arXiv subjects

Vincent L. Fish

Publications and source records attributed to Vincent L. Fish.

At least 73 records · Page 4Linked to original sources

arXiv2010

First Interferometric Images of the 36 GHz Methanol Masers in the DR21 Complex

Class I methanol masers are believed to be produced in the shock-excited environment around star-forming regions. Many authors have argued that the appearance of various subsets of class I masers may be indicative of specific evolutionary stages of star formation or excitation conditions. Until recently, however, no major interferometer was capable of imaging the important 36 GHz transition. We report on Expanded Very Large Array observations of the 36 GHz methanol masers and Submillimeter Array observations of the 229 GHz methanol masers in DR21(OH), DR21N, and DR21W. The distribution of 36 GHz masers in the outflow of DR21(OH) is similar to that of the other class I methanol transitions, with numerous multitransition spatial overlaps. At the site of the main continuum source in DR21(OH), class I masers at 36 and 229 GHz are found in virtual overlap with class II 6.7 GHz masers. To the south of the outflow, the 36 GHz masers are scattered over a large region but usually do not appear coincident with 44 GHz masers. In DR21W we detect an "S-curve" signature in Stokes V that implies a large value of the magnetic field strength if interpreted as due to Zeeman splitting, suggesting either that class I masers may exist at higher densities than previously believed or that the direct Zeeman interpretation of S-curve Stokes V profiles in class I masers may be incorrect. We find a diverse variety of different maser phenomena in these sources, suggestive of differing physical conditions among them.

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arXiv2010

Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter VLBI Observations of Sagittarius A*

Millimeter very-long baseline interferometry (mm-VLBI) provides the novel capacity to probe the emission region of a handful of supermassive black holes on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, this provides access to the region in the immediate vicinity of the horizon. Broderick et al. (2009) have already shown that by leveraging spectral and polarization information as well as accretion theory, it is possible to extract accretion-model parameters (including black hole spin) from mm-VLBI experiments containing only a handful of telescopes. Here we repeat this analysis with the most recent mm-VLBI data, considering a class of aligned, radiatively inefficient accretion flow (RIAF) models. We find that the combined data set rules out symmetric models for Sgr A*'s flux distribution at the 3.9-sigma level, strongly favoring length-to-width ratios of roughly 2.4:1. More importantly, we find that physically motivated accretion flow models provide a significantly better fit to the mm-VLBI observations than phenomenological models, at the 2.9-sigma level. This implies that not only is mm-VLBI presently capable of distinguishing between potential physical models for Sgr A*'s emission, but further that it is sensitive to the strong gravitational lensing associated with the propagation of photons near the black hole. Based upon this analysis we find that the most probable magnitude, viewing angle, and position angle for the black hole spin are a=0.0(+0.64+0.86), theta=68(+5+9)(-20-28) degrees, and xi=-52(+17+33)(-15-24) east of north, where the errors quoted are the 1-sigma and 2-sigma uncertainties.

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arXiv2010

1.3 mm Wavelength VLBI of Sagittarius A*: Detection of Time-Variable Emission on Event Horizon Scales

Sagittarius A*, the ~4 x 10^6 solar mass black hole candidate at the Galactic Center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength Very Long Baseline Interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the ARO/SMT on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924-292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3 mm VLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

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arXiv2010

Discovery of the first methanol (CH3OH) maser in the Andromeda galaxy (M31)

We present the first detection of a 6.7 GHz Class II methanol (CH3OH) maser in the Andromeda galaxy (M31). The CH3OH maser was found in a Very Large Array (VLA) survey during the fall of 2009. We have confirmed the methanol maser with the new Expanded VLA (EVLA), in operation since March 2010, but were unsuccessful in detecting a water maser at this location. A direct application for this methanol maser is the determination of the proper motion of M31, such as was obtained with water masers in M33 and IC10 previously. Unraveling the three-dimensional velocity of M31 would solve for the biggest unknown in the modeling of the dynamics and evolution of the Local Group of galaxies.

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arXiv2010

Global VLBI Observations of the 6.0 GHz Hydroxyl Masers in Onsala 1

We present global VLBI observations of the first-excited state OH masers in the massive star-forming region Onsala 1 (ON 1). The 29 masers detected are nearly all from the 6035 MHz transition, and nearly all are identifiable as Zeeman pair components. The 6030 and 6035 MHz masers are coincident with previously published positions of ground-state masers to within a few milliarcseconds, and the magnetic fields deduced from Zeeman splitting are comparable. The 6.0 GHz masers in ON 1 are always found in close spatial association with 1665 MHz OH masers, in contrast to the situation in the massive star-forming region W3(OH), suggesting that extreme high density OH maser sites (excited-state masers with no accompanying ground-state maser, as seen in W3(OH)) are absent from ON 1. The large magnetic field strength among the northern, blueshifted masers is confirmed. The northern masers may trace an outflow or be associated with an exciting source separate from the other masers, or the relative velocities of the northern and southern masers may be indicative of expansion and rotation. High angular resolution observations of nonmasing material will be required to understand the complex maser distribution in ON 1.

