VERA and ALMA Observations of the H2O Supermaser Burst in Orion KL

Abstract

The 22 GHz H2O maser in Orion KL has shown extraordinary burst events in 1979-1985 and 1998-1999, sometimes called supermaser. We have conducted monitoring observations of the supermaser in Orion KL using VERA, VLBI Exploration of Radio Astrometry, in the current third burst since 2011 March. Three flux maxima are detected in 2011 and 2012 with rising and falling timescales of 2-7 months. Time variations of the supermaser seem symmetric for all of the active phases. The maximum total flux density of 135000 Jy is observed in 2012 June while it is still one order of magnitude lower than those in previous bursts. The supermaser consists of two spatially different components at different velocities. They are elongated along a northwest-southeast direction perpendicular to the low-velocity outflow driven by Source I. Proper motions of the supermaser features with respect to Source I are measured toward west and southwest directions, almost parallel to the low-velocity outflow. The flux density and linewidth show an anti-correlation as expected for an unsaturated maser emission. The supermaser is located close to the methylformate (HCOOCH3) line and continuum emission peaks in the Orion Compact Ridge detected by ALMA. The broader velocity range of the weak HCOOCH3 emission at the supermaser position would be an evidence of a shock front. On the other hand, the 321 GHz H2O line is not detected at the position of the supermaser. It can be explained qualitatively by one of the theoretical H2O excitation models without extraordinary conditions. Our results support a scenario that the supermaser is excited in the dense gas interacting with the low-velocity outflow in the Compact Ridge. The extremely high flux density and its symmetric time variation for rising and falling phases could be explained by a beaming effect during the amplification process rather than changes in physical conditions.

arXiv comment29 pages, 11 figures, Accepted by PASJ VERA special issue

References in corpus (9)

• Distance to Orion KL Measured with VERA
• A Feature Movie of SiO Emission 20-100 AU from the Massive Young Stellar Object Orion Source I
• Tracing the Bipolar Outflow from Orion Source I
• CH3OCH3 in Orion-KL: a striking similarity with HCOOCH3
• Identifying the outflow driving sources in Orion-KL
• ALMA observations of the outflow from the Source I in the Orion-KL region
• Physical Relation of Source I to IRc2 in the Orion KL Region
• Identification of Bursting Water Maser Features in Orion KL
• Non--local radiative transfer in strongly inverted masers

Cited by in corpus (13)

• The Extraordinary Outburst in the Massive Protostellar System NGC6334I-MM1: Emergence of Strong 6.7 GHz Methanol Masers
• The extraordinary outburst in the massive protostellar system NGC6334I-MM1: Flaring of the water masers in a north-south bipolar outflow driven by MM1B
• ALMA Imaging of Millimeter/Submillimeter Continuum Emission in Orion KL
• Water maser variability in a high-mass YSO outburst -- VERA and ALMA observations of S255 NIRS 3
• ALMA observations of the archetypal "hot core" that isn't: Orion KL
• VLBI observations of the G25.65+1.05 water maser superburst
• ALMA Observation of the 658 GHz Vibrationally Excited H${}_2$O Maser in Orion KL Source I
• Evidence for dense gas heated by the explosion in Orion KL
• JVLA overview of the bursting H${}_2$O maser source G25.65+1.05
• The Science Case for Simultaneous mm-Wavelength Receivers in Radio Astronomy
• ALMA imaging study of methyl formate (HCOOCH${}_3$) in the torsionally excited states towards Orion KL
• Discovery of 22 GHz Water Masers in the Serpens South Region
• Identifying the Mechanisms of Water Maser Variability During the Accretion Burst in NGC6334I