Uranus flux calibration

[ste616]

During an attempt to determine the flux density of 1934-638 at frequencies above 30 GHz, Mark W. and I found that the Uranus brightness temperature varies in a way that is not accounted for by MIRIAD.

MIRIAD has a brightness temperature for Uranus that varies by frequency, but does not account for any temporal variation. It does account for the Earth-Uranus distance, but the angle of Uranus' pole to Earth is also an important factor as the pole regions are at a different temperature to the rest of the atmosphere. It is also thought that Uranus' atmosphere changes its temperature over time.

If a model cannot be created that accounts for these variations, it will be necessary to calibrate Uranus before using it as a flux calibrator. This may be possible by observing it using the 3mm receiver which is absolutely calibrated to temperature with its paddle. If the Uranus atmospheric temperature models can be shifted up/down (ie. they hold when the atmosphere warms/cools) then the Uranus brightness temperature may then be determined at other frequencies.

This possibility is now under investigation.

[emonts]

Jamie, do you know roughly how big this uncertainty in Uranus calibration -- is it more than the 30% flux error generally assumed for mm observing?

Also, are we sure that 1934-638 has a constant flux (at mm)? (Is it an idea to use several known calibrators for check-up?)

(great forum, thanks)

[ste616]

Good question Bjorn. I know from my recent work that the brightness temperature MIRIAD wants to fit Uranus with is about 50% too high. That is, straight after a paddle scan, Uranus' flux is measured, and mfboot wants to scale such an observation by approximately 1.5. Ideally, if we can trust that the paddle scan has a known temperature (and I hope we can assume this is the case), then the mfboot scaling at 3mm should be essentially 1.

To evaluate models, I will first assume that 1934-638 has a constant flux over time, but I will check if this makes sense by looking at other calibrators as well.

[ste616]

Just an update on what I'm doing.

There are two papers that discuss this variability and calibration issue, and which have enough information to make a pretty good model. These papers are:

Kramer et al. (2008), A&A 482, p359, http://adsabs.harvard.edu/abs/2008A%26A...482..359K" onclick="window.open(this.href);return false. This paper discusses how Uranus' brightness temperature at 90 GHz is dependent on its orientation towards Earth.

Weiland et al. (2010), http://fr.arxiv.org/abs/1001.4731" onclick="window.open(this.href);return false. This paper uses WMAP data to provide absolute brightness temperature calibration for Uranus at frequencies very near to all the bands available at the ATCA.

My current plan is to use the WMAP data as an absolute calibration, and then use the Kramer et al. results to make a temporal model and install it in MIRIAD.

[baerbel]

Hi Jamie,

can you us an idea how much the flux of Uranus would change depending on its pole position ?
I assume we know how the flux varies with distance from Earth, but this effect then introduces
an additional uncertainty.

Baerbel (and Catarina)

[ste616]

Hi Baerbel,

Please feel free to look at my personal work page for this topic at http://jamiestevens-atca.wikidot.com/ur ... alibration" onclick="window.open(this.href);return false

In summary, to answer your question, compared to the average Uranus temperature from Kramer et al. (2008), the observed Uranus temperature varies from 95% of this value to 108% depending on the orientation of Uranus. This effect can be seen in the figure below, which also gives an idea of the uncertainties in these observations.