4 PI SKY ON THE OPTICAL COUNTERPARTS OF RADIO TRANSIENTS

4 Pi Sky has today published a study of the optical and radio flux density properties of a wide range of astrophysical transient phenomena – from Galactic flare stars to extragalactic gamma-ray bursts (GRBs). The aim of the investigation was to answer the question of whether a radio transient source could be classified from an optical flux density measurement obtained through swift follow-up. In fact, this scenario may suddenly become very frequent with the recent inauguration of MeerLICHT, a robotic optical telescope that will track the new radio telescope MeerKAT, providing optical photometric coverage of any night-time radio transients discovered by MeerKAT!

By analysing approximately 12,000 pairs of radio and optical flux density measurements, we found that there was a clear separation in the parameter space of Galactic and extragalactic objects, as seen in Figure 1 from the publication reproduced below. We also saw that objects from specific populations generally clustered together as evidenced by the distribution of supernovae (SNe) and GRBs.

fig01

Figure 1 from the publication that shows the optical and radio flux densities of different transient phenomena plotted against one another. It shows how the majority of Galactic and extragalactic sources, such as stars and quasars respectively, are well separated.

The measurements included in the plot above are recorded at a single time, but the flux density of transients change significantly as they evolve. We explored how this would appear in the radio and optical parameter space by including a dynamic data sample that followed the radio and optical evolution over months or even years. Below is a plot showing a result of that study, specifically for SNe. It shows that many events follow a similar anti-clockwise trajectory, a feature that could help in classifying an object if an initial classification is unclear. In the paper we also explore how GRBs, X-ray binaries, cataclysmic variable and quasars evolve over time.

fig06c

The evolution of a selection of supernova events in radio and optical flux density over time. The green and red circles signify the beginning and end of the measurements for the respective object, where as the numbers denote the time in days after the explosion date of the event.

The study is the first step in using the radio and optical flux density information of a transient as part of a classification pipeline – and will already provide a very useful reference for the MeerKAT/MeerLICHT project. As more simultaneous radio and optical data is collected thanks to projects such as that one, our understanding of the radio-optical transient sky will substantially improve and, importantly, allow us to ‘fill-in’ areas of the plot to see how populations appear beyond our current sensitivity limits.

The full paper can be found on arXiv here: https://arxiv.org/abs/1806.09815.

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.

NCP-transient-grid-new

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.

2minlightcurve

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.