Is the Crab Nebula a Standard Candle for Polarimetry ?
For many years, the flux from the whole Crab nebula was presumed to be constant,
in such a way that the Crab was considered a "standard candle". Some doubts were
cast on this status from high energy gamma-ray and hard X-ray observations made
by the Fermi and INTEGRAL satellites. Since 2007 strong high energy flaring
activities have been detected by the Agile and Fermi gamma-ray telescopes at a
rate of about 1 per year. Although, currently they have no clear origin, these
high energy flares show the complex timing behavior of this source.
In this context, Moran et al. (2015) published observations of the polarization
of optical and hard X-ray photons from the Crab nebula and pulsar system using
the Galway Astronomical Stokes Polarimeter (GASP) installed on the 200" Hale
telescope at Mount Palomar, the Hubble Space Telescope/Advanced Camera for
Surveys (HST/ACS) and the International Gamma-Ray Astrophysics Laboratory
satellite (Integral). These, when combined with other optical polarization
observations, suggest that the optical and gamma-ray polarized emission,
measured in the latest case with the so-called IBIS/Compton mode, have changed
in a similar manner, demonstrating that the Crab is also not a "polarimetric
calibration standard".
Indeed, a change in the optical polarization angle has been observed by this
work, from 109.5° in 2005 to 85.3° in 2012. On the other hand, the gamma-ray
polarization angle changed from 115° to 80° during a similar period. Strong flaring
activities at higher gamma-ray energies have been detected in the Crab nebula during
this period and magnetic reconnection processes have been suggested to explain these
observations. The change in the polarized optical and gamma-ray emission of the Crab
nebula/pulsar system as observed, for the first time, by GASP and Integral may
indicate that magnetic reconnection is possibly at work in the Crab nebula. Moran
et al. report also, for the first time, a non-zero measure of the optical circular
polarization from the Crab pulsar + knot system. These results outline the strong
scientific potential of polarimetric studies in particular in systems like the Crab
nebula where magnetic fields play a key role.
Legend: On the left, this HST image shows the inner part of the Crab nebula with
the Crab pulsar and its near-by knot located 0.65" (1300 AU) south-east of the pulsar
inside the red box. On the right, a zoomed view of the image is shown. The arrows
indicate at different periods in time the polarization angle in the optical
(HST in 2005 and GASP in 2012) and in hard X-rays (Integral in the period 2003-2007
and 2012-2014). Also indicated are the directions of the proper motion (PM) and spin
axis (SA) of the pulsar.
Reference :
"A recent change in the optical and γ-ray polarization of the Crab nebula
and pulsar"
P. Moran, G. Kyne, C. Gouiffès, P. Laurent, G. Hallinan, M. Redfern,
A. Shearer
http://arxiv.org/pdf/1511.07641v1.pdf, to be published in MNRAS
