The Most Powerful H2O Maser Flares

Mechanisms for the Most Powerful H2O Maser Flares M.D. Gray (NARIT) With B. Pimpanuwat (NARIT) A.M.S. Richards, S. Etoka (University of Manchester, UK) Maser Flares ●Significant brightening on timescale short compared to source evolution ●Several possible environments ●Much can be learned from a small number of key parameters ●Various mechanisms, including shocks, radiation, rotation, line-of-sight overlap Very Powerful H2O Flares IRAS183160602 130kJy, 2-3 months, var. index ~1000, quasi-periodic (Volvach et al. 2019) W49 Orion W49: flare 1, single spectral feature; flare2 several spectral features. Inverse square of line width ∝ ln(F) → unsaturated? Bright background → overlap? Shocks and superradiance also suggested. 2019MNRAS.487L..77V Orion: flares 197985, 1998-99,201112 (x10 weaker than earlier flares). Rise 7 months, fall 3. 2 spectral components. Unsaturated? Shock and overlap suggested. 2014PASJ...66..10 6H Key Parameters ●Variability index (and absolute flux density) ●Timescales (duty cycle, rise & decay) ●Periodicity ●Correlations (usually with IR radiation, WISE, NEOWISE) ●Light curve shape and symmetry Likely Mechanisms for H2O ●Short rise times (<100d): not rotation ●Aperiodic (complicated, but brightest flares not periodic) ●Correlation (may follow long-term brightening, but not on same timescale as flare) ●Variability index (high hundreds → many thousands, huge absolute flux density) ●Together: shock excitation or single-pass line-ofsight overlap Observational Support ●VERA, March 2011-August 2012, 0.4mas accuracy ●ALMA 320-340GHz continuum and methyl formate ●2 spectral components also 2 spatial components ●Elongation NW-SE ⊥ proper motion ●No 321GHz H2O maser near flare ●Position correlation with methylformate → shock? ●Shock from Source-I outflow interaction with compact ridge? ●Duration of spatial structures < 2yr Theoretical Aspects ●Shocks provide a collisional pump ●J and C-type shocks ●Models in 2024MNRAS.530.3342G ●Shocks are ‘isothermal’ ●Complicated optical depth/H2O abundance relation ●Final abundance must not enter quenching zone ●C-type enhances H2O abundance before significant compression Collisional quenching compression Sample Light Curves ●Unknown decay form, but symmetric unlikely ●Masers in post-shock gas highly saturated ●Initial rise time of order 30d achievable ●Hydrodynamic model too slow: faster shock reduces peak flux density (best output with compression factor 15-20). ●C-type behaviour limited to speeds <~40km/s: can’t get shorter rise times Viewing Angle ●J-type (top): F decays fast from view || to shock front ●C-type: slower decay → more visible ●Visibility solid angle contains ½ flux density ●ΩV = 0.45sr (J-type) ●ΩV = 3.1sr (C-type) Conclusions ●Overlaps can match var. index and timescales, but model range very incomplete ●Unsaturated system (near peak overlap) not possible ●Shocks will work but suffer more from quenching limit ●Overlap seems less ‘special’? ●Overlap has VLBI consequences: shrinkage of apparent feature size. ●Also narrowing of spectra (without unsaturated state)