No evidence of millimetre excess emission in the Crab Nebula
Résumé
We present a comprehensive study of the Crab Nebula spectral energy distribution (SED) over more than 6 decades in frequency ranging from 1 to 106 GHz. We focus on the millimetre regime where an excess of flux has been claimed. For this purpose we use new radio and submillimetre data from the WMAP satellite between 23 and 94 GHz and from the Archeops balloon experiment between 143 and 545 GHz, and a compendium of already published Crab Nebula observations. The Crab SED has been compared to models including three main components : synchrotron which is responsible for the emission at low and at high frequencies, dust which explains the excess of flux observed by the IRAS satellite and an extra component to account for a possible millimetre excess. For this extra component we have considered both a low-temperature dust and a low-energy cutoff synchrotron. In the case of extra dust component the best fit model requires extremely low dust temperatures of 5-6 K and therefore a large dust mass of $\sim$110-230~M$_\odot$, making the model fully unrealistic. In any case the extra dust component does not significantly improve the fit to the data in the millimetric regime from 100 to 1000 GHz. For the case of an extra synchrotron component we find that the spectral index of the energy distribution and the cutoff frequency are not constrained by the model. More importantly the best-fit to the data has larger residuals in the millimetric regime that when assuming no extra component. The currently available data in the unpolarised emission of the Crab Nebula show no evidence of millimetric excess with respect to the known synchrotron emission. This is important for the Planck satellite mission which will use the Crab nebula for polarisation cross-checks in the range 30 to 857 GHz.
Origine : Fichiers produits par l'(les) auteur(s)