ISDC Data Centre for Astrophysics

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

We reanalyzed the INTEGRAL Earth occultation observations of early 2006 to derive IBIS spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range. The potential of such observations is demonstrated by the state-of-the-art hard X-ray spectra we derived for three fundamental components: the CXB, the GRXE and the Earth emission.

Türler et al. 2010, A&A 512, 49

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_⊙ 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.

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.
Credits: ISDC/M. Türler

Spectra of the CXB obtained with INTEGRAL
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).
Credits: ISDC/M. Türler

Recent determinations of the spectrum of the CXB
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.
Credits: ISDC/M. Türler

Spectrum of the GRXE in the 3-300 keV range
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).
Credits: ISDC/M. Türler

Various determinations of the Earth's hard X-ray emission
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 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.
Credits: ISDC/M. Türler

INTEGRAL observed the Earth in the night of 24-25 Jan 2006 (rev. 401), 2-3 Feb 2006 (rev. 404), 5-6 Feb 2006 (rev. 405) and 8-9 Feb 2006 (rev. 406). These data were immediately public and can be retrieved using the W3 Browse facility by following the steps below:

Push "More options"

In the table of catalogues, select only the "SCW - Science Window Data" and push "Specify Additional Parameters"

In the form, enter in the "obs_id" field the values e.g.: 8860092..8860096 (for all Earth observations listed below) 8860093* (for the SCWs from revolution 404 only)

Select SCWs you want to obtain and push "Request"

Enter your e-mail address and push "Submit request" and follow the instructions.

Revolutions	Observation pattern	Observation ID
401	Staring	88600920001 (pre-Earth obs.)¹ 88600920002 (Earth obs.) 88600920003 (post-Earth obs.)
404	Staring	88600930001 (pre-Earth obs.)¹ 88600930002 (Earth obs.) 88600930003 (post-Earth obs.)
405	Staring	88600940001 (pre-Earth obs.)¹ 88600940002 (Earth obs.) 88600940003 (post-Earth obs.)
406	Staring	88600950001 (pre-Earth obs.)¹ 88600950002 (Earth obs.) 88600950003 (post-Earth obs.)

¹ The pre-Earth obs. took place always in the previous revolution (e.g. 400), the real Earth obs and the post-Earth obs took place in the current revolution (e.g. 401).

Caution for data analysis:

Note that the observations are cut into Science Windows of ~20 minutes and are marked as Slews rather than Pointings. If you want to do an image analysis with OSA software (e.g. to identify point sources in the background field of view), make sure to change first the OBS1_ScwType parameter to 'ANY' (or 'SLEW') instead of 'POINTING'. For JEM-X analysis one has to change the value of the SW_TYPE keyword from 'SLEW' to 'POINTING' in the Science Window group. To avoid doing it by hand for each Science Window use the FTool fkeypar in a loop.