Absolute Amplitude (Flux) Calibration Sources Discussion

TIP Last Update: JeffMangum - 08 May 2013



Flux Calibration Sources

Uranus and Neptune

Summary: Current models for Uranus are good to about 5% in the millimeter/submillimeter. Neptune has issues as its spectral line emission is only now being incorporated into models.

Background Documentation

Orton presentation summary March 2013.
Glenn Orton gave a presentation on the more current Uranus and Neptune models used for Herschel calibration at a conference on Herschel calibration in March 2013. His summary slide above tells the current story as to the accuracy of flux models for these two planets.
Plots of those models as shown in the following two diagrams:
Uranus flux model (Orton 2013).
Neptune flux model (Orton 2013).

Measurement Plan

The following from Glenn Orton

Two things are important priorities for Uranus:

One is to determine the variability of temperatures "at depth" (something like the 1-bar pressure level). For this purpose, getting a resolved image at any frequency longward of something like 25 cm-1 (750 GHz) would provide a map of temperatures, because the spectrum in this region is overwhelmingly dominated by emission from the H2 collision-induced absorption. So, changes in the radiance can only be the result of changes of temperature, as the H2 abundance is uniform across the planet. "Confusion" by small amounts of stratospheric H2O are tiny. This is important in order to determine the variability of the disk-averaged spectrum of the planet as it went through changes of its aspect as a result of seasonal variability (recall Uranus' axial tilt is 98 deg. ). Over the course of the Herschel mission, this could have been responsible for as much as 1.5% variability, or less than 0.2%, depending on how far apart temperatures in different regions are.

This is also important for Neptune in general, although over the course of the Herschel mission, its flux is unlikely to have changed much.

Another, arguably more important measurement - but possibly not in the aegis of ALMA, is to get a moderate-resolution but very precise spectrum over the 5-15 cm-1 (150–450 GHz) range in order to detect any broad spectroscopic features belonging to H2S or whatever else is creating the (currently unknown) absorption in that region. Note that the green filled circles are from Gene Serabyn's nice work, but they are still too noisy to detect any of the subtle features of the H2S lines. I note again that H2S or PH3 are the most likely candidates for absorption in this region, but those are guesses based on cosmogonic arguments from the expected inventory of elements and the chemistry they form.

A little lower in priority (because we may be getting this information from the SMA) is getting spatial variability in this region, which is important for the same reason as temperatures, to track variability over the course of the mission.

Spatial variability for Neptune in this region is useful, as well, although the additional absorption in Neptune doesn't appear to be necessary. Measurements of spatial variability in Neptune would detect variability in temperatures at pressures greater than 1 bar (maybe 2-3 bars), which is also useful and pretty extraordinary for understanding Neptune's dynamics.

The general measurement requirements for these observations are:
  1. Resolution: A few tenths of an arcsec spatial resolution would be good. Uranus is ~3.3-3.8", so resolving it with 5-6 independent elements would be minimal ~0.5" or so. Higher resolution is always better, of course. This will at least distinguish the major bands. (For Neptune, only 2.8", we'd like to get at least down to 0.4")
  2. Sensitivity: A brightness temperature sensitivity of 1.0 K would be a minimum target, either in spatial differentiation for Uranus (or Neptune). It's also a minimum to detect broad H2S lines (see the model spectrum (slide 14) between 5 and 15 cm-1). Much better would be 0.5 K, if that's possible.
  3. Wavelength/Frequency Ranges: 5-12 cm-1 / 150 – 360 GHz. Lines are expect to be several GHz wide (maybe as wide as the entire 8 GHz available bandwidth). Or we look for narrower (but probably rather faint) emission lines of H2S if any of it survives into the stratosphere. Or look for alternative lines, like PH3. I can't predict these; this would truly be a exploratory observation for spectroscopy.


Background Documentation

Need information from Mueller

The following from Thomas Mueller

Subsurface Emission: Subsurface emission is not well modeled at the moment and needs further investigation. The wavelength where the subsurface emission starts to play a role is not clear and it might even be object dependent. I would guess it is around 200-500 micron. This will be investigated via the combined PACS and SPIRE data. But since both instruments have different calibration schemes (SPIRE: Uranus & Neptune; PACS: stars) we first have to get more confidence in the absolute levels and the related cross-calibration aspects. I plan to write up what I presented in ESAC in end March for the calibration workshop proceedings. I expect to receive the final(?) SPIRE fluxes in May, hopefully on time for a submission in end June.

