A revised view of the linear polarization in the subparsec core of M87 at 7 mm

TL;DR

This study reanalyzed 7 mm polarization data of M87's core using an advanced calibration pipeline, revealing a more accurate and compact polarization structure, and demonstrating the importance of improved calibration methods for weakly polarized sources.

Contribution

The paper introduces GPCAL, a new polarization calibration pipeline that improves accuracy over traditional methods like LPCAL, enabling better imaging of weak polarization signals in astrophysical sources.

Findings

GPCAL provides more accurate D-terms than LPCAL.

The core polarization is about 0.2-0.6% with angles 66-92°.

The polarization structure is more compact and aligned with the intensity peak.

Abstract

The linear polarization images of the jet in the giant elliptical galaxy M87 have previously been observed with Very Long Baseline Array at 7 mm. They exhibit a complex polarization structure surrounding the optically thick and compact subparsec-scale core. However, given the low level of linear polarization in the core, it is required to verify that this complex structure does not originate from residual instrumental polarization signals in the data. We have performed a new analysis of the same data sets observed in four epochs by using the Generalized Polarization CALibration pipeline (GPCAL). This novel instrumental polarization calibration pipeline overcomes the limitations of LPCAL, a conventional calibration tool used in the previous M87 studies. The resulting images show a compact linear polarization structure with its peak nearly coincident with the total intensity peak, which is significantly different from the results of previous studies. The core linear polarization is characterized as fractional polarization of $\sim0.2-0.6$% and polarization angles of $\sim66-92^\circ$, showing moderate variability. We demonstrate that, based on tests with synthetic data sets, LPCAL using calibrators having complex polarization structures cannot achieve sufficient calibration accuracy to obtain the true polarization image of M87 due to a breakdown of the "similarity approximation". We find that GPCAL obtains more accurate D-terms than LPCAL by using observed closure traces of calibrators that are insensitive to both antenna gain and polarization leakage corruptions. This study suggests that polarization imaging of very weakly polarized sources has become possible with the advanced instrumental polarization calibration techniques.