A Radio Changing-state Jet in the Narrow-line Seyfert 1 Galaxy J1105+1452

Abstract We report the discovery of a radio-quiet to radio-loud transition in the narrow-line Seyfert 1 galaxy J1105+1452. The source has undergone a long-term evolution from a radio-quiet state in the 1990s to a persistently radio-bright state after 2017. Post-2017 flux densities in the 0.8–7 GHz range cluster between 32 and 43 mJy, whereas the 144 MHz flux density is only 1.94 ± 0.23 mJy. This indicates strong low-frequency suppression from a compact, absorbed component. Modeling the radio spectral energy distribution with a synchrotron self-absorption model yields a turnover frequency ν p = 0.48 ± 0.03 GHz and a peak flux density S p = 38.9 ± 4.7 mJy. These parameters classify J1105+1452 as a megahertz peaked-spectrum source, consistent with the new episode of an early-stage compact jet. Under the assumption of equipartition, we derive an intrinsic physical radius R ∼ 0.68 pc and an average apparent expansion velocity β app ≈ 0.64. The observed brightness temperature T b ≈ 6.0 × 10 11 K necessitates a Doppler factor δ ≈ 12, implying a relativistic jet viewed at θ ≲ 5°. Despite the dramatic radio evolution, the X-ray spectrum remains stable and steep (Γ ≃ 3.0), suggesting that the X-ray emission remains dominated by the disk/corona, while the radio band has become jet dominated. Our results identify J1105+1452 as a rare radio changing-state NLSy1, providing a unique laboratory for studying the birth and early evolution of relativistic jets at high Eddington ratios.