4 Pi Sky VOEvent Broker becomes the standard for rapid-response triggering

Due to the success of the Arcminute Microkelvin Imager Large-Array Rapid-Response Mode (ALARRM) observing program, the 4 Pi Sky VOEvent Broker and the Comet VOEvent client are fast becoming the go-to software standard for receiving, parsing and filtering VOEvent transient alerts. These software allow for the full automation and timely follow-up of transient events using telescopes and facilities with rapid-response observing modes.

Recently the “Radio-Gamma-ray: Transient Alert Mechanisms” meeting was held in Amsterdam (26 – 28 September), in an effort to push for a standardisation of transient astronomy infrastructure and techniques, such as the generation, dissemination, distribution, and reaction to multi-messenger events.

At this meeting, several facilities including the Low Frequency Array (LOFAR), the Australia Telescope Compact Array (ATCA), and the High Energy Stereoscopic System (H.E.S.S) reported they were using Comet and the 4 Pi Sky VOEvent Broker to conduct rapid-response triggering on transient events. The International Virtual Observatory Alliance (IVOA), who manage and edit the VOEvent protocol, recognise both Comet and the 4 Pi Sky VOEvent tools as key software for implementing a VOEvent response network (see slide images below).


Experiments on the Australia Telescope Compact Array, led by Gemma Anderson, use the 4 Pi Sky VOEvent broker to trigger on Swift transient events


Stefan Ohm explains that H.E.S.S. triggers on ASASSN and GAIA transients using the 4 Pi Sky VOEvent broker


Dave Morris at the International Virtual Observatory Alliance (IVOA) mentions that Comet and the 4 Pi Sky VOEvent broker are key software for VOEvent triggering

A peculiar supernova with an explosive past

Radio observations made with the Arcminute Microkelvin Imager (AMI) Large Array as part of the 4 PI SKY project have demonstrated that the massive stellar progenitor of the supernova SN 2014C experienced two very different mass-loss episodes before it finally exploded, These results have been presented in the recent paper Anderson et al. (2017, link below).



X-rays from SN 2014C in nearby galaxy NGC 7331. The insert shows images taken with the Chandra X-ray Observatory, showing the position of SN 2014C before and after the supernova explosion.   Image credit: X-ray images: NASA/CXC/CIERA/R.Margutti et al; Optical image: SDSS

The inset images are from NASA’s Chandra X-ray Observatory, showing a small region of the galaxy before the supernova explosion (left) and after it (right). Red, green and blue colors are used for low, medium and high-energy X-rays, respectively.


Mass-loss is an important ingredient in the evolution of massive stars (which are at least 8 times as massive as our Sun), and has a significant impact on their final stellar death known as supernovae. A star looses its mass through strong stellar winds with speeds between 10s to 1000s km/s. However, other factors such as the interaction with a binary companion star, or the rapid ejection of a large amount of stellar material, are likely the biggest contributors to a massive star shedding its mass.

The expanding shock-wave produced by a supernova, likely travelling at ~10% of the speed of light, impacts the surrounding gas that was lost from the massive stellar progenitor during its lifetime. This interaction produces radio radiation, and the denser the surrounding environment, the brighter the radio emission will be. Radio observations of supernovae can therefore directly track the mass-loss history of its progenitor, illuminating past eras of strong stellar winds or eruptive events just prior to explosion.


Figure 1: The radio emission from SN 2014C monitored for nearly 600 days following the explosion. 

A steady brightening and fading in the radio emission over time demonstrates that most supernovae are surrounding by environments with densities that drop off steadily with distance, thus illustrating that the progenitor had an uneventful past. However, this was not the case for the supernova SN 2014C, discovered on 5 January 2014 in the nearby galaxy NGC 7331, which lies nearly 50 million light years away. Shortly following its discovery, AMI detected the radio emission from SN 2014C. AMI monitored its radio emission, watching it brighten to a peak at 80 days post-burst, before it began to fade. However, around 200 days post-explosion the radio emission unexpectedly began to re-brighten, peaking a second time at 400 days with a luminosity 4 times brighter than the first peak. This double bump morphology is shown in Figure 1. Such behaviour is extremely unusual and has only been seen from a small number of supernovae.

