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.

T_2024+3352..ICL001.5

 

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.

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

4 PI HACKFEST

4 PI SKY belatedly joins the “hackfest” bandwagon…. the day this ball started rolling properly

20140701_120921

Gemma, Rob, Tim, Jess, Adam, Gosia, Teo … there at day one…