Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey

Neal Jackson; Shruti Badole; Rajan Chhetri; Kaspars Prusis; Atvars Nikolajevs; Leah Morabito; Michiel Brentjens; Frits Sweijen; Marco Iacobelli; Emanuela Orrù; J. Sluman; R. Blaauw; H. Mulder; P. van Dijk; Sean Mooney; Adam Deller; Javier Moldon; J. R. Callingham; Jeremy Harwood; Martin Hardcastle; George Heald; Alexander Drabent; J. P. McKean; A. Asgekar; I. M. Avruch; M. J. Bentum; A. Bonafede; W. N. Brouw; M. Brüggen; H. R. Butcher; B. Ciardi; A. Coolen; A. Corstanje; S. Damstra; S. Duscha; J. Eislöffel; H. Falcke; M. Garrett; F. de Gasperin; J. -M. Griessmeier; A. W. Gunst; M. P. van Haarlem; M. Hoeft; A. J. van der Horst; E. Jütte; L. V. E. Koopmans; A. Krankowski; P. Maat; G. Mann; G. K. Miley; A. Nelles; M. Norden; M. Paas; V. N. Pandey; M. Pandey-Pommier; R. F. Pizzo; W. Reich; H. Rothkaehl; A. Rowlinson; M. Ruiter; A. Shulevski; D. J. Schwarz; O. Smirnov; M. Tagger; C. Vocks; R. J. van Weeren; R. Wijers; O. Wucknitz; P. Zarka; J. A. Zensus; P. Zucca

doi:10.1051/0004-6361/202140756

Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey

Neal Jackson, Shruti Badole, Rajan Chhetri, Kaspars Prusis, Atvars Nikolajevs, Leah Morabito, Michiel Brentjens, Frits Sweijen, Marco Iacobelli, Emanuela Orrù, J. Sluman, R. Blaauw, H. Mulder, P. van Dijk, Sean Mooney, Adam Deller, Javier Moldon, J. R. Callingham, Jeremy Harwood, Martin HardcastleGeorge Heald, Alexander Drabent, J. P. McKean, A. Asgekar, I. M. Avruch, M. J. Bentum, A. Bonafede, W. N. Brouw, M. Brüggen, H. R. Butcher, B. Ciardi, A. Coolen, A. Corstanje, S. Damstra, S. Duscha, J. Eislöffel, H. Falcke, M. Garrett, F. de Gasperin, J. -M. Griessmeier, A. W. Gunst, M. P. van Haarlem, M. Hoeft, A. J. van der Horst, E. Jütte, L. V. E. Koopmans, A. Krankowski, P. Maat, G. Mann, G. K. Miley, A. Nelles, M. Norden, M. Paas, V. N. Pandey, M. Pandey-Pommier, R. F. Pizzo, W. Reich, H. Rothkaehl, A. Rowlinson, M. Ruiter, A. Shulevski, D. J. Schwarz, O. Smirnov, M. Tagger, C. Vocks, R. J. van Weeren, R. Wijers, O. Wucknitz, P. Zarka, J. A. Zensus, P. Zucca

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Abstract

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper, we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30 006 observations of 24 713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥200 km baselines is identified by the detection of compact flux density, for declinations north of 30° and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2′, for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 min varying by a few tens of percent between the shortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2′ and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the MurchisonWidefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

Original language	English
Article number	A2
Number of pages	14
Journal	Astronomy & Astrophysics
Volume	658
Early online date	25 Jan 2022
DOIs	https://doi.org/10.1051/0004-6361/202140756
Publication status	Published - 1 Feb 2022

Keywords

astro-ph.GA
Surveys
Techniques: interferometric
Instrumentation: interferometers
Galaxies: active
Radio lines: galaxies

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10.1051/0004-6361/202140756Licence: Unspecified

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© ESO 2021. This is the accepted manuscript version of an article which has been published in final form at 10.1051/0004-6361/202140756
Accepted author manuscript, 1.18 MBLicence: Unspecified

