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

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.

Polarization and long-term variability of Sgr A* X-ray echo

“…Below we use the effective area and the modulation factor from Soffitta et al (2013). A similar question has already been addressed in Marin et al (2015) based on the assumed positions of several clouds. In our simulations the uncertainties in the choice of the 3D gas 6 Note, that the degree of polarization depends on the cosine of the scattering angle, i.e.…”

“…There were many studies that consider the reflection scenarios in the GC region (e.g., Murakami et al 2000;Molaro, Khatri, & Sunyaev 2016;Marin et al 2015). Here we return to this problem to emphasize the power of long-term monitoring of the reflected emission.…”

Probing the clumping structure of giant molecular clouds through the spectrum, polarisation and morphology of X-ray reflection nebulae

“…Scattered flux from Sgr B2 in hard X-rays has also been detected using Integral (Revnivtsev et al 2004). Similarly, other GMCs in the CMZ (Murakami et al 2001;Marin et al 2015) have since been shown to act as XRNe. An alternative scenario to the reflection of past Sgr A * flares, which explains the 6.4 keV emission in Sgr B2 as the result of the interaction of low-energy cosmic ray electrons with matter in the GMCs, has also been proposed (Valinia et al 2000;Yusef-Zadeh et al 2002;Yusef-Zadeh 2013;Dogiel et al 2009).…”

The Growth of Interest in Astronomical X-Ray Polarimetry

“…In the latter case, the very low X-ray fluxes, coupled with relatively low or inexistent polarization from galaxy clusters, naturally explains why the publication rate in this field is the lowest. Until the advent of extra-sensitive X-ray polarimeters, it is not mandatory to explore in greater detail the expected polarization of quiescent galaxies (save the Milky Way, where important and feasible observations await us [22][23][24]). Several subcategories are highlighted: the black solid line represents general publications on X-ray polarimetry (reviews, historical notes); violet stands for publications concerning theory, instrumentation, and satellites, red for papers on solar, stellar, and exoplanet science; orange for papers on objects dominated by strong gravity effects (e.g., black holes); yellow for objects dominated by strong magnetic fields (e.g., neutron stars); the green solid line stands for papers related to galaxies (e.g., galaxy clusters or the Milky Way).…”