Flux Calibrator Models

Descriptions of flux calibrator models for flux density scaling

There are two categories of flux calibrator models available to determine flux density scales: compact extra-galactic sources and solar system objects. The models for bright extragalactic sources are described in the form of polynomial expressions for spectral flux densities and clean component images for spatial information. The flux density scales based on the solar system objects are commonly used to establish flux density scales for mm and sub-mm astronomy. These models consist of brightness temperature models and their ephemeris data.

Compact extragalactic sources

For the VLA, the default source models are customarily point sources defined by the ’Baars’, ’Perley 90’, ’Perley-Taylor 99’, ’Perley-Butler 2010’,  ’Perley-Butler 2013’ (time-variable), 'Perley-Butler 2017' (time-variable) or ’Scaife-Heald 2012’ flux density scales (’Perley-Butler 2017’ is the current standard by default), or point sources of unit flux density if the flux density is unknown. 'Stevens-Reynolds 2016' currently contains only one source, 1934-638, which is primarily used for flux calibrator for the ACTA.

The model images (CLEAN component images) are available for the sub set of the sources listed below. setjy task provides listing of the available model images included in the CASA package's data directory ( You can find the path to the directory containing your list of VLA Stokes I models by typing (inside CASA) print os.getenv('CASAPATH').split(' ')[0] + '/data/nrao/VLA/CalModels/' ). These models can be plotted in plotms.

Alternatively, the user can provide a model image at the appropriate frequency in Jy/pixel units, typically the .model made by clean (which is a list of components per pixel, as required, although the restored .image is in Jy/beam). For unknown calibrators, however, the spectral flux distribution has to be explicitely specified in setjy. If you do not specify the correct path to a model (and you have not provided your own model), the default model of a point sources of unit flux density will be adopted.

 3C/Common Name B1950 Name J2000 Name Alt. J2000 Name Standards – – – J0133-3629 9 3C48 0134+329 0137+331 J0137+3309 1,2,3,4,5,6,7,9 FORNAX X – – J0322-3712 9 3C123 0433+295 0437+296 J0437+2940 2, 9 3C138 0518+165 0521+166 J0521+1638 1,3,4,5,6 PICTOR A – – J0519-4546 9 3C144 (TAURUS A/CRAB) – – J0534+2200 9 3C147 0538+498 0542+498 J0542+4951 1,3,4,5,6, 7, 9 3C196 0809+483 0813+482 J0813+4813 1,2,7,9 3C218(HYDRA A) – – J0918-1205 9 3C274 (VIRGO A) – – J1230+1223 9 3C286 1328+307 1331+305 J1331+3030 1,2,3,4,5,6,7,9 3C295 1409+524 1411+522 J1411+5212 1,2,3,4,5,6,7,9 3C438 (HERCULES A) – – J1651+0459 9 3C353 – – J1720-0059 9 – 1934-638 – J1939-6342 1,3,4,5,6,8 3C380 1828+487 1829+487 J1829+4845 7,9 3C405 (CYGNUS A) – – J1959+4044 9 3C444 – – J2214-1701 9 3C461 (CASSIOPEIA A) – – J2323+5848 9

Standards are: (1) Perley-Butler 2010, (2) Perley-Butler 2013, (3) Perley-Taylor 99, (4) Perley-Taylor 95, (5) Perley 90, (6) Baars, (7) Scaife-Heald 2012, (8) Stevens-Reynolds 2016 (9) Perley-Butler 2017

Type Table fluxcalmodels-tab-1 Known sources and their alternative names recognized by setjy task

ALMA also uses a few dozen compact QSO as flux standards, monitored 2-4 times a month at bands 3, 6 and 7 (90 - 345 GHz). Due to rapid variability these data are not packaged with CASA, but can be accessed via https://almascience.eso.org/alma-data/calibrator-catalogue

Baars

The only standard to not have the year in the name. It is 1977. The models are second order polynomials in log(ν), valid between 408 MHz and 15 GHz.

Reference: Baars et al. (1977) [1] with a commentary by Kellermann, K. I. (1999) [2]

Citation Number 1 Baars, J. W. M. et al. 1977, A&A, 61, 99 (ADS)
Citation Number 2 Kellermann, K. I. 2009, A&A 500, 143 (ADS)

Perley 90

This standard also includes 1934-638 from Reynolds (7/94) and 3C138 from Baars et al. (1977) [1] .

Details can be found at http://www.vla.nrao.edu/astro/calib/manual/baars.html.

Perley-Taylor 95

Perley and Taylor (1995.2); plus Reynolds (1934-638; 7/94) Details can be found at http://www.vla.nrao.edu/astro/calib/manual/baars.html.

