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R.Terrier APC, Paris SAp CEA/Saclay

Nature, March 18 2004. Point source contribution to the soft  -ray Galactic diffuse emission. R.Terrier APC, Paris SAp CEA/Saclay.

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R.Terrier APC, Paris SAp CEA/Saclay

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  1. Nature, March 18 2004 Point source contribution to the soft -ray Galactic diffuse emission R.Terrier APC, Paris SAp CEA/Saclay F. Lebrun, A. Bazzano, G. Bélanger, A. Bird, L. Bouchet, A. Dean, M. del Santo, A. Goldwurm, N. Lund, H. Morand, A. Parmar, J. Paul, J.-P. Roques, V. Schönfelder, A.W. Strong, P. Ubertini, R. Walter, C. Winkler

  2. Sources detected with ISGRI during GCDE Use all GCDE + 2 ToO data (~2000 scw) Maps in the 20-40, 40-60, 60-120, 120-220 keV energy bands Filtered to remove residual background non-uniformities significance computed from local fluctuations in the image Find excesses in 20-60 keV map 91 sources above 6 Fit source position in 20-60 keV map Determine flux in each energy band Look for possible counterparts Look in X, , radio sources as well as pulsars, SNRs catalogs

  3. Sources detected with ISGRI during GCDE

  4. ~40 LMXB

  5. 7 HMXB

  6. SNR, pulsars, SGR, CV

  7. AGNs

  8. sources of unknown type

  9. New sources ~15 new sources

  10. Total of 91 detected sources

  11. Statistics (as in Nature paper). 40 LMXB 7 HMXB 2 pulsars 1 ms pulsar 2 SNR 1 SGR 1 Seyfert 1 --------------- 54 identified 11 unknown at other wavelengths 26 new -------------- 91 total

  12. Using IBIS as a collimator IBIS imagery suppresses structures larger than the PSF (12') but large FOV (19° FWHM) Use IBIS as a collimator experiment Compare measured count rate to expected count rate from detected sources: • Cosmic-ray induced BKG (especially time varying BKG) • Isotropic & constant component (Cosmic diffuse BKG + internal BKG) • Point sources & diffuse Galactic emission Need to: • Correct for isotropic and time varying backgrounds • Estimate precisely source count rate in the detector Residuals: • Should be distributed along Galactic plane = additional constraint • Give the maximal level of interstellar Galactic emission

  13. On background subtraction Strong BKG variations need to be corrected for: High energy (>500 keV) count rate dominated by CR induced events (~60 c/s) -ray events <0.3 c/s for 1 Crab Correct for BKG variation using HE information Determine correlation with >500 keV count rate using high latitude observations |b|>30° (no sources, no Galactic emission) Uncertainties in this relation limits the accuracy of flux correction Allows to evaluate isotropic & constant BKG

  14. Estimating the sources count rate To estimate the total point sources flux through 'ISGRI collimator': For each 2.2ks pointing: • For all detected sources • Use the estimated count rate based on imaging • Correct for acceptance & absorption using the source position in the FOV • Sum all sources count rates • Normalize total flux using crab observations to correct for inaccuracies in imaging flux calibration • Use Crab observations from rev 102-103 Systematics of ~10% are introduced by this calibration

  15. Longitude profiles |b|<5°

  16. Longitude profiles |b|<5°

  17. Fitting the Galactic emission To further constraint the diffuse emission in the residuals: Assume: Galactic interstellar emission distributed as: • 5° FWHM gaussian along Galactic plane In central regions (|l|<20° & |b|<25°): Subtract source count rate from corrected count rate Fit the latitude distribution of residuals with Galactic emission profile cor(b) - s(b) = GB(b) +  Isotropic component Width dominated by FOV

  18. Latitude profiles |l|<20° Sco-X1: large angle acceptance pb

  19. Latitude profiles |l|<20° Shielding transparency larger

  20. Remaining diffuse flux 20 – 40 keV 8.6 ± 2.9 c/s 14 ± 4.5% of total Galactic emission 40 – 60 keV 0.2 ± 1.1 c/s < 19% (3) 60 -120 keV 0.7 ± 1.3 c/s < 27% (3) 120 – 220 keV -0.3 ± 0.6 c/s < 52% (3) sources Consistent with SPI. SPI more sensitive at high energies diffuse

  21. Summary & future prospects Galactic emission is dominated by point sources • Fraction ~ 14% unaccounted for by detected point sources ( 20- 40 keV) • No significant residual Galactic emission detected by IBIS at higher energies • But IBIS Constraint not very strong above 100 keV • Need to reduce the systematics: Use a better angular response for ISGRI Take into account source variability • Need to understand better the connection with low energy diffuse process: Study of the 15-20 keV band in progress • logN-logS studies to evaluate undetected sources contribution • High energy studies using simultaneously IBIS & SPI Precise determination of truly diffuse emission in the reach of INTEGRAL

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