*6.1. X-ray Flares*

X-ray flares—flaring and re-brightening behavior in the X-ray emission from GRBs were discovered by the Neil Gehrels *Swift* space observatory [243] and are detected in about a third of *Swift* GRBs [244–249]. They typically display a characteristic shape with a sharp rise in flux followed by a smoother decay, eventually fading back to the pre-flare flux level, and also show a different spectrum (typically harder) compared to the underlying emission. X-ray flares typically occur at times 10<sup>2</sup> s . *t* . 10<sup>5</sup> s after the GRB onset. Their temporal and spectral properties appear to be a smooth continuation of the prompt GRB emission spikes [247,248,250]. While during the prompt GRB emission the typical width or spectrum of the different spikes typically does not show a clear systematic evolution, the X-ray flares gradually become wider (with FWHM ∆*t* satisfying ∆*t*/*t* ∼ 0.1 − 0.3), less luminous (h*L*i ∝ *t* <sup>−</sup>2.7±0.1), and softer with time *t*. Their overall properties strongly suggest that X-ray flares have a common origin with the prompt GRB emission and likely share similar dissipation and/or emission mechanisms.

Therefore, studying the polarization properties of X-ray flares may provide new insights both for their origin, as well as on the emission and/or dissipation mechanisms that are common with the prompt emission. There are some theoretical predictions for their polarization properties e.g., [251,252], but there is still much room for more detailed and realistic predictions that could be tested against future observations. Their observed similarities to prompt GRB pulses suggests that many of the models for prompt GRB polarization may be generalized to apply also for X-ray flares. The fact that X-ray flares last up to hours or sometimes even days after the GRB onset allows pointed observations by sensitive instruments, while their softer spectrum makes them prime targets for future pointed X-ray polarimeters such as eXTP with a polarimetry focusing array at 2–10 keV energies e.g., [198,253].
