A recent change in the optical and γ-ray polarization of the Crab nebula and pulsar

Abstract

We report on observations of the polarization of optical and γ-ray photons from the Crab nebula and pulsar system using the Galway Astronomical Stokes Polarimeter (GASP), 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 polarized optical emission and γ-ray polarization changes in a similar manner. A change in the optical polarization angle has been observed by this work, from 109.5 ± 0.7 • in 2005 to 85.3 ± 1.4 • in 2012. On the other hand, the γ-ray polarization angle changed from 115 ± 11 • in 2003-2007 to 80 ± 12 • in 2012-2014. Strong flaring activities have been detected in the Crab nebula over the past few years by the high energy γ-ray missions Agile and Fermi, and magnetic reconnection processes have been suggested to explain these observations. The change in the polarized optical and γ-ray emission of the Crab nebula/pulsar as observed, for the first time, by GASP and Integral may indicate that reconnection is possibly at work in the Crab nebula. We also report, for the first time, a non-zero measure of the optical circular polarization from the Crab pulsar+knot system.

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

“…Our combined 2015 observing campaign indicates that in both optical and hard X-rays a polarisation angle with a value higher than previously measured (see Figure 4).The statistical significance of the INTEGRAL point is modest (estimated to have an equivalent 3 σ detection level (see (6) for a discussion on the INTEGRAL polarigram errors), but consistent with the value derived from the WHT+GASP observation. It is worthwhile to mention that concerning the INTEGRAL/IBIS polarisation angle value, data taken by the ASTROSAT mission at the same epoch and in a similar energy range similar, is consistent with our result (D. Bhattacharya, this conference).…”

“…X-ray, Optical and Radio observations did not show any significant flux variation across the nebula (5) during these flaring events and no change of the pulsar's temporal properties was observed. A change in the polarised optical and hard X-rays/γ-ray emission of the Crab nebula/pulsar was recently mentioned thanks to a joint study in Optical with the GASP polarimeter and in hard X-rays with the INTEGRAL/IBIS telescope (6). The similar change observed possibly indicates a common origin and triggered new observing campaigns.…”

The PoGO+ Balloon-Borne Hard X-ray Polarimetry Mission

“…Taken at face value, published polarimetric results for the Crab from different instruments indicate an increase of polarization fraction as a function of energy. However, recent results from AstroSat [15] are in agreement with PoGO results [14], which do not show a drastic increase as seen by INTEGRAL IBIS [12,13]. This strain illustrates the difficulty of deriving polarimetric results, polarization fraction in particular, from instruments not calibrated as polarimeters.…”

“…At low energies, instruments on the OSO-8 satellite measured polarization at 2.6 and 5.2 keV already in the 1970s [9]. For the higher energies of several hundred keV, results are reported from the INTEGRAL SPI [10,11], and INTEGRAL IBIS [12,13] instruments. The reliability of the INTEGRAL results may, however, be limited by the fact that these instruments were not designed as polarimeters and were not calibrated for polarimetry in ground testing campaigns.…”

e-ASTROGAM mission: a major step forward for gamma-ray polarimetry

“…53,54 Combined polarimetric observations of the Crab pulsar/nebula system with INTEGRAL in hard X-rays and in the optical band with the Galway Astronomical Stokes Polarimeter (GASP) optical polarimeter have led to the suggestion that the polarization angle of both the visible and high energy light changed in a similar way over a year-long timescale. 59 This result was tentatively attributed to magnetic reconnection events as observed in the solar corona or detected in AGN 30 during bright high-energy flares. The e-ASTROGAM performance will definitively permit simultaneous MeV-optical observations, allowing a detailed study of these variations on a shorter timescale (Figure 8).…”