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arXiv2010

Expanded VLA Detection of 36.2 GHz Class I Methanol Masers in Sagittarius A

We report on the interferometric detection of 36.2 GHz Class I methanol emission with the new 27-40 GHz Ka band receivers available on the Expanded Very Large Array (EVLA). The brightness temperatures of the interferometric 36 GHz detections unambiguously indicate for the first time that the emission is maser emission. The 36 GHz methanol masers are not co-spatial with 1720 MHz OH masers, indicating that the two species trace different shocks. The 36 GHz and 44 GHz methanol masers, which both are collisionally pumped, do not necessarily co-exist and may trace different methanol gas. The methanol masers seem correlated with NH_3(3,3) density peaks. We favor an explanation in which the 36 GHz Class I methanol masers outline regions of cloud-cloud collisions, perhaps just before the onset of the formation of individual massive stars. The transition of the Very Large Array (VLA) to the EVLA is well under way, and these detections demonstrate the bright future of this completely renewed instrument.

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arXiv2009

Detecting Changing Polarization Structures in Sagittarius A* with High Frequency VLBI

Sagittarius A* is the source of near infrared, X-ray, radio, and (sub)millimeter emission associated with the supermassive black hole at the Galactic Center. In the submillimeter regime, Sgr A* exhibits time-variable linear polarization on timescales corresponding to <10 Schwarzschild radii of the presumed 4 million solar mass black hole. In previous work, we demonstrated the potential for total-intensity (sub)millimeter-wavelength VLBI to detect time-variable -- and periodic -- source structure changes in the Sgr A* black hole system using nonimaging analyses. Here we extend this work to include full polarimetric VLBI observations. We simulate full-polarization (sub)millimeter VLBI data of Sgr A* using a hot-spot model that is embedded within an accretion disk, with emphasis on nonimaging polarimetric data products that are robust against calibration errors. Although the source-integrated linear polarization fraction in the models is typically only a few percent, the linear polarization fraction on small angular scales can be much higher, enabling the detection of changes in the polarimetric structure of Sgr A* on a wide variety of baselines. The shortest baselines track the source-integrated linear polarization fraction, while longer baselines are sensitive to polarization substructures that are beam-diluted by connected-element interferometry. The detection of periodic variability in source polarization should not be significantly affected even if instrumental polarization terms cannot be calibrated out. As more antennas are included in the (sub)mm-VLBI array, observations with full polarization will provide important new diagnostics to help disentangle intrinsic source polarization from Faraday rotation effects in the accretion and outflow region close to the black hole event horizon.

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arXiv2009

Observing a Black Hole Event Horizon: (Sub)Millimeter VLBI of Sgr A*

Very strong evidence suggests that Sagittarius A*, a compact radio source at the center of the Milky Way, marks the position of a super massive black hole. The proximity of Sgr A* in combination with its mass makes its apparent event horizon the largest of any black hole candidate in the universe and presents us with a unique opportunity to observe strong-field GR effects. Recent millimeter very long baseline interferometric observations of Sgr A* have demonstrated the existence of structures on scales comparable to the Schwarzschild radius. These observations already provide strong evidence in support of the existence of an event horizon. (Sub)Millimeter VLBI observations in the near future will combine the angular resolution necessary to identify the overall morphology of quiescent emission, such as an accretion disk or outflow, with a fine enough time resolution to detect possible periodicity in the variable component of emission. In the next few years, it may be possible to identify the spin of the black hole in Sgr A*, either by detecting the periodic signature of hot spots at the innermost stable circular orbit or parameter estimation in models of the quiescent emission. Longer term, a (sub)millimeter VLBI "Event Horizon Telescope" will be able to produce images of the Galactic center emission to the see the silhouette predicted by general relativistic lensing. These techniques are also applicable to the black hole in M87, where black hole spin may be key to understanding the jet-launching region.

