Radio object with continuous optical spectrum
Astrophysical objects with featureless optical spectra, distance unknown.
ROCOSes, short for Radio Objects with Continuous Optical Spectra, are a set of roughly 80 astronomical bodies. Their optical spectra are unusually flat, lacking any emission or absorption lines—except for those caused by our galaxy’s interstellar medium. This feature prevents astronomers from measuring their distances or determining whether they lie inside or outside the Milky Way. They belong to the blazar subclass and share spectral traits with DC-dwarfs and single stellar-mass black holes.
Discovered in the 1970s, these objects were identified by a team of Soviet astrophysicists working at the Crimean Astrophysical Observatory and the Special Astrophysical Observatory of the Russian Academy of Science. The researchers used the Crimean 2.6-meter optical telescope and the 6-meter BTA-6 telescope, along with a 1000-channel photon counter and photometers. Their findings appeared in a series of papers in *Astronomy Letters* and *Astronomy Reports*.
To qualify as a ROCOS, an astronomical radio object must meet two criteria: it must have an optical image that looks stellar and matches a known radio source, and its optical spectrum must show no emission or absorption features (beyond those from galactic interstellar material), with a signal-to-noise ratio comparable to that used for quasar candidates. About 8% of known radio objects satisfy these conditions.
Because ROCOSes lack distinct spectral lines, they resemble highly polarized quasars (HPQ), BL Lac objects, and single stellar-mass black holes. This absence also rules out using redshift to find their distances or even to tell if they are inside or outside our galaxy.
- type
- Astrophysical object group
- number_known
- About 80
- discovery_period
- 1970s
- classification
- Subclass of blazars
- key_telescopes
- Crimean Astrophysical Observatory 2.6-meter, Special Astrophysical Observatory 6-meter (BTA-6)
- criteria
- Stellar optical image identified with a radio source; no emission or absorption features except from galactic interstellar medium
Lore & Background
Radio Objects with Continuous Optical Spectra, or ROCOSes, were discovered in the 1970s. Among the discoverers was a group of Soviet astrophysicists, who studied them at the Crimean Astrophysical Observatory and the Special Astrophysical Observatory of the Russian Academy of Science, using the former's 2.6-meter optical telescope and the latter's 6-meter optical telescope (BTA-6), along with a 1000-channel photon counter and photometers. The group published their findings in a series of articles in the Russian scientific journals Astronomy Letters and Astronomy Reports.
An astronomical radio object is classified as a ROCOS if it possesses an optical image with stellar appearance, which is identified with a radio source, and no emission or absorption features in its optical spectrum, except for those due to galactic interstellar medium, with a signal-to-noise ratio at the level of those observable for quasar candidates. About 8% of the known astronomical radio objects satisfy these two criteria and are considered ROCOSes.
The absence of distinct emission or absorption lines in the ROCOSes' spectra makes them very similar in this regard to highly polarized quasars (HPQ), BL Lac objects, and single stellar-mass black holes. The absence of optical spectral features also makes it impossible to use red shift for determining their distances or even ascertaining if they are located within or outside our galaxy.
Reader's Guide
Radio Objects with Continuous Optical Spectra represent a small but significant subset of astronomical radio sources, comprising about 8% of known radio objects. Their defining characteristic—a complete lack of emission or absorption lines in their optical spectra—renders standard distance-measurement techniques like redshift unusable, leaving their locations and whether they lie within or beyond the Milky Way unknown. This featureless spectral profile aligns them with highly polarized quasars, BL Lac objects, and single stellar-mass black holes, placing ROCOSes within the broader blazar classification. Discovered in the 1970s by Soviet astrophysicists using major telescopes at the Crimean and Special Astrophysical Observatories, their study relied on advanced instrumentation such as a 1000-channel photon counter. The inability to determine their distances poses a fundamental challenge to understanding their nature and cosmic distribution, making them a persistent puzzle in extragalactic astronomy. Their significance lies in highlighting the limits of observational techniques and the diversity of objects that evade conventional classification.
Did You Know?
- ROCOSes are a group of about 80 astrophysical objects.
- They were discovered in the 1970s by a group of Soviet astrophysicists.
- About 8% of known astronomical radio objects satisfy the criteria to be considered ROCOSes.
- The absence of spectral features makes it impossible to determine if ROCOSes are located within or outside our galaxy.
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