The polarized gamma-ray burst GRB 061122

Abstract

We report on the polarization measure, obtained with IBIS on board INTEGRAL, of the prompt emission of GRB 061122. Over an 8 s interval containing the brightest part of the gamma-ray burst (GRB) we put a lower limit on its polarization fraction of 60 per cent at 68 per cent confidence level (c.l.) and of 33 per cent at 90 per cent c.l. on the 250–800 keV energy range.

We performed late time optical and near-infrared imaging observations of the GRB field using the Telescopio Nazionale Galileo and the Canada–France–Hawaii Telescope. Our multiband (ugrizYJHK) photometry allowed us to identify the host galaxy of GRB 061122 and to build its spectral energy distribution. Using a photometric redshift code we fitted these data, and derived the basic properties of the galaxy, including its type and redshift, that we could constrain to the interval [0.57, 2.10] at a 90 per cent c.l., with a best-fitting value of z = 1.33.

The polarization measurement in different energy bands, together with the distance determination, allowed us to put the most stringent limit (ξ ≲ 3.4 × 10−16) to date to a possible Lorentz invariance violation based on the vacuum birefringence effect, predicted by some quantum-gravity theories.