INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

Fig. on the right: Simulation of INTEGRAL's Earth observation (EO 1) of January 2006. The animation shows a reconstruction of the sky as seen by the IBIS/ISGRI instrument at hard X-ray energies (27.4 keV). It is shown on a logarithmic scale of intensity - increasing from dark and blue to red and bright areas - to reveal the glow of the Earth (blue globe) and the diffuse emission from the sky background (purple) and the galactic ridge (red strip), which are much fainter than the point sources (bright spots). The Earth coordinates (latitudes and longitudes) are overlayed in white with the position of the magnetic pole (circle). The Earth appears to move through the field-of-view and its apparent size decreases as the INTEGRAL spacecraft moves away from the planet on its high-altitude orbit.

Abstract

Aims. We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations of early 2006.
Methods. We analyse the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model instead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of each component is obtained by fitting the normalization of the model lightcurves to the observed modulation in different energy bins. Because of degeneracy among some components, a realistic choice of the input parameters with an additional constraint for spectral smoothness is used to guide the fits.
Results. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT. A 10% higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the presence of a minimum around 80 keV with improved statistics and yields an estimate of ~0.6 M_Sun for the average mass of white dwarfs in the Galaxy. The analysis also provides updated normalizations for the spectra of the Earth's albedo and the cosmic-ray induced atmospheric emission.
Conclusions. This study demonstrates the potential of INTEGRAL Earth-occultation observations to derive the hard X-ray spectrum of three fundamental components: the CXB, the GRXE and the Earth emission. Further observations would be extremely valuable to confirm our results with improved statistics.

CXB Results

Fig. on the right: Comparison of the IBIS/ISGRI CXB spectrum obtained here (red circles) with the previous INTEGRAL results of IBIS/ISGRI (black diamonds), JEM-X (blue squares) and SPI (green triangles) published by Churazov et al. (2007).

Fig. on the left: Comparison of the CXB spectrum obtained by INTEGRAL - JEM-X measurements (blue squares) from Churazov et al. (2007) and our IBIS/ISGRI results (red circles) - with the HEAO-1 spectra and analytical model by Gruber et al. (1999) (error bars and solid line). The results for the A-2 and A-4 instruments of HEAO-1 are shown in orange and green, respectively. The grey dashed line shows the effect of increasing the intensity of the HEAO-1 model by 10% as proposed by Churazov et al. (2007).

Fig. on the right: Comparison of the INTEGRAL IBIS/ISGRI (red circles, this work) and JEM-X (magenta diamonds,Churazov et al. 2007) spectra with the other recent CXB measurements by Swift and BeppoSAX. The Swift/XRT error box (orange shaded area) and the Swift/BAT results (green triangles) are from Moretti et al. (2008) and Ajello et al. (2008), respectively. The best-fit model of Moretti et al. (2008) for the combined Swift dataset is shown with a black line and grey uncertainty area. The original BeppoSAX/PDS measurements of Frontera et al. (2007) have been scaled by +13% in intensity (blue squares) to correct for the difference in Crab normalization with respect to INTEGRAL.

GRXE Results

Fig. on the right: Comparison of the obtained GRXE spectrum (blue squares) with recent other determinations all renormalized to the central radian of the Milky Way defined by |l| < 30° and |b| < 15°. The previous INTEGRAL/IBIS data (red circles) and the RXTE/PCA measurements (black diamonds) are from Krivonos et al. (2007, Fig. 14). The INTEGRAL/SPI spectrum (green triangles) is from Bouchet et al. (2008, Fig. 9).

Fig. on the left: Comparison of the obtained GRXE spectrum (blue error bars and long-dashed line) with the higher-energy emission of the Galactic ridge in the region defined by |l| < 30° and |b| < 15° as observed by INTEGRAL/SPI (green triangles) from Bouchet et al. (2008, Fig. 9) and by CGRO/COMPTEL (back diamonds) and CGRO/EGRET (red error bars). The CGRO data are from Porter et al. (2008, Fig. 3), which is based on the analysis by Strong et al. (1999, 2004). The green solid line shows the model fitting the SPI observations above 100 keV (Bouchet et al. 2008). It is the sum of a powerlaw continuum (dotted line), the positronium continuum (short-dashed line) and a narrow electron-positron annihilation line at 511 keV.

Earth emission results

Fig. on the right: Resulting spectrum of the total Earth emission (green triangles, solid line) with separated contributions from the Earth reflection of the CXB (cyan triangles, short-dashed line) and the cosmic-ray induced atmospheric emission (magenta triangles, long-dashed line). The thin grey curves are normalized as derived by Churazov et al. (2007), whereas the thick colored lines are normalized to match our measurements.

Fig. on the left: Comparison of the obtained Earth emission spectrum (green triangles) with previous determinations by various missions. The thin grey line is the INTEGRAL spectrum of Churazov et al. (2007) as described in the Fig. above. The obtained IBIS/ISGRI spectrum lies well in between the OSO-3 (black diamonds) and the Swift/BAT (red circles) measurements of Schwartz & Peterson (1974) and Ajello et al. (2008), respectively. The values of the \emph{BeppoSAX}/PDS measurements of Frontera et al. (2007) have been increased by 13% (blue squares) to correct for the difference in Crab normalization with respect to INTEGRAL.