New hard X-rays and optical polarimetric observations of the Crab nebula and pulsar

Polarisation is a powerful tool to study compact objects where high-energy particle acceleration processes and magnetic fields play a major role. Joining optical and high-energy polarisation data, whilst challenging, should develop a better understanding of the source acceleration processes and magnetic fields. A recent study of the polarisation from the Crab nebula and pulsar in hard X-ray with the International Gamma-Ray Astrophysics Laboratory satellite (INTEGRAL) and in optical with the Galway Astronomical Stokes Polarimeter (GASP) indicated similar variations in the phase average polarisation signal observed between 2005 and 2012. This result was tentatively proposed to evoke that magnetic reconnection was at work in the inner part of the Crab nebula, a physical process suggested to explain the strong flaring activities observed in the past years with the high energy gamma-ray missions Agile and Fermi. We describe here new and higher quality GASP optical data obtained in December 2015 at the WHT telescope at La Palma during nearly simultaneous hard X-rays INTEGRAL/IBIS observations. Our result points to a polarisation angle value higher than that perviously measured, confirming that the inner core of the Crab nebula is variable and cannot be considered as a polarimetric standard.