An observational test of the plasma lensing effect using QSOs with and without MgII absorption

TL;DR

This study investigates plasma lensing effects on quasars with MgII absorbers across radio, infrared, and optical wavelengths, finding subtle evidence of lensing and dust extinction influencing observed fluxes and number counts.

Contribution

It combines multi-wavelength data to test plasma lensing effects on QSOs with MgII absorption, providing new insights into intervening medium properties.

Findings

Moderate excess of bright QSOs with MgII absorbers in radio flux consistent with plasma lensing.

Steeper decline at faint optical fluxes for MgII absorbers suggests possible lensing or dust effects.

Residual excess at bright end indicates weak lensing contribution cannot be ruled out.

Abstract

Radio wave propagation can be perturbed by compact ionized gas clumps through plasma lensing, which induces frequency dependent magnification and may distort the observed number counts of background sources. The quasar (QSO) number densities are a powerful probe for understanding the effects of intervening material. Absorption lines in QSO spectra reveal the presence of interstellar and intergalactic gas, which can change observed fluxes through dust extinction and plasma lensing. By combining observations from radio (VLASS), infrared (WISE), and optical bands (DESI), we assembled a sample of QSOs: ~4000 sources with MgII absorbers, and ~12, 000 non-absorbers. In the radio band, the MgII sample shows a moderate excess at the bright end of the flux distribution, which is broadly consistent with plasma lensing predications. In the optical, the MgII sample turns over at higher g-band fluxes and exhibits a steeper decline at the faint end than the non-MgII sample. Control samples were constructed by matching in redshift, infrared (W1), and optical (g) luminosities. In these comparisons, the radio excess becomes less prominent, suggesting that the apparent magnification may not be robust evidence for plasma lensing. Nevertheless, a weak contribution cannot be ruled out, especially given residual excess observed at the bright end relative to the non-MgII sample. Dust extinction along the line-of-sight remains a plausible alternative. Regardless of the dominant mechanism, the multi-wavelength differences offer a valuable probe of the physical state of the intervening medium.