  Magnetars are neutron stars having extreme magnetic field strengths. Study of
their emission properties in quiescent state can help understand effects of a
strong magnetic field on neutron stars. SGR 0501+4516 is a magnetar that was
discovered in 2008 during an outburst, which has recently returned to
quiescence. We report its spectral and timing properties measured with new and
archival observations from the Chandra X-Ray Observatory, XMM-Newton, and
Suzaku. We found that the quiescent spectrum is best fit by a power-law plus
two blackbody model, with temperatures of kT_low~0.26 keV and kT_high~0.62 keV.
We interpret these two blackbody components as emission from a hotspot and the
entire surface. The hotspot radius shrunk from 1.4 km to 0.49 km since the
outburst, and there was a significant correlation between its area and the
X-ray luminosity, which agrees well with the prediction by the twisted
magnetosphere model. We applied the two temperature spectral model to all
magnetars in quiescence and found that it could be a common feature among the
population. Moreover, the temperature of the cooler blackbody shows a general
trend with the magnetar field strength, which supports the simple scenario of
heating by magnetic field decay.
