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).

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

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

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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).

 

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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.

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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.

 

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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

Introducing the AMI-GRB Database

The Arcminute Microkelvin Imager (AMI) Large Array robotically triggers on Swift transients (ALARRM mode), majority of which are gamma-ray bursts. GRBs in the northern hemisphere are followed up on logarithmic timescales between 1 hour and 10 days post-burst to look for radio afterglows at 15 GHz. As a resource to the GRB community, we have put together the AMI-GRB database, which maintains a log of the AMI observations carried out March 2016 onwards. This systematic study with the AMI will significantly advance our understanding of radio emission from GRBs.

ami-grb

e-MERLIN detection of compact radio emission from V404 Cyg

Renewed activity in the Black hole binary V404 Cyg has been reported in December 2015 (e.g. ATels #8453, #8454, #8455, #8457, #8458, #8459, #8462), following on from a giant outburst seen early in the year. Radio monitoring of the source suggested renewed jet activity (Atel #8454) with a short radio flare appearing to reach over 70 mJy with RATAN-600 around MJD 57387.4 (2015-Dec-31; Atel #8482). Sub-mm emission also showed new activity and was detected at a level of ~41 mJy (at 350 GHz) on 2015-Jan-01 (Atel #8499). Furthermore, around these observations, it was reported that a possible hard to soft transition might be occurring (Atel #8500). To investigate the presence of resolved ejecta, we triggered high-resolution radio observations using the e-MERLIN telescope.

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Radio observations of V404 Cyg were taken with e-MERLIN on 2016-Jan-05 between 06:30-22:00 UTC at a central frequency of 5.07 GHz and bandwidth of 444 MHz. A compact point-like source was detected with a peak flux density of 0.95 +/- 0.05 mJy/bm; this is a factor of ~2 above the long-term quiescent radio level of V404, which is ~0.4 mJy (Gallo, Fender & Hynes 2005). The synthesised beam had a minimum FWHM of 48 by 35 mas, suggesting most (or all) of the radio flux was constrained to within ~50 mas or ~100 AU (at a distance of 2.4 kpc).

 

We thank the eMERLIN staff for their rapid response to the event and to observatory’s director for approval of the project. eMERLIN is an STFC facility that has been built and operated by the University of Manchester.

 

We also thank the ERC for supporting this project through the 4 PI SKY grant.

Discovery of a Low-frequency Radio Transient near the North Celestial Pole with LOFAR

4 Pi Sky Authors: Adam Stewart / Rob Fender / Jess Broderick / Tom Hassall / Teo Muñoz-Darias / Tim Staley / Gosia Pietka / Rene Breton

See the full publication on astro-ph


Until a few years ago, the low-frequency radio transient sky was a relatively unexplored area of science. However, this is fast changing with new, low-frequency radio telescopes now fully operational and performing frequent transient surveys. 4 Pi Sky has led one of the first transient searches using one of these telescopes, LOFAR, where the North Celestial Pole (NCP) was monitored for around 400 hours over a period of four months. This resulted in the discovery of a new transient event, which is detailed in a paper due to be published in MNRAS and announced today on astro-ph.

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Caught: The transient as it appeared in the images generated by LOFAR, showing the transient appearing, and subsequently disappearing, from view. The lower panels display a zoom-in of the transients location.

The transient, named ILT J225347+862146, was detected in only one of 1897 60 MHz observations, with a brightness of approximately 20 Jy. It was discovered by using the LOFAR Transients Pipeline (TraP), a piece of software 4 Pi Sky helped develop. Each of these observations was 11 minutes in duration, so it was possible to probe the transient at a higher time resolution by splitting the data into two minute observations. In doing this, it was found that the transient only appears to be active for only 4-6 minutes of this observation.

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A brief burst: The light curve of the transient object during the 11 minute period of the observation, showing a sudden appearance along with a just as fast decline. The different light curves denote slightly different processing methods of the data, but both show the same trend.

But what is ILT J225347+862146? No objects at the transients location have been detected in historical radio catalogues, nor were there any obvious candidates in optical follow-up observations performed with the Liverpool Telescope. Possibilities were explored; from extragalactic fast radio bursts, to perhaps a nearby flare star, but while some of the characteristics of this transient were consistent with previously detected events from these objects, others were not. One feasible explanation is that could be from a nearby substellar object, for example a brown dwarf, which are difficult to detect. However, at this stage, the true origin of ILT J225347+862146 remains a mystery.

With the continual, and rapid advance in technology and techniques of low-frequency radio astronomy, then ILT J225347+862146 may be the first of many such transients of this nature to be discovered.