Odd Radio Circles (ORC’s) — Reading the Paper
They are odd indeed…
So I decided to start a series where I breakdown an astrophysics paper that I read and my thoughts on it from the perspective of an undergraduate. I obviously can’t understand a lot of the technical details that go into such papers but I can talk about the basics, results and conclusions.
The paper referred to for this article is Unexpected Circular Radio Objects at High Galactic Latitude by Norris et. al.
The Basic Idea
Using the Australian Square Kilometre Array Pathfinder telescope, Norris et. al. found circular radio objects that are about one arcminute (the full moon is about 30 arcminutes so these ORCs would appear to be 1/30th the size in the night sky) in diameter and unlike anything seen before. ORCs are also only visible in radio waves i.e if you were to point an optical or infrared telescope at it you wouldn’t see it. The paper goes over several possible explanations for these mysterious objects and especially over what they are NOT which is actually really interesting.
Observations
The objects were first discovered in the Pilot Survey of the Evolutionary Map of the Universe (EMU), which is an all-sky survey done using the ASKAP (Australian Square Kilometre Array Pathfinder) telescope.
3 of the 4 ORC’s were discovered by visual inspection of the images from this survey. The 4th ORC was found in archival data taken with the Giant MetreWave Radio Telescope (GMRT) in March of 2013.
In order to ensure that these ORC’s weren’t just artefacts or errors in instrumentation, two seperate telescopes- Australian Telescope Compact Array (ATCA) and the Murchison Widefield Array (MWA) -were used to independently confirm the discovery.
Results
From what the authors have seen so far, ORC’s have some common properties such as:
- A strong circular symmetry (the shape is very distinct circle)
- All 4 ORC’s have similar diameters
- They have a steep spectral index
- They can be found at high galactic latitude
They also have a few things that are different such as:
- Two ORC’s have a central galaxy while the other 2 don’t
- 3 of the ORC’s are partly filled rings while one is more of a uniform disk
Another thing mentioned in the paper is that 2 of the ORC’s are close together which could indicate a common orgin. (ORC 2 and 3)
The Discussion
In such cases, the authors have to answer 2 questions.
- Are they real?
From what we can tell so far at least ORC 1 and 2 are real because they have been independently verified by different telescopes using different softwares.
- Are they new?
The authors also propose that these objects are new due to the lack of any mentions of them in past papers.
So can we explain their existence with any already known phenomena?
What it isn’t…
The authors of this paper rule out 11 different phenomena that could explain these ORC’s!
Some of the ruled out options are:
Most supernova remnants that we see are concentrated in the Milky Way band because that is where most of the stars can be found. These ORC’s are located pretty out of the way making them unlikely to be relics from supernovas.
- Galactic Planetary Nebula
Galactic Planetary Nebula end up getting ruled out because they have thermal free emissions and a positive spectral index while ORC’s are quite different.
- Ring around Wolf- Rayet Star
Wolf- Rayet Stars can eject bubbles of material that are visible in both radio and optical wavelengths. They also are usually a few arcseconds or less in size which rules them out as explanations for ORC’s that are only visible in radio wavelength and are about an arcmin in diameter.
- Face-on star-forming galaxy or ring galaxy
Ring shaped star forming galaxies like the Cartwheel galaxy are well known, located nearby and bright in the optical wavelengths. If the ORC’s were emissions corresponding to these galazies, they would be close enough to detect using DES imaging and are therefore ruled out too.
- Cluster halo
Clusters of galaxies often show diffused radio halos that are an arcmin or more in diamter however they have irregular shapes. The brightness of their halos tends to peak in the center while ORC’s have bright edges.
- Einstein Ring
Gravitational lensing of background sources can produce arcs of emission. This occurs when an object with a lot of mass bends the path of the light ray from a source behind it and makes it appear like a ring shaped lens. These Einstein rings are typically seen in the optical range. They are also never more than a few arcsec in diameter which differs from the properties of ORC’s and is also not a possible explanation.
Conclusion
Well I guess the only option left is that ORC’s are new class of radio-astronomical objects. We know that they are real and new. But the authors can’t for sure confirm what they are, just yet. We know they are unlikely to be any of the 11 suggested phenomena but the authors acknowledge that they could also be explained by more than 1 particular phenomena.
Since this is the only paper on ORC’s so far we will just have to wait and see what else radio astronomers can come up with to explain these werid and super mysterious radio circles.
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