The LOFAR Two-metre Sky Survey (LoTSS) V. Second data release (P334+18)

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) we present 120-168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44◦30′ and 1h00m +28◦ 00′ and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451 hrs (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 μ Jy/beam; 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. 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 μ Jy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8mJy/beam at 4′ and 2.2mJy/beam 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. This is field P334+18.

本数据集来自正在进行的低频阵列（LOw-Frequency ARray, LOFAR）两米巡天（Two-metre Sky Survey, LoTSS）数据发布项目，我们提供了覆盖北天27%天区的120–168 MHz射电图像。本次观测覆盖范围分为两个天区，中心位置分别约为赤经12时45分、赤纬+44°30′，以及赤经1时00分、赤纬+28°00′，对应天区面积分别为4178平方度与1457平方度。该批图像源自3451小时（7.6拍字节，PB）的LOFAR高带宽天线观测数据，在经过全面且完全自动化的数据处理流程后，我们对数据进行了与方向无关的仪器特性校正，以及与方向相关的电离层畸变校正。我们从总强度（斯托克斯I，Stokes I）图像中构建了包含4396228个射电源的源表，其中绝大多数在此前从未被射电波段探测到。在6角秒（6′′）分辨率下，中心频率为144 MHz的全带宽斯托克斯I连续谱图像具有如下参数：均方根（rms）灵敏度中位数为83微央斯基每波束（μJy/beam）；流量密度定标精度约为10%；天体测量精度达0.2角秒；我们估算，在峰值亮度为0.8毫央斯基每波束（mJy/beam）时，点源完整性可达90%。通过在全波段范围内生成三张带宽为16 MHz的图像，我们得以测量多数射电源的带内谱指数，但受限于流量密度定标精度与较小的相对带宽，所得谱指数的误差大于±0.2。我们的圆偏振（斯托克斯V，Stokes V）图像采用20角秒分辨率，覆盖120–168 MHz连续谱波段，其均方根灵敏度中位数为95微央斯基每波束；经估算，斯托克斯I到斯托克斯V的泄漏率为0.056%。我们的线偏振（斯托克斯Q、斯托克斯U，Stokes Q/U）图像数据立方体包含480个带宽为97.6 kHz的频谱平面；在4角分（4′）分辨率下，单平面均方根灵敏度中位数为10.8毫央斯基每波束，20角秒分辨率下则为2.2毫央斯基每波束；经估算，斯托克斯I到斯托克斯Q/U的泄漏率约为0.2%。本文中，我们对该数据集的各项参数进行了表征，并公开发布了斯托克斯I、Q、U、V图像与定标后的uv数据，以期推动该独特数据集的全面科学应用。本数据集所属天区编号为P334+18。