The Nature of the Nuclear H₂O Masers of NGC 1068: Reverberation and Evidence for a Rotating Disk Geometry

Abstract

We report new (1995) Very Large Array observations and (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999) Effelsberg 100m monitoring observations of the 22 GHz H 2 O maser spectrum of the Seyfert 2 galaxy NGC 1068. The sensitive VLA observations provide a registration of the 22 GHz continuum emission and the location of the maser spots with an accuracy of ∼ 5 mas. Within the monitoring data, we find evidence that the nuclear masers vary coherently on time-scales of months to years, much more rapidly than the dynamical time-scale. We argue that the nuclear masers are responding in reverberation to a central power source, presumably the central engine. Between October and November 1997, we detected a simultaneous flare of the blue-shifted and red-shifted satellite maser lines. Reverberation in a rotating disk naturally explains the simultaneous flaring. There is also evidence that near-infrared emission from dust grains associated with the maser disk also responds to the central engine. We present a model in which an X-ray flare results in both the loss of maser signal in 1990 and the peak of the near-infrared lightcurve in 1994. In support of rotating disk geometry for the nuclear masers, we find no evidence for centripetal accelerations of the redshifted nuclear masers; the limits are ±0.006 km s −1 year −1 , implying that the masers are located within 2 • of the kinematic line-of-nodes. We also searched for high velocity maser emission like that observed in NGC 4258. In both VLA and Effelsberg spectra, we detect no high velocity lines between ∼ ±350 km s −1 -±850 km s −1 relative to systemic, arguing that masers only lie outside a radius of ∼ 0.6 pc (1.9 light-years) from the central engine (assuming a distance of 14.4 Mpc). We also consider possible models for the jet masers near radio continuum component C. We favor a shock-precursor model, in which the molecular gas surrounding the jet is heated by X-ray emission from a shock front between the jet and a molecular cloud.

Non-Keplerian rotation in the nucleus of NGC 1068: Evidence for a massive accretion disk?

“…Baan & Hashhick (1996) claimed to have observed a drift in the velocity of the water masers, indicating that the maser spots are subject to large accelerations, incompatible with the disk interpretation, according to which the maser centripetal acceleration should be perpendicular to the line of sight. Those large accelerations have not been confirmed by subsequent work by Gallimore et al (2001), who, monitoring the velocity drift, find that the maser spots between 0.6 pc and 1 pc should lie within θ 2 • from the line of nodes.…”

Water vapour at high redshift: Arecibo monitoring of the megamaser in MG J0414+0534

“…For Mrk 348, strong variation of both maser and continuum flux densities are reported, with a close temporal correlation between them (Peck et al 2003). The peak flux density of the jet-maser component in NGC 1068 is also variable, although the variability is not outstanding (Gallimore et al 2001). For the third jet-maser case, that of NGC 1052, the variability is mainly caused by changes in the line profile (Braatz et al 2003).…”

Mid-infrared imaging- and spectro-polarimetric subarcsecond observations of NGC 1068

“…The kinematics of the maser clouds indicate that the masers are located in a nonKeplerian disk with inner radius ∼0.65 pc and outer radius ∼1.1 pc at a P.A. of −40 • (Gallimore et al 2001). Through MIR interferometric observations, Jaffe et al (2004) estimated a warm (T = 320 K) dust structure with a 2.1 pc thick and 3.4 pc diameter, surrounding a hot (T > 800 K) compact (∼1 pc) structure.…”

“…North Knot) (e.g. Gallimore et al 2001). The radio-jet emission between the central few parsecs and the molecular cloud is dominated by synchrotron emission, while the jet-molecular cloud interaction radiates at all wavelengths.…”