The Submillimeter Array


The SWARM Development Team is very pleased to announce that the SMA now offers up to 32 GHz of processed bandwidth — twice the processed bandwidth of ALMA — which can be configured for 32 GHz of instantaneous on sky frequency coverage or 16 GHz of frequency coverage with dual polarization. Moreover, all observations automatically produce 140 kHz uniform spectral resolution across that full band—yielding about 13 million channels for each scan—all of which are archived at the CfA Radio Telescope Data Center (RTDC). The availability of 32 GHz of processed bandwidth greatly improves observing efficiency or sensitivity for both the SMA and the SMA baselines of the Event Horizon Telescope VLBI. The high spectral resolution across the entire bandwidth improves the flexibility of post observing analysis and potentially the usefulness of the archived data for future scientific investigations.

Options for configuring the improved bandwidth include: 16 GHz single polarization in each of two distinct atmospheric windows; 32 GHz in a single contiguous window; or dual polarization with 16 GHz of sky coverage in either the 230 or the 345 GHz window. A spectrum of Orion BN/KL, shows how we can obtain 32 GHz of contiguous sky coverage by choosing an offset of 8 GHz between the center frequencies of our double-sideband receivers. A similar setting is available for the 345 and 400 GHz receivers. We expect this to be very useful for line surveys. For dual polarization, two receivers are tuned with exactly overlapping bands. The VLBI mode for the EHT array covers 16 GHz (64 Gbps).

SWARM: A 32 GHz Correlator and VLBI Beamformer for the Submillimeter Array


The top figure shows the full 32 GHz instantaneous spectral coverage we obtain from the completed, four quadrant SWARM correlator. To obtain continuous spectral coverage, the 240 GHz receiver set was tuned 8 GHz higher in sky frequency than the 230 GHz receiver set. Since each sideband of each receiver produces 8 GHz of spectral coverage, and since the hole in coverage between the LSB and USB of each receiver set is also 8 GHz, tuning the two receiver sets in this way interleaves the sidebands, and produces gap-free coverage. The 345 GHz and 400 GHz receiver sets can be similarly offset, to provide 32 GHz of coverage in the 345 GHz window. The red and blue portion of the top gure represents the USB of a single receiver and SWARM quadrant. That quadrant is shown alone in the middle figure. The bottom figure shows a portion of that same quadrant, and highlights the high (~140 KHz) resolution SWARM provides throughout its 32 GHz coverage. Spectral lines labelled A are from 13CH3OH. SWARM produces nearly 1/4 million usable spectral channels on each baseline.