1. The X-ray Universe 2008 Granada, 29 May 2008
INTEGRAL Spectrum of the Cosmic X-ray Background from Occultation by the Earth
Marc Türler (ISDC, Geneva)
(Collaborators: M. Chernyakova, A. Neronov,
N. Produit, R. Walter, T. Courvoisier)

2. INTEGRAL's Earth Observation
4 similar public observations of ~8 hours at start of revol , 405 & 406 in Jan-Feb 2006
Earth moving through the FoV at ~fixed attitude
Pointing includes the galactic plane close to the bulge (Norma arm)
Earth diameter ~10°, but decreasing with time (+ rotation, etc.)
Churazov et al. (2007)

3. Two different approaches
Churazov et al. (2007) use Monte-Carlo simulations to model the spectral shape of the Earth emission and its reflection of the CXB (HEAO-1 spectrum). They find a CXB normalization exceeding that of HEAO-1 by about 10%, but cannot constrain the CXB spectral shape.
Our approach uses the spatial distribution of the Earth emission, the galactic ridge and point sources to derive the CXB spectral shape.
Churazov et al. (2007)

4. Sky component model
We model the spatial distribution of 4 components in the IBIS/ISGRI FoV modulated by the Earth occultation:
The Galactic ridge emission
All (>2) point sources
An uniform CXB
The Earth emission as a smooth transition between
CXB reflection (at low E.)
Atmospheric emission (due to cosmic rays)

5. Instrumental components
Additional components are due to the instrument: an uniform (time variable) instrumental background, as well as the modulation by the instrumental responses:
IBIS efficiency map
(Nomex) at 30 keV
IBIS exposure map

6. Model of detector lightcurves
The convolution with the exposure and efficiency maps modifies the detector lightcurve profile:

7. Fitting the model lightcurves
Model detector lightcurves are generated for each EO and all spectral channels and are then fitted to the data.

8. Using all 4 Earth observations
We do separate fits to all four EOs which cannot be averaged because of different:
s/c attitude
point sources (fixed count rate extracted for each EO)
instr. background (same variations as seen in the SPI-ACS)

9. Resulting count spectra
To obtain smoother spectra for each EO, we do iterative fits with additional minimization of the dispersion of counts in adjacent energy bins (maximum entropy approach).
The final count spectra are the average of all 4 EOs with uncertainties based on their dispersion.

10. The resulting CXB spectrum
Unfolded spectra are obtained with XSpec using OSA 7.0 ISGRI ARF & RMF responses

11. Conclusions
The obtained CXB spectrum in the keV range is consistent with the HEAO-1 spectrum.
We do not confirm the about 10% higher normalization found by Churazov et al. (2007) in agreement with the BeppoSAX results of Frontera et al. (2007).
The Galactic ridge emission is also very well constrained and is in qualitative agreement with Krivonos et al. (2007).
Krivonos et al. (2007)