Measurement Plan

The following from Thomas Mueller

Possible target list: Ceres, Pallas, Vesta, Lutetia.

The ESAC calibration conference proceedings paper might be a good starting point for sketching out an ALMA observing plan for these asteroids. I would see two key elements for ALMA:
  1. Observe a single object at all possible frequencies very close in time to obtain a first spectrum and to see if the fluxes follow the model predictions. Here we should select a target with a very shallow lightcurve (well below 5%).
  2. Observe one of the targets at a given frequency over a full rotation period (at least 10 measurement during the 5-10 h rotation period). We should select a target where the expected lightcurve amplitude is more than 5%.
This would allow us to access the frequency related aspects as well as the time-stability aspects. It would also allow us to establish a procedure on how to exchange information and to learn about the possibilities and limitations on both sides.

Giant Stars

  • All of the Herschel giant star calibrators have excess emission at millimeter/submillimeter wavelengths.
  • Need to find a parameter space in the HR diagram that is populated by stars which are not predicted to have excess flux at millimeter/submillimeter wavelengths.
  • If a star is shown to have excess millimeter/submillimeter emission due to an active chromosphere it just cannot be used as a flux standard. Emission models for stars just cannot do a good enough job of predicting active chromospheric emission.
  • Remember too that to be observable from ALMA that the declination must be less than 23 degrees.

Background Documentation


Measurement Plan

  • Measurement Sequence: The following measurement sequence should be use to measure MS stars as potential flux standards. The idea is that we compare these potential standards with the primary millimeter planetary calibrators, Uranus and Neptune. We would like to observe these two planets both before and after each target star, and three times, evenly spaced, within each designated stellar integration period.:
...previous STAR+planets sequence of observations – Uranus 10sec - Neptune 10sec
- STAR - 37sec
Uranus 10sec - Neptune 10sec
- STAR 37sec
Uranus 10sec - Neptune 10sec
- STAR 37sec
Uranus 10sec - Neptune 10sec – NEXT STAR+planets sequence of observations.
Similarly for the faintest star, HD12929 using 8 antennas: ...previous STAR+planets sequence of observations
– Uranus 10sec - Neptune 10sec
- STAR - 286sec
Uranus 10sec - Neptune 10sec - STAR 286sec
Uranus 10sec - Neptune 10sec - STAR 286sec
Uranus 10sec - Neptune 10sec – NEXT STAR+planets sequence of observations

  • Target List: See the following table for the proposed initial target list of potential MS stellar flux calibrators:

NOTE: 2013/04/18: 3mm flux predictions from Cohen are wrong. Cohen has recalculated, yielding the results listed in the table below.

Star ICRS RA ICRS Dec Predicted Band 3 (3mm) Flux (mJy)Sorted ascending Predicted Band 7 (1mm) Flux (mJy)
Sigma Libra (HD133216)
Alpha Cen A/B
14 39 36.204
-60 50 08.23
1.8 / 0.8
16.4 / 7.4
HD71129 (J0822-5930)
Gamma Cru (J1231-5706)

Bold = Measured to have excess millimeter/submillimeter continuum flux (i.e. active chromosphere)


Gamma Crux (J1231-5706)

A good model for the absolute flux of Gamma Crux assumes Rayleigh-Jeans emission extrapolated from a 90 micron flux of 18.16+-0.65 Jy (derived from AKARI measurements). This model will have an absolute accuracy of 3.6% (dictated by the uncertainty on the AKARI flux). Using this model, Gamma Crux has a flux of 756 mJy (with an uncertainty of 3.6%) at 680 GHz.

Measured 2012/11 at 105, 220, and 340 GHz. Measured/predicted fluxes...

105 GHz
220 GHz
340 GHz
... / 8.6
37(2) / 52.5
79(1) / 125

Looks like the October 2012 ALMA measurement at 105 GHz is foobar (too large by a factor of ~2).

Sigma Libra

Herschel flux standard analyzed in Structure of the Outer Layers of Cool Standard Stars (Dehaes etal. 2011). No evidence for an active chromosphere, but fluxes at wavelengths longer than 1mm are only upper limits. Flux predictions at 1mm and 3mm based on measured MAMBO flux at 1.2mm of 12.1+-2.0 mJy quoted in Structure of the Outer Layers of Cool Standard Stars (Dehaes etal. 2011).


Measured 2012/11 at 105, 220, and 340 GHz. Measured/predicted fluxes in mJy...

