In astronomy, an odd radio circle (ORC) is a very large (over 50 thousand times the diameter of our

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

~ 3 Million Light-years) unexplained astronomical object that, at radio wavelengths, is highly circular and brighter along its edges. As of 27 April 2021, there have been five such objects (and possibly six more) observed. The observed ORCs are bright at radio wavelengths, but are not visible at

visible Visibility, in meteorology, is a measure of the distance at which an object or light can be seen. Visibility may also refer to: * A measure of turbidity in water quality control * Interferometric visibility, which quantifies interference contrast ...

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

wavelengths. This is due to the physical process producing this radiation, which is thought to be

synchrotron radiation Synchrotron radiation (also known as magnetobremsstrahlung radiation) is the electromagnetic radiation emitted when relativistic charged particles are subject to an acceleration perpendicular to their velocity (). It is produced artificially in ...

. Three of the ORCs contain optical

galaxies A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

in their centers, suggesting that the galaxies might have formed these objects.

Description

CSIRO ScienceImage 2161 Close up of a radio astronomy telescope with several more in the background

The ORCs were detected in late 2019 after astronomer Anna Kapinska studied a Pilot Survey of the

Evolutionary Map of the Universe Evolutionary Map of the Universe, or EMU, is a large project which will use the new Australian Square Kilometre Array Pathfinder, ASKAP telescope to make a census of Astronomical radio source, radio sources in the sky. EMU is expected to detect abo ...

(EMU), based on the

Australian Square Kilometre Array Pathfinder The Australian Square Kilometre Array Pathfinder (ASKAP) is a radio telescope array located at Murchison Radio-astronomy Observatory (MRO) in the Mid West region of Western Australia. The facility began as a technology demonstrator for the int ...

(ASKAP) radio telescope array. All of the ORCs are about 1

arcminute A minute of arc, arcminute (arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of one degree. Since one degree is of a turn (or complete rotation), one minute of arc is of a turn. The na ...

in diameter, and are some distance from the

galactic plane The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usuall ...

, at high

galactic latitude The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an ap ...

s. The possibility of a spherical shock wave, associated with

fast radio burst In radio astronomy, a fast radio burst (FRB) is a transient radio pulse of length ranging from a fraction of a millisecond to 3 seconds, caused by some high-energy astrophysical process not yet understood. Astronomers estimate the average FRB rel ...

gamma-ray burst In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies. They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milli ...

s, or

neutron star merger A neutron star merger is a type of stellar collision. It occurs in a fashion similar to the rare brand of type Ia supernovae resulting from merging white dwarf stars. When two neutron stars orbit each other closely, they gradually spiral inw ...

s, was considered, but, if related, would have to have taken place in the distant past due to the large

angular size The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

of the ORCs, according to the researchers. Also, according to the astronomers, "Circular features are well-known in radio astronomical images, and usually represent a spherical object such as a

supernova remnant A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar mat ...

, a

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated to ...

, a circumstellar shell, or a face-on disc such as a

protoplanetary disc A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, be ...

or a

star-forming galaxy The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have gen ...

, ... They may also arise from imaging artefact around bright sources caused by calibration errors or inadequate

deconvolution In mathematics, deconvolution is the operation inverse to convolution. Both operations are used in signal processing and image processing. For example, it may be possible to recover the original signal after a filter (convolution) by using a deco ...

. This class of circular feature in radio images does not seem to correspond to any of these known types of object or artefact, but rather appears to be a new class of

astronomical object An astronomical object, celestial object, stellar object or heavenly body is a naturally occurring physical entity, association, or structure that exists in the observable universe. In astronomy, the terms ''object'' and ''body'' are often us ...

References

External links

Images of ORCs
( CTA Observatory /

Western Sydney University Western Sydney University, formerly the University of Western Sydney, is an Australian multi-campus university in the Greater Western region of Sydney, Australia. The university in its current form was founded in 1989 as a federated network ...

; 04/20/2020) * (Anton Petrov; 07/15/2020) * (Mr. Researcher; 07/10/2020) * (World Today; 07/09/2020) {{portal bar, Astronomy, Physics, Space, Science Astronomical events Radio astronomy Stellar phenomena Unexplained phenomena Unsolved problems in astronomy 2020 in science

Description

See also

References

External links