The Crab Nebula at sub-arcsecond resolution with the International LOFAR Telescope

We present International LOw Frequency ARray (LOFAR) Telescope (ILT) observations of the Crab Nebula, the remnant of a core-collapse supernova explosion observed by astronomers in 1054. The field of the Crab Nebula was observed between 120 and 168 MHz as part of the LOFAR Two-Metre Sky Survey (LoTSS), and the data were re-processed to include the LOFAR international stations to create a high-angular-resolution ($0.43'' 0.28''$) map at a central frequency of 145 MHz. Combining the ILT map with archival centimetre-range observations of the nebula with the Very Large Array (VLA) and LOFAR data at 54 MHz, we become sensitive to the effects of free-free absorption against the synchrotron emission of the pulsar wind nebula. This absorption is caused by the ionised filaments visible in optical and infrared data of the Crab Nebula, which are the result of the pulsar wind nebula expanding into the denser stellar ejecta that surrounds it and forming Rayleigh-Taylor fingers. The LOFAR observations are sensitive to two components of these filaments: their dense cores, which have electron densities of ≳1,000 cm^-3, and the diffuse envelopes, with electron densities of ∼50-250 cm^-3. The denser structures have widths of ∼0.03 pc, whereas the diffuse component is at one point as large as $0.2$ pc. The morphology of the two components is not always the same. These findings suggest that the layered temperature, density, and ionisation structure of the Crab optical filaments extends to larger scales than previously considered.