LOTSS DR2 - Where to find the data

User Interfaces

The LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) Data Release 2 (LoTSS DR2) is hosted on the SURF Data Repository, and exposed to the community through the standard Virtual Observatory protocols. This section describes how the LoTSS DR2 data products can be queried and accessed by using different applications.

Introduction

The science-ready data products as described in Available data products are exposed through standard Virtual Observatory protocols to facilitate their access and exploration.

The LoTSS DR2 data is hosted on the SURF Data Repository (sdr) system. We recommend astronomers to use the VO interfaces described below for data discovery. Some of the data is stored on tape. This is indicated in the data description in the DataLink document that is returned by the VO query. To access files on tape, users need to obtain an account which gives them the possibility to "stage" the data from the tape to a disk from which it can be downloaded. Data that is stored on disk is directly accessible through the VO.

In particular, the protocols offered are the Tabular Access Protocol (TAP), Simple Application Messaging Protocol (SAMP) and the Simple Image Access protocol (SIA). TAP and SAMP enables queries to explore the data in a tabular form using tools such as TOPCAT. TOPCAT is an interactive graphical viewer and editor for tabular data, it enables the interactive exploration of large tables performing several types of plotting, statistics, editing and visualization of tables. SIA enables the rapid display of images and cubes through all sky atlas tools such as ALADIN. ALADIN is an interactive sky atlas allowing the user to visualize digitized astronomical images/cubes and superimpose entries from astronomical catalogues or databases.

The data published in the VO can also be accessed using a web browser at https://vo.astron.nl. This web interface provides a page on which all the collections present in the registry are listed, including the published LoTSS DR2 data sets:

Selecting a data collection allows the user to perform a cone search through a webform (Fig. 40). The result is either a source or Gaussian list, or a table of data products of that given class overlapping a given pointing. The size of the continuum images as well as the cubes extend beyond the 10% primary beam level for cleaning the secondary lobes of bright offset sources. To ensure that the search is done in the area of maximum sensitivity the search is performed on a maxim radius of 0.75 degrees from the center (this represents the average value of where the sensitivity drops). This value can be modified using the Max distance from center. A different output with respect to the default can be customized using More output fields selection button.

Fig. 40 Query search form for continuum images.

The result is a table in the requested output format in which every row corresponds to a data product (Fig. 41).

In each row there is a column, Product key, which is a link that allows the user to download the fits file of the image of interest. The column titles should generally be self-descriptive. However, the long human-readable description of the content of each column is a tooltip that will appear when hovering over the column name.

The selected target and the position of the individual pointings can be visualized using the Quick plot button at the top of the window of the results of the search query (Fig. 41).

In the column Related products another link connects to a page containing a list of links to additional related data that can be useful to interpret or reanalyze that given product, for which a preview is provided (Fig. 42).

Fig. 41 Result of an image search query. Click for a bigger image.

Fig. 42 Links to data products related to the target of interest. The top two items represent the primary data product (ie the one that can be directly downloaded from the table view) and a link to the thumbnail of that data product. The other products are the anciliary data. Click for a bigger image. Note that the number of related products is too long to fit readably in a single screen shot.

Some data products are directly accessible, in which case the link on this table will initiate a direct download. However, the larger (and rawer) data products are stored on tape (in which case the text 'on tape' appears in the description). The link will then bring you to the entry of the corresponding pointing in the SURF Data repository (Fig. 43/1). If data is on tape, the status is either "online", in which case it can be downloaded by pressing the "Download" button, or "offline" in which case it can be requested to be put online by pressing the "Request" button. Please note that you need to be logged in to perform the request. If a request has been correctly performed, the status of the file will change to "staging" unill it becomes "online". 

Fig. 43/1 Surf Data Repository view of the data from a pointing. Click for a bigger image.

The source and Gaussian cone search forms each return a table with source positions and properties. As before, the long descriptions are available using tool tips. The columns "Mosaic_URL" links to the anciliary data product page of the mosaic from where the Gaussian or source was extracted (like e.g. Fig 42).

Data Usage Policy

Papers making use of this data should include the following acknowledgement:

This work makes use of data from the Apertif system installed at the Westerbork Synthesis Radio Telescope owned by ASTRON. ASTRON, the Netherlands Institute for Radio Astronomy, is an institute of the Dutch Research Council (“De Nederlandse Organisatie voor Wetenschappelijk Onderzoek, NWO).

In addition, they should include the LoTSS DR2 PID:

CHECK DOI

and cite the imaging survey data release paper. This is not yet available but should be soon. Until then, it can be referenced as:

Shimwell, T.W., et al. “The LOFAR Two-metre Sky Survey (LoTSS). V. Second data release" accepted for publication in Astronomy & Astrophysic