Subaru constraint on circular polarization in I-band emission from the Magnetar 4U 0142+61

TL;DR

This study reports the first optical circular polarimetry of the magnetar 4U 0142+61, setting constraints on its polarization and informing models of magnetar emission mechanisms.

Contribution

It provides the first imaging circular polarimetry measurement of an optical magnetar, offering new observational constraints.

Findings

Measured circular polarization V=1.1±2.0%, with an upper limit of 4.3% at 90% confidence.

Result is inconclusive for discriminating emission models due to large uncertainties.

Highlights the need for linear polarimetry to better understand magnetar emission mechanisms.

Abstract

We present the first imaging circular polarimetry of the anomalous X-ray pulsar (AXP) 4U 0142+61 at optical wavelengths. The AXP is the only magnetar that has been well studied at optical and infrared wavelengths and is known to have a complicated broad-band spectrum over the wavelength range. The optical polarimetric observation was carried out with the 8.2-m Subaru telescope at I-band. From the observation, the degree of circular polarization V was measured to be V=1.1+/-2.0%, or |V|<4.3% (90% confidence). The relatively large uncertainty was due to the faintness of the source (I=23.4-24.0). Considering the currently suggested models for optical emission from magnetars, our result is not sufficiently conclusive to discriminate the models. We suggest that because linear polarization is expected to be strong in the models, linear polarimetry of this magnetar should be conducted.