On the Polar Caps of the Three Musketeers

XMM-Newton observations of PSR B0656+14, PSR B1055-52 and Geminga havesubstantially increased the statistics available for these three isolatedneutron stars, so apparently similar to deserve the nickname of "ThreeMusketeers" (Becker & Truemper, 1997). Here we shall take advantage of the EPICstatistics to perform phase resolved spectroscopy for all three objects. Thephase-averaged spectrum of the three musketeers is best described by a threecomponent model. This includes two blackbody components, a cooler one, possiblyoriginating from the bulk of the star surface, and a hotter one, coming from asmaller portion of the star surface (a "hot spot"), plus a power law. Therelative contributions of the three components are seen to vary as a functionof phase, as the stars' rotation bring into view different emitting regions.The hot spots, which have very different apparent dimensions (in spite of thesimilarity of the three neutron stars polar cap radii) are responsible for thebulk of the phase variation. The amplitude of the observed phase modulation isalso markedly different for the three sources. Another striking aspect of ourphase-resolved phenomenology is the apparent lack of any common phasealignement between the observed modulation patterns for the two blackbodycomponents. They are seen to vary in phase in the case of PSR B1055-52, but inanti-phase in the case of PSR B0656+14. These findings do not support standardand simplistic models of neutron star magnetic field configuration and surfacetemperature distribution.Comment: 42 pages, 17 postscript figures. Accepted for publication in Ap