The LOFAR Two-metre Sky Survey V. Second data release

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120-168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44 degrees 30' and 1h00m +28 degrees 00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6 '' resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 mu Jy beam(-1); a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2 ''; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beam(-1). By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of > +/- 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20 '' resolution 120-168 MHz continuum images have a median rms sensitivity of 95 mu Jy beam(-1), and we estimate a Stokes Ito Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 x 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mu Jy beam(-1) at 4' and 2.2 mu Jy beam(-1) at 20 ''; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.

Automated summary

This article presents the data release of LOFAR Two-metre Sky Survey, which covers 27% of the northern sky with 120-168 MHz images. The catalogue includes 4,396,228 radio sources, most of which have never been detected at radio wavelengths before. The images were derived from extensive and automated data processing, including correction for instrumental properties and ionospheric distortions. The release of Stokes I, Q, U, and V images, as well as calibrated uv-data, enables the thorough scientific exploitation of this unique dataset.