– Observation of three multiple-imaged quasars –

Article by Dr. Gerold Holtkamp, ​​March 15, 2025

The observation of the three quasars Q0957+561, RXJ0921+4529, and SDSS J1029+2623 is described. In these cases, multiple images are generated by galaxies or galaxy clusters in the foreground, which act as gravitational lenses. It is shown that these images can be resolved even at an urban observation site.

Introduction

Quasars look like stars. This is why they weren't recognized as special non-stellar objects until 1963. Maarten Schmidt found that the already known radio source 3C 273 was far too distant (z = 0.158 –> 1.9 billion years of light travel time) to have such apparent brightness as a star. The source was therefore designated as a quasi-stellar, i.e., a quasar. [1]

Due to their extremely high brightness, quasars are observable by us on Earth even at enormous distances. On their way to us, their light may have to pass through large accumulations of matter, such as galaxies or even galaxy clusters, which warp space and thus act as a gravitational lens. As a result, we on Earth may observe two or more images of the same quasar. Observations of three of these quasars are described below.

Double Quasar Q0957+561

The double quasar Q0957+561 was discovered in 1979 [2]. It was clearly proven that these are two images of the same quasar created by gravitational lensing. It was also shown that the light from the B component arrives at Earth 417 days later than that from the A component. [3] The quasar has a redshift of z = 1.41, which corresponds to a light travel time of 9.3 billion years. The light from the galaxy responsible for the lensing has a redshift of z = 0.36, which corresponds to a light travel time of 3.9 billion years. [4] [5] The apparent magnitude (V-band) of the A component is 16.7 mag, that of the B component 16.5 mag, and that of the galaxy cluster 21.9 mag, so that the latter is not visible in the images shown here. The apparent separation of the two images of the quasar is 6 arcsec. [6]

Double-Quasar RXJ0921+4529

The quasar was discovered in 2000 and described as a double image created by gravitational lensing, with the caveat that it could also be a true double quasar. In 2010, another research group presented the exact opposite. [8]

Its light took about 9.9 billion years (redshift Z=1.66) to reach Earth, in my garden in the Sonnenhügel district of Osnabrück. [5] According to Simbad, the bright component A is 18.7 mag bright (V), while component B is said to be over 1 mag dimmer. [9] In my own image from February 16/17, 2025, it is still very faintly visible. The apparent distance is 6.97 arcsec. [8]

Triple Quasar SDSS J1029+2623
The quasar SDSS J1029+2623, which is imaged multiple times by gravitational lensing, was discovered in 2006. [11] It is currently my personal garden-distance-record. The image was taken on March 2, 2025. The redshift of the quasar is z=2.197, which corresponds to a light travel time of 10.83 billion years. The redshift of the galaxies (clusters) (gravitational lensing) is z=0.58. Their light thus traveled "only" 5.64 billion years to reach Osnabrück. [12] [5]

The apparent separation of images A and B/C is 22.6 arcsec. This quasar has the most separated images. The light from component B arrives 744 days later than that from component A. The light from component C arrives another 2-3 days later than that from component B. [12]

Gravitational lenses, which consist of a cluster of galaxies, are well-suited for testing cosmological models, and research on these objects is therefore currently underway. [13]

Ending

Gravitational lensing, first proposed by Einstein over 100 years ago, is now a generally accepted reality. Admittedly, our own images of quasars, which are double or more imaged by such gravitational lensing, are not as colorful as the images we are used to from astronomers. But there is a very special appeal to imaging these phenomena from an urban garden, completely incomprehensible to our everyday experience.

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[1] For general information on quasars see: https://de.wikipedia.org/wiki/Quasar
If you want to go into detail, you can do so here: https://www.open.edu/openlearn/science-maths-technology/introduction-active-galaxies/content-section-0?intro=1

[2] Walsh, D., Carswell, R. & Weymann, R., 0957 + 561 A, B: twin quasistellar objects or gravitational lens?, Nature 279, 381–384 (1979), https://doi.org/10.1038/279381a0

[3] V. N. Shalyapin, L. J. Goicoechea, E. Koptelova, A. Ullán and R. Gil-Merino, New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations, Astronomy & Astrophysics  492, 401-410 (2008), https://doi.org/10.1051/0004-6361:200810447

[4] L. J. Goicoechea, V. N. Shalyapin and A. Oscoz, Apparent correlation between extrinsic and intrinsic flux variations in the first gravitationally lensed quasar, Monthly Notices of the Royal Astronomical Society, Volume 530, Issue 2, May 2024, Pages 2273–2281, https://doi.org/10.1093/mnras/stae952

[5] https://astro.goblack.de/Theorie/t_tools.html
Als Hubble-Kontante wurde der Wert 74,2 der NASA/IAU genommen.

[6] Wikipedia, ausgelesen am 13.3.2025
https://de.wikipedia.org/wiki/Q0957%2B561

[7] https://kosmos-os.de/der-zwillingsquasar-qso-0957561ab/

[8] RXJ 0921+4529: A BINARY QUASAR ORAGRAVITATIONALLENS?
L. C. Popovic et al, The Astrophysical Journal Letters, 721:L139–L142, 2010 October 1
RXJ 0921+4529: A BINARY QUASAR OR A GRAVITATIONAL LENS? – IOPscience

[9] Multi-Frequency Analysis of the New Wide-Separation Gravitational Lens Candidate RXJ0921+4529
J. A. Munoz et al, August 2000, The Astrophysical Journal 546:769-774
https://www.researchgate.net/publication/222711948_Multifrequency_Analysis_of_the_New_Wide-Separation_Gravitational_Lens_Candidate_RX_J09214529

 [10] Read from Aladin on 15.3.2025 with the coordinates: 09 21 22.87 +45 29 58.6
https://aladin.cds.unistra.fr/AladinLite/

[11] SDSS J1029 2623: A GRAVITATIONALLY LENSED QUASAR WITH AN IMAGE SEPARATION OF 22.51
Naohisa Inada et al, The Astrophysical Journal, 653: L97–L100, 2006 December 20
https://iopscience.iop.org/article/10.1086/510671/pdf

[12] A TWO-YEAR TIME DELAY FOR THE LENSED QUASAR SDSSJ1029+2623
Janine Fohlmeister et al
The Astrophysical Journal, 764:186 (8pp), 2013 February 20
https://iopscience.iop.org/article/10.1088/0004-637X/764/2/186

[13] The Next Step in Galaxy Cluster Strong Lensing: Modeling the Surface Brightness of  Multiply Imaged Sources
Ana Acebron et al, The Astrophysical Journal, 976:110 (14pp), 2024 November 20
https://iopscience.iop.org/article/10.3847/1538-4357/ad8343/pdf