Title: Discovery of ASKAP J173608.2–321635 as a Highly Polarized Transient Point Source with the Australian SKA Pathfinder

Authors: Ziteng Wang, David L. Kaplan, Tara Murphy, Emil Lenc, Shi Dai, Ewan Barr, Dougal Dobie, B. M. Gaensler, George Heald, James K. Leung, Andrew O’Brien, Sergio Pintaldi, Joshua Pritchard, Nanda Rea, Gregory R. Sivakoff, B. W. Stappers, Adam Stewart, E. Tremou, Yuanming Wang, Patrick A. Woudt, Andrew Zic

First Author’s Institution: The University of Sydney, Australia

Status: Published in ApJ [open access]

There seems to be a never-ending list of exciting radio sources within and outside of our galaxy. There are pulsars, a type of rapidly rotating neutron star. There are magnetars, neutron stars with extraordinarily powerful magnetic fields. There are flare stars, stars whose brightness will rapidly vary or “flare” in just a few minutes. There are things like supernovae and jets in active galactic nuclei. There are the Galactic Center Radio Transients, radio transients towards the Galactic Center with unknown origins. There are fast radio bursts, extremely energetic bursts of radio emission originating at extragalactic distances. And, on top of all of these, today’s authors present a possible new type of radio source, ASKAP J173608.2−321635.

So what’s so cool about this source?

ASKAP J173608.2−321635 was first detected by a group of astronomers using the Australian Square Kilometre Array Pathfinder Variables and Slow Transients (ASKAP VAST) survey, a survey that focuses on finding transient objects whose emission changes on timescales down to ~5 seconds. After the initial detection, they continued to monitor it and detected it a total of six times over a nine month period (see Fig. 1). They found that the flux was fairly steady on the timescale of a few hours and that it was persistent yet variable on the longer timescale of a few weeks (see Fig. 2). They then followed the source up using the radio telescope MeerKAT in South Africa since MeerKat: a) operates in a different frequency regime allowing them to study the frequency dependence of the source and b) has options for both searching for pulsar-like emission (pulsed emission) and continuum (constant) emission.

In their first five observations with MeerKAT, they did not detect anything (sad!). However, they hit gold on their sixth observation, finding strong continuum emission from the source (see Fig. 1). They looked for pulsed-emission from the source as well, but couldn’t find any.

The continuum emission that they detected had a number of interesting features such as: 

1. significant circular polarization and high levels of linear polarization
2. a steep, negative spectrum, meaning that the flux decreased quickly as a function of frequency
3. a rapidly decreasing flux with a characteristic timescale of decay of ~26 hours (see lower right panel of Fig. 2) 
4. no variability on the timescale of ~minutes 

In addition to radio observations with ASKAP and MeerKAT, the authors also looked for non-radio emission from the source. They looked at both archival X-ray and near-IR data, and performed their own observations using the Neil Gehrels Swift Observatory and the Chandra X-ray Observatory. They didn’t find any significant emission in their searches though. 

So what could it be… 

This source seems to have a lot of really interesting features, but there are already a lot of interesting radio sources out there. Let’s take a look at the similarities and differences between our source and other radio sources to try to figure out a possible origin.

Could it be a…. Star? Stars are one of a few sources which can emit polarized emission. However, X-ray and radio luminosities are typically correlated for these objects, so we would have expected to detect X-rays from the source. Additionally, near-IR emission would also likely be significant and detectable. So, unless our source has a really really small ratio of X-ray/IR to radio emission, we can probably rule out a star. Buh bye stars! 

Could it be a…. Pulsar? The steep spectrum and circular polarization of our source are very reminiscent of pulsars. However, the source shows no pulsed emission. It is possible that it is a highly scattered pulsar, a pulsar with an ultra-long period, or a pulsar in an eccentric binary system, but it doesn’t like look it could be your everyday pulsar.

Could it be a…. Magnetar? Magnetars tend to have very flat spectra (e.g. the intensity of the emission does not vary a lot as a function of frequency), which is very unlike our source! Magnetars also typically have periods of ~1-10 seconds, which is ruled out by the MeerKAT periodicity searches. However, there is the possibility that it is a special ultra-long period magnetar.

Could it be a…. Jet, Gamma-ray Burst, Supernova, or Tidal Disruption Event? Again, not looking so likely. The levels of circular and linear polarization, the steep spectra, and the short timescale over which it decayed make all of these very unlikely. 

Could it be a…. Galactic Center Radio Transient? We actually might have something here! Galactic Center Radio Transits (GCRTs) similarly have steep spectra, are highly polarized, and have no X-ray emission. However, only three GCRTs have so far been detected and their origins, if they are even the same, are still uncertain. Additionally, the timescale over which the emission varies for ASKAP J173608.2−321635 doesn’t match the timescales of other GCRTs.

So how should we walk away thinking about this source? 

It seems like this source is definitely not our everyday pulsar, magnetar, or star. The authors seem to be steering towards a new type of source, which might be related to GCRTs, or might be the first of its kind! The only way to find out is to keep observing! 

Astrobite edited by Ishan Mishra

Featured image credit: Sebastian Zentilomo/University of Sydney