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

Abstract

We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142+61 in the K' band ($λ_{\rm eff} = 2.11$ $μ$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 ($F_ν\propto ν^α$) was constrained to $α> -0.87$ (90% C.L.) for an interstellar extinction of $A_{V} = 3.5$.