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Astron. Astrophys. 335, 12-18 (1998)

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4. Predictions for the EURD observations

We now predict the intensity, the wavelength and the width of the decay line due to neutrinos near the sun. The uncertainties which will be quoted are not formal errors but represent reasonable ranges for the values of these parameters. An attempt to detect this line is currently being made by the EURD detector (Bowyer et al 1995, Morales et al 1997, Bowyer, Edelstein & Lampton 1997) which is on board the orbiting Spanish satellite MINISAT 01.

To determine the intensity of the line one must know the opacity of the medium surrounding the sun for photons just beyond the Lyman limit. In fact the sun is known to be immersed in a partially neutral hydrogen cloud, which is the central part of what is called the Local Interstellar Medium (Cox & Reynolds 1987). The best determinations of the volume density [FORMULA] near the sun have been derived from HST observations of the Lyman [FORMULA] absorption line in the spectra of Procyon [FORMULA] (Linsky et al 1995) and of [FORMULA] (Wood, Alexander & Linsky 1996). They obtained for the lines of sight to these two stars [FORMULA] cm-3 and [FORMULA] cm-3 respectively. The two stars lie in rather different directions, so it is comforting that they lead to the same value of [FORMULA], namely 0.1 cm-3. The corresponding mean free path l for a photon at the Lyman limit [FORMULA] pc, which is substantially less than the distances to the two stars. Accordingly the flux in the line at the sun, which is [FORMULA], will be [FORMULA] cm-2 sec-1, since [FORMULA] cm-3 and [FORMULA] sec.

The wavelength [FORMULA] of the line for our updated value of [FORMULA] is


The error quoted is the uncertainty in the central wavelength, not the linewidth, which, as we shall see, is about [FORMULA]. Unfortunately our predicted wavelength falls right inside the position of a much stronger nightglow emission feature which stretches from about [FORMULA] to [FORMULA], and is due to the recombination of OII in the Earth's outer atmosphere (Chakrabarti 1984, Chakrabarti, Kimble & Bowyer 1984). This emission feature was detected by the EUV spectrometer on board the STP78-1 satellite which was launched in 1979. The minimum intensity of the detected feature is 15 Rayleighs, which is [FORMULA] times greater than our predicted flux for the decay line. Nevertheless it may be possible to observe this line if sufficient data are available (Bowyer 1997).

Finally we consider the expected width of the decay line. This width is due to the velocity dispersion v of the neutrinos producing the line. For a simple isotropic isothermal sphere model of the neutrino halo of our Galaxy one would have [FORMULA], where the asymptotic rotation velocity [FORMULA] of the Galaxy can be taken to be about 220 km. sec-1 (Binney & Tremaine 1987). Thus [FORMULA] km. sec-1 and so [FORMULA], which is much less than the wavelength resolution of EURD. However, recently Cowsik, Ratnam & Bhattacharjee (1996) have claimed to have constructed a self-consistent model of the dark matter halo of our Galaxy which requires v to lie between 600 and 900 km. sec-1. This claim has been challenged by Evans (1997), Gates, Kamionkowski & Turner (1997) and Bienaymé & Pichon (1997), and Cowsik et al (1997) have replied to the first two criticisms.

We do not wish to enter into this controversy here, and merely note that, if the decay line could be detected and its width measured, one would be able to deduce directly the velocity dispersion of the neutrinos. In this connexion it should be noted that in our strongly flattened model for the neutrino halo (Sciama 1997b), referred to in Sect. 2, the velocity dispersion, and so the linewidth, would depend strongly on direction. One could imagine measuring this anisotropic effect in a future mission with adequate wavelength resolution, if the decay line could be disentangled from the OI emission feature. In addition, if the neutrino halo itself has little or no rotation, one might be able to observe the Doppler effect associated with the sun's rotation in the Galaxy, which would shift the central wavelength of the line by nearly [FORMULA] in directions parallel and antiparallel to the sun's motion.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998