Mark 4 VLBI correlator: Architecture and algorithms

Whitney, A. R.; Cappallo, R. J.; Aldrich, W.; Anderson, Ben; Boś, A.; Casse, J. L.; Goodman, John M.; Parsley, Steve; Pogrebenko, S. V.; Schilizzi, R. T.; Smythe, D. L.

doi:10.1029/2002rs002820

Cited by 43 publications

(39 citation statements)

References 8 publications

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“…We correlated our data using the Haystack Mark4 correlator (36). For subsequent fringe-fitting, we used the Haystack Observatory Post-processing System.…”

Section: Correlation Proceduresmentioning

confidence: 99%

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*

Johnson

Fish

Doeleman

et al. 2015

Science

200

157

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Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizonscale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered fields near the event horizon, on scales of ∼6 Schwarzschild radii, and we have detected and localized the intra-hour variability associated with these fields.Sagittarius A* (Sgr A * ) emits most of its ∼10 36 erg/s luminosity at wavelengths just short of one millimeter, resulting in a distinctive "submillimeter bump" in its spectrum (1). A diversity of models attribute this emission to synchrotron radiation from a population of relativistic thermal electrons in the innermost accretion flow (2-4). Such emission is expected to be strongly linearly polarized, ∼70% in the optically thin limit for a highly ordered magnetic field configuration (5), with its direction tracing the underlying magnetic field. At 1.3-mm wavelength, models of magnetized accretion flows predict linear polarization fractions > ∼ 30% (6-9), yet connected-element interferometers measure only a 5−10% polarization fraction for Sgr A * (10, 11), typical for galaxy cores (12). However, the highest resolutions of these instruments, ∼0.1−1 , are insufficient to resolve the millimeter emission region, and linear polarization is not detected from Sgr A * at the longer wavelengths where facility verylong-baseline interferometry (VLBI) instruments offer higher resolution (13). Thus, these low polarization fractions could indicate any combination of low intrinsic polarization, depolarization from Faraday rotation or opacity, disordered magnetic fields within the turbulent emitting 2 plasma, or ordered magnetic fields with unresolved structure leading to a low beam-averaged polarization. The higher polarization seen during some near-infrared flares may support the last possibility (14, 15), but the origin and nature of these flares is poorly understood, and they may probe a different emitting electron population than is responsible for the energetically dominant submillimeter emission.To definitively study this environment, we are assembling the Event Horizon Telescope (EHT), a global VLBI array operating at 1.3-mm wavelength. Initial studies with the EHT have spatially resolved the ∼40 microarcsecond (µas) emission region of Sgr A * (16, 17), suggesting the potential for polarimetric VLBI with the EHT to resolve its magnetic field structure. On longer baselines,m mixes information about the spatial distribution of polarization with information about the strength and direction of polarization and must be in...

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“…We correlated our data using the Haystack Mark4 correlator (36). For subsequent fringe-fitting, we used the Haystack Observatory Post-processing System.…”

Section: Correlation Proceduresmentioning

confidence: 99%

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*

Johnson

Fish

Doeleman

et al. 2015

Science

200

157

View full text Add to dashboard Cite

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“…After correlation, post-processing was conducted with the fringe-fitting program fourfit 1 of the HOPS package (Whitney et al 2004). …”

Section: Initial Vlbi Observations and Data Reductionmentioning

confidence: 99%

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

Bourda

Charlot

Porcas

et al. 2010

A&A

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Context. The European space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the Gaia and VLBI frames with the highest possible accuracy. It has been found that only 70 (10%) of the sources from the International Celestial Reference Frame (ICRF) are suitable for establishing this link, either because they are not bright enough at optical wavelengths or because they have significant extended radio emission which precludes reaching the highest astrometric accuracy. Aims. In order to improve the situation, we have initiated a VLBI survey dedicated to finding additional suitable radio sources for aligning the two frames. Methods. The sample consists of 447 optically-bright (magnitude ≤18) extragalactic radio sources, typically 20 times weaker than the ICRF sources, which have been selected by cross-correlating an optical quasar catalog with the NRAO VLA Sky Survey (NVSS).Results. This paper presents the observing strategy to detect, image, and measure accurate radio positions for these sources. It also provides results on the VLBI detectability of the sources, as derived from initial observations with the European VLBI Network in June and October 2007. Based on these observations, a high detection rate of 89% is found, which is promising for the continuation of this project. This high VLBI detection rate for sources from the NVSS catalog is probably due to the selection process, suggesting that optically-bright quasars have compact radio structures.

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“…The HOBART26, PARKES and DSS45 have a Mark5 recording system and a Mark4 (Whitney et al 2004) baseband conversion rack. However, for the first epoch, the LBADR system (Phillips et al 2009) was used.…”

Section: Observationsmentioning

confidence: 99%

The LBA Calibrator Survey of southern compact extragalactic radio sources - LCS1

Petrov

Phillips

Bertarini

et al. 2011

Monthly Notices of the Royal Astronomical Society

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We present a catalogue of accurate positions and correlated flux densities for 410 flat‐spectrum, compact extragalactic radio sources previously detected in the Australia Telescope 20 GHz (AT20G) survey. The catalogue spans the declination range [−90°, −40°] and was constructed from four 24‐h very long baseline interferometry (VLBI) observing sessions with the Australian Long Baseline Array at 8.3 GHz. The VLBI detection rate in these experiments is 97 per cent, the median uncertainty of the source positions is 2.6 mas and the median correlated flux density on projected baselines longer than 1000 km is 0.14 Jy. The goals of this work are (1) to provide a pool of southern sources with positions accurate to a few milliarcsec, which can be used for phase‐referencing observations, geodetic VLBI and space navigation; (2) to extend the complete flux‐limited sample of compact extragalactic sources to the Southern hemisphere; and (3) to investigate the parsec‐scale properties of high‐frequency selected sources from the AT20G survey. As a result of this VLBI campaign, the number of compact radio sources south of declination −40° which have measured VLBI correlated flux densities and positions known to milliarcsec accuracy has increased by a factor of 3.5. The catalogue and supporting material is available at http://astrogeo.org/lcs1.

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Mark 4 VLBI correlator: Architecture and algorithms

Cited by 43 publications

References 8 publications

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*

Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*

VLBI observations of optically-bright extragalactic radio sources for the alignment of the radio frame with the future Gaia frame

The LBA Calibrator Survey of southern compact extragalactic radio sources - LCS1

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