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.

eVLBI_autoV3

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.

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

“TraP”, a transient detection pipeline, gets its first public release

We’re happy to announce that the TraP, an image-plane transients-detection pipeline, has reached version 2.0 and been made publicly available for the first time, along with an accompanying paper (to be published in Astronomy and Computing). The TraP project was born out of the UvA LOFAR-transients group, with close collaboration from the 4 Pi Sky team over the past few years.

The TraP is what’s known as a source-cataloguing pipeline, which means that it processes images one-by-one, extracting source positions and flux intensities from each image, then attempts to match up these measurements into lightcurves spanning the entire dataset. If something new pops up, or if a source shows significant variability in its lightcurve, the TraP will flag this up for further investigation. This approach has been used before (e.g. for the CRTS project), but the TraP is novel for a few reasons. Key among these is fact that TraP can be run on a plain-old stack of images, without the need for a carefully preprocessed ‘deep’ image (an image of unusually high quality made by summing the best images from a dataset) to seed the catalogue beforehand. This makes it possible to run the TraP on a sequence of images as they are observed in near real-time. It can also collate data from across a range of frequency bands, which is vital in the current era of wide-band observatories and multi-observatory observing campaigns.

A Banana screenshot

A screenshot from Banana, the web-based graphical interface for browsing TraP results.

Additionally, the TraP project has embraced some ways of working that are still quite unusual in astronomy. For starters, the graphical interface used to give an overview of results is a Django-powered web-interface. This means that while all the software and results are stored and run on a heavy-duty central server, end-users can still browse through the results in a intuitive manner using the web front-end, Banana (don’t ask). The TraP has grown into a significant software-project by most standards (~20,000 lines of Python and SQL code, ~350 unit-tests), with a diverse group of contributors. To ensure things kept running smoothly we employed comprehensive unit-testing, continuous integration, issue-tracking and code-review, which should put the project in good stead for a more open development model.

The TraP has already been put to good use on data from the LOFAR RSM, ALARRM, and JVLA-CHILES surveys (see our projects page). Hopefully this open release will allow profitable use with many more datasets.

Simultaneous independent measurements of a truncated inner accretion disc

4PiSky Authors: Daniel Plant / Rob Fender

One question has polarised the black hole X-ray binary community for many years: is the inner accretion disc truncated in the low-hard state?

The inner radius of the accretion disc, fitted by the disc (x-axis; Method 2) and Fe line (y-axis; Method 1) components. The dot-dash line indicates the innermost stable circular orbit for a zero spin black hole, showing that both methods, from all three observations, are consistent with a truncated accretion disc.

The inner radius of the accretion disc, fitted by the Fe line (y-axis; Method 1) and disc (x-axis; Method 2) components. The dot-dash line indicates the innermost stable circular orbit for a zero spin black hole, showing that both methods, from all three observations, are consistent with a truncated accretion disc.

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