The Impact of Gravitational Redshift on Systemic Water Maser Emission in NGC 4258

Abstract

NGC 4258 is well known for its 22 GHz water maser features that appear to reside in a thin Keplerian disk orbiting at R_0=0.13 pc. The influence of gravitational redshift on the level of maser saturation and the line strength and width of systemic water maser emission is studied. The effective gravitational velocity shift along the gain path through the disk amounts to about twice the thermal velocity width of H2O molecules at 500 K for a masing ring of thickness 0.2R_0. This GR induced frequency shift has a sign opposite to the shift caused by the projected velocity difference vector of Keplerian motion on the blue side of the systemic maser emission and has the same sign on the red systemic side. Consequently, the negative optical depth that a line photon sees as it traverses the masing disk along a path of minimal Keplerian velocity gradient is larger on the blue side. Note that one is not concerned here with an overall shift in the maser spectrum caused by the fact that the masing ring as a whole is located in the black hole's potential well, i.e., it is the change in the gravitational potential along the gain path that matters. It is found, for a Schwarzschild metric, that saturation is enhanced (suppressed) for systemic blue-shifted (red-shifted) maser features by gravitational redshift of line photons. Approaching peak flux systemic features are, on average, brighter, due to saturation, and wider (~2 km/s), due to rebroadening, than their receding counterparts (~1 km/s). The effects are consistent with observations that span several years, i.e., over periods longer than needed for disk material to move through the systemic emission region. Further confirmation of this effect can follow by combining all available systemic 22 GHz data over the past two decades (abridged).