Multifrequency Very Long Baseline Interferometry Study of Emission and Absorption in the Two-sided Jets of NGC 3998

Abstract We present the multifrequency, multi-epoch very long baseline interferometry (VLBI) study of the two-sided jets in the low-luminosity active galactic nucleus NGC 3998, where physical properties of the jets on parsec scales remain poorly understood. Using Very Long Baseline Array data observed at 1.4, 1.7, 2.3, and 5 GHz, we detect symmetric twin jets aligned along the north–south direction, with a total extent of ∼5.3 pc. Notably, the position angle of the parsec-scale jets differs by 26°–30° from that of the kiloparsec-scale jets, suggesting the possibility of jet precession. Based on the frequency-dependent core shift and north/south jet brightness ratio, we identify the northern jet as the approaching jet and the southern jet as the counterjet. Measurements of the radial intensity profile on both sides indicate a change in the counterjet emission from rapid fading to a slower decline at 1.4, 1.7, and 2.3 GHz. Spectral analysis shows that the approaching jet exhibits an optically thin spectrum, while the counterjet is dominated by an optically thick, inverted spectrum. These findings tentatively suggest free–free absorption in NGC 3998, which should be verified in future studies. Finally, our observations also reveal a flat-spectrum VLBI core, showing significant radio variability that is likely linked to a jet ejection event.