The evolution of the H₂O maser emission in the accretion burst source G358.93−0.03

Abstract

Context. The massive young stellar object (MYSO) G358.93−0.03-MM1 showed an extraordinary near-infrared-to (sub-)millimetredark and far-infrared-loud accretion burst, which is closely associated with flares of several class II methanol maser transitions, and, later, a 22 GHz water maser flare. Aims. Water maser flares provide an invaluable insight into ejection events associated with accretion bursts. Although the short timescale of the 22 GHz water maser flare made it impossible to carry out a very long baseline interferometry observation, we could track it with the Karl G. Jansky Very Large Array (VLA). Methods. The evolution of the spatial structure of the 22 GHz water masers and their association with the continuum sources in the region is studied with the VLA during two epochs, pre-and post-H 2 O maser flare. Results. A drastic change in the distribution of the water masers is revealed: in contrast to the four maser groups detected during epoch I, only two newly formed clusters are detected during epoch II. The 22 GHz water masers associated with the bursting source MM1 changed in morphology and emission velocity extent. Conclusions. Clear evidence of the influence of the accretion burst on the ejection from G358.93−0.03-MM1 is presented. The accretion event has also potentially affected a region with a radius of ∼2 ′′ (∼13 500 AU at 6.75 kpc), suppressing water masers associated with other point sources in this region.

New Methanol Maser Transitions and Maser Variability Identified from an Accretion Burst Source G358.93-0.03

“…On April 20, at the same velocity component, the peak-flux density reaches 24 Jy (MacLeod et al 2019). Furthermore, on May 7, some new maser features were detected in the velocity range of −22 to −17 km s −1 (with peak at ∼-20 km s −1 ) and −16 to −13 km s −1 (with a peak at ∼-14.5 km s −1 ), which are close to that detected with the Very Large Array (VLA) in June by Bayandina & Burns (2022b) with the exception of a new maser component peaked at -21.5 km s −1 that appeared in June from the VLA detection. These new water components have the similar velocity ranges to those detected in both the 36.17 and 44.07 GHz methanol transitions, supporting the idea that they have the same pumping origination and are associated with accretion burst actions on the shocked regions where they are excited (see above).…”

“…Both observational surveys and theoretical considerations show that the maser emission in 36.17 GHz and 44.07 GHz transitions comes from generally the same regions but their spectra do not coincide in detail, due to the different sensitivity to the pumping conditions (Sobolev et al 2007;Voronkov et al 2014;Leurini et al 2016;Sobolev & Parfenov 2018). The locations of these two transitions ) are close to the position of the water maser cluster components II −3−4 which occurred in 2019 May−June (Bayandina & Burns 2022b). Velocity of the 44.07 GHz line emission is close to the velocity of the water masers detected in May, suggesting that these masers might also have close locations.…”

“…This means that the water maser emission burst occurred later than the methanol masers, likely because the water maser is distributed in regions far away from the burst source G358-MM1 compared to the methanol masers. This statement is supported by the VLA images of methanol and water masers (Chen et al 2020a;Bayandina & Burns 2022b), thus there is an expected time delay between the heat-wave propagation to the methanol and to water maser regions.…”

“…Velocity of the 44.07 GHz line emission is close to the velocity of the water masers detected in May, suggesting that these masers might also have close locations. Occurrence of new water maser clusters is likely associated with the accretion burst in G358 (Bayandina & Burns 2022b). Both water masers and class I methanol masers are pumped by collisions, so the variability of the 36.17 GHz and 44.07 GHz is also likely associated with the accretion burst phenomenon.…”

Physical Environments of the Luminosity Outburst Source NGC 6334I Traced by Thermal and Maser Lines of Multiple Molecules

“…It is worth mentioning that a direct link between the 6.7 GHz class II CH 3 OH maser flaring and accretion burst has recently been established only in a small number of HMYSOs, e.g., G358.93-0.03-MM1 (Breen et al 2019;MacLeod et al 2019;Sugiyama et al 2019;Burns et al 2020), S255IR-NIRS3 (Fujisawa et al 2015;Carattio Garatti et al 2017;Moscadelli et al 2017), and NGC 6334I-MM1 (Hunter et al 2017(Hunter et al , 2018MacLeod et al 2018). Besides, 22 GHz H 2 O (water) maser emission from protostars has exhibited flare phenomena toward Orion KL (Abraham et al 1981;Omodaka et al 1999;Hirota et al 2014), W49N (Liljeström & Gwinn 2000;Honma et al 2004), G25.65 + 1.05 (Volvach et al 2017a(Volvach et al , 2017bLekht et al 2018), G358.93-0.03 (Bayandina et al 2022;Miao et al 2022), and W51D (Zhang et al 2022).…”