X-ray emission from middle-aged pulsars

We present a simple unified model that accounts for properties of X-ray emi ssion from the three middle-aged pulsars PSR 1055-52, PSR 0656-14, and PSR 0630+18 (Geminga). The X-ray radiation from these objects is pulsed more st rongly at energies above a transition point around 0.5 keV. In addition, th e phase of the pulses shifts by about 80 degrees -100 degrees around the sa me point. Geminga also has the peculiarity that its pulsed fraction decreas es in the 0.3-0.5 keV energy range, attaining a minimum near 0.5 keV. We sh ow that a two-component hydrogen atmosphere is able to account for these di sparate features. In our model, the hotter component is powered by particle bombardment and is restricted to the polar regions, while the cooler one c overs the entire stellar surface. The two components also differ in their e mission patterns, with the hard and soft contributions coming from areas ra diating into fan and pencil beams, respectively.