Optical I-band Linear Polarimetry of the Magnetar 4U 0142+61 with Subaru

TL;DR

This study presents optical I-band polarimetry of the magnetar 4U 0142+61, finding low polarization levels that suggest magnetar optical emission may not be strongly polarized, contrasting with radio pulsars.

Contribution

First optical I-band polarimetric measurement of the magnetar 4U 0142+61 using Subaru, providing constraints on its polarization and emission mechanisms.

Findings

Polarization degree P ≤ 5.6% at 90% confidence level.

Optical emission from the magnetar likely not strongly polarized.

Results imply differences in emission mechanisms compared to radio pulsars.

Abstract

The magnetar 4U~0142+61 has been well studied at optical and infrared wavelengths and is known to have a complicated broad-band spectrum over the wavelength range. Here we report the result from our linear imaging polarimetry of the magnetar at optical $I$-band. From the polarimetric observation carried out with the 8.2-m Subaru telescope, we determine the degree of linear polarization $P=1.0\pm$3.4\%, or $P\leq$5.6\% (90\% confidence level). Considering models suggested for optical emission from magnetars, we discuss the implications of our result. The upper limit measurement indicates that different from radio pulsars, magnetars probably would not have strongly polarized optical emission if the emission arises from their magnetosphere as suggested.