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arXiv2009

Estimating the Parameters of Sgr A*'s Accretion Flow Via Millimeter VLBI

Recent millimeter-VLBI observations of Sagittarius A* (Sgr A*) have, for the first time, directly probed distances comparable to the horizon scale of a black hole. This provides unprecedented access to the environment immediately around the horizon of an accreting black hole. We leverage both existing spectral and polarization measurements and our present understanding of accretion theory to produce a suite of generic radiatively inefficient accretion flow (RIAF) models of Sgr A*, which we then fit to these recent millimeter-VLBI observations. We find that if the accretion flow onto Sgr A* is well described by a RIAF model, the orientation and magnitude of the black hole's spin is constrained to a two-dimensional surface in the spin, inclination, position angle parameter space. For each of these we find the likeliest values and their 1-sigma & 2-sigma errors to be a=0(+0.4+0.7), inclination=50(+10+30)(-10-10) degrees, and position angle=-20(+31+107)(-16-29) degrees, when the resulting probability distribution is marginalized over the others. The most probable combination is a=0(+0.2+0.4), inclination=90(-40-50) degrees and position angle=-14(+7+11)(-7-11) degrees, though the uncertainties on these are very strongly correlated, and high probability configurations exist for a variety of inclination angles above 30 degrees and spins below 0.99. Nevertheless, this demonstrates the ability millimeter-VLBI observations, even with only a few stations, to significantly constrain the properties of Sgr A*.

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arXiv2008

Using Millimeter VLBI to Constrain RIAF Models of Sagittarius A*

The recent detection of Sagittarius A* at lambda = 1.3 mm on a baseline from Hawaii to Arizona demonstrates that millimeter wavelength very long baseline interferometry (VLBI) can now spatially resolve emission from the innermost accretion flow of the Galactic center region. Here, we investigate the ability of future millimeter VLBI arrays to constrain the spin and inclination of the putative black hole and the orientation of the accretion disk major axis within the context of radiatively inefficient accretion flow (RIAF) models. We examine the range of baseline visibility and closure amplitudes predicted by RIAF models to identify critical telescopes for determining the spin, inclination, and disk orientation of the Sgr A* black hole and accretion disk system. We find that baseline lengths near 3 gigalambda have the greatest power to distinguish amongst RIAF model parameters, and that it will be important to include new telescopes that will form north-south baselines with a range of lengths. If a RIAF model describes the emission from Sgr A*, it is likely that the orientation of the accretion disk can be determined with the addition of a Chilean telescope to the array. Some likely disk orientations predict detectable fluxes on baselines between the continental United States and even a single 10-12 m dish in Chile. The extra information provided from closure amplitudes by a four-antenna array enhances the ability of VLBI to discriminate amongst model parameters.

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arXiv2008

Methods for detecting flaring structures in Sagittarius A* with high frequency VLBI

The super massive black hole candidate, Sagittarius A*, exhibits variability from radio to X-ray wavelengths on time scales that correspond to < 10 Schwarzschild radii. We survey the potential of millimeter-wavelength VLBI to detect and constrain time variable structures that could give rise to such variations, focusing on a model in which an orbiting hot spot is embedded in an accretion disk. Non-imaging algorithms are developed that use interferometric closure quantities to test for periodicity, and applied to an ensemble of hot-spot models that sample a range of parameter space. We find that structural periodicity in a wide range of cases can be detected on most potential VLBI arrays using modern VLBI instrumentation. Future enhancements of mm/sub-mm VLBI arrays including phased array processors to aggregate VLBI station collecting area, increased bandwidth recording, and addition of new VLBI sites all significantly aid periodicity detection. The methods described herein can be applied to other models of Sagittarius A*, including jet outflows and Magneto-Hydrodynamic accretion simulations.

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arXiv2008

Detecting Flaring Structures in Sagittarius A* with (Sub)Millimeter VLBI

Multiwavelength monitoring observations of Sagittarius A* exhibit variability on timescales of minutes to hours, indicating emission regions localized near the event horizon. (Sub)Millimeter-wavelength VLBI is uniquely suited to probe the environment of the assumed black hole on these scales. We consider a range of orbiting hot-spot and accretion-disk models and find that periodicity in Sgr A* flares is detectable using closure quantities. Our methods are applicable to any model producing source structure changes near the black hole, including jets and magnetohydrodynamic disk instabilities, and suggest that (sub)millimeter VLBI will play a prominent role in investigating Sgr A* near the event horizon.

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arXiv2007

Excited-state OH Masers and Supernova Remnants

The collisionally pumped, ground-state 1720 MHz maser line of OH is widely recognized as a tracer for shocked regions and observed in star forming regions and supernova remnants. Whereas some lines of excited states of OH have been detected and studied in star forming regions, the subject of excited-state OH in supernova remnants -- where high collision rates are to be expected -- is only recently being addressed. Modeling of collisional excitation of OH demonstrates that 1720, 4765 and 6049 MHz masers can occur under similar conditions in regions of shocked gas. In particular, the 6049 and 4765 MHz masers become more significant at increased OH column densities where the 1720 MHz masers begin to be quenched. In supernova remnants, the detection of excited-state OH line maser emission could therefore serve as a probe of regions of higher column densities. Using the Very Large Array, we searched for excited-state OH in the 4.7, 7.8, 8.2 and 23.8 GHz lines in four well studied supernova remnants with strong 1720 MHz maser emission (SgrAEast, W28, W44 and IC443). No detections were made, at typical detection limits of around 10 mJy/beam. The search for the 6 GHz lines were done using Effelsberg since the VLA receivers did not cover those frequencies, and are reported on in an accompanying letter (Fish, Sjouwerman & Pihlstrom 2007). We also cross-correlated the positions of known supernova remnants with the positions of 1612 MHz maser emission obtained from blind surveys. No probable associations were found, perhaps except in the SgrAEast region. The lack of detections of excited-state OH indicates that the OH column densities suffice for 1720 MHz inversion but not for inversion of excited-state transitions, consistent with the expected results for C-type shocks.

