Dataset Open Access

Allen Telescope Array polarimetric observation of 3C286

Estévez, Daniel; Farah, Wael

This dataset contains a polarimetric interferometry observation of the quasar 3C286 done with two dishes from Allen Telescope Array on 2020-10-31. The observation was done during two hours around transit, so it has large parallactic angle coverage. Eight different frequency bands between 1 and 8 GHz were observed in turns. Scans of 100 seconds were done with a frequency change between each scan.

The frequency bands observed are 1413 MHz, 1665 MHz, 2675 MHz, 4820 MHz, 6663 MHz, and 8665 MHz. A total of 30.72 MHz of spectrum divided in 256 frequency bins were observed in each band. This data set contains the same data in two formats: the raw measurements produced with GNU Radio, and the observations in UVFITS format.

The raw measurements consists of power spectra and cross-spectra for the X and Y linear polarizations of each antenna, using a coherent averaging interval of 0.1 seconds.

The observation was done with antennas 1h and 4g from the array, using two USRP N32x to digitize the signals, and GNU Radio to perform the digital signal processing. Both of the antennas were connected to LO d in the RF converter board, which downconverted the signals to a 512 MHz IF. The two channels of each USRP were used to digitize the two polarization channels of each antenna. The USRPs were configured to share the LO between all the channels, and were synchronized to the 10 MHz and 1PPS signals from the telescope timing distribution system.

The raw measurements are not phased to the source: instrumental and geometric delays are not corrected in any way.

All the raw measurements are stored in GNU Radio Metdata files with detached headers, contained in a flat .tar.xz archive. These files have names such as 3c286_2020-10-31T17:47:32.114581_cross_0. The beginning of the filename indicates the target of the observation, then there is the UTC timestamp corresponding to the beginning of the observation, and finally a code that indicates the measurement channel.

Measurement channels are divided in "power" for (real-valued) PSD and "cross" for (complex-valued) cross spectra. The "power" channels are numbered 0 through 3 and correspond to |X1|^2, |Y1|^2, |X2|^2 and |Y2|^2, where Xj and Yj denote the X and Y polarizations of antenna j (here j = 1 corresponds to antenna 1h and j = 2 corresponds to antenna 4g). The "cross" channels are numbered 0 through 5 and correspond to <X1,Y1>, <X1,X2>, <X1,Y2>, <Y1,X2>, <Y1,Y2>, <X2,Y2>.

Each file corresponds to one channel and one scan. The metadata detached header contains useful information, such as the precise timestamping of the samples and the observation frequency. Note that some of the files have invalid timestamps in the metadata (they are easy to distinguish because their first timestamp is zero). The timestamp in the filename is only a coarse estimate, and is typically within 1 second error of the precise timestamp in the metadata.

The UVFITS files contain only the cross-spectra between different antennas (correlations XX, YY, XY and YX). The data is phased to the source, but the same number of frequency channels and integration interval is maintained. There is one UVFITS file per frequency band.

More information about this observation and dataset can be found in the post "Polarimetric observation of 3C286 with Allen Telescope Array" by the first author.

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