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The linear polarization images of the jet in the giant elliptical galaxy M87have previously been observed with Very Long Baseline Array at 7 mm. Theyexhibit a complex polarization structure surrounding the optically thick andcompact subparsec-scale core. However, given the low level of linearpolarization in the core, it is required to verify that this complex structuredoes not originate from residual instrumental polarization signals in the data.We have performed a new analysis of the same data sets observed in four epochsby using the Generalized Polarization CALibration pipeline (GPCAL). This novelinstrumental polarization calibration pipeline overcomes the limitations ofLPCAL, a conventional calibration tool used in the previous M87 studies. Theresulting images show a compact linear polarization structure with its peaknearly coincident with the total intensity peak, which is significantlydifferent from the results of previous studies. The core linear polarization ischaracterized as fractional polarization of $\sim0.2-0.6$% and polarizationangles of $\sim66-92^\circ$, showing moderate variability. We demonstrate that,based on tests with synthetic data sets, LPCAL using calibrators having complexpolarization structures cannot achieve sufficient calibration accuracy toobtain the true polarization image of M87 due to a breakdown of the "similarityapproximation". We find that GPCAL obtains more accurate D-terms than LPCAL byusing observed closure traces of calibrators that are insensitive to bothantenna gain and polarization leakage corruptions. This study suggests thatpolarization imaging of very weakly polarized sources has become possible withthe advanced instrumental polarization calibration techniques.

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