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arXiv2007

Effelsberg Observations of Excited-State (6.0 GHz) OH in Supernova Remnants and W3(OH)

While masers in the 1720 MHz transition of OH are detected toward many supernova remnants (SNRs), no other OH transition is seen as a maser in SNRs. We present a search for masers at 6049 MHz, which has recently been predicted to produce masers by pure collisional excitation at conditions similar to that required for 1720 MHz masing. The Effelsberg 100 m telescope was used to observe the excited-state 6016, 6030, 6035, and 6049 MHz lines of OH toward selected SNRs, most of which have previously-detected bright 1720 MHz masers. No excited-state masers are found toward SNRs, consistent with previous observations of the 6049 MHz and other excited-state transitions. We do not see clear evidence of absorption toward SNR target positions, although we do see evidence of absorption in the molecular cloud at +50 km/s near Sgr A East. Weak absorption is detected at 6016 MHz toward W3(OH), while stronger, narrower emission is seen at 6049 MHz, suggesting that the 6049 MHz emission is a low-gain maser. We conclude that conditions in SNRs are not conducive to excited-state maser emission, especially in excited-state satellite lines.

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arXiv2007

EVLA Observations of OH Masers in ON 1

This Letter reports on initial Expanded Very Large Array (EVLA) observations of the 6035 MHz masers in ON 1. The EVLA data are of good quality, lending confidence in the new receiver system. Nineteen maser features, including six Zeeman pairs, are detected. The overall distribution of 6035 MHz OH masers is similar to that of the 1665 MHz OH masers. The spatial resolution is sufficient to unambiguously determine that the magnetic field is strong (~ -10 mG) at the location of the blueshifted masers in the north, consistent with Zeeman splitting detected in 13441 MHz OH masers in the same velocity range. Left and right circularly polarized ground-state features dominate in different regions in the north of the source, which may be due to a combination of magnetic field and velocity gradients. The combined distribution of all OH masers toward the south is suggestive of a shock structure of the sort previously seen in W3(OH).

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arXiv2007

Proper Motions of OH Masers and Magnetic Fields in Massive Star-Forming Regions

We present data of proper motions of OH masers in the massive star-forming regions ON 1, K3-50, and W51 Main/South. OH maser motions in ON 1 are consistent with expansion at approximately 5 km/s, likely tracing the expanding ultracompact H II region. Motions in K3-50 are faster and may be indicating the final stages of OH maser emission in the source, before the OH masers turn off as the H II region transitions from the ultracompact to the compact phase. W51 South shows indications of aspherical expansion, while motions in W51 Main are more difficult to interpret. Nevertheless, it appears that the relative projected separation between W51 Main and W51 South is decreasing, corresponding to an estimate of enclosed mass of at least 1500 solar masses, consistent with estimates derived from millimeter-wavelength dust emission. We confirm the ~20 mG magnetic fields previously seen in W51 Main, which may represent the upper end of the density range allowable for 1665 MHz maser emission. Magnetic field strengths and directions, obtained from Zeeman splitting, in each source are consistent with values obtained in the first epoch four to nine years ago.

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arXiv2007

Excited-state OH Mainline Masers in AU Geminorum and NML Cygni

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz mainline OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH mainline transitions in NML Cyg. We detect masers in both mainline transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg outlined below), or possibly an artifact in the data, and that the excited state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass-loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.

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arXiv2007

OH Masers in G11.904-0.141

The massive star-forming region G11.904-0.141 is one of only 11 sources to show maser emission in the highly-excited 13441 MHz transition of OH. VLBA observations of the 1665, 1667, 4765, and 13441 MHz transitions of OH toward G11.904-0.141 are presented. Masers are detected at 1665, 1667, and 4765 MHz, but the 13441 MHz masers are not detected. Consistent magnetic field strengths of approximately +3.5 mG are detected in the ground-state masers, in contrast with a possible -3.0 mG magnetic field previously detected at 13441 MHz. The variable 13441 MHz masers may be associated with an outflow.

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