# Features of Structure and Absorption in the Jet-Launching Region of M87
## 2 Observations and Data Reduction  
In 28 March 2015, we observed M87 at 22 GHz (\(\lambda=1.3\) cm) and 43 GHz (\(\lambda=7\) mm) with VLBA. To optimize the \(u-v\) coverage, the two observing frequencies were cycled in each of the observing runs. Six days later, we observed M87 at 86 GHz (\(\lambda=3\) mm) with VLBA. For all three frequencies, some antennas were absent during the observations due to some technical problems. The details of the problems are described in the footnote of Table 1.  
An automatic reference pointing control was used for the antenna-pointing. 5-min scans on the nearby bright source 3C 273 and 3C279 as fringe finders, delay and bandpass calibrators, were inserted during the observations, every 35 min for 22/43 GHz and 75 min for 86 GHz. The received signals were sampled with 2-bit quantization and recorded with an aggregate data rate of 2048 Mbit s\({}^{-1}\). See details of observations in Table 1.  
The initial data calibration was made in AIPS (Astronomical Image Processing System) software package. For 22 and 43 GHz, we followed the standard procedure of VLBA data reduction. For 86 GHz, the procedure is slightly different: the global fringe-fitting was first performed on scans of 3C273 and 3C279 with point source models to derive time evolutions of the residual delay, rate, and phase for each IF separately; the derived residual delay varies slowly with time, so the solutions of 3C 273 and 3C279's delay were used as a first-order approximation for M87's residual delay; then the global fringe-fitting was performed twice on M87, the first was performed with a point source model, the solution interval was set to 30 s and the threshold of signal to noise ratio (SNR) was set to 3.0, to avoid false signals, a tight delay and rate search window (10 nanoseconds and 20 mHz for delay and rate respectively) were used, the IFs are averaged to increase the SNR by a factor of 2.8; then we produce an initial image, and perform the second one with the CLEAN models obtained from the initial map to increase the fringe detection rate (the detection rate was increased slightly for 0.2%).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & **Frequency** & \multirow{2}{*}{**Telescopes**} & **On-Source Time** & **Beam Size \({}^{\mathrm{a}}\)** & \(I_{\mathrm{peak}}\)\({}^{\mathrm{b}}\)** & \(\sigma\)\({}^{\mathrm{c}}\) \\  & **(GHz)** & & **(Min)** & **(mas \(\times\) mas, deg)** & **(mJy beam\({}^{-1}\))** & **(mJy beam\({}^{-1}\))** \\ \hline
28 March 2015 & 22.72 & VLBA \({}^{\mathrm{d}}\) & 124 & 0.93 \(\times\) 0.45, 12.6 & 1054 & 0.360 \\  & 43.12 & VLBA \({}^{\mathrm{d}}\) & 124 & 0.56 \(\times\) 0.23, 16.7 & 726 & 0.224 \\
3 April 2015 & 86.28 & VLBA \({}^{\mathrm{e}}\) & 289 & 0.24 \(\times\) 0.12, \(-\)17.7 & 501.4 & 0.217 \\ \hline \end{tabular}  
* Notes: \({}^{\mathrm{a}}\) The Synthesized beam FWHM size of the images. \({}^{\mathrm{b}}\) The peak flux density on the images. \({}^{\mathrm{c}}\) The root-mean-square noise of the images. \({}^{\mathrm{d}}\) Nine antennas were used, MK was absent due to the “bad diskpack” problem.
* Seven antennas were used, HN and SC have no 86 GHz receiver while NL was “disc failure”.  
\end{table}
Table 1: Observations.  
After calibration, multi-source data are split into single-source files and imported into DIFMAP for self-calibration and imaging. Self-calibration/Imaging loops are performed for multiple iterations to obtain images with high dynamical ranges. The final image was produced in DIFMAP with natural weighting.