R.TerrierAPC, 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

Point source contribution to the soft -ray Galactic

diffuse emission

Nature, March 18 2004

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 bandsFiltered to remove residual background non-

uniformitiessignificance computed from local fluctuations in the image

Find excesses in 20-60 keV map 91 sources above 6Fit source position in 20-60 keV map

Determine flux in each energy band

Look for possible counterpartsLook in X, , radio sources as well as pulsars, SNRs catalogs

Sources detected with ISGRI during GCDE

~40 LMXB

7 HMXB

SNR, pulsars, SGR, CV

AGNs

sources of unknown type

~15 new sourcesNew sources

Total of 91 detected sources

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

Using IBIS as a collimatorIBIS 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 emissionNeed 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

On background subtractionStrong 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 CrabCorrect for BKG variation using HE information

Determine correlation with >500 keV count rateusing 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

Estimating the sources count rateTo 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

Longitude profiles

|b|<5°

Longitude profiles

|b|<5°

Fitting the Galactic emissionTo 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 rateFit the latitude distribution of residuals with Galactic emission profile

cor(b) - s(b) = GB(b) +

Width dominated by FOV

Isotropic component

Latitude profilesSco-X1:large angle acceptance pb

|l|<20°

Latitude profiles

Shielding transparency larger

|l|<20°

Remaining diffuse flux20 – 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

diffuse

Consistent with SPI.

SPI more sensitive at high energies

Summary & future prospectsGalactic 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 ISGRITake 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