realfast

realfast is a commensal observing system that searches VLA data for millisecond-timescale dispersed transient signals in real-time, in parallel with normal VLA observations.  The primary science goal is to detect and localize extragalactic fast radio burst (FRB) signals.  Starting in semester 2019A, the realfast system will begin to be used alongside some VLA observations.  For interested VLA observers, this page provides details about how realfast works, and the current policies regarding its use and data products.

Technical Description

Typical VLA observations are recorded with a time resolution of several seconds.  The WIDAR correlator integrates visibility data in time in two places; initial integration is done in the correlator FPGA hardware, and additional integration is done in the correlator backend (CBE) computer cluster.  The realfast system receives a copy of the fast-sampled (typically between 5 ms and 20 ms) visibility data stream from the CBE.  The CBE then integrates the visibility data to the few-second timescale requested for the primary observation.

The fast data stream is sent to a dedicated compute cluster with 160 graphical processing units (GPUs) running custom data processing code.  This system receives the fast visibility data, excises RFI and applies calibration, then performs interferometric gridding and imaging for each time slice at a range of trial dispersion measures (DM), all in real time.  If a candidate signal above threshold is detected, a time cutout of the original raw data (typically ~one second) around the event is saved for further offline analysis.

For further details about the realfast project and software, see the realfast project website.

Observing with realfast Resources

realfast requires no change to observing setup procedures and results in no difference in the primary VLA data;  its operation is completely transparent to the primary observers.  Eventually realfast is expected to operate in parallel with nearly all VLA continuum observations.  During commissioning its use is being limited to certain bands and correlator configurations. The following default continuum modes will operate commensally with realfast (all using full polarization):

While encouraged, use of commensal realfast is currently optional.  Observers with questions about how to set up their scheduling blocks or instrument configurations should contact the NRAO Science Helpdesk.

Data Access Policies

When it enters regular operation, all realfast results will be immediately public, via the VLA data archive.  Currently, the procedures for archival, quality assurance, and data access are still being developed.  During the commissioning period, primary observers will be invited to participate in any publications resulting from realfast data taken during their observations.