Evolution of Low-Frequency Radio Emission of the Ionised Gas around S255IR NIRS3 after the Accretion Luminosity Burst

We present the results of analyzing the radio variability of the source S255IR NIRS3 following an accretion event. The source was observed at two epochs: in January–March 2017 at frequencies of 0.3–10 GHz and in March 2019 at 1.32–1.42 GHz using the GMRT and VLA observatories. The 2017 data show that the spectrum is well described by a power-law dependence, Fν~ν0.50± 0.02 , over the entire frequency range. Such a spectral index is typically attributed to a combination of thermal bremsstrahlung emission from the ionized gas with both optically thick and optically thin components. No evidence for nonthermal emission or emission solely from an optically thick component is found at the low-frequency end of the spectrum. In contrast, the 2019 observations in the range of 1.32–1.42 GHz yield a spectral index of 1.58± 0.03 , which is characteristic of predominantly optically thick thermal emission from the ionized gas. This change in behavior may be explained by a transition from jet-dominated emission in 2017 to emission dominated by an ultracompact H II region in 2019. The observed flux density is 3.93± 0.10 mJy in 2017 (at 1.42 GHz) and 3.58± 0.16 mJy in 2019 (1.37 GHz). The emission measured in 2017 is EM 3× 10⁸ pc cm ⁻⁶ .