Perley-Taylor 99

Perley and Taylor (1999.2); plus Reynolds (1934-638; 7/94) Details can be found at http://www.vla.nrao.edu/astro/calib/manual/baars.html.

Perley-Butler 2010

A preliminary version of Perley-Butler 2013. This version also has coefficients for sources that showed some degree of variability (see Perley & Butler (2013) [3]) but they are treated as the steady sources (i.e. no time dependent models are used).

Citation Number 3 Perley, R. A., & Butler, B. J. 2013, ApJS, 204, 19 (ADS)

Perley-Butler 2013

Flux scale for the constant flux sources 3C123, 3C196, 3C286, and 3C295 as well as variable sources (3C48, 3C138, and 3C147). The models for the variable sources are time-dependent.
Reference: Perley & Butler (2013)  [3] .

Scaife-Heald 2012

Low frequency, 30-300MHz, calibrators 3C48, 3C147, 3C196, 3C286, 3C295, and 3C380.

Reference: Scaife & Heald (2012)  [4]

Citation Number 4 Scaife, A. M., & Heald, G. H. 2012, MNRAS, 423, 30 (ADS)

Stevens-Reynolds 2016

Low frequency (<11GHz) polynomial from Reynolds and updated high frequecy polynomial from Stevens.

Reference: Partridge et al. (2016)  [5]

Citation Number 5 Partridge et al. 2016, ApJ 821,1 (ADS)

Perley-Butler 2017

The flux density scale of Perley-Butler 2013 extended downward to ~50 MHz. Twenty sources were drawn from the Baar, Perley-Butler 2013, and Scaife-Heald 2012. Flux scale for the constant flux sources Fornax A, 3C123, J0444-2809, Pictor A, 3C144, (Taurus A or Crab), 3C196, 3C218 (Hydra A),  3C274 (Virgo A or  M87), 3C286, 3C295,  3C348 (Hercules A), 3C353, 3C380, 3C405 (Cygnus A), 3C444, and 3C461 (Cassiopeia A) as well as variable sources (3C48, 3C138, and 3C147). The models for the variable sources are time-dependent. The frequency range valid for the model for each source is also listed below.

Source Valid frequency range in GHz
J0133-3649 0.2-4
3C48 0.05-50
Fornax X 0.2-0.5
3C123 0.06-50
J0444-2809 0.2-2.0
3C138 0.2-50
Pictor A 0.2-4.0
Taurus A 0.05-4.0
3C147 0.05-50
3C196 0.050-50
Hydra A 0.05-12
Virgo A 0.05-3
3C286 0.05-50
3C295 0.05-50
Hercules A 0.2-12
3C353 0.2-4
3C380 0.05-4.0*
Cygnus A 0.05-12
3C444 0.2-12
Cassiopeia A 0.2-4

* The corrected frequency range for 3C380 is noted here based on  B. J.  Butler 2018, private comunication (CAS-9538)
Reference: Perley & Butler (2017)  [7]

Citation Number 7 Perley, R. A. & Butler, B. J. 2017, ApJS, 230,7(ADS)

Solar System objects

The usual approach in mm and sub-mm regimes is to use models that are, to first order, thermal sources in the Solar System. Their apparent brightness varies in time with their distance from the Earth (and Sun), and orientation if they are not perfect spheres with zero obliquity. However, most of them have almost constant surface properties, so once those properties are measured their apparent brightness distributions, they can in principle be predicted for any time, given an ephemeris. Planets, in particular, have more complex spectra and effects such as atmospheric lines, magnetic fields, seasons, polar caps and surface features that need to be taken into account when they are available and significant. In CASA, the Solar System objects supported by setjy are available through the ‘Butler-JPL-Horizons 2010’, and ’Butler-JPL-Horizons 2012’ standards. It is recommended to use 'Butler-JPL-Horizons 2012' as it contains updated models. The 2012 models are described in ALMA Memo 594, which is available on https://science.nrao.edu/facilities/alma/~aboutALMA/Technology/ALMA_Memo_Series/alma594/abs594 . Models can be found  by typing (in CASA) print os.getenv('CASAPATH').split(' ')[0] + '/data/alma/SolarSystemModels'.

The following objects are supported based on models from Butler-JPL-Horizons 2012, updated where necessary as mentioned under each object. Please refer ALMA Memo594 for the detailed comparisons with the models in Butler-JPL-Horizons-2010.

Venus

The model spans the frequencies from ~300MHz to 1THz. No atmospheric lines such as CO,H2O, HDO, and etc are included. Modeled based on Clancy et al. (2012)  [6].

Citation Number 6 Clancy, R.T. et al. 2012, Icarus, 217, 779 (ADS)

Mars

Full implementation of the model of Rudy et al. (1987) [7], tabulated as a function of time and frequency (30-1000GHz). No atmospheric lines are included.

