Prospects for a polarimetric mapping of the Sgr A molecular cloud complex with IXPE

Abstract

Context. The X-ray polarization degree of the molecular clouds surrounding Sgr A* is expected to be greatly reduced due to the mixing of the polarized reflection emission with the unpolarized thermal emission that pervades the Galactic center region. This makes this type of observation a challenging experiment for the upcoming Imaging X-ray Polarimeter Explorer (IXPE), whose launch is expected in 2021. Aims. We aim to determine the detectability of four molecular clouds of the Sgr A complex (MC2, Bridge B2, Bridge E, and G0.11-0.11) in a realistic IXPE pointing of the Sgr A field of view. We assess the minimum increase in the detectable polarization when a molecular cloud is off-axis. We provide two different strategies for reconstructing the intrinsic cloud polarization once the data is available. Methods. We used the Monte Carlo tool ixpeobssim to simulate IXPE observations of the Sgr A molecular cloud complex. We used Chandra maps and spectra to model the diffuse emission in the Galactic center region along with a realistic model of the instrumental and diffuse background. We created synthetic polarization products of the unpolarized emission and combined them with a test data set obtained from a simulation of a 2 Ms long IXPE observation to retrieve the intrinsic polarization degree of the molecular clouds. Results. We find that for the molecular clouds considered here, the minimum detectable polarization (MDP) increases by ~1–15% with respect to the case in which a cloud is observed on-axis. We successfully retrieve the intrinsic polarization degree in the 4.0–8.0 keV band and line-of-sight distance of one of them taken as an example, namely, G0.11-0.11, by correcting the observed (i.e., for a 2 Ms-long simulation) polarization degree map using either a synthetic dilution map or a Stokes intensity map of the unpolarized emission. With both methods, the position of the cloud along the line-of-sight is derived from the reconstructed polarization degree with an uncertainty of 7 and 4 pc, respectively. Conclusions. We confirm the results of previous studies, finding that G0.11-0.11 is the most promising target. For the Sgr A molecular complex region, we propose an observation strategy that may permit detection up to three clouds in the 4.0–8.0 keV band, depending on their true line-of-sight position. We demonstrate that by using simulated data products of the unpolarized components, it is possible to clean up the observed polarization maps from the environmental contamination. The methods we present here are potentially useful for the analysis of X-ray polarimetric data of any extended source that is affected by environmental dilution of the polarized signal. To accurately measure (i.e, with uncertainties of the order of a few parsec) the distance of the cloud along the line-of-sight, a high-quality spectrum and image of the clouds quasi simultaneous to the IXPE pointing are needed.

X-Ray Polarimetry and IXPE mission

“…It becomes more deep in the proximity of IXPE launch. [28][29][30]. Eventually a field in the complex Sgr A was selected as the most promising from the point of view of signal to noise ratio.…”

Testing particle acceleration models for BL Lac jets with the Imaging X-ray Polarimetry Explorer

“…For the shock acceleration model, the polarization angle is constant over time. In the case of bright point-like sources such as blazars, the sky and instrumental background are negligible (see e.g., Di Gesu et al 2020;Ferrazzoli et al 2021), thus, these parameters were not included in our simulations.…”

When the Milky Way hosted a quasar

“…1 (technical details about the data reduction of the IXPE and Chandra observations are described in the Methods section). We excluded the 2-4 keV band as it is entirely dominated by unpolarized plasma emission [21][22][23].…”