New Observation Method Proposed for Discovering Gravitational Lens Quasars

- Cheonmunyeon and International Joint Research Team Develop Method to Discover by Reconstructing Light Curves
- Expected to Solve Hubble Constant Discrepancy Issue

[Asia Economy Reporter Kim Bong-su] Quasars, which are supermassive black hole celestial objects, are important for studying the formation of early universe celestial bodies, but they are difficult to observe because they are too far from Earth. A domestic research team has proposed a new method to discover many quasars through international collaborative research.

The Korea Astronomy and Space Science Institute announced on the 25th that an international research team led by Dr. Arman Shafieloo and Dr. Satadru Bag of the Theoretical Astronomy Center published these research results in The Astrophysical Journal on the 15th of last month.

Gravitational lensing is a phenomenon where a massive celestial object warps the surrounding spacetime, acting like a lens. Due to the gravitational lensing effect, light is refracted, causing the image of the light source celestial object to appear multiple times or distorted, resulting in it becoming brighter or dimmer. A representative gravitational lensing phenomenon is when a point light source quasar appears multiple times. When gravitational lensing occurs on a quasar, multiple images of the quasar appear at different positions. In such cases, it is difficult to distinguish the images without using a large telescope, making it hard to clearly determine whether gravitational lensing has occurred. On the other hand, by using the quasar’s light curve, it is possible to determine the occurrence of gravitational lensing without long-term observation with a large telescope. However, it is difficult to standardize the pattern of brightness variation of quasars, making it hard to understand the original quasar light curve.

To overcome this limitation, the research team proposed an observational method to discover gravitationally lensed quasars without knowing the original shape of the quasar’s light curve. First, assuming the quasar has undergone gravitational lensing, they adjusted the time delay between the two signals separated by the gravitational lens. If the time delay between the two signals differs from the actual time, severe distortion occurs in the arbitrarily reconstructed quasar light curve. Using this method, they found the time with the least distortion in the light curve and discovered gravitationally lensed quasars.

The method used by the research team increases the possibility of discovering gravitationally lensed quasars by at least twice. It is also expected to significantly reduce the chance of mistakenly identifying normal quasars as gravitationally lensed quasars. This method can be most effectively used in observational surveys that scan the sky widely and quickly, tracking how numerous celestial objects change over time. Therefore, it is expected to be importantly used not only in the ongoing Zwicky Time Domain Facility (ZTF) survey but also in the upcoming Vera C. Rubin Observatory’s Large Synoptic Survey Telescope (LSST) survey.

The Hubble constant, which indicates how fast the universe is expanding, is a crucial value for understanding the nature of our universe. However, over the past decade, the values of the Hubble constant measured by various methods have not matched, causing the “Hubble constant discrepancy” to be one of the major debates in modern astronomy. To resolve this discrepancy, it is essential to accurately measure the distances to numerous distant celestial objects using various methods. In particular, gravitationally lensed quasars contain diverse information such as brightness differences between images and time delays in light curves, which can be combined to measure distances accurately. If a significant number of gravitationally lensed quasars are discovered based on the new observational method proposed by the research team, it is expected that the Hubble constant discrepancy problem can be resolved.

Dr. Shafieloo said, “We propose a method to effectively discover gravitationally lensed quasars without using large telescopes for extended periods,” adding, “By discovering gravitationally lensed quasars in the ZTF survey and verifying this study first, domestic researchers will be able to lead the discovery of gravitationally lensed quasars in the upcoming LSST survey.”

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