VERA Observation of the W49NH2O Maser Outburst in 2003 October

Abstract

We report on a strong outburst of the W 49N $\mathrm{H}_2 \mathrm{O}$ maser observed with VERA. Single-dish monitoring with VERA 20m telescopes detected a strong outburst of the maser feature at $V_\mathrm{LSR} =-30.7 \,\mathrm{km} \,\mathrm{s}^{-1}$ in 2003 October. The outburst had a duration of $\sim 100$ days and a peak intensity of $7.9 \times 10^4 \,\mathrm{Jy}$, being one of the strongest outbursts in W 49N observed so far. VLBI observations with the VERA array were also carried out near to the maximum phase of the outburst, and the outburst spot was identified in the VLBI map. While the map was in good agreement with previous studies, showing three major concentrations of maser spots, we found a newly formed arc-like structure in the central maser concentration, which may be a shock front powered by a forming star or a star cluster. The outburst spot was found to be located on the arc-like structure, indicating a possible connection of the present outburst to a shock phenomenon.

VLBI observations of the G25.65+1.05 water maser superburst

“…The term 'superburst' refers to a particularly extreme class of maser bursts, where a maser emitting region exhibits a sudden increase in flux density of several orders of magnitude. There are 3 recognised water maser superburst star forming regions (SFRs): Orion KL (Matveenko et al 1988;Garay et al 1989;Shimoikura et al 2005;Hirota et al 2011Hirota et al , 2014a, W49N (Honma et al 2004;Kramer et al 2018;Volvach et al 2019c) and G25.65+1.05 (Volvach et al 2017;Ashimbaeva et al 2017;Volvach et al 2019b). Sudden enhancement in maser flux density, though less extreme, has also been seen in species other than water (MacLeod et al 2018;Szymczak et al 2018) and in evolved stars (Esipov et al 1999;Vlemmings et al 2014;Gómez et al 2015;Etoka et al 2017).…”