VLBI imaging of a flare in the Crab nebula: more than just a spot

We report on very long baseline interferometry (VLBI) observations of the radio emission from the inner region of the Crab nebula, made at 1.6 GHz and 5 GHz after a recent high-energy flare in this object. The 5 GHz data have provided only upper limits of 0.4 milli-Jansky (mJy) on the flux density of the pulsar and 0.4 mJy/beam on the brightness of the putative flaring region. The 1.6 GHz data have enabled imaging the inner regions of the nebula on scales of up to approximate 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 +/- 0.3 mJy 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 +/- 0.2 mJy 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 approximate to 0.5 mG for the magnetic field and a total minimum energy of 1.2 x 10(41) erg vested in the flare (corresponding to using about 0.2% of the pulsar spin-down power). The 1.6 GHz observations provide upper limits for the brightness (0.2 mJy/beam) and total flux density (0.4 mJy) 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 (mu(alpha) = -13.0 +/- 0.2 mas/yr, mu(delta) = +2.9 +/- 0.1 mas/yr) is obtained.