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| Format: | Preprint |
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2025
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| Online Access: | https://arxiv.org/abs/2503.09084 |
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| _version_ | 1866910871698014208 |
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| author | Yan, Xi Cui, Lang Ho, Luis C. |
| author_facet | Yan, Xi Cui, Lang Ho, Luis C. |
| contents | 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 $\sim 5.3$ pc. Notably, the position angle of the pc-scale jets differs by $26^\circ$-$30^\circ$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2503_09084 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Multi-frequency Very Long Baseline Interferometry Study of Emission and Absorption in the Two-Sided Jets of NGC 3998 Yan, Xi Cui, Lang Ho, Luis C. Astrophysics of Galaxies High Energy Astrophysical Phenomena 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 $\sim 5.3$ pc. Notably, the position angle of the pc-scale jets differs by $26^\circ$-$30^\circ$ 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. |
| title | Multi-frequency Very Long Baseline Interferometry Study of Emission and Absorption in the Two-Sided Jets of NGC 3998 |
| topic | Astrophysics of Galaxies High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2503.09084 |