Presentation #100.72 in the session AGN.
Some theories of quantum gravity predict that Lorentz invariance may be broken at the Planck scale, with tiny residual deviations at attainable energy scales. Possibly observable effects include a wavelength-dependence of the speed of light and vacuum birefringence. Astrophysical searches for violations of Lorentz symmetry present some of the most powerful tests, and amongst those measurements of the polarization of light from distant objects are most sensitive. In fact, the most stringent constraints come from measurements of X-ray polarization of gamma-ray bursts. However, individual observations cannot reveal the full picture because any CPT-preserving model (with massless photons) that induces birefringence must necessarily be anisotropic. Therefore, multiple observations of different objects are required to fully constrain a given model. The Standard-Model Extension provides an effective field theory framework to characterize different Lorentz and CPT violating models.
The Imaging X-ray Polarimetry Explorer satellite has observed a large number of sources during its first year of operation, including several extragalactic objects with significant X-ray polarization. Here, I present limits on Lorentz invariance violation derived from published results, and combine them with previously derived constraints at other wavelengths. This results in the most stringent constraints on Lorentz invariance violation of the d=4,5,6 coefficients of the Standard-Model Extension.