X-ray polarimetry of faint extended sources: the case of the molecular clouds in the Galactic center

The NASA/ASI Imaging X-ray Polarimetry Explorer (IXPE) will be launched in Spring 2021, and will allow to perform for the first time, thanks to its imaging-capable detectors, spatially resolved X-ray polarimetry of faint, extended sources. One of the most challenging, yet rewarding, targets are expected to be the molecular clouds in the Galactic center. X-ray polarimetry could be indeed the key to understanding the origin of the observed X-ray reflection spectra of these clouds, that is thought to be the echo of a past outburst from the supermassive black hole Sgr A*. In this scenario, the X-ray emission of the molecular clouds is predicted to be strongly polarized through Compton scattering, with the polarization angle carrying a memory of the direction of the external illuminating source. The clouds are however embedded in a diffuse, collisionally-ionized plasma that is expected to lower the overall observed polarization fraction. Morover, due to their low luminosity, the effect of the instrumental and diffuse background must be carefully evaluated. We present simulations of IXPE observations of the molecular clouds in the Galactic center. Our simulations include the plasma component as well as the instrumental background and cosmic X-ray background. We obtain realistic estimates of the dilution of the polarized signal and of the necessary exposure times as well as the impact of the background on the observability of the targets by IXPE and eXTP. These pieces of information are crucial for designing any future X-ray polarization observations of this intriguing Galactic region.