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 $1.94\pm 0.23$ 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 $\hat{I}{½}_{\mathrm{p}}=0.48\pm 0.03$ GHz and a peak flux density $S_{\mathrm{p}}=38.9\pm 4.7$ 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 $R\sim 0.68$ pc and an average apparent expansion velocity ${\hat{I}}_{\mathrm{app}}^2\approx 0.64$. The observed brightness temperature $T_b\approx 6.0\times 10^{11}$ K necessitates a Doppler factor $\hat{I}´\approx 12$, implying a relativistic jet viewed at $\hat{I}¸\lesssim 5^{\circ }$. Despite the dramatic radio evolution, the X-ray spectrum remains stable and steep ($\hat{I}“\simeq 3.0$), 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.

Accepted for publication in The Astrophysical Journal Letters