Search for Mid-IR Flux Variations from The Anomalous X-ray Pulsar 4U 0142+61

TL;DR

This study used Spitzer to observe the anomalous X-ray pulsar 4U 0142+61 after an X-ray burst, finding no significant mid-infrared flux variations, which suggests the source was in a quiescent state during observations.

Contribution

First mid-infrared observations of 4U 0142+61 post-X-ray burst showing no flux variation, highlighting the importance of multi-wavelength monitoring for understanding variability.

Findings

No significant mid-infrared flux variations detected.

Flux levels consistent with previous observations from 2005.

Source was in a quiescent state during observations.

Abstract

We report on our Spitzer observations of the anomalous X-ray pulsar 4U 0142+61, made following a large X-ray burst that occurred on 2007 February 7. To search for mid-infrared flux variations, four imaging observations were carried out at 4.5 and 8.0 $\mu$m with the Infrared Array Camera from February 14 to 21. No significant flux variations were detected, and the average fluxes were 32.1$\pm$2.0 $\mu$Jy at 4.5 $\mu$m and 59.8$\pm8.5$ $\mu$Jy at 8.0 $\mu$m, consistent with those obtained in 2005. The non-detection of variability is interesting in light of reported rapid variability from this source in the near-infrared, but consistent with the fact that the source already went back to its quiescent state before our observations began, as indicated by contemporaneous X-ray observations. In order to understand the origin of the near-infrared variability, frequent, simultaneous multi-wavelength observations are needed.