Reflection nebulae in the Galactic center: soft X-ray imaging polarimetry

¹ Astronomical Institute of the Academy of Sciences, Boční II 1401, 14100 Prague, Czech Republic
e-mail: frederic.marin@asu.cas.cz
² INAF/IAPS, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 14 November 2014
Accepted: 17 February 2015

Abstract

Context. The origin of irradiation and fluorescence of the 6.4 keV bright giant molecular clouds surrounding Sgr A^∗, the central supermassive black hole of our Galaxy, remains enigmatic despite numerous attempts to decipher it with spectroscopic and timing analyses.

Aims. Testing the theory of a past active period of Sgr A^∗ requires opening a new observational window: X-ray polarimetry. In this paper, we aim to show how modern imaging polarimeters could revolutionize our understanding of the Galactic center (GC).

Methods. Through Monte Carlo modeling, we produced a 4−8 keV polarization map of the GC. We focused on the polarimetric signature produced by Sgr B1, Sgr B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and Sgr C1. We estimated the resulting polarization that arises from these scattering targets, included polarized flux dilution by the diffuse plasma emission detected toward the GC, and simulated the polarization map that modern polarimetric detectors would obtain assuming the performances of a mission prototype.

Results. The eleven reflection nebulae we investigated present a variety of polarization signatures, ranging from nearly unpolarized to highly polarized (~77%) fluxes. Their polarization position angle is found to be normal to the scattering plane, as expected from previous studies. A major improvement in our simulation is the addition of a diffuse, unpolarized plasma emission that strongly affects soft X-ray polarized fluxes. The dilution factor is in the range 50%−70%, making the observation of the Bridge structure unlikely even in the context of modern polarimetry. The best targets are the Sgr B and Sgr C complexes and the G0.11-0.11 cloud, arranged in the order of decreasing detectability.

Conclusions. An exploratory observation of a few hundred kilo-seconds of the Sgr B complex would allow a significant detection of the polarization and be sufficient to derive indications of the primary radiation source. A more ambitious program (few Ms) of mapping the giant molecular clouds could then be carried out to probe the turbulent history of Sgr A^∗ with great precision and place important constraints on the composition and three-dimensional position of the surrounding gas.