105 GHz
220 GHz
340 GHz
18(4) / 3
27(3) / 15
27(2) / 26

Looks like HD71129 has excess flux at longer wavelengths, a hallmark of active chromospheric excess. As this star is only a early giant (K0), one might expect it to have an active chromosphere.

Alpha Boo

The Structure of the Outer Layers of Cool Standard Stars (Dehaes etal. 2011) analysis suggests that Alpha Boo has excess continuum emission at wavelengths greater than about 800 micron.

Alpha Ceti

The Structure of the Outer Layers of Cool Standard Stars (Dehaes etal. 2011) analysis suggests that Alpha Ceti has excess continuum emission at wavelengths greater than about 800 micron.

Alpha Centauri

The reference "Alpha Centauri A in the Far Infrared: First Measurement of the Temperature minimum of a Star Other Than the Sun (Liseau etal 2013)" presents measurements of Alpha Cen A and B out to 1mm wavelength. Suggests that this star might be a good candidate for a flux calibrator. Extrapolating the measured PACS 70 micron flux for Alpha Cen A from Liseau etal (2013) of 3.35 Jy to 1mm and 3mm, one gets S(1mm) = S(70)*(70/1000)^2 = 3.35*(70/1000)^2 = 16.4 mJy, while a similar calculation for a wavelength of 3mm yields S(3mm) = 1.82 mJy. Note that Liseau etal (2013) measured S(870) = 28+-7 mJy for Alpha Cen A using LABOCA on APEX.

As quoted in CSV-1021, on 2013-02/12 Ed measured:
  • Alpha Cen A: S(850 micron) = 22.9 (SB X906) and 23.5 (SB X9f7) mJy
  • Alpha Cen B: S(850 micron) = 10.5 (SB X906) and 11.0 (SB X9f7) mJy

This compares to a prediction of S(850) = 3.35*(70/850)^2 = 22.7 mJy for Alpha Cen A and S(850) = 3.35*0.45*(70/850)^2 = 10.2 mJy for Alpha Cen B (I used the scaling Sa/(Sa + Sb) = 0.69 for lambda > 10 micron assumed by Liseau etal (2013) to calculate the Alpha Cen B flux).

This leaves us with the following measured/predicted fluxes for Alpha Cen A and B:

850 micron
1 mm
3 mm
22.2 / 22.7
... / 16.4
... / 1.82
10.8 / 10.2
... / 7.4
... / 0.82

One thing to note about the Liseau etal (2013) Alpha Cen measurements, the 870 micron measurement, when compared to the model prediction at that wavelength, is a bit high (though agrees within one sigma). Should investigate further as this might be an indicator of poorly-modeled chromospheric emission.

Historical Discussion of Flux Calibration Sources for ALMA

In the following I repeat a now ancient, though still relevant, discussion regarding options for flux calibration sources. For a lucid discussion of this topic see the presentation on Astronomical Calibration Sources for ALMA given by Bryan Butler at the 2003 ALMA/HIFI Calibration Meeting.

A good flux calibrator has the following properties:
  1. Unresolved size
  2. Constant or theoretically predictable flux
  3. Bright
At millimeter and submillimeter wavelengths, few if any sources meet all of these criteria. The current generation millimeter interferometers calibrate flux using variants of the following procedure:

  • Observe a planet with a low-spatial resolution array. Combine these measurements with a model of the resolved planetary flux to scale the observed planetary visibilities to the total power flux scale. The planet is the "primary flux calibrator".
  • Observe a bright quasar with the same low-spatial resolution array. The quasar is the "secondary flux calibrator".
  • Observe the same bright quasar, now of known flux, with all antennas in interferometric mode to set the interferometric flux scale.
  • Correct these observations for elevation-dependent antenna and atmospheric effects such as the gain curves and time dependent atmospheric attenuation.

This calibration procedure is just an extension of the flux calibration system used with millimeter and submillimeter single dishes. The key step in this calibration scheme is the determination of the flux of the primary calibrator (the planets in the above). Unfortunately, determination of the flux of a planet is not straight forward. As is the case for the VLBA, the high spatial resolution achievable with ALMA presents a fundamental problem in that most of these possible primary flux calibrators are highly resolved at the maximum resolution of the array -- for example, the 3 km baseline corresponds to 8.5 X 10^6 \lambda at 850 GHz or an angular resolution of 24 mas. Even most quasars show structures at these scales and are highly variable.

Historical Summary of Flux Calibration Sources

This topic: ALMA > WebHome > AlmaSci > AlmaCal > CalAmp > CalAmpSources
Topic revision: 2013-05-08, JeffMangum
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