INTEGRAL observes the Earth occulting GX 301-2 and the CXB
Back in 2006, INTEGRAL performed a series of four Earth-occultation
observations (EO) to measure the intensity of the cosmic X-ray background
(CXB). The CXB is the overall radiation of unresolved active galactic nuclei
located at cosmological distances. The strategy of these observations was to
allow exceptionally the Earth to pass through the field of view (FoV) of
INTEGRAL. The occultation of the CXB by the Earth would then result in a
decrease of detector counts (see the
Picture of the Month of
March 2006). This
strategy was found to be very successful and allowed to determine the CXB
spectrum between 3 and 100 keV [1,2].
In order to refine the measurements, a new series of observations was started
in 2012. The goal was to increase the observing time and to reduce
uncertainties due to the presence of several point sources and the diffuse
Galactic ridge emission in the region observed by INTEGRAL in 2006. Unfortunately,
the results of the 2012 Earth-observation campaign were disappointing [3]. In
addition to perturbations from higher solar activity, the data were found to
be strongly affected by radioactive decay resulting from the spacecraft
activation in the proton belts at perigee passage. To overcome this problem
due to the change of the INTEGRAL orbit over the years, the Mission Operation
Center scheduled the first EO of 2013, EO 3.1, no more at the start, but at
the end of the revolution when the spacecraft approaches the perigee of its
orbit.
The animation shows a simulation of EO 3.1 performed in July 2013. The image
spans the ~30 degree wide FoV of the IBIS instrument with the central white
circle indicating the FoV of the JEM-X instrument. The color coding represents
the radiation as seen on the detector plane. The decrease of intensity towards
the borders is a vignetting effect due to the reduced exposure in the
partially-coded FoV. The apparent size of the Earth rapidly increases when the
spacecraft approaches perigee. The simulation includes the expected reflection
of the CXB by the Earth atmosphere and some X-ray auroral emission.
The point source at the bottom left is the high-mass X-ray binary GX 301-2,
which was found to be very active during the observation. The white lightcurve
drawn at the top of the image is the observed modulation of the IBIS/ISGRI
detector counts. The variability on short timescales is entirely due to GX
301-2, which was also briefly occulted by the Earth disc. This extra
contribution could however be removed to reveal the actual CXB occultation
signal (yellow curve).
The success of this first pre-perigee observation is encouraging to pursue the
EO program in the future.
References:
[1] INTEGRAL observations of the cosmic X-ray background in the 5-100 keV range via occultation by the Earth,
Churazov E., Sunyaev R., Revnivtsev M., et al. A&A 467, 529, 2007
[2] INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission,
Türler M., Chernyakova M., Courvoisier T. J.-L., et al. A&A 512, A49, 2010
[3] Analysis of the new INTEGRAL Earth observations to measure the cosmic X-ray background,
Türler M., Produit N., Pavan L., et al. PoS (INTEGRAL 2012) 067, 2013 arXiv:1302.6493