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Jackson, N., Badole, S., Chhetri, R., Prusis, K., Nikolajevs, A., Morabito, L., Brentjens, M., Sweijen, F., Iacobelli, M., Orrù, E., Sluman, J., Blaauw, R., Mulder, H., Dijk, P. V., Mooney, S., Deller, A., Moldon, J., Callingham, J. R., Harwood, J., ... Zucca, P. (2022). Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey. Astronomy & Astrophysics, 658, Article A2. https://doi.org/10.1051/0004-6361/202140756

@article{1ea30a7bf49748f586d9ebc0ae56636f,

title = "Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey",

abstract = "The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper, we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30 006 observations of 24 713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥200 km baselines is identified by the detection of compact flux density, for declinations north of 30° and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2′, for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 min varying by a few tens of percent between the shortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2′ and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the MurchisonWidefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.",

keywords = "astro-ph.GA, Surveys, Techniques: interferometric, Instrumentation: interferometers, Galaxies: active, Radio lines: galaxies",

author = "Neal Jackson and Shruti Badole and Rajan Chhetri and Kaspars Prusis and Atvars Nikolajevs and Leah Morabito and Michiel Brentjens and Frits Sweijen and Marco Iacobelli and Emanuela Orr{\`u} and J. Sluman and R. Blaauw and H. Mulder and Dijk, {P. van} and Sean Mooney and Adam Deller and Javier Moldon and Callingham, {J. R.} and Jeremy Harwood and Martin Hardcastle and George Heald and Alexander Drabent and McKean, {J. P.} and A. Asgekar and Avruch, {I. M.} and Bentum, {M. J.} and A. Bonafede and Brouw, {W. N.} and M. Br{\"u}ggen and Butcher, {H. R.} and B. Ciardi and A. Coolen and A. Corstanje and S. Damstra and S. Duscha and J. Eisl{\"o}ffel and H. Falcke and M. Garrett and Gasperin, {F. de} and Griessmeier, {J. -M.} and Gunst, {A. W.} and Haarlem, {M. P. van} and M. Hoeft and Horst, {A. J. van der} and E. J{\"u}tte and Koopmans, {L. V. E.} and A. Krankowski and P. Maat and G. Mann and Miley, {G. K.} and A. Nelles and M. Norden and M. Paas and Pandey, {V. N.} and M. Pandey-Pommier and Pizzo, {R. F.} and W. Reich and H. Rothkaehl and A. Rowlinson and M. Ruiter and A. Shulevski and Schwarz, {D. J.} and O. Smirnov and M. Tagger and C. Vocks and Weeren, {R. J. van} and R. Wijers and O. Wucknitz and P. Zarka and Zensus, {J. A.} and P. Zucca",

note = "{\textcopyright} ESO 2021. This is the accepted manuscript version of an article which has been published in final form at 10.1051/0004-6361/202140756",

year = "2022",

month = feb,

day = "1",

doi = "10.1051/0004-6361/202140756",

language = "English",

volume = "658",

journal = "Astronomy & Astrophysics",

issn = "0004-6361",

publisher = "EDP Sciences",

}

Jackson, N, Badole, S, Chhetri, R, Prusis, K, Nikolajevs, A, Morabito, L, Brentjens, M, Sweijen, F, Iacobelli, M, Orrù, E, Sluman, J, Blaauw, R, Mulder, H, Dijk, PV, Mooney, S, Deller, A, Moldon, J, Callingham, JR, Harwood, J, Hardcastle, M, Heald, G, Drabent, A, McKean, JP, Asgekar, A, Avruch, IM, Bentum, MJ, Bonafede, A, Brouw, WN, Brüggen, M, Butcher, HR, Ciardi, B, Coolen, A, Corstanje, A, Damstra, S, Duscha, S, Eislöffel, J, Falcke, H, Garrett, M, Gasperin, FD, Griessmeier, J-M, Gunst, AW, Haarlem, MPV, Hoeft, M, Horst, AJVD, Jütte, E, Koopmans, LVE, Krankowski, A, Maat, P, Mann, G, Miley, GK, Nelles, A, Norden, M, Paas, M, Pandey, VN, Pandey-Pommier, M, Pizzo, RF, Reich, W, Rothkaehl, H, Rowlinson, A, Ruiter, M, Shulevski, A, Schwarz, DJ, Smirnov, O, Tagger, M, Vocks, C, Weeren, RJV, Wijers, R, Wucknitz, O, Zarka, P, Zensus, JA & Zucca, P 2022, 'Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey', Astronomy & Astrophysics, vol. 658, A2. https://doi.org/10.1051/0004-6361/202140756