The radio re-brightening that AMI detected 200 days post-explosion was produced by the supernova shock-wave encountering a dense shell of Hydrogen gas (see Figure 2), which was thrown off by the massive stellar progenitor at an earlier point during its evolution. This Hydrogen shell was likely lost during an extreme eruptive event or through interaction with a binary stellar companion. The progenitor of SN 2014C therefore experienced at least two very different episodes of mass-loss during its lifetime, which was illuminated through radio observations.



Figure 2: A schematic of the environment surrounding the supernova likely produced by the massive stellar progenitor before it exploded. The darker areas indicate regions of higher gas density surrounding the supernova site.

4 PI SKY team members Gemma Anderson, Kunal Mooley, Rob Fender, and Tim Staley are all co-authors on the paper.

Link to paper: https://arxiv.org/abs/1612.06059

Probing the bright radio flare and afterglow of GRB 130427A with AMI

4PiSky Authors: Gemma Anderson / Tim Staley / Rob Fender

ADS Link: http://adsabs.harvard.edu/abs/2014MNRAS.440.2059A

AMI-LA Rapid Response Mode (ALARRM) observations of the nearby bright gamma-ray burst GRB 130427A allowed the 4 Pi Sky team to obtain one of the earliest radio detections of a GRB to date. As soon as this GRB had risen above the horizon the AMI-LA quickly slewed to its position detecting radio emission within 8 hours post-burst. Further follow-up AMI observations showed the radio flux to increase in brightness before rapidly declining one day later. Such a sudden decline in radio emission is very rare and has only been observed from a few GRBs.


The AMI 15.7 GHz and VLA 14 GHz light curve of GRB 130427A overplotted with the afterglow model derived by Perley et al. (2014, solid line) showing the individual contributions from the reverse shock (short dashed line) and forward shock (long dashed line). The AMI peak at 16 hrs is one of the earliest radio peaks ever observed from a GRB.

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4 Pi Sky at the National Astronomy Meeting 2014

The 4 Pi Sky group hit Portsmouth for the 2014  National Astronomy Meeting (NAM) . Rob Fender presented the Wednesday morning plenary talk on the Extreme Universe where he discussed the growing field of transient astronomy and how identifying these events could leads us to discover some of the most extreme astrophysical objects in our universe. In the photo below Fender is pointing at a picture of LOFAR, which is a key telescope in our search for transients and one of the processor facilities to the SKA.


Members of the 4 Pi Sky team featured in both the “Jets in Astrophysics” and “Bumps, Burps and Bangs” parallel sessions where they presented talks and posters of our latest results.

Anthony Rushton presented his results in the “Jets in Astrophysics” session where he discussed the relativistic jet production in the nearby universe. Jess Broderick showed us highlights from the LOFAR monitoring of the X-ray binaries SS433 and GRS1915+105. During this talk the UK community was able to have a sneak peak at the deep radio maps of these regions in the Galactic plane that LOFAR is capable of obtaining.

Gemma Anderson presented a talk in the “Bumps, Burps and Bangs”, which was a session dedicated to transient and time domain astronomy in the UK. Anderson discussed the robotic radio follow-up of GRBs we have been conducting with the Arcminute Microkelvin Imager (AMI), which appropriately followed a talk on the highlights of robotic optical follow-up of GRB with the Liverpool Telescope. This AMI dedicated talk focused on our exciting detection of the reverse shock radio emission from GRB 130427A and some of our initial results of the overall AMI GRB catalogue.  Adam Stewart’s poster showed the results surrounding the first detection of a short duration radio transient candidate with LOFAR. The transient was detected at the Northern Celestial Pole. Aidan Glennie’s poster discussed his discovery of fast X-ray transients in Chandra data archive.

The 4 Pi Sky group also enjoyed a delicious conference dinner aboard the HMS Warrior, one of the Heritage Ships is the Portsmouth Historic Dockyards.