A Commensal Radio-Only Cosmic Ray Detector at the Owens Valley Radio Observatory Long Wavelength Array

Abstract

The brief (10 nanoseconds) transient radio emission from cosmic ray air showers carries key information about the energy and mass composition of high energy cosmic rays, but anthropogenic radio frequency interference has historically prevented radio-based cosmic ray studies from being carried out independently from other types of detectors. We describe a cosmic ray detection system for the Owens Valley Radio Observatory Long Wavelength Array that searches for radio emission from cosmic ray air showers without relying on an external trigger, and runs alongside the other observing modes of the array. The OVRO-LWA, located in Eastern California, recently completed an expansion to 352 dual-polarization antennas and new signal processing infrastructure. In order to detect cosmic rays in the presence of radio frequency interference (RFI), initial event classification and RFI rejection is performed on Field Programmable Gate Array boards, which each process a sampled voltage timeseries from both polarizations of a subarray of 32 antennas. Each board uses dedicated RFI veto antennas outside the air shower radio footprint to reject RFI events. We present the trigger design, RFI flagging strategy, and candidate cosmic rays.

Figures (23)

• Figure 1: (a) OVRO-LWA antenna. Front-end electronics sit under the white square cover at the top of the antenna. (b) Typical OVRO-LWA spectrum from both polarizations of one antenna stand (the same antenna shown in panel a), including the response of the antenna and the entire analog signal path, with the spectra from the East-West-aligned dipole and the North-South-aligned dipole in orange and blue, respectively. (c) Layout of the 352 dipole pairs. (d) Inset showing the layout of the dense core array.
• Figure 2: Left: Simplified diagram of the OVRO-LWA signal path. Signals from 352 dual polarization antennas are transmitted to a central location, where 44 analog circuit boards filter 16 signals each, passing the signals to 44 digitization (analog-to-digital conversion [ADC]) boards, each of which samples 16 signals each. Eleven SNAP2 boards process the output of the ADCs, and the cosmic ray trigger logic runs on these boards. A 40 Gb switch allows cosmic ray data to be transmitted to a separate computer from the correlator.
• Figure 3: SNAP2 boards with important features labeled: A) 40 Gb QSFP+ Ethernet ports. B) 1 Gb Ethernet ports. C) The Xilinx Kintex Ultrascale FPGA is under this heat sink and fan--- the fan is used in the lab only, since in the OVRO-LWA cooled air flows through the entire electronics rack. D) One of two FPGA-mezzanine-card (FMC) connectors. A stacked pair of ADC cards (not pictured) attaches to each FMC connector. E) Power supply connector. F) Pin header for programming the board via JTAG. After using JTAG to load an initial firmware configuration, subsequent re-programmings use the 1 Gb Ethernet interface. G) SMA input for pulse-per-second (PPS) signal.
• Figure 4: Flowchart summarizing the trigger logic for the cosmic ray radio-only trigger. This process is implemented in firmware running on the SNAP2 FGPA boards.
• Figure 5: Flowchart summarizing the information flow in the cosmic ray detection system. Configuration settings and system monitoring information travel between the FGPAs and a central computer on a 1 Gb Ethernet network (gold arrows). Timeseries data travel from the FGPAs to the central computer on a 40 Gb Ethernet network (purple arrows). Trigger signals travel from each FPGA to its neighbor in a loop on a dedicated wire between general purpose input output pins on the FPGA boards (green arrows).