14 research outputs found
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The ParsecâScale Accretion Disk in NGC 3393
We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC 3393. The maser emission has a linear distribution oriented at a position angle of ~â34°, perpendicular to both the kiloparsec-scale radio jet and the axis of the narrow-line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of ( within 0.36 ± 0.02 pc (1.48 ± 0.06 mas), which corresponds to a mean mass density of . We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of in the maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of 0.17 ± 0.02 pc, which is smaller than the inner disk radius (0.36 ± 0.02 pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC 4258 and NGC 1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.Astronom
Probing the Magnetic Field at Sub-Parsec Radii in the Accretion Disk of NGC 4258
We present an analysis of polarimetric observations at 22 GHz of the water
vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect
any circular polarization in the spectrum indicative of Zeeman-induced
splitting of the maser lines of water, a non-paramagnetic molecule. We have
improved the 1-sigma upper limit estimate on the toroidal component of the
magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from
300 mG to 90 mG. We have developed a new method for the analysis of spectra
with blended features and derive a 1-sigma upper limit of 30 mG on the radial
component of the magnetic field at a radius of 0.14 pc. Assuming thermal and
magnetic pressure balance, we estimate an upper limit on the mass accretion
rate of ~10^(-3.7) M_sun/yr for a total magnetic field of less than 130 mG. We
discuss the ramifications of our results on current maser models proposed to
explain the observed maser emission structure and the consequences for current
accretion theories. We find from our magnetic field limits that the thin-disk
model and the jet-disk model are better candidates for accounting for the
extremely low-luminosity nature of NGC 4258, than models that include
advection-dominated accretion flows.Comment: 20 pages, including 10 figures and 2 tables. Accepted for publication
in the Astrophysical Journa
Using VLBI to Probe the Orion-KL Outflow on AU Scales
We present the first contemporaneous 43GHz and 86GHz VLBI images of the v=1
J=2-1 and J=1-0 SiO masers in the Orion-KL nebula. Both maser species exhibit
the same general morphology of earlier J=1-0 maser images which appear to trace
the edges of a bi-polar conical outflow. Surprisingly, the J=2-1 masers form
further from the central protostar than the J=1-0 masers, a fact not readily
explained by current SiO maser pumping models. The average magnitude of offsets
between corresponding regions of the two masing transitions is approximately
14% of the total radial extent of the SiO maser emission. This offset indicates
that each transition must trace different physical conditions.Comment: 20 pages, 4 figure
Discovery of Candidate HO Disk Masers in AGN and Estimations of Centripetal Accelerations
Based on spectroscopic signatures, about one-third of known HO maser
sources in active galactic nuclei (AGN) are believed to arise in highly
inclined accretion disks around central engines. These "disk maser candidates"
are of interest primarily because angular structure and rotation curves can be
resolved with interferometers, enabling dynamical study. We identify five new
disk maser candidates in studies with the Green Bank Telescope, bringing the
total number published to 30. We discovered two (NGC1320, NGC17) in a survey of
40 inclined active galaxies (v_{sys}< 20000 kms^{-1}). The remaining three disk
maser candidates were identified in monitoring of known sources: NGC449,
NGC2979, NGC3735. We also confirm a previously marginal case in UGC4203. For
the disk maser candidates reported here, inferred rotation speeds are 130-500
kms^{-1}. Monitoring of three more rapidly rotating candidate disks (CG211,
NGC6264, VV340A) has enabled measurement of likely orbital centripetal
acceleration, and estimation of central masses (2-7x10^7 M_\odot) and mean disk
radii (0.2-0.4pc). Accelerations may ultimately permit estimation of distances
when combined with interferometer data. This is notable because the three AGN
are relatively distant (10000<v_{sys}<15000 kms^{-1}). As signposts of highly
inclined geometries at galactocentric radii of \sim0.1-1pc, disk masers also
provide robust orientation references that allow analysis of (mis)alignment
between AGN and surrounding galactic stellar disks, even without
interferometric mapping. We find no preference among published disk maser
candidates to lie in high-inclination galaxies, providing independent support
for conclusions that central engines and galactic plane orientations are not
correlated. (ABRIDGED)Comment: 7 figures, accepted for publication in ApJ, Dec. 10, 200
Evidence for a Geometrically Thick Self-Gravitating Accretion Disk in NGC 3079
We have mapped, for the first time, the full velocity extent of the water
maser emission in NGC 3079. The largely north-south distribution of emission,
aligned with a kpc-scale molecular disk, and the segregation of blue- and
red-shifted emission on the sky are suggestive of a nearly edge-on molecular
disk on pc-scales. Positions and line-of-sight velocities of blue- and
red-shifted maser emission are consistent with a central mass of ~2 x 10^6 Msun
enclosed within a radius of ~0.4 pc. The corresponding mean mass density of
10^6.8 Msun pc^-3 is suggestive of a central black hole, which is consistent
with the detection of hard X-ray excess (20-100 keV) and an Fe Kalpha line from
the nucleus. Because the rotation curve traced by the maser emission is flat,
the mass of the pc-scale disk is significant with respect to the central mass.
Since the velocity dispersion of the maser features does not decrease with
radius and constitutes a large fraction of the orbital velocity, the disk is
probably thick and flared. The rotation curve and the physical conditions
necessary to support maser emission imply a Toomre Q-parameter that is << 1.
