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  Current Research Highlight :: April 2006 all highlights

Hard X-ray glow of the Milky Way is now mapped with INTEGRAL

Scientists of the Max Planck Institute for Astrophysics (MPA) for the first time obtained the map and the spectrum of hard X-ray glow of the Galaxy. The emission, which origin remained a mystery for almost 30 years, is now believed to originate from millions of accreting white dwarf binaries.

Fig. 1: The map of the hard X-ray (17-60 keV) glow of the Milky Way obtained by INTEGRAL (false color map image) very well agrees with the near infrared image of the Galaxy (black countours). It strongly supports the idea that the bulk of this glow emission is provided by weak compact sources.

Fig. 2: Brightness distribution of the Galactic ridge hard X-ray emission (red dashed area) along the Galactic plane overplotted with the brightness distribution of the Galaxy in near infrared spectral band (blue line).

Emission of our Galaxy in different spectral bands has very different origin. For example, in near infrared, optical, and ultraviolet spectral bands the Galactic emission is dominated by stars. In hard gamma rays the Galactic emission is due to interactions of high energy cosmic rays with the Galactic interstellar matter (ISM). In X-ray and soft gamma ray energy bands the emission of the Galaxy is dominated by some hundred bright compact sources (accreting neutron stars and black holes). An additional weak X-ray component was detected, which was not resolved into point sources in existing observations. This glow was called the Galactic ridge X-ray emission (GRXE), because it concentrates towards the Galactic plane forming something like a ridge of X-ray glow.

Recently it was shown that there are good reasons to believe that the GRXE (at least a dominant fraction of it) is due to a large number of very weak X-ray sources — accreting white dwarf binaries and coronally active stars (see e.g. the linkPfeil.gifresearch highlight of march). The implication of this conclusion is a definitive prediction of the shape of the ridge spectrum and the spatial distribution of the GRXE. The spectrum of the ridge emission should have an exponential cutoff at energies higher than ~20 keV, which approximately corresponds to the proton binding energy of the white dwarf. The spatial distribution of the GRXE should trace the density of Galactic stars.

These predictions are now verified by a team of MPA astronomers using the data of one of the most advanced hard X-ray imagers on orbit — the IBIS telescope of the INTEGRAL observatory. Data collected by this instrument over its 4 years of operation allowed them to map the weak hard X-ray glow of the Galaxy and measure its spectrum. The map does not correlate well with the Gamma-ray map of the Mikly Way, strongly suggesting that X-ray emission in not generated by the cosmic rays interaction with ISM. Instead the intensity of the GRXE in hard X-rays traces well the stellar mass density distribution, thus providing strong support to the idea that the bulk of GRXE is provided by weak compact sources. In particular, for the considered 17-100 keV energy band, the dominant contribution to the GRXE should come from accreting white dwarf binaries. This result was further strengthened by the detection of a high energy cutoff in the rigde spectrum.

At higher energies (200-500 keV) the diffuse emission of annihilating positrons start to dominate over all other Galactic emission (see e.g. the linkPfeil.gifresearch highlight of may 2005). At even higher energies (>MeV) cosmic ray induced emission unambigousely appears.

R.Krivonos, M.Revnivtsev, E. Churazov, S. Sazonov, S. Grebenev and R. Sunyaev

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