Introduction

The MWA is capable of performing triggered observations. This facilitates automated and rapid follow-up of astronomical transients and other time-critical space-based events. It is possible to perform rapid-response observations using the standard MWAX correlator and the associated Voltage Capture System (VCS) and Voltage Buffer.

Two possible pathways access the back-end of the MWA triggering services, which schedule and perform the rapid-response observations. The user can directly send triggers to MWA (recommended when requiring the lowest latencies) or respond to public event streams using the TRACE-T web application. The back-end and original MWA triggering front-end system are described in Hancock et a. (2019).

Rapid-response observing requests sent to MWA will only override the schedule if the target is above the horizon. If the scheduled programs are interruptable, the trigger overrides the observing schedule for the requested length of observing time.

Triggering with the MWAX standard correlator

• All triggered observations are pointed observations (not drift scans). TRACE-T automatically requests that the observer-specified observation length be divided into snapshots of 120 seconds (this is the default for the ‘exptime’ parameter for MWAX standard correlator observations in the Triggering web services). This ensures the target stays near the centre of the primary beam.

• If the trigger occurs during the day, the MWA triggering service engages the Sun avoidance subroutine, which optimises for the best pointing direction that places the Sun into a primary beam null. Note that this can add up to 10 seconds to the telescope response time (see Hancock et al. 2019 for details).

Triggering with the Voltage Capture System

• For MWAX Voltage Capture System (VCS) triggers, TRACE-T automatically requests that the observer-specified observation length be divided into 5-minute files (this is the default for the ‘exptime’ parameter for MWAX VCS observations in the Triggering web services), which makes data processing more manageable.

Triggering with the Voltage Buffers

• The Voltage Buffers can provide up to 4 minutes of voltage data before the trigger from wherever the telescope was pointed during that period.

• The Voltage Buffer data is saved with the same observation ID(s) and project ID(s) as what was scheduled before the trigger.

• If the Voltage Buffer trigger is to be followed by further observations with the VCS, a second trigger request must be sent to the telescope simultaneously.

• Idle array configurations or shadowing for when the array is not in use can be requested in your observing proposal to give some control over the telescope pointing at the time of the buffer dump.

Getting Started

1. Choose the triggering pathway

The next section outlines the two possible pathways. Choose one and set up the code.

2. Act on events

Start a live instance of the chosen triggering pathway. Check if the code can listen to the correct alert streams and filter them correctly.

3. Set up a test account

Contact Andrew Williams to set up a test account, which enables the user to send real triggers to MWA without overriding the observing schedule. This will demonstrate how the telescope will respond to the triggering requests.

4. Submit MWA proposal

Write an MWA proposal for the next observing semester. Make sure it outlines the science case for requiring rapid-response observations.

Triggering Pathways

Direct Triggering

Andrew to fill out

TRACE-T Web Application

TRACE-T is the Transient RApid-response using Coordinated Event Triggering software triggers on General Coordiates Network (GCN) VOEvents (XML packets) sent over Kafka. It filters these alerts based on user-defined logic and then sends observation requests to MWA. The user has full oversight of their event type of interest, the observing requests sent to the telescope and whether or not the observing request was successful. TRACE-T can also send email, SMS and phone call alerts when triggering an observation.

TRACE-T Version 1 - Current version

https://github.com/ADACS-Australia/TraceT

The current version of TRACE-T was written by the Australian Data and Computing Services (ADACS), which is based on the original front-end systems described in Hancock et al. (2019) and Anderson et al. (2021). Please reference both publications if you use this software.

Functionality

The current TRACE-T functionality was implemented based on the interests of the MWA collaboration. It ingests VOEvents sent by the Neil Gehrels Swift Observatory, the Fermi Gamma-ray Space Telescope, LIGO-Virgo-Kagra (LVK) and Monitor of All-sky X-ray Image (MAXI) to perform triggered observations on gamma-ray bursts (GRBs), gravitational wave events (GWs), and flare stars. When setting up a triggering program, the user selects the transient type (GRB, GW or flare star) and then chooses the event telescope (Swift, Fermi, LVK or MAXI).

TRACE-T can send triggered observation requests to both the MWA and the Australia Telescope Compact Array (ATCA; see Anderson et al. 2021a). Both required an accepted proposal and access credentials supplied by the respective observatory.

Latencies

Important note: If running multiple triggering programs from your TRACE-T instance, each ingested transient event notice will be filtered by each program individually (in series) rather than simultaneously, taking 2-5 seconds each. Each triggering program allows you to specify a priority, which will ensure that those requiring the lowest latencies are processed first.

A detailed break-down of the latencies associated with MWA triggering is provided in Section 3.1.1. of Hancock et al. (2019). The key latencies to be aware of are the following:

• The theoretical hardware minimum for a triggered MWA observation to begin is 8 seconds.

• The event filtering conducted by TRACE-T and web back-end systems provides an additional 2 seconds of latency.

• If the trigger occurs during the day, an additional 10 seconds of latency is accrued by the Sun-avoidance algorithm.

Latency updates that differ from Hancock et al. (2019) due to system upgrades and following the installation of MWAX include:

• If requesting a calibrator observation to take place following the trigger, the back-end now calculates the nearest calibrator after scheduling the triggered observation of the transient.

See Hancock et al. (2019), Anderson et al. (2021b), Tian et al. (2022a), Xu et al. (2025) for a breakdown of latencies associated with real triggered observations of gamma-ray bursts (GRBs) with MWA. This demonstrates that in practice the MWA response time is usually between 30-90 seconds. Note that most of these GRBs observations were triggered before the TRACE-T program prioritisation upgrade described above.

Triggering setup

Gamma-ray Bursts (GRBs)

Gravitational Wave events (GWs)

See Tian et al. (2023) for further details on the observing mode and strategy

In Progress

A more user-friendly version of TRACE-T is now being developed by ADACS. This next iteration will allow for more free-form triggering on any transient event broadcast via NASA GCN. Upgrades will include: