arXiv subjects

R. C. Walker

Publications and source records attributed to R. C. Walker.

At least 19 recordsLinked to original sources

arXiv2023

Imaging the ring and jet in M87 at 85 GHz with the ngEHT and ngEHT+ngVL

The Global mm-VLBI Array (GMVA) has demonstrated the ability to resolved what may be the general relativistic shadow of the supermassive black hole in M87 at 86 GHz, as well as delineate the inner jet to $\sim 1$~mas distance. We investigate the ability of the planned ngEHT, and the ngEHT + ngVLA at 85 GHz, to image such a nuclear 'ring', and the associated jet, using a constructed model based on the current estimate of the ring size, and a scaled version of the best VLBA image of the M87 jet at 43 GHz. While the resolution does not improve due to the limit set by the diameter of the Earth, the ngEHT alone should provide both a higher fidelity image of the ring on scales $\le 0.1$~mas, and a good image of a more extended jet to $\sim 1$~mas. Adding the ngVLA improves substantially the dynamic range (factor 3.5), as well as adds the ability to image structures on larger scales, in this case out to at least 5~mas, and potentially to much larger scales given the $\sim 10^5$ range in spatial scales covered by the ngVLA itself. Both arrays provide good image fidelity ($\le 0.1$), in the inner $\sim 1$~mas, but the ngEHT-only image does not reproduce the outer jet well, or at all, with fidelity values greater than unity. The combined array reproduces much of the outer jet with good fidelity ($\le 0.3$). Adding the ngVLA also decreases the susceptibility to antenna-based phase errors by a similar factor, and improves the ability for fringe fitting and subsequent phase and amplitude self-calibration. As for scales $< 100~μ$as, ie. the ring itself, adding the ngVLA makes little change for very bright sources, where uniform weighting can be employed. But for faint sources, adding the ngVLA adds potentially an order-of magnitude sensitivity improvement (Issaoun et al. 2023).

astro-ph.HE↗

arXiv2023

A Suggested Final Configuration for the Very Large Array

If the construction of the ngVLA begins in 2026, its sensitivity is expected to match that of the VLA by late 2029. At that juncture it is anticipated that open-skies observing will cease on the VLA and commence on the ngVLA. We suggest that during 2026-2029 the VLA be held in a customized final configuration encompassing portions of its standard A, B, C and D configurations. Such a final VLA configuration would (1) help minimize the cost of VLA operations and maximize the pace of ngVLA construction and commissioning; (2) help VLA users pivot to the high-resolution, high-frequency research topics that are projected to headline the ngVLA science program; and (3) help mitigate the effects of source confusion during responses to transients in the era of the Rubin Observatory and LIGO A+.

astro-ph.IM↗

arXiv2020

Linear polarization in the nucleus of M87 at 7 mm and 1.3 cm

We report on high angular resolution polarimetric observations of the nearby radio galaxy M87 using the Very Long Baseline Array at 24 GHz ($λ=$1.3 cm) and 43 GHz ($λ=$7 mm) in 2017-2018. New images of the linear polarization substructure in the nuclear region are presented, characterized by a two-component pattern of polarized intensity and smooth rotation of the polarization plane around the 43 GHz core. From a comparison with an analogous dataset from 2007, we find that this global polarization pattern remains stable on a time interval of 11 yr, while showing smaller month-scale variability. We discuss the possible Faraday rotation toward the M87 nucleus at centimeter to millimeter wavelengths. These results can be interpreted in a scenario where the observed polarimetric pattern is associated with the magnetic structure in the confining magnetohydrodynamic wind, which also serves as the source of the observed Faraday rotation.

astro-ph.GA↗

arXiv2016

Resolving the base of the relativistic jet in M87 at 6$R_{\rm sch}$ resolution with global mm-VLBI

M87 is one of the nearest radio galaxies with a central SMBH and a prominent relativistic jet. Due to its close distance to the observer and the large SMBH mass, the source is one of the best laboratories to obtain strong observational constraints on the theoretical models for the formation and evolution of the AGN jets. In this article, we present preliminary results from our ongoing observational study about the innermost jet of M87 at an ultra-high resolution of $\sim$50$μ$as achieved by the Global Millimeter-VLBI Array (GMVA). The data obtained between 2004 and 2015 clearly show limb-brightened jets at extreme resolution and sensitivity. Our preliminary analysis reveals that the innermost jet expands in an edge-brightened cone structure (parabolic shape) but with the jet expansion profile slightly different from the outer regions of the jet. Brightness temperatures of the VLBI core obtained from cm- to mm-wavelengths show a systematic evolution, which can be interpreted as the evolution as a function of distance from the BH. We also adopt an alternative imaging algorithm, BSMEM, to test reliable imaging at higher angular resolution than provided by the standard CLEAN method (i.e. super-resolution). A demonstration with a VLBA 7mm example data set shows consistent results with a near-in-time 3mm VLBI image. Application of the method to the 2009 GMVA data yields an image with remarkable fine-scale structures that have been never imaged before. We present a brief interpretation of the complexity in the structure.

astro-ph.GA↗

arXiv2016

Kinematics of the jet in M87 on scales of 100 -- 1000 Schwarzschild radii

Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows. The two-dimensional structure and kinematics of the jet in M\,87 (NGC\,4486) have been studied by applying the Wavelet-based Image Segmentation and Evaluation (WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array (VLBA) observations made in January-August 2007 at 43 GHz ($λ= 7$ mm). The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M\,87 at sub-parsec scales. The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion. A linear acceleration regime is observed up to $z_{obs} \sim 2$\,mas. The acceleration is reduced at larger scales, which is consistent with saturation of Poynting flux conversion. Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow. The flow consists of a slow, mildly relativistic layer (moving at $β\sim 0.5\,c$), associated either with instability pattern speed or an outer wind, and a fast, accelerating stream line (with $β\sim 0.92$, corresponding to a bulk Lorentz factor $γ\sim 2.5$). A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected, providing evidence for jet rotation. The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk, at a distance $r_0 \sim 5\, R_\mathrm{s}$ from the central engine. The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M\,87.

astro-ph.HE↗

arXiv2014

Evidence from the Very Long Baseline Array that J1502SE/SW are Double Hotspots, not a Supermassive Binary Black Hole

SDSS J150243.09+111557.3 is a merging system at z = 0.39 that hosts two confirmed AGN, one unobscured and one dust-obscured, offset by several kiloparsecs. Deane et al. recently reported evidence from the European VLBI Network (EVN) that the dust-obscured AGN exhibits two flat-spectrum radio sources, J1502SE/SW, offset by 26 mas (140 pc), with each source being energized by its own supermassive black hole (BH). This intriguing interpretation of a close binary BH was reached after ruling out a double-hotspot scenario, wherein both hotspots are energized by a single, central BH, a configuration occuring in the well-studied Compact Symmetric Objects. When observed with sufficient sensitivity and resolution, an object with double hotspots should have an edge-brightened structure. We report evidence from the Very Long Baseline Array (VLBA) for just such a structure in an image of the obscured AGN with higher sensitivity and resolution than the EVN images. We thus conclude that a double-hotspot scenario should be reconsidered as a viable interpretation for J1502SE/SW, and suggest further VLBA tests of that scenario. A double-hotspot scenario could have broad implications for feedback in obscured AGNs. We also report a VLBA detection of high-brightness-temperature emssion from the unobscured AGN that is offset several kiloparsecs from J1502SE/SW.

astro-ph.GA↗

arXiv2012

Rapid TeV Gamma-Ray Flaring of BL Lacertae

We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a 34.6-minute exposure, when the integral flux above 200 GeV reached $(3.4\pm0.6) \times 10^{-6} \;\text{photons}\;\text{m}^{-2}\text{s}^{-1}$, roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be $13\pm4$ minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of $3.6\pm 0.4$, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array (VLBA) revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results.

astro-ph.HE↗

arXiv2011

The 2010 M 87 VHE flare and its origin: the multi-wavelength picture

The giant radio galaxy M 87, with its proximity (16 Mpc) and its very massive black hole ((3 - 6) \times 10^9 M_solar), provides a unique laboratory to investigate very high energy (E>100 GeV; VHE) gamma-ray emission from active galactic nuclei and, thereby, probe particle acceleration to relativistic energies near supermassive black holes (SMBH) and in relativistic jets. M 87 has been established as a VHE gamma-ray emitter since 2005. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In 2008, a rise in the 43 GHz Very Long Baseline Array (VLBA) radio emission of the innermost region (core; extension of < 100 Rs ; Schwarzschild radii) was found to coincide with a flaring activity at VHE. This had been interpreted as a strong indication that the VHE emission is produced in the direct vicinity of the SMBH. In 2010 a flare at VHE was again detected triggering further multi-wavelength (MWL) observations with the VLBA, Chandra, and other instruments. At the same time, M 87 was also observed with the Fermi-LAT telescope at MeV/GeV energies, the European VLBI Network (EVN), and the Liverpool Telescope (LT). Here, preliminary results from the 2010 campaign will be reported.

astro-ph.CO↗

arXiv2009

Pinpointing the TeV gamma-ray emission region in M87 using TeV and 43 GHz radio monitoring

The TeV radio galaxy M87 is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing towards our line of sight, is spatially resolved in X-ray (by Chandra), optical and radio observations. In 2008, the three main Atmospheric Cherenkov Telescope observatories VERITAS, MAGIC and H.E.S.S. coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. In February, strong and rapid day-scale TeV flares were detected. VLBA monitoring observations during the same period showed that the 43 GHz radio flux density of the unresolved core began to rise at the time of the TeV flares and eventually reached levels above any previously seen with VLBI. New jet components appeared during the flare. The localization accuracy of the TeV instruments of many arcseconds, even for strong sources, is inadequate to constrain the origin of the emission in the inner jets of AGNs. For M87, with a 6 billion solar mass black hole and a distance of 16.7 Mpc, the VLBA resolution instead corresponds to 30 by 60 Schwarzschild radii. This is starting to resolve the jet collimation region. The temporal coincidence of the TeV and radio flares indicates that they are related and provides the first direct evidence that the TeV radiation from this source is produced within a few tens of RS of the radio core, thought to be coincident to within the VLBA resolution with the black hole.

astro-ph.HE↗

arXiv2009

A first joint M87 campaign in 2008 from radio to TeV gamma-rays

M87, the central galaxy of the Virgo cluster, is the first radio galaxy detected in the TeV regime. The structure of its jet, which is not pointing toward the line of sight, is spatially resolved in X-ray (by Chandra), in optical and in radio observations. Time correlation between the TeV flux and emission at other wavelengths provides a unique opportunity to localize the very high energy gamma-ray emission process occurring in AGN. For 10 years, M87 has been monitored in the TeV band by atmospheric Cherenkov telescopes. In 2008, the three main atmospheric Cherenkov telescope observatories (H.E.S.S., MAGIC and VERITAS) coordinated their observations in a joint campaign from January to May with a total observation time of approx. 120 hours. The campaign largely overlapped with an intensive VLBA project monitoring the core of M87 at 43 GHz every 5 days. In February, high TeV activities with rapid flares have been detected. Contemporaneously, M87 was observed with high spatial resolution instruments in X-rays (Chandra). We discuss the results of the joint observation campaign in 2008.

astro-ph.CO↗

arXiv2008

A Statistical Description of AGN Jet Evolution from the VLBA Imaging and Polarimetry Survey (VIPS)

A detailed analysis of the evolution of the properties of core-jet systems within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a power-law relationship between jet intensity and width that suggests for the typical jet, little if any energy is lost as it moves away from its core. Using VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be aligned with the the direction of emission on kiloparsec scales. We also found that this alignment improves as the jets move farther from their cores on projected scales as small as ~50-100 pc. This suggests that realignment of jets on these projected scales is relatively common. We typically find a modest amount of bending (a change in jet position angle of ~5 deg.) on these scales, suggesting that this realignment may typically occur relatively gradually.

astro-ph↗

arXiv2007

Characteristics of EGRET Blazars in the VLBA Imaging and Polarimetry Survey (VIPS)

We examine the radio properties of EGRET-detected blazars observed as part of the VLBA Imaging and Polarimetry Survey (VIPS). VIPS has a flux limit roughly an order of magnitude below the MOJAVE survey and most other samples that have been used to study the properties of EGRET blazars. At lower flux levels, radio flux density does not directly correlate with gamma-ray flux density. We do find that the EGRET-detected blazars tend to have higher brightness temperatures, greater core fractions, and possibly larger than average jet opening angles. A weak correlation is also found with jet length and with polarization. All of the well-established trends can be explained by systematically larger Doppler factors in the gamma-ray loud blazars, consistent with the measurements of higher apparent velocities found in monitoring programs carried out at radio frequencies above 10 GHz.

astro-ph↗

arXiv2007

Progress Toward a VLBA Movie of the Jet Collimation Region in M87

With its high black hole mass, proximity, and bright jet, M87 provides the best prospect for a direct imaging study of the acceleration and collimation region of a jet. Previous VLBI observations have shown an edge brightened structure with a wide opening angle at small scales. An effort to measure component speeds in this region using existing VLBA data at 43 GHz gave tentative results of 0.25c to 0.4c but also indicated that faster sampling is needed. Here we provide a progress report on a project to make a properly sampled movie of motions in the inner jet using the VLBA at 43 GHz. A pilot project during 2006 measured speeds of about 0.6c and was used to set a frame interval of 3 weeks for the movie. The movie observations began in January 2007. Results from the pilot and from the first frame of the movie are presented. The goal of the project is to provide observations of the structure and dynamics of the jet on scales from under 100 to a few hundred Schwarzschild radii that can be compared with expectations from theoretical studies and numerical modeling.

astro-ph↗

arXiv2006

The VLBA Imaging and Polarimetry Survey at 5 GHz

We present the first results of the VLBA Imaging and Polarimetry Survey (VIPS), a 5 GHz VLBI survey of 1,127 sources with flat radio spectra. Through automated data reduction and imaging routines, we have produced publicly available I, Q, and U images and have detected polarized flux density from 37% of the sources. We have also developed an algorithm to use each source's I image to automatically classify it as a point-like source, a core-jet, a compact symmetric object (CSO) candidate, or a complex source. The mean ratio of the polarized to total 5 GHz flux density for VIPS sources with detected polarized flux density ranges from 1% to 20% with a median value of about 5%. We have also found significant evidence that the directions of the jets in core-jet systems tend to be perpendicular to the electric vector position angles (EVPAs). The data is consistent with a scenario in which ~24% of the polarized core-jets have EVPAs that are anti-aligned with the directions of their jet components and which have a substantial amount of Faraday rotation. In addition to these initial results, plans for future follow-up observations are discussed.

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arXiv2005

A Redshift Determination for XRF 020903: First Spectroscopic Observations of an X-Ray Flash

We report the discovery of optical and radio afterglow emission from the extremely soft X-ray flash, XRF 020903. Our spectroscopic observations provide the first redshift for an X-ray flash, thereby setting the distance scale for these events. At $z=0.251$, XRF 020903 is one of the nearest cosmic explosions ever detected, second only to the recent GRB 030329 and the unusual GRB 980425/SN 1998bw. Moreover, XRF 020903 is the first X-ray flash for which we detect an optical afterglow. The luminosity of the radio afterglow of XRF 020903 is 1000 times greater than that of Ibc supernovae but similar to those of GRB afterglows. From broadband afterglow modeling we show that the explosion energy of XRF 020903 is not dissimilar from values inferred for typical gamma-ray bursts, suggesting that these cosmological explosions may derive from a similar mechanism.

astro-ph↗

arXiv2005

VLBA Imaging Polarimetry of Active Galactic Nuclei - An Automated Approach

We present full polarization Very Long Baseline Array (VLBA) observations at 5 GHz and 15 GHz of 24 compact active galactic nuclei (AGN). These sources were observed as part of a pilot project to demonstrate the feasibility of conducting a large VLBI survey to further our understanding of the physical properties and temporal evolution of AGN jets. The sample is drawn from the Cosmic Lens All-Sky Survey (CLASS) where it overlaps with the Sloan Digital Sky Survey at declinations north of 15 degrees. There are 2100 CLASS sources brighter than 50 mJy at 8.4 GHz, of which we have chosen 24 for this pilot study. All 24 sources were detected and imaged at 5 GHz with a typical dynamic range of 500:1, and 21 of 24 sources were detected and imaged at 15 GHz. Linear polarization was detected in 8 sources at both 5 and 15 GHz, allowing for the creation of Faraday rotation measure (RM) images. The core RMs for the sample were found to have an average absolute value of 390 +/- 100 rad/m^2. We also present the discovery of a new Compact Symmetric Object, J08553+5751. All data were processed automatically using pipelines created or adapted for the survey.

astro-ph↗

arXiv2004

VLBI observations of weak sources using fast frequency switching

We have developed a new phase referencing technique for high frequency VLBI observations. In conventional phase referencing, one interleaves short scans on a nearby phase calibrator between the target source observations. In fast frequency switching described here, one observes the target source continuously while switching rapidly between the target frequency and a lower reference frequency. We demonstrate that the technique allows phase calibration almost reaching the thermal noise limit and present the first detection of the AGN in the FR I radio galaxy NGC 4261 at 86 GHz. Although point-like, this is the weakest source ever detected with VLBI at this frequency.

astro-ph↗

arXiv2004

Modeling the 3C 120 Radio Jet from 1 to 30 Milliarcseconds

The predicted spatial development of helical structures along an expanding jet are used to model observed structures and motions in the 3C 120 jet. Proper motions of helical components associated with the pattern and of other components that move through the pattern provide estimates of flow and helical pattern speeds. Theoretical modeling allows determination of sound speeds as a function of jet viewing angle. Jet morphology argues against viewing angles larger than 12 deg. At a 12 deg viewing angle the most likely scenario involves a decline in jet sound speed from c/3 < a_j < c/1.7 at ~0.5 mas from the core to 0.1c < a_j < 0.25c at ~25 mas from the core accompanied by some acceleration in the jet flow from Lorentz factor < 5 to Lorentz factor > 7. At smaller viewing angles the jet sound speed is lower and at a 6 deg viewing angle the jet sound speed is about a factor 2 less but the flow Lorentz factor is comparable. The decline in radio intensity is on the order of what would be associated with isothermal jet expansion. Knot interknot intensity variations are greater than would be expected from adiabatic compressions associated with the helical twist and we infer the presence of a shock along the leading edge of the helical twist. Our results imply that the macroscopic heating of the expanding jet fluid is less than the microscopic energization of the synchrotron radiating relativistic electrons.

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