The Discovery of Time Variability in OH Megamasers

Abstract

We report the discovery of variability in the OH megamaser in IRAS 21272+2514 at z=0.15. This is the first OH megamaser (OHM) observed to vary in time. The variation is broadband, spanning 0.5-1.5 MHz or 100-300 km/s in the rest frame, and strong, showing rms modulations of 10%-15% while the largest change in flux density between extremal epochs is 34%. Timescales sampled range from 39 to 821 days, and the characteristic modulation timescale is likely to be less than the shortest time baseline. The source of modulation is currently ambiguous, although we favor interstellar scintillation. Best estimates of the size scales of the 1667 MHz OH line features obtained from refractive interstellar scintillation models constrain variable features to be smaller than 2 pc, and quiescent features to be larger than a few pc. Compact masers account for roughly 27%-58% of the total OH emission in this source. Accelerations in spectral lines are generally constrained to be less than 3 km/s/yr, but acceleration of this magnitude is suggested in one of the quiescent spectral components and merits further study.