle More analysis: stacking spectra image analysis etc. 17