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.



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.


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.


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.

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.


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: 





4 PI SKY goes hunting for more transients

This week, members from the 4 PI SKY team visited the AMI telescope in Cambridge, UK, in search of even more transients.

Members of the 4 PI SKY team visiting the AMI teelscope

Members of the 4 PI SKY team visiting the AMI telescope. Shown (from left to right): Clare Rumsey, Richard Saunders, Anthony Rushton, Tim Staley, Kunal Mooley, Rob Fender and Richard Armstrong.

The Universities of Oxford and Cambridge already have a very succesful partnership of following up astronomical transients at 15 GHz using the AMI large array. Gamma-ray alerts from the Swift-BAT space telescope robotically send messages back to earth-based servers, which in-turn automatically command AMI to slew to transient location in the sky (effectively eliminating the need of human intervention). However, when the array isn’t chasing high-energy explosions it spends a significant amount of time surveying galaxy clusters looking for Sunyaev-Zel’dovich (SZ) effects.

AMI recently completed the Tenth Cambridge Survey (10C; AMI Consortium: Davies et al. 2011; AMI Consortium: Franzen et al. 2011) at 15.7 GHz creating the deepest high-frequency (10 GHz) radio survey, complete to 1mJy in 10 different fields covering a total of≈27 deg^2. These data could contain radio transients that haven’t previously been found at other wavelengths and it is our goal to search the entire archive for historic events.

AMI-SA correlator

A new correlator that will power high-spectral resolution observations with AMI

In the mean-time, the AMI telescope is undergoing a major upgrade to the correlator. The original correlator was a lag-based system, which suffered from large errors in correlator lag spacing  and was prone to man-made radio frequency interferences (RFI) particularly at low declinations due to geostationary satellites.

The new AMI Digital Correlator (AMIDC), pictured right, will have a highly channelized digital correlator system giving more flexibility within the radio band and a much more uniform response across it, which would provide the potential to avoid or mitigate to a large extent many of the problems with the current system. This will significantly improve the sensitivity of the array.

Ultimately, we would like to use the new system to detect radio transients in near real-time and produce rapid VOEvent alerts that can help coordinate follow-up observations.

Robotizing high-resolution radio observations to transient alerts

The 4 PI SKY team has been granted time on the European VLBI Network (EVN) to use a new mode to rapidly hunt transient events (PI Rushton). From 2015 we will use Automated triggers to override e-VLBI sessions if we detect outbursts from objects like flare stars and Gamma-ray bursts (GRBs). The system will quickly generate new scheduling programs for each participating EVN antenna and we aim to robotically slew the whole array to the event after few 10’s of minutes.

Radio emission from transients can start to rise after just a few minutes of being detected at high-energy. It is important to rapidly slew to the source and begin monitoring at high-resolution in order to localise the emission. We will also be able to search for outflows and proper motion caused by jets of plasma. This will give us unique insight to the causes of the most energetic events in our Galaxy and the wider Universe.


Credit: Paul Boven (JIVE), AMI (Uni. Cambridge), Swift and VisibleEarth (NASA)

Our network currently uses VOevent alerts from NASA’s Swift space-based telescope to robotically trigger the Arcminute Microkelvin Imager large array (AMI-LA) in Cambridge, UK. The AMI-LA telescope in rapid response mode (ALARRM) can respond to a new outburst of gamma-rays after a few minutes and will help trigger observations at high-resolution with the EVN.

Developing automatic generic triggers for the EVN has been a joint effort including JIVE, Onsala Space Observatory, EVN TOG CBD and PC. In particular we thank the efforts of Simon Casey, Mark Kettenis and Zsolt Paragi. The project includes team members from University Oxford, Instituto de Astrofísica de Andalucía, Space Telescope Science Institute and Caltech.