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Release of Band-to-Band High-Frequency Long-Baseline ALMA Test Data

Mar 31, 2020

ALMA is releasing data acquired as part of the Extension and Optimisation of Capabilities effort (EOC). These data were taken as part of the High-Frequency Long-Baseline Campaign (HF-LBC-2017) during Cycle 5, which was organised to test the calibration and imaging capability of ALMA at high-frequencies (>284GHz, Band 7) and using long baselines (>8.5km).

One main priority was to test and implement the band-to-band (B2B) observation mode. This technique allows the calibration of high-frequency observations by using a phase calibrator observed at a lower frequency, e.g. pairing Band 7 target observations with a Band 3 phase calibrator. The campaign conducted tests comparing standard In-Band phase referencing with the B2B mode using quasars, along with exploring the effects of phase calibrator to target separation angles, and phase referencing cycle times. A summary of the campaign is detailed by Asaki Y. et al. 2020 ApJS 247 23A. The successful campaign helped lead to the opening of Band 7 long-baselines during Cycle 7 and has also paved the way for Band 9 and 10 B2B mode observations that will be tested in the coming cycle.

As part of the HF-LBC-2017 effort observations were also conducted of previously targeted spatially extended sources, HL Tau and VY CMa in Band 9 which are available for download.

The released data consists of:

1) Continuum only (TDM) observations in Band 9 of the protostellar disk source HL Tau. Two single pointing executions blocks were taken. The tests included Walsh Switching enabling an aggregate bandwidth of ~15GHz and used phase referencing with <60s cycle times and a calibrator observed at Band 4.  B2B phase transfer was used to test the accuracy in imaging the disk around HL Tau in comparison to the 2014 SV data. A resolution <20mas was achieved and the inner ring structure could be discerned. However in using only the long-baseline array configuration at Band 9 the images resolve out most of the disk structure larger than 0.5” in size.. These data are available here.

2) Continuum and spectral line observations of the oxygen-rich red supergiant VY CMa. Walsh switching was enabled resulting in an aggregate bandwidth of 15GHz, with 6 of the 8 spectral windows (SPWs) using high-spectral resolution FDM mode and cover the 658GHz water maser and SiO line. The single execution block observation used a phase calibrator observed at Band 4 and a cycle time of ~76s. These observations use B2B calibration and subsequent self-calibration using a single channel covering the bright water maser. A comparisons was made to the 2014 SV data indicating that most of the North plume and Clump C is resolved out, while the central source of VY CMa remains unresolved in the continuum. The continuum (line-free) image achieved a resolution of 12x11mas. These data are available here.