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.