arXiv subjects

Lincoln Greenhill

Publications and source records attributed to Lincoln Greenhill.

22 records · Page 2Linked to original sources

arXiv2007

The structure of the accretion disk in NGC 4258 derived from observations of its water vapor masers

A wealth of new information about the structure of the maser disk in NGC 4258 has been obtained from a series of 18 VLBA observations spanning three years, as well as from 32 additional epochs of spectral monitoring data from 1994 to the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk has been defined precisely. The thickness of the maser disk has been measured to be 12 microarcseconds (FWHM), which is slightly smaller than previously quoted upper limits. Under the assumption that the masers trace the true vertical distribution of material in the disk, from the condition of hydrostatic equilibrium the sound speed is 1.5 km/s, corresponding to a thermal temperature of 600K. The accelerations of the high velocity maser components have been accurately measured for many features on both the blue and red side of the spectrum. The azimuthal offsets of these masers from the midline (the line through the disk in the plane of the sky) and derived projected offsets from the midline based on the warp model correspond well with the measured offsets. This result suggests that the masers are well described as discrete clumps of masing gas, which accurately trace the Keplerian motion of the disk. However, we have continued to search for evidence of apparent motions caused by ``phase effects.'' This work provides the foundation for refining the estimate of the distance to NGC 4258 through measurements of feature acceleration and proper motion. The refined estimate of this distance is expected to be announced in the near future.

astro-ph↗

arXiv2005

X-ray Emission from Megamaser Galaxy IC 2560

Observation of the H2O megamaser galaxy IC 2560 with the Chandra Observatory reveals a complex spectrum composed of soft X-ray emission due to multi-temperature thermal plasma, and a hard continuum with strong emission lines. The continuum is most likely a Compton reflection (reprocessing) of primary emission that is completely absorbed at least up to 7 keV. The lines can be identified with fluorescence from Si, S and Fe in the lowest ionization stages. The equivalent widths of the Si and S lines are broadly compatible with those anticipated for reprocessing by optically thick cold plasma of Solar abundances, while the large equivalent width of the Fe line requires some overabundance of iron. A contribution to the line from a transmitted component cannot be ruled out, but the limits on the strength of the Compton shoulder make it less likely. From the bolometric luminosity of the nuclear region, we infer that the source radiates at 1 - 10% of its Eddington luminosity, for an adopted central mass of 3 million Solar masses. The overall spectrum is consistent with the hypotheses that the central engines powering the detected megamsers in accretion disks are obscured from direct view by the associated accretion disk material itself, and that there is a correlation between the occurrence of megamaser emission and Compton-thick absorption columns. For the 11 known galaxies with both column density measurements and maser emission believed to arise from accretion disks, eight AGN are Compton thick.

astro-ph↗

arXiv2004

Q0906+6930: The Highest-Redshift Blazar

We report the discovery of a radio-loud flat-spectrum QSO at z=5.47 with properties similar to those of the EGRET gamma-ray blazars. This source is the brightest radio QSO at z>5, with a pc-scale radio jet and a black hole mass estimate >10^{10}M_\odot. It appears to be the most distant blazar discovered to date. High energy observations of this source can provide powerful probes of the background radiation in the early universe.

astro-ph↗

arXiv2002

Water Masers as Tracers of Protostellar Disks and Outflows in the Intermediate Mass Star Forming Region NGC 2071

We have mapped the water maser emission associated with the infrared centers IRS1 and IRS3 of the NGC 2071IR star forming region at four epochs over ~4 months with the Very Long Baseline Array (VLBA). We detected 269 maser features with ~1 km/sec linewidths and measured 30 proper motions. In each infrared center, the water maser emission appears to trace parts of a protostellar disk and collimated outflow. The disk components are ~9 and ~17 AU long, in IRS3 and IRS1 respectively, and ~2 AU wide. They are identified as disks by their compact size, elongation parallel to the direction of known IR polarization, central location in the maser maps, small internal proper motions, and proximity to 1.3 cm continuum emission. The outflows have axes perpendicular to the disks and exhibit proper motions of up to ~42 km/sec. They are outlined by maser emission up to ~260 AU from the protostars. The IRS3 outflow appears to be conical on one side, while the IRS1 outflow comprises a narrowly collimated bipolar flow surrounded by outward-facing, funnel-shaped cavities. The detection of water maser emission tracing such compact disk components and specifically conical or funnel-shaped structures is unusual. The fact that the distributions are similar in IRS3 and IRS1 may indicate the two infrared centers are roughly coeval. NGC 2071IR provides a rare opportunity to resolve the structures and dynamics of disks and outflows together, and to do so for two protostars that are only ~2000 AU apart (in projection) in a deeply embedded star forming region of intermediate luminosity.

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