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

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 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 ≈0.5 mG for the magnetic field and a total minimum energy of 1.2 × 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 (μ α = −13.0 ± 0.2 mas/yr, μ δ = +2.9 ± 0.1 mas/yr) is obtained.

H.E.S.S. observations of the Crab during its March 2013 GeV gamma-ray flare

“…The upper limits on the enhancement of the Crab flux are far above what is expected for the TeV energy range from some models, which predict enhancement factors of at most 1.01, as described above referring to Lobanov et al (2011). In scenarios as in Bednarek & Idec (2011) or Kohri et al (2012, however, enhancement factors of 2 or more are possible, exceeding the upper limits presented here.…”

“…Since a flare in the MeV energy band is expected to be accompanied by an enhanced flux at tens of TeV (Lobanov et al 2011), a search for variations in the flux above different energy thresholds was conducted. Integral fluxes above 1 TeV and 5 TeV were calculated for the night-wise samples (see Fig.…”

“…While in the latter case, the inverse-Compton (IC) component will largely remain unchanged, in the former, the flare observed at a synchrotron energy E syn is accompanied by a flare at a corresponding energy E IC of IC scattered ambient photons. The apparent observed energy E syn of a few hundred MeV exceeds the maximum achievable energy of synchrotron radiation from shock-accelerated electrons/positrons (Guilbert et al 1983;de Jager et al 1996;Lyutikov 2010). This observation indicates the presence of a mild Doppler boost or a different acceleration mechanism altogether (Lyutikov 2010;Cerutti et al 2013).…”

“…The apparent observed energy E syn of a few hundred MeV exceeds the maximum achievable energy of synchrotron radiation from shock-accelerated electrons/positrons (Guilbert et al 1983;de Jager et al 1996;Lyutikov 2010). This observation indicates the presence of a mild Doppler boost or a different acceleration mechanism altogether (Lyutikov 2010;Cerutti et al 2013). Observations at the VHE band during flaring episodes provide additional information on the conditions in the emission region (e.g.…”

Pulsar-Wind Nebulae

“…In particular, Komissarov and Lyutikov (2011) argued that the flares may originate from the emission from the oblique shock Doppler-boosted towards the observer, that they associate with the well-known bright compact structure near the pulsar (the so-called "inner knot"). Unfortunately, the brightness of the knot does not show any variations correlated with the gamma-ray flares (see Tavani et al 2011;Lobanov et al 2011;Weisskopf et al 2013 and references therein) contrary to what the model predicts.…”