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

Abstract

Abstract We have searched for pulsation of the anomalous X-ray pulsar (AXP) 4U 0142$+$61 in the $K^\prime$ band ($\lambda_{\rm eff}$$=$ 2.11$\mu$m) using the fast-readout mode of Infrared Camera and Spectrograph (IRCS) at the Subaru 8.2 m telescope. We found no significant signal at the pulse frequency expected by the precise ephemeris based on an X-ray monitoring observation with RXTE. Nonetheless, we obtained the best upper limit of 17% (90% C.L.) for the root-mean-square pulse fraction in the $K^\prime$ band. Combined with the $i^\prime$ band pulsation (Dhillon et al. 2005, MNRAS, 363, 609), the slope of the pulsed component ($F_\nu \propto \nu^\alpha$ in units of Jy) was constrained to $\alpha$$>$$-$0.87 (90% C.L.) for an interstellar extinction of $A_{V}$$=$ 3.5.