Citation Number 7 Rudy, D.J. et al. 1987, Icarus, 71, 159 (ADS)

Jupiter

Model for 30-1020GHz (from Glenn Orton, private communication), does not include synchrotron emission.

Uranus

Model for 60-1800GHz (from Glenn Orton and Raphael Moreno, private communication), contains no rings or synchrotron.

Neptune

Model for 2-2000 GHz (from Glenn Orton and Raphael Moreno, private communication), contains no rings or synchrotron.

Io

Spline interpolation of data points from 15 to 29980 GHz (references: please refer to the ALMA memo 594 Table 1).  Strongly not recommended to use for the primary flux calibrator for ALMA observations.

Europa

Spline interpolation of data points from 15 to 29980 GHz (references: please refer to the ALMA memo 594 Table 1).  Strongly not recommended to use for the primary flux calibrator for ALMA observations.

Ganymede

Spline interpolation of data points from 5 to 29980 GHz (references: please refer to the ALMA memo 594 Table 1).

Callisto

Spline interpolation of data points from 5 to 29980 GHz (references: please refer to the ALMA memo 594 Table 1).

Titan

Model from Mark Gurwell, from 53.3-­1024.1 GHz. Contains surface and atmospheric emission. The atmosphere includes N2-­N2 and N2-­CH4 Collision-­Induced Absorption (CIA), and lines from minor species CO, 13CO, C18O, HCN, H13CN and HC15N. See, e.g., Gurwell & Muhleman (2000) [8]; Gurwell (2004) [9].

Citation Number 8 Gurwell, M.A. & D.O. Muhleman 2000, Icarus, 145, 65w (ADS)
Citation Number 9 Gurwell, M.A. 2004, ApJ, 616, L7 (ADS)

Asteroids

Some asteroids, namely Ceres, Pallas, Vesta, and Juno are included in the Butler-JPL-Horizons 2012. The models consists of the constant brightness temperature in frequency. For Ceres, Pallas, and Vesta, updated models based on thermophysical models (TPM) (T. Mueller, private communication) which are tabulated in time and frequency, are available for the observations taken after January 1st 2015, 0:00 UT. setjy task will automatically switch to the new models for the observations taken on and after that date. The TPM are also available for Lutetia but it is not advised to use for the absolute flux calibration for ALMA. Each of the tabulated models contains the flux density at 30, 80, 115, 150, 200, 230, 260, 300, 330, 360, 425, 650, 800, 950, and 1000 GHz. The time resolution is 1 hour for Ceres and 15 min for Lutetia, Pallas, and Vesta. The cubic interpolation is employed to obtain the flux densities at other frequencies.

Ceres

Model with a constant $T_b$ = 185K over frequencies (Moullet et al. 2010 [10], Muller & Lagerros 2002 [11], Redman et al. 1998 [12], Altenhoff et al. 1996 [13]) if time of the observations took place ($t_{obs}$) is before 2015.01.01, 0:00 UT, TPM if $t_{obs}$ $\ge$ 2015.01.01, 0:00 UT.

Citation Number 10 Moullet, A. et al. 2010, A&A, 516, L10 (ADS)
Citation Number 11 Muller, T.G. & J.S.V. Lagerros 2002, A&A, 381, 324 (ADS)
Citation Number 12 Redman, R.O. et al. 1998, AJ, 116, 1478 (ADS)
Citation Number 13 Altenhoff, W.J. et al. 1996, A&A, 309, 953 (ADS)

Pallas

Model with a constant $T_b$ = 189K (Chamberlain et al. 2009 [14], Altenhoff et al. 1994 [15]) for $t_{obs}$ $\lt$ 2015.01.01, 0:00 UT, and TPM for $t_{obs}$ $\ge$ 2015.01.01, 0:00 UT

Citation Number 14 Chamberlain, M.A. et al. 2009, Icarus, 202, 487 (ADS)
Citation Number 15 Altenhoff, W.J. et al. 1994, A&A, 287, 641 (ADS)

Vesta

Model with a constant $T_b$ = 155K (Leyrat et al. 2012 [16], Chamberlain et al. 2009 [14], Redman et al. 1998 [12], Altenhoff et al. 1994 [15]) for $t_{obs}$ $\lt$ 2015.01.01, 0:00 UT, and TPM for $t_{obs}$ $\ge$ 2015.01.01, 0:00 UT

Citation Number 16 Leyrat, C. et al. 2012, A&A, 539, A154 (ADS)

Juno

Model with a constant $T_b$ = 153K (Chamberlain et al. 2009 [14], Altenhoff et al. 1994 [15])