Search for Near-Infrared Pulsation of the Anomalous X-ray Pulsar 4U 0142+61

TL;DR

This study searched for near-infrared pulsations from the anomalous X-ray pulsar 4U 0142+61 but found no significant signals, setting upper limits on pulsation amplitude and constraining the spectral slope of the pulsed emission.

Contribution

First near-infrared pulsation search for 4U 0142+61, establishing upper limits and spectral constraints on its pulsed emission.

Findings

No significant near-infrared pulsation detected.

Upper limit of 17% on pulse fraction in K' band.

Pulsed component spectral slope constrained to > -0.87.

Abstract

We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142+61 in the K' band ($\lambda_{\rm eff} = 2.11$ $\mu$m) using the fast-readout mode of IRCS at the Subaru 8.2-m telescope. We found no significant signal at the pulse frequency expected by the precise ephemeris obtained by the X-ray monitoring observation with RXTE. Nonetheless, we obtained a best upper limit of 17% (90% C.L.) for the root-mean-square pulse fraction in the K' band. Combined with i' band pulsation (Dhillon et al. 2005), the slope of the pulsed component ($F_\nu \propto \nu^\alpha$) was constrained to $\alpha > -0.87$ (90% C.L.) for an interstellar extinction of $A_{V} = 3.5$.