Light-Bending Scenario for Accreting Black Holes in X-Ray Polarimetry

Muleri

Soffitta

et al. 2015

A&A

Self Cite

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.

Section: Polarization Dilution By the Gc Diffuse Plasma Emission And mentioning

confidence: 99%

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

Muleri

Soffitta

et al. 2015

A&A

Self Cite

“…Estimations on the detected polarization P detect. and the error on ψ are calculated from Monte Carlo simulations, such as presented in Dovčiak et al (2011), and are derived from the generation of test modulation curves. The expected initial polarization P source is taken from our modelling of the GC examined in Section 2.2.…”

Section: Detectability With Available Technologymentioning

confidence: 99%

Prospects of 3D mapping of the Galactic Centre clouds with X-ray polarimetry

Monthly Notices of the Royal Astronomical Society

Karas

Kunneriath

et al. 2014

Self Cite

Despite past panchromatic observations of the innermost part of the Milky Way, the overall structure of the Galactic Centre (GC) remains enigmatic in terms of geometry. In this paper, we aim to show how polarimetry can probe the three-dimensional position of the molecular material in the central ∼100 pc of the GC. We investigate a model where the central supermassive black hole Sgr A * is radiatively coupled to a fragmented circumnuclear disc (CND), an elliptical twisted ring representative of the central molecular zone (CMZ), and the two main, bright molecular clouds Sgr B2 and Sgr C. 8-35 keV integrated polarization mapping reveals that Sgr B2 and Sgr C, situated at the two sides of the CMZ, present the highest polarization degrees (66.5 and 47.8 per cent, respectively), both associated with a polarization position angle ψ = 90• (normal to the scattering plane). The CND shows a lower polarization degree, 1.0 per cent with ψ = −20.• 5, tracing the inclination of the CND with respect to the Galactic plane. The CMZ polarization is spatially variable. We also consider a range of spatial locations for Sgr A * and the reprocessing media, and investigate how the modelled three-dimensional geometry influences the resulting GC polarization. The two reflection nebulae are found to always produce high polarization degrees ( 10 per cent). We show that a 500 ks observation with a broad-band polarimeter could constrain the location and the morphology of the scattering material with respect to the emitting source, revealing the past activity of Sgr A * .

“…According to Connors & Stark (1977) and Connors et al (1980), the polarization position angle of radiation will change non-linearly according to the physical parameters of the model. Using general relativistic routines (Dovčiak et al 2011), we will investigate to what degree a polarized primary source will affect the net polarization detected by a future X-ray polarimeter.…”

Section: Discussionmentioning

confidence: 99%

X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model

Goosmann

Petrucci

2016

A&A

Self Cite

Context. Obscuring circumnuclear dust is a well-established constituent of active galactic nuclei (AGN). Traditionally referred to as the receding dusty torus, its inner radius and angular extension should depend on the photo-ionizing luminosity of the central source. Aims. We quantify the expected time-dependent near-infrared (NIR), optical, ultraviolet (UV) and X-ray polarization of a receding dusty torus as a function of the variable X-ray flux level and spectral shape. Methods. Using a Monte Carlo approach, we simulate the radiative transfer between the multiple components of an AGN adopting model constraints from the bright Seyfert galaxy NGC 4151. We compare our model results to the observed NIR to UV polarization of the source and predict its X-ray polarization. Results. We find that the 2−8 keV polarization fraction of a standard AGN model varies from less then a few percent along polar viewing angles up to tens of percent at equatorial inclinations. At viewing angles around the type-1/type-2 transition, there is a different X-ray polarization variability in a static or a receding torus scenario. In the former case, the expected 2−8 keV polarization of NGC 4151 is found to be 1.21% ± 0.34% with a constant polarization position angle, while in the latter scenario it varies from 0.1% to 6% depending on the photon index of the primary radiation. Additionally, an orthogonal rotation of the polarization position angle with photon energy appears for very soft primary spectra. Conclusions. Future X-ray polarimetry missions will be able to test whether the receding model is valid for Seyfert galaxies seen at a viewing angle close to the torus horizon. The overall stability of the polarization position angle for photon indexes softer than Γ = 1.5 ensures that reliable measurements of X-ray polarization are possible. We derive a long-term observational strategy for NGC 4151 assuming observations with a small to medium-sized X-ray polarimetry satellite.