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

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: $\sim 4000$ sources with Mg II absorbers, and $\sim 12\,000$ non-absorbers. In the radio band, the Mg II 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 Mg II sample turns over at higher g-band fluxes and exhibits a steeper decline at the faint end than the non-Mg II 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-Mg II sample. Dust extinction along the line-of-sight remains a plausible alternative. Regardless of the dominant mechanism, the multiwavelength differences offer a valuable probe of the physical state of the intervening medium.