Optical polarimetry of the inner Crab nebula and pulsar

Abstract

Time-resolved polarisation measurements of pulsars offer an unique insight into the geometry of their emission regions. Such measurements provide observational constraints on the different models proposed for the pulsar emission mechanisms. Optical polarisation data of the Crab Nebula was obtained from the HST archive. The dataset consists of a series of observations of the nebula taken with the HST/ACS. We produced polarisation vector maps of the inner nebula and measured, for the first time, the degree of linear polarisation (P.D.) and the position angle (P.A.) of the pulsar's integrated pulse beam, and of its nearby synchrotron knot. This yielded P.D. = 5.2 ± 0.3% and P.A. = 105.1 ± 1.6 • for the pulsar, and P.D. = 59.0 ± 1.9% and P.A. = 124.7 ± 1.0 • for the synchrotron knot. This is the first high-spatial resolution multi-epoch study of the polarisation of the inner nebula and pulsar. None of the main features in the nebula show evidence of significant polarisation evolution in the period covered by these observations. The results for the pulsar are consistent with those obtained by S lowikowska et al. (2009) using the high-time resolution photo-polarimeter OPTIMA, once the DC component has been subtracted. Our results clearly prove that the knot is the main source of the DC component.

Pulsar-Wind Nebulae

“…This emission comes from the immediate vicinity of the termination shock, it is highly Doppler beamed and originates in the high-speed part of the post-shock flow. A correlation was found between the knot position and the flux, such that brighter states correspond to a smaller offset between the knot and the location of the pulsar, which is in excellent agreement with the recent optical observations of Crab's knot (Moran et al 2013). The simulated polarization degree and polarization angle in the knot also agree with observations.…”

“…This naturally explains the observed high degree of polarization in X-rays, optical, and radio (Moran et al 2013;Bietenholz et al 2004;Weisskopf et al 1978). Multiwavelength observations and modeling show that IC scattering on Cosmic Microwave Background and NIR/IR background starts to dominate PWN spectra in GeV (Acero et al 2013) and completely dominates synchrotron at TeV energies (Atoyan and Aharonian 1996, Volpi et al 2008, Gelfand et al 2009, and Olmi et al 2014; see the Crab and Vela PWN spectra in Fig.…”

VLT polarimetry observations of the middle-aged pulsar PSR B0656+14

“…The value (125.8 • ± 13.2 • ) is close to the PA of the pulsar proper motion (93.12 • ± 0.38 • ; Brisken et al 2003). Since the difference between the two angles is within 3σ, an alignment between the two vectors is possible, as clearly seen in the Crab pulsar (Moran et al 2013) and, approximately, in the Vela pulsar also (Moran et al 2014). Evidence of alignment between the polarisation and proper motion vectors is also found at radio wavelengths (Johnston et al 2005;Noutsos et al 2012).…”

“…We investigated possible correlations between the phase-averaged optical PD and the pulsar characteristic age τ, the spin period P s and its derivativeṖ s , the spin-down powerĖ, and the pulsar magnetic field measured both at the surface (B s ) and at the light cylinder (B LC ). For both the Crab and Vela pulsars, we assumed the recent HST measurements (Moran et al 2013(Moran et al , 2014 as a reference. We did not include the VLT polarisation measurements of PSR B0540−69 and PSR B1509−58 (Wagner & Seifert 2000), which have no associated errors.…”

“…Owing to their faintness, linear optical polarisation measurements only exist for a handful of pulsars, whereas the circular polarisation was only measured for the Crab pulsar (Wiktorowicz et al 2015). The young (∼960 year old) and bright (V ∼ 16.5) Crab pulsar (PSR B0531+21) is the only one for which precise measurements have been repeatedly obtained, both phase-resolved (e.g., Słowikowska et al 2009) and phase-averaged (e.g., Moran et al 2013). Interestingly, the phase-averaged optical polarisation position angle is similar to that of the pulsar proper motion direction and close to that of the axis of symmetry of the X-ray arcs and jets of the pulsar-wind nebula (PWN) observed by Chandra (e.g.…”