The MeerKAT core

OLYMPUS DIGITAL CAMERA

This a photo of the MeerKAT core, taken on Oct 17 (2017) [credit Thomas Abbott, SKA South Africa]. All 64 MeerKAT primary antennas are now in place. In this picture 44 antennas are visible.

4 PI SKY co-leads the ThunderKAT Large Survey Programme (LSP) on MeerKAT, which has 1380 hours of guaranteed time during the MeerKAT survey phase (~2018-2023) as well as an agreement to search all other LSP data commensally to search for transients.

http://www.thunderkat.uct.ac.za/

Exciting times ahead!

4 PI SKY involved in jets breakthrough

The 4 PI SKY research group was involved in a significant measurement of the size and associated timescales very close to the base of a black hole relativistic jet, in a project led by collaborators at The University of Southampton.

Picture of black hole jet

High time resolution X-ray and infrared observations during flares from the black hole V404 Cyg in 2015 revealed a ~0.1 sec time delay between the emission in the two bands. There are good arguments already that the infrared emission arises from the ‘base’ of the relativistic jet, and AMI-LA radio observations provided simultaneously by the 4 PI SKY group confirmed this. This allowed the team, led by Dr Poshak Gandhi from Southampton, to establish a size scale of ~0.1 light seconds between the X-ray emitting region (inner parts of the accretion flow) and the first synchrotron emitting zone (the jet base). The research is published in Nature Astronomy, and the article can be found on the arXiv as arXiv:1710.09838.

For the full Southampton press release (including movie!) go to:

https://www.southampton.ac.uk/news/2017/10/black-hole-jets.page

4 PI SKY observe the radio counterpart to merging neutron stars

The recent detection of a neutron star-neutron star merger by aLIGO+VIRGO, combined with the associated electromagnetic counterpart have generated a huge amount of excitement in the astrophysical community.

GW170817

The 4 PI SKY research group has been strongly involved in this, in particular in the radio regime. The JAGWAR project, which led to the discovery of the radio counterpart (arXiv:1710.05435), is led by Kunal Mooley, a member of 4 PI SKY. Kunal and Rob Fender were also involved in observations of the field with MeerKAT (still in commissioning) and Rob was also part of the effort to find a low frequency radio counterpart with LOFAR. Kunal and Rob are furthermore also involved in a project, GWAMI, to try and chase the radio counterparts to future GW events with AMI-LA.

jvla

 

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).

4pisky_atca

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

HESS_TransientsSystem_Amsterdam_slide

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

ivoa-voevent-201709_slide

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).

 

sn2014_fullres

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.

lightcurve_v2

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.

 

shell

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