Spitzer Mid-Infrared Upper Limits on Anomalous X-Ray Pulsars 1E 1048.1–5937, 1RXS J170849.0–400910, and XTE J1810–197

We report on mid-infrared imaging observations of the anomalous X-ray pulsars (AXPs) 1E 1048.1-5937, 1RXS J170849.0-400910, and XTE J1810-197. The observations were carried out at 4.5 and 8.0 μm with the Infrared Array Camera and at 24 μm 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 upper limits are above the flux ratio for 4U 0142+61, indicating that if AXPs have similar infrared/X-ray flux ratios, our observations were not sufficiently deep to detect our AXP targets. For XTE J1810-197, the upper limits set a constraint on its rising radio energy spectrum, suggesting a spectral break between 4.2 × 1010 and 6 × 1013 Hz.

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