ARTS tracking beams

The Apertif Radio Transient System (ARTS) archive contains tied-array beam (TAB) data. The TABs have a time-dependent and frequency-dependent pointing. This tool is able to convert the TAB data to a tracking beam (TB), which tracks a fixed point on the sky over the course of an observation. Additionally, it can convert TAB data to Synthesised Beams (SBs), which are suitable for transient searches.

Dependencies

• python >= 3.6
• numpy >= 1.17
• astropy
• tqdm

Installation

To install the latest release:

pip install arts_tracking_beams

To install the latest master branch:

pip install git+https://github.com/loostrum/arts_tracking_beams

Usage

Basic usage of this package is described below. Tutorials are available at https://loostrum.github.io/artstrackingbeams.

Input data

First download the data set of interest from the Apertif Long-Term Archive (ALTA). Tools to find which pulsars are in the field-of-view of a given Apertif pointing and to download the data are available as a separate python package.

A data file from the archive is identified by three parameters: the task ID, compound beam (CB) index, and TAB index. The file ARTS200102003_CB00_TAB00.fits would be the observation identified by task ID 200102003 (that is, the third observation on January 2nd, 2020), CB zero, TAB zero. A TB is created from the TABs of a single CB.

Creating a tracking beam

A tracking beam (TB) is created from the TAB data with arts_create_tracking_beam.

The simplest use case is to create a tracking beam from a folder which contains only one data set (i.e. the TABs of one CB of one observation), for a source with known coordinates. For example, to create a tracking beam towards the Crab pulsar:

arts_create_tracking_beam --input_folder /path/to/data/ --source 'PSR B0531+21'

If there are multiple data sets in the input data folder, specify the task ID and/or CB index. Instead of the source name, it is also possible to provide a RA and Dec. The name of the output FITS file is determined automatically from the input source name or RA/Dec, but can also be specified manually. Using all of these options, an example command is:

arts_create_tracking_beam --input_folder /path/to/data/ --taskid 200102003 --cb 0 --ra 05:34:32 --dec 22:00:52 --output tracking_beam.fits

The TB creation consists of two steps: 1. Calculate the required TABs at each frequency and time 2. Reorder the data from the input TAB FITS files and create a new FITS file containing the TB.

The results of step 1 can be saved to disk with --save_tab_indices. To only calculate the TAB indices and disable step 2 completely, use --no_fits_output. To generate the FITS output from a TAB indices file on disk, use--load_tab_indices /path/to/tab/index/file.txt. The script then loads the TAB indices and immediately goes to step 2.

There are a few more settings that can be customized. Run arts_create_tracking_beam -h for an overview of all options.

Creating a synthesised beam

A synthesised beam (SB) is a type of beam that reorders the TABs as function of frequency, but not as function of time. A single CB is covered by 71 SBs. Each SB is always made out of the same TABs. The SBs are used in the real-time transient search that ARTS runs. The brightest transients may also be detectable in the archival data, so we here include a tool to create the synthesised beams as well.

The synthesised beam tool, arts_create_synthesised_beam, works in a very similar fashion as the tracking beam tool. An example command:

arts_create_synthesised_beam --input_folder /path/to/data --sb 35

Run arts_create_synthesised_beam -h for more options.