Abstract
The water masers in NGC 4258 delineate the structure and dynamics of a subparsec-diameter accretion disk around a supermassive black hole. Very Long Baseline Array (VLBA) observations provide precise information about the positions in the plane of the sky and the three-dimensional velocity vectors for the maser emission, but the positions along the line of sight must be inferred from models. Previous measurements placed an upper limit on the accelerations of the high-velocity spectral features of 1 km s-1 yr-1, suggesting that they are located near the midline (the diameter perpendicular to the line of sight), where they would have exactly zero acceleration. From similar measurements, the accelerations of the systemic velocity spectral features have been estimated to be about 9 km s-1 yr-1, indicating that they lie toward the front of the disk where the acceleration vector points directly away from the line of sight. We report acceleration measurements for 12 systemic velocity spectral features and 19 high-velocity spectral features using a total of 25 epochs of observations from Effelsberg (five epochs), the Very Large Array (15 epochs), and the VLBA (five epochs) spanning the years 1994-1997. The measured accelerations of the systemic velocity features are between 7.5 and 10.4 km s-1 yr-1, and there is no evidence for a dip in the spectrum at the systemic velocity. Such a dip has been attributed in the past to an absorbing layer of noninverted H2O. The accelerations of the high-velocity features, measured here for the first time, range from -0.77 to 0.38 km s-1 yr-1. From the line-of-sight accelerations and velocities, we infer the positions of these high-velocity masers with a simple edge-on disk model. The resulting positions fall between -136 and 93 in azimuth (measured from the midline). A model that suggests a spiral shock origin of the masers, in which changes in maser velocity are due to the outward motion of the shock wave, predicts apparent accelerations of -0.05(θp/25) km s-1 yr-1, where θp is the pitch angle of the spiral arms. Our data are not consistent with these predictions. We also discuss the physical properties of the high-velocity masers. Most notably, the strongest high-velocity masers lie near the midline, where the velocity gradient is smallest, thereby providing the longest amplification path lengths.
Highlights
Water masers were first detected in the galaxy NGC4258 (M106) by Claussen, Heiligman, & Lo (1984) and Henkel et al (1984)
We have measured accelerations for seventeen redshifted high-velocity features and two blueshifted high-velocity features and find them to range between −0.77 and 0.38 km s−1yr−1 (Figure 5, Table 2)
In the model the high-velocity masers are produced in thin post-shock regions, and we should observe maser emission where the proposed spiral arms are parallel to the line of sight, which occurs along a diameter that is at an angle to the midline equal to the pitch angle of the spiral
Summary
Water masers were first detected in the galaxy NGC4258 (M106) by Claussen, Heiligman, & Lo (1984) and Henkel et al (1984). The peak flux density varies in time but is generally between 2 and 10 Jy, with a typical value of about 4 Jy. Early Very Long Baseline Interferometry (VLBI) observations of these features revealed that the systemic emission is quite compact, on the order of one-hundredth of a parsec (Claussen et al 1988), but is elongated with a velocity gradient along its major axis (7970 ± 40 km s−1pc−1), most likely indicative of circular motion seen edge-on (Greenhill et al 1995a). Later studies found the whole of the systemic emission to be increasing in velocity at a rate of about 10 km s−1yr−1 (Haschick, Baan, & Peng 1994; Greenhill et al 1995b; Nakai et al 1995)
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