Smooth or spotty? - Two 1.809 MeV images of our Galaxy

Smooth or spotty ? - Two 1.809 MeV images of our Galaxy

(J. Knödlseder et al., 1999, A&A, 345, 813)

What is the difference between these two skymaps? The algorithm that has been used to obtain the images. Both maps show the distribution of 1.809 MeV gamma-ray line emission throughout the sky, which originates from the radioactive decay of 26Al. The COMPTEL telescope aboard the Compton Gamma-Ray Observatory (CGRO) has observed the entire sky during its nine years of mission lifetime, and has produced an enormous amount of data that were the basis of these images. Yet, most of the data recorded by COMPTEL were not due to photons coming from the sky but due to interactions of cosmic-ray particles with the telescope and the satellite. The 1.809 MeV photons comprise only a tiny fraction of all recorded events, and in particular, they are extremely rare. In addition, the data encode the information about the origin of the 1.809 MeV photons in a complex way, requiring non-linear reconstruction algorithms to extract images of gamma-ray emission.
The top map has been obtained using the Maximum Entropy (ME) algorithm that iteratively determines a skymap that is compatible with the data. Against a common believe, the ME image is not the smoothest image that is compatible with the data! The bottom map is much smoother, but is equally well consistent with the COMPTEL 1.809 MeV data. This map has been obtained using the newly developped Multiresolution Expectation Maximisation (MREM) algorithm, that considers also the spatial structure of the emission features; in contrast, ME splits the skymap into individual pixels which are treated as independent. In fact, this presents a major problem in the reconstruction of the 1.809 MeV skymap since source intensities are not sufficiently significant in individual pixels to allow for a reliable extraction of diffuse emission features from the data. And 1.809 MeV appears to be diffuse, at least at the angular resolution of four degrees of the COMPTEL telescope.
However, the question remains whether the 1.809 MeV is really so smooth as indicated by the MREM skymap. COMPTEL just can't tell, more sensitive gamma-ray telescopes such as SPI on INTEGRAL are needed to answer this question. What is clear, however, is that if the 1.809 MeV sky is spotty it will not be identical to the ME image, since the spots in this image are produced by statistical noise that is artifically amplified by the reconstruction algorithm.

Last update : 5-6-2002