|The Twin Quasar Q0957+561|
The Twin Quasar QSO 0957+561, which lies 7.8 billion light-years from Earth, is seen right in the center of this picture.
|Observation data (Epoch J2000)|
|Right ascension||10h 01m 20.99s|
|Declination||+55° 53′ 56.5″|
|Distance||8,700,000,000 ly (2,400,000,000 pc)|
|Apparent dimensions (V)||6" distance|
|Apparent magnitude (V)||16.7|
|Twin Quasar, Double Quasar, Twin QSO, QSO 0957+561, Q0957+561, SBS 0957+561, TXS 0957+561, 8C 0958+561, PGC 2518326, A: USNO-A2 1425-7427021 B:USNO-A2 1425-7427023|
|See also: Quasar, List of quasars|
The Twin Quasar (also known as Twin QSO, Double Quasar, SBS 0957+561, TXS 0957+561, Q0957+561 or QSO 0957+561 A/B), was discovered in 1979 and was the first identified gravitationally lensed object. It is a quasar that appears as two images, a result from gravitational lensing caused by the galaxy YGKOW G1 that is located in the line of sight between Earth and the quasar.
QSO 0957+561 A (SBS 0957+561 A) and QSO 0957+561 B (SBS 0957+561 B) are the two components of a double-imaged quasar, meaning that an intervening mass concentration between Earth and the quasar bends light so that two images of the quasar appear in the sky. This is known as gravitational lensing, and is a consequence of Einsteinian warped space-time. The quasar lies at redshift z = 1.41 (8.7 billion ly), while the lensing galaxy lies at redshift z = 0.355 (3.7 billion ly). The lensing galaxy with apparent dimension of 0.42×0.22 arcminutes lies almost in line with the B image, lying 1 arcsecond off. The quasar lies 10 arcminutes north of NGC 3079, in the constellation Ursa Major. The astronomical data services SIMBAD and NASA/IPAC Extragalactic Database (NED) list several other names for this system.
The Twin Quasar's two images are separated by 6 arcseconds. Both images have an apparent magnitude of 17, with the A component having 16.7 and the B component having 16.5. There is a 417 ± 3-day time lag between the two images.
The quasars QSO 0957+561A/B were discovered in early 1979 by an Anglo-American team around Dennis Walsh, Robert Carswell and Ray Weyman, with the aid of the 2.1 m Telescope at Kitt Peak National Observatory in Arizona, United States. The team noticed that the two quasars were unusually close to each other, and that their redshift and visible light spectrum were surprisingly similar. They published their suggestion of "the possibility that they are two images of the same object formed by a gravitational lens".
The Twin Quasar was one of the first directly observable effects of gravitational lensing, which was described in 1936 by Albert Einstein as a consequence of his 1916 General Theory of Relativity, though in that 1936 paper he also predicted "Of course, there is no hope of observing this phenomenon directly."
Critics however identified a difference in appearance between the two quasars in radio frequency images. In mid 1979 a team led by David Roberts at the VLA (Very Large Array) near Socorro, New Mexico/USA discovered a relativistic jet emerging from quasar A with no corresponding equivalent in quasar B. Furthermore, the distance between the two images, 6 arcseconds, was too great to have been produced by the gravitational effect of the galaxy G1, a galaxy identified near quasar B.
Young et al. discovered that galaxy G1 is part of a galaxy cluster which increases the gravitational deflection and can explain the observed distance between the images. Finally, a team led by Marc V. Gorenstein observed essentially identical relativistic jets on very small scales from both A and B in 1983 using VLBI (Very Long Baseline Interferometry). Subsequent, more detailed VLBI observations demonstrated the expected (parity reversed) magnification of the image B jet with respect to image A jet.  The difference between the large-scale radio images is attributed to the special geometry needed for gravitational lensing, which is satisfied by the quasar but not by all of the extended jet emission seen by the VLA near image A.
30 years of observation made it clear that image A of the quasar reaches earth about 14 months earlier than the corresponding image B, resulting in a difference of path length of 1.1 ly.
In 1996, a team at Harvard-Smithsonian Center for Astrophysics led by Rudy E. Schild discovered an anomalous fluctuation in one image's lightcurve, which led to a controversial and unconfirmable theory that there is a planet approximately three Earth masses in size in the lensing galaxy. The results remain speculative because the chance alignment that led to its discovery will never happen again. If it could be confirmed, however, it would make it the most distant known planet, 4 billion ly away.
In 2006, R. E. Schild suggested that the accreting object at the heart of Q0957+561 is not a supermassive black hole, as is generally believed for all quasars, but a magnetospheric eternally collapsing object. Schild's team at the Harvard-Smithsonian Center for Astrophysics asserted that "this quasar appears to be dynamically dominated by a magnetic field internally anchored to its central, rotating supermassive compact object" (R. E. Schild).
- "Seeing double". ESA/Hubble Picture of the Week. Retrieved 20 January 2014.
- Kundic, T.; Turner, E.L.; Colley, W.N.; Gott, III; Rhoads, J.E. (1997). "A robust determination of the time delay in 0957+561A,B and a measurement of the global value of Hubble's constant". Astrophys. J. 482 (1): 75–82. arXiv:astro-ph/9610162. Bibcode:1997ApJ...482...75K. doi:10.1086/304147.
- Nomenclature of Celestial Objects (Result I)
- Nature 279, S.381–384: 0957 + 561 A, B: twin quasistellar objects or gravitational lens? D.Walsh, R.F.Carswell, R.J.Weymann 31 May 1979
- Einstein, Albert (1936). "Lens-like action of a star by the deviation of light in the gravitational field". Science. 84 (2188): 506–507. Bibcode:1936Sci....84..506E. doi:10.1126/science.84.2188.506. PMID 17769014.
- TIME (1 October 1979). "Science: The Mysterious Celestial Twins". Time.
- Young, P.; Gunn, J.E.; Oke, J.B.; Westphal, J.A. & Kristian, J. (1980). "The double quasar Q0957 + 561 A, B – A gravitational lens image formed by a galaxy at Z = 0.39". Astrophysical Journal. 241: 507–520. Bibcode:1980ApJ...241..507Y. doi:10.1086/158365.
- M.V. Gorenstein; et al. (1984). "The milli-arcsecond images of Q0957 + 561". Astrophysical Journal. 287: 538–548. Bibcode:1984ApJ...287..538G. doi:10.1086/162712.
- M.V. Gorenstein; et al. (1988). "VLBI Observations of the Gravitational Lens System 0957+561: Structure and Relative Magnification of the A and B Images". Astrophysical Journal. 334: 42–58. Bibcode:1988ApJ...334...42G. doi:10.1086/166816.
- "Quasare im Doppelpack" aus "Astro-Lexikon" Andreas Müller August 2007
- New Scientist (issue 2037), Do alien worlds throng faraway galaxy? Govert Schilling 6 July 1996
- "Research Sheds New Light on Quasars". SpaceDaily.com. 26 July 2006.
- Q0957+561: Die historisch erste Linse mit Quasar – The University of Cologne.
- Q0957+561 – CCD image based on 45-min total exposure – March 2007.
- Q0957+561 A,B.