2011A&A...533A..10L

We report on very long baseline interferometry (VLBI) observations of the radio emission from the inner region of the Crab nebula, made at 1.6GHz and 5GHz after a recent high-energy flare in this object. The 5GHz data have provided only upper limits of 0.4milli-Jansky (mJy) on the flux density of the pulsar and 0.4mJy/beam on the brightness of the putative flaring region. The 1.6GHz data have enabled imaging the inner regions of the nebula on scales of up to ≃40". The emission from the inner ``wisps'' is detected for the first time with VLBI observations. A likely radio counterpart (designated ``C1'') of the putative flaring region observed with Chandra and HST is detected in the radio image, with an estimated flux density of 0.5±0.3mJy and a size of 0.2"-0.6". Another compact feature (``C2'') is also detected in the VLBI image closer to the pulsar, with an estimated flux density of 0.4±0.2mJy and a size smaller than 0.2". Combined with the broad-band SED of the flare, the radio properties of C1 yield a lower limit of ≃0.5mG for the magnetic field and a total minimum energy of 1.2x10⁴¹ erg vested in the flare (corresponding to using about 0.2% of the pulsar spin-down power). The 1.6GHz observations provide upper limits for the brightness (0.2mJy/beam) and total flux density (0.4mJy) of the optical Knot 1 located at 0.6" from the pulsar. The absolute position of the Crab pulsar is determined, and an estimate of the pulsar proper motion (µ_α=-13.0±0.2mas/yr, µ_δ= +2.9±0.1mas/yr) is obtained.