Spitzer Mid-infrared Upper Limits on Anomalous X-Ray Pulsars 1E   1048.1-5937, 1RXS J170849-400910, and XTE J1810-197
  We report on mid-infrared imaging observations of the anomalous X-ray pulsars (AXPs) 1E 1048.1-5937, 1RXS J170849-400910, and XTE J1810-197. The observations were carried out at 4.5 and 8.0 microns with the Infrared Array Camera and at 24 microns with the Multiband Imaging Photometer on the Spitzer Space Telescope. No mid-infrared counterparts were detected. As infrared emission from AXPs may be related to their X-ray emission either via the magnetosphere or via a dust disk, we compare the derived upper limits on the infrared/X-ray flux ratios of the AXPs to the same ratio for 4U 0142+61, an AXP previously detected in the mid-infrared range. The comparison indicates that our three non-detections are consistent with their relatively low X-ray fluxes. For XTE J1810-197, our upper limits set a constraint on its rising radio/millimeter energy spectrum, suggesting a spectral break between 1.5$\times 10^{11}$--6$\times10^{13}$ Hz. 
Z. Wang, V. M. Kaspi, and S. J. U. Higdon