2025/03/12 by Yan, Xi, Cui, Lang, Ho, Luis C. · 1 citation
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2503.09084
We present the multi-frequency, 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 pc-scale jets differs by 26^∘-30^∘ from that of the kpc-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 counter-jet. Measurements of the radial intensity profile on both sides indicate a change in the counter-jet 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 counter-jet 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.