A Multitransition Methanol Maser Study of the Accretion Burst Source G358.93–0.03-MM1

Abstract

We present the most complete to date interferometric study of the centimeter-wavelength methanol masers detected in G358.93−0.03 at the burst and post-burst epochs. A unique, near-IR/(sub)millimeter-dark and far-IR-loud massive young stellar object accretion burst was recently discovered in G358.93−0.03. The event was accompanied by flares of an unprecedented number of rare methanol maser transitions. The first images of three of the newly discovered methanol masers at 6.18, 12.23, and 20.97 GHz are presented in this work. The spatial structure evolution of the methanol masers at 6.67, 12.18, and 23.12 GHz is studied at two epochs. The maser emission in all detected transitions resides in a region of ∼0.″2 around the bursting source and shows a clear velocity gradient in the north–south direction, with redshifted features to the north and blueshifted features to the south. A drastic change in the spatial morphology of the masing region is found: a dense and compact “spiral” cluster detected at epoch I evolved into a disperse, “round” structure at epoch II. During the transition from the first epoch to the second, the region traced by masers expanded. The comparison of our results with the complementary Very Large Array, very long baseline interferometry, Submillimeter Array, and Atacama Large Millimeter/submillimeter Array maser data is conducted. The obtained methanol maser data support the hypothesis of the presence of spiral arm structures within the accretion disk, which was suggested in previous studies of the source.

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

“…Data reduction was performed as described in Bayandina et al (2022). Here we repeat two remarks detailed in Bayandina et al (2022): (1) the observations were made during a multi-frequency maser flare that led to a leakage of a few maser lines into continuum spectral windows (see the notes in Table 1); (2) a positional shift was introduced into the maser data to compensate for a discrepancy in the absolute coordinates of the maser and continuum data obtained with different facilities and beams; for example, the maser data obtained with the VLA in the present work is compared with the Atacama Large Millimeter/submillimeter Array (ALMA) data from Brogan et al (2019).…”

“…The study of the maser emission during the burst performed by the Maser Monitoring Organization (M2O 1 ; a global community for maser-driven astronomy; Burns et al 2022) has provided the most complete picture of an accretion burst in MYSOs to date (MacLeod et al 2019;Brogan et al 2019;Breen et al 2019;Burns et al 2020;Stecklum et al 2021;Bayandina et al 2022). The MM1 burst was accompanied with flares of maser emission (e.g.…”

“…The MM1 burst was accompanied with flares of maser emission (e.g. Sugiyama et al 2019;MacLeod et al 2019;Brogan et al 2019;Breen et al 2019;Bayandina et al 2022). Among the detected masers were rare and even previously undiscovered methanol maser transitions, some of which were unpredicted (MacLeod et al 2019;Breen et al 2019;Brogan et al 2019).…”

“…Interferometric observations, conducted after the methanol maser flare, revealed the presence of an accretion disk and were able to follow the propagation of a heatwave across it (Burns et al 2020;Chen et al 2020;Bayandina et al 2022). The next important step was to study the spatial structure of the water maser.…”

“…Additionally, with the VLA we were able to study the centimetrecontinuum sources in the region. The general technical details of the observations are described in Bayandina et al (2022). Here we mention only the parameters of the water maser study.…”