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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: alobanov@mpifr-bonn.mpg.de
² Institut für Experimentalphysik, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, Alan Turing Building, University of Manchester, Manchester M13 9PL, UK

Received: 14 April 2011
Accepted: 1 July 2011

Abstract

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  ≈ 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 02–06. 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 02. Combined with the broad-band SED of the flare, the radio properties of C1 yield a lower limit of  ≈ 0.5 mG for the magnetic field and a total minimum energy of 1.2 × 10⁴¹ 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 06 from the pulsar. The absolute position of the Crab pulsar is determined, and an estimate of the pulsar proper motion (μ_α =  −13.0 ± 0.2 mas/yr, μ_δ =  + 2.9 ± 0.1 mas/yr) is obtained.