TY - JOUR

T1 - Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey

AU - Jackson, Neal

AU - Badole, Shruti

AU - Chhetri, Rajan

AU - Prusis, Kaspars

AU - Nikolajevs, Atvars

AU - Morabito, Leah

AU - Brentjens, Michiel

AU - Sweijen, Frits

AU - Iacobelli, Marco

AU - Orrù, Emanuela

AU - Sluman, J.

AU - Blaauw, R.

AU - Mulder, H.

AU - Dijk, P. van

AU - Mooney, Sean

AU - Deller, Adam

AU - Moldon, Javier

AU - Callingham, J. R.

AU - Harwood, Jeremy

AU - Hardcastle, Martin

AU - Heald, George

AU - Drabent, Alexander

AU - McKean, J. P.

AU - Asgekar, A.

AU - Avruch, I. M.

AU - Bentum, M. J.

AU - Bonafede, A.

AU - Brouw, W. N.

AU - Brüggen, M.

AU - Butcher, H. R.

AU - Ciardi, B.

AU - Coolen, A.

AU - Corstanje, A.

AU - Damstra, S.

AU - Duscha, S.

AU - Eislöffel, J.

AU - Falcke, H.

AU - Garrett, M.

AU - Gasperin, F. de

AU - Griessmeier, J. -M.

AU - Gunst, A. W.

AU - Haarlem, M. P. van

AU - Hoeft, M.

AU - Horst, A. J. van der

AU - Jütte, E.

AU - Koopmans, L. V. E.

AU - Krankowski, A.

AU - Maat, P.

AU - Mann, G.

AU - Miley, G. K.

AU - Nelles, A.

AU - Norden, M.

AU - Paas, M.

AU - Pandey, V. N.

AU - Pandey-Pommier, M.

AU - Pizzo, R. F.

AU - Reich, W.

AU - Rothkaehl, H.

AU - Rowlinson, A.

AU - Ruiter, M.

AU - Shulevski, A.

AU - Schwarz, D. J.

AU - Smirnov, O.

AU - Tagger, M.

AU - Vocks, C.

AU - Weeren, R. J. van

AU - Wijers, R.

AU - Wucknitz, O.

AU - Zarka, P.

AU - Zensus, J. A.

AU - Zucca, P.

N1 - © ESO 2021. This is the accepted manuscript version of an article which has been published in final form at 10.1051/0004-6361/202140756

PY - 2022/2/1

Y1 - 2022/2/1

N2 - The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper, we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30 006 observations of 24 713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥200 km baselines is identified by the detection of compact flux density, for declinations north of 30° and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2′, for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 min varying by a few tens of percent between the shortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2′ and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the MurchisonWidefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

AB - The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper, we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30 006 observations of 24 713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥200 km baselines is identified by the detection of compact flux density, for declinations north of 30° and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2′, for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 min varying by a few tens of percent between the shortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2′ and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the MurchisonWidefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

KW - astro-ph.GA

KW - Surveys

KW - Techniques: interferometric

KW - Instrumentation: interferometers

KW - Galaxies: active

KW - Radio lines: galaxies

UR - http://www.scopus.com/inward/record.url?scp=85124079583&partnerID=8YFLogxK

U2 - 10.1051/0004-6361/202140756

DO - 10.1051/0004-6361/202140756

M3 - Article

SN - 0004-6361

VL - 658

JO - Astronomy & Astrophysics

JF - Astronomy & Astrophysics

M1 - A2

ER -

Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey

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