The variability of the Crab nebula in radio: no radio counterpart to gamma-ray flares

Abstract

We present new Jansky Very Large Array (VLA) radio images of the Crab Nebula at 5.5 GHz, taken at two epochs separated by 6 days about two months after a gamma-ray flare in 2012 July. We find no significant change in the Crab's radio emission localized to a region of < 2 light-months in radius, either over the 6-day interval between our present observations or between the present observations and ones from 2001. Any radio counterpart to the flare has a radio luminosity of < ∼ 2 × 10 −4 times that of the nebula. Comparing our images to one from 2001, we do however find changes in radio brightness, up to 10% in amplitude, which occur on decade timescales throughout the nebula. The morphology of the changes is complex suggesting both filamentary and knotty structures. The variability is stronger, and the timescales likely somewhat shorter, nearer the centre of the nebula. We further find that even with the excellent u-v coverage and signal-to-noise of the VLA, deconvolution errors are much larger than the noise, being up to 1.2% of peak brightness of the nebula in this particular case.

Pulsar-Wind Nebulae

“…Similar to the SE X-ray jet, the counter-jet appears to be hardly discernible in the radio images (see e.g., right panel in Fig. 3) but it stands out in the 2-epoch difference image produced by Bietenholz et al (2014) and shown in the bottom right panel of Figure 1. Finally, the counter-jet is so faint in X-rays (if preset at all) that it cannot be discerned from the torus emission.…”

New expansion rate measurements of the Crab nebula in radio and optical

“…Since they are poorly localized in gammarays, we obtained radio observations of the Crab following a gamma-ray flare in 2012 August to look for a radio counterpart. Our initial results are reported in Bietenholz et al (2015): we did not find any such radio counterparts. We did, however, obtain high-quality radio images of the Crab.…”

“…As already mentioned, we use the 2012 radio observations described in Bietenholz et al (2015). We give here only a brief description of the observations: there were two sessions of VLA observations on 2012 August 20 and 26, spaced 6 days apart (observing code 12A-486).…”

The dust content of the Crab Nebula

“…We updated the VLA 1.4 GHz image from B1950 to J2000 coordinates. Due to the expansion of the supernova remnant, we have furthermore expanded the SNR's emission spatially by 3.11 % accounting for the expansion rate of 0.135 % per year (Bietenholz et al 2015) between the observing date (1987) and the reference epoch of WISE, Herschel and Planck observations (2010) 7 . Total integrated flux densities of 599.3 Jy and 833.77 Jy were inferred at 4.8 and 1.4 GHz, respectively.…”