The Broadband Spectrum and Infrared Variability of the Magnetar AXP 1E 1048.1−5937

Abstract

We present photometry of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 in the infrared and optical, taken at Magellan and the VLT. The object is detected in the I, J, and Ks bands under excellent conditions. We find that the source has varied greatly in its infrared brightness and present these new magnitudes. No correlation is found between the infrared flux and spin-down rate, but the infrared flux and X-ray flux may be anticorrelated. Assuming nominal reddening values, the resultant spectral energy distribution (SED) is found to be inconsistent with the only other AXP SED available (for 4U 0142+61). We consider the effect of the uncertainty in the reddening to the source on its SED. We find that although both the X-ray and infrared fluxes have varied greatly for this source, the most recent flux ratio is remarkably consistent with what is found for other AXPs. Finally, we discuss the implications of our findings in the context of the magnetar model.