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 / AMI radio data reveal jet from Tidal Disruption Event

Radio observations made with AMI-LA as part of the 4 PI SKY project have revealed the presence of a relativistic jet from a Tidal Disruption Event (TDE), as presented in a new paper published in Science (van Velzen et al., link below). In a TDE, a more or less normal star strays too close to a supermassive black hole, is tidally pulled apart and ~50% of it accreted by the black hole. The other ~50% gets ejected from the system.

Artists impression of the tidal disruption of a star by a supermassive black hole, subsequent accretion and jet formation.

The radio observations with AMI-LA were performed rapidly after the All -Sky Automated Survey for Supernovae (ASAS-SN) team classified the optical transient as a likely TDE. The novel aspect of the data is that for the first time a relativistic jet, implied by the radio flaring, has been found from a TDE which was discovered optically, where the optical emission arises from the accretion flow. Previous radio-detected TDEs have been entirely dominated in their emission by the jet, implying they are being viewed down the barrel of the relativistic outflow. This detection, likely to be off-axis, suggests that a large number, maybe all, of TDEs will be associated with radio emission. This in turn implies that, despite this jet being relatively weak compared to the first TDE jets discovered, the Square Kilometre Array should find over one per week such events when it starts taking data in the early 2020s.

vv

X-ray, near-UV, and radio light curves of the Tidal Disruption Event ASAS-SN 14li, from van Velzen et al. (Science, 2015). The 15.7 GHz data, crucial to the jet interpretation, are from our AMI-LA programme. 

The final intriguing aspect about these observations is the fact that the supermassive black hole into which the tidally disrupted star was accreted, was already active, as indicated by earlier radio observations. This implies that the star entered on a orbit towards the supermassive black hole through the pre-existing accretion flow, disrupting it as it went.

We plan to chase all future bright optical TDE candidates to try and repeat this success and prepare the ground for MeerKAT and SKA. 4 PI SKY team members Gemma Anderson, Tim Staley and Rob Fender are all co-authors on the paper.

Link to paper: http://arxiv.org/abs/1511.08803

Link to ASAS-SN: http://www.astronomy.ohio-state.edu/~assassin/index.shtml

Links to some of the press releases: 

http://hub.jhu.edu/2015/11/26/black-hole-eats-a-star

http://www.icrar.org/news/news_items/media-releases/star-snacking-black-hole

https://astronomynow.com/2015/11/26/high-speed-flare-observed-from-supermassive-black-hole-eating-star/

http://www.abc.net.au/news/2015-11-27/star-torn-apart-by-black-hole-feeding-frenzy/6977188

V404 Cyg: The Kraken Wakes

After 26 years dormant, the nearby black hole V404 Cyg went into outburst on Monday last week (June 15). Many of the world’s premier observatories are following this spectacular outburst, which is likely to yield the textbook event for black hole accretion for many years to come.

The 4 PI SKY team are at the forefront of efforts to collect data and understand this source. Most notably, the AMI-LA telescope in rapid response mode (ALARRM), was triggered by a Swift X-ray alert on the source and obtained radio data only two hours after the trigger, revealing already a bright and declining radio flare (see figure below).

burstplot

Since this initial observation we have been following the source intensely with AMI and have detected a large number of radio flares, which are probably associated with relativistic ejection events. 4 PI SKY team members are also involved in radio observations with other major facilities such as eMERLIN and LOFAR, and are leading aspects of the X-ray and optical data analysis.

The ALARRM robotic observations of V404 were reported in this press release from the European Space Agency:

ESA press release on V404 Cyg