This year the Murchison Widefield Array annual December meeting was hosted by Arizona State University, here in Tempe. In celebration of over a year of active observing the meeting was opened to attendance and contributions from the wider community. The result was a rare convocation of the low frequency radio astronomy community. Topics ranged from transients and radio galaxies to 21 cm at cosmological distances. One of the highlights of the meeting was the announcement of the discovery of low altitude flux tubes in the ionosphere (Physics Today article). Remember friends, they aren’t image artifacts if they line up with the earths magnetic field lines!

The full list of participants along with most of the talk slides can be found over on mwatelescope.org.

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The meeting was also preceded by a 5 day intensive work session by the Epoch of Reionization team.

 

pictor_spectrum

With the new PAPER data (‘x’s) and a refined fitting procedure we can accurately model the EoR band flux to better than 2%.

Paper Title: A Flux Scale for Southern Hemisphere 21cm EoR Experiments

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If you look at typical catalogs of sources in the southern hemisphere epoch of reionization band you soon realize why measurements often disagree by 20%.  Even the brightest sources one would use for calibration are uncertain.  As you can see from the plot to the right, most of the old data (dots) are pretty uncertain. This source (Pictor A) is one of the brightest in the southern sky and is often use to set the flux scale.  The flux scale standard everyone uses to compare measurements is based on Cygnus A, which is not really even visible in the south, isn’t even defined at 150MHz.

So thats why we took a bunch of PAPER data and measured a bright source (Pictor A) several thousand times. This let us get rid of a lot systematics to get a nice spectrum. Then we fit a spectrum to old and new data using newish fitting procedure that accounts for error bars and nicely estimates the fit uncertainty. Previous estimates put the model uncertainty at about 20% (that number again!) but using the new fitter on the old data we found that we could predict the flux at about the 5% level, folding in the new PAPER data the model precision went below 2%.  An order of magnitude improvement!  we also verified this method on a couple dozen other sources with good success. Below is a nice large PAPER image of the area we were looking at.

psa64_pic_strip_positions_psa747_v2_annotated_cropped

A PAPER mosaic of the southern EoR band sky. Sources measured in this paper have ‘x’s, only two (orange dots) seemed to disagree with other measurements.

mwa32_psa32_ff_all_matches_long_v2_cropped

Comparing PAPER and MWA fluxes to come up with a model relating the two. The blue dots are all equally possible to within 76%.

Title: The precision and accuracy of early Epoch of Reionization foreground models: comparing MWA and PAPER 32-antenna source catalogs

This paper compares the fluxes in the first PAPER and MWA catalogs.  These sources are the brightest foregrounds in front of the EoR HI emission.  Various estimates suggest that we have to subtract these guys to anywhere from 0.1 to 0.01%. Percent!! Also we need to know the flux precisely so we can calibrate our power spectrum.

These first catalogs turn out to be accurate to about 20%.  We also looked at sources that were measured twice by the MWA and found that they didn’t agree very well (20-50% or more) away from the center of the image.  This means that the primary beam model used to flatten the flux scale was off.  This is probably true for both experiments.

In the end we noted that though 20% is no where near where we need to be for precise EoR foreground subtraction or flux calibration, its not too shabby for two experimental arrays at first light!

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