Thus, the disk is most likely clumpy, and we argue that it is probably forming
stars. Overall, the accretion disk in NGC 3079 stands in contrast to the
compact, thin, warped, differentially rotating disk in the archetypal maser
galaxy NGC 4258 (abridged).Comment: 41 pages, 13 figures, to appear in the 2005 January 10 issue of the
Astrophysical Journal. High resolution versions of the figures and of the
paper are available at
http://cfa-www.harvard.edu/~pkondratko/publications/NGC3079
Using VLBI to Probe the Orion-KL Outflow on AU Scales
We present the first contemporaneous 43GHz and 86GHz VLBI images of the v=1
J=2-1 and J=1-0 SiO masers in the Orion-KL nebula. Both maser species exhibit
the same general morphology of earlier J=1-0 maser images which appear to trace
the edges of a bi-polar conical outflow. Surprisingly, the J=2-1 masers form
further from the central protostar than the J=1-0 masers, a fact not readily
explained by current SiO maser pumping models. The average magnitude of offsets
between corresponding regions of the two masing transitions is approximately
14% of the total radial extent of the SiO maser emission. This offset indicates
that each transition must trace different physical conditions.Comment: 20 pages, 4 figure
Discovery of Water Maser Emission in Five AGN and a Possible Correlation Between Water Maser and Nuclear 2-10 keV Luminosities
We report the discovery of water maser emission in five active galactic
nuclei (AGN) with the 100-m Green Bank Telescope (GBT). The positions of the
newly discovered masers, measured with the VLA, are consistent with the optical
positions of the host nuclei to within 1 sigma (0.3 arcsec radio and 1.3 arcsec
optical) and most likely mark the locations of the embedded central engines.
The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618
NED02, display the characteristic spectral signature of emission from an
edge-on accretion disk with maximum orbital velocity of ~700, ~800, and ~1300
km s^-1, respectively. We also present a GBT spectrum of a previously known
source MRK 0034 and interpret the narrow Doppler components reported here as
indirect evidence that the emission originates in an edge-on accretion disk
with orbital velocity of ~500 km s^-1. We obtained a detection rate of 12
percent (5 out of 41) among Seyfert 2 and LINER systems with 10000 km s^-1 <
v_sys < 15000 km s^-1. For the 30 nuclear water masers with available hard
X-ray data, we report a possible relationship between unabsorbed X-ray
luminosity (2-10 keV) and total isotropic water maser luminosity, L_{2-10}
proportional to L_{H2O}^{0.5+-0.1}, consistent with the model proposed by
Neufeld and Maloney in which X-ray irradiation and heating of molecular
accretion disk gas by the central engine excites the maser emission.Comment: 16 pages, 5 tables, 3 figures, to appear in the November 10, 2006,
v651n2 issue of the Astrophysical Journa
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Evidence for a Geometrically Thick SelfâGravitating Accretion Disk in NGC 3079
We have mapped, for the first time, the full velocity extent of the water maser emission in NGC 3079. The largely north-south distribution of emission, aligned with a kpc-scale molecular disk, and the segregation of blue- and red-shifted emission on the sky are suggestive of a nearly edge-on molecular disk on pc-scales. Positions and lineof-sight velocities of blue- and red-shifted maser emission are consistent with a central mass of ⌠2 Ă 106 Mâ enclosed within a radius of ⌠0.4 pc. The corresponding mean mass density of 106.8 Mâ pcâ3 is suggestive of a central black hole, which is consistent with the detection of hard X-ray excess (20 â 100 keV) and an Fe Kα line from the nucleus. Because the rotation curve traced by the maser emission is flat, the mass of the pc-scale disk is significant with respect to the central mass. Since the velocity dispersion of the maser features does not decrease with radius and constitutes a large fraction of the orbital velocity, the disk is probably thick and flared. The rotation curve and the physical conditions necessary to support maser emission imply a Toomre Qparameter that is âȘ 1. Thus, the disk is most likely clumpy, and we argue that it is probably forming stars. Overall, the accretion disk in NGC 3079 stands in contrast to the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258. We have also mapped radio continuum emission in the vicinity of the disk and identify a new, time-variable, non-thermal component (E) that is not collinear with the previously imaged putative jet. Based on the large luminosity and the unusually steep spectrum (α < â2.1), we exclude a radio supernova as the progenitor of E. However, because its spectrum is consistent with an aging electron energy distribution, E might be a rapidly cooling remnant, which may indicate that the jet axis wobbles. Alternatively, considering its location, the component might mark a shock in a wide-angle outflow that is interacting with a dense ambient medium. In this context, masers at high latitudes above the disk, mapped in this and previous studies, may be tracing an inward extension of the kpc-scale bipolar wide-angle outflow previously observed along the galactic minor axis.AstronomyPhysic
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Discovery of Water Maser Emission in Five AGNs and a Possible Correlation Between Water Maser and Nuclear 2â10 keV Luminosities
We report the discovery of water maser emission in five active galactic nuclei (AGN) with the 100-m Green Bank Telescope (GBT). The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1Ï (0.âČâČ3 radio and 1.âČâČ3 optical) and most likely mark the locations of the embedded central engines. The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618 NED02, display the characteristic spectral signature of emission from an edge-on accretion disk with maximum orbital velocity of ⌠700 km sâ1, ⌠800 km sâ1, and ⌠1300 km sâ1, respectively. We also present a GBT spectrum of a previously known source MRK 0034 and interpret the narrow Doppler components reported here as indirect evidence that the emission originates in an edge-on accretion disk with orbital velocity of ⌠500 km sâ1. We obtained a detection rate of 12% (5 out of 41) among Seyfert 2 and LINER systems with 10000 km sâ1 < vsys < 15000 km sâ1. For the 30 nuclear water masers with available hard X-ray data, we report a possible relationship between unabsorbed X-ray luminosity (2 â10 keV) and total isotropic water maser luminosity, L2â10 â L 0.5±0.1 H2O, consistent with the model proposed by Neufeld and Maloney in which X-ray irradiation and heating of molecular accretion disk gas by the central engine excites the maser emission.Astronom