Polarization of reflected X-ray emission from the Sgr A molecular complex: multiple flares, multiple sources?

TL;DR

This study tests the X-ray echo model for CMZ molecular clouds by combining IXPE polarization measurements with Chandra imaging and spectroscopy. The large Sgr A region shows polarization consistent with illumination by Sgr A*, constraining the flare age to ~190 years and the source direction; however, the central bright region exhibits an almost perpendicular polarization direction, suggesting either a different illuminating source or multiple simultaneous fronts. The results motivate a multi-source, multi-flare illumination picture and highlight the need for deeper, higher-resolution polarimetry and micro-calorimetric spectroscopy to disentangle geometry, timing, and polarization effects. If confirmed, this complex illumination would map the 3D CMZ structure and the past activity of the Galactic Center with unprecedented detail, guiding future high-energy polarimetry missions.

Abstract

The extended X-ray emission observed in the direction of several molecular clouds in the central molecular zone of our Galaxy exhibits spectral and temporal properties consistent with the X-ray echo scenario. This concept postulates that the observed signal is a light-travel-time delayed reflection of a short ($δt<$1.5 yr) and bright ($L_{\rm X}>10^{39} {\rm erg s^{-1}}$) flare that was most probably produced a few hundred years ago by Sgr A*. This scenario predicts a distinct polarization signature for the reflected X-ray continuum, with the polarization vector being perpendicular to the direction toward the primary source and the polarization degree being determined by the scattering angle. We report the results of two deep observations of the currently brightest (in reflected emission) molecular complex Sgr A taken with the Imaging X-ray Polarimetry Explorer in 2022 and 2023. We confirm the previous polarization measurement for a large region encompassing the Sgr A complex with high significance. We reveal an inconsistent polarization pattern for the brightest reflection region in its center. The X-ray polarization from this region is almost perpendicular to the expected direction in the case of Sgr A* illumination and shows smaller degree of polarization compared to the large region. This could indicate the simultaneous propagation of several illumination fronts throughout the CMZ, with the origin of one of them not being Sgr A*. The primary source could be associated with the Arches stellar cluster or a currently unknown source located closer to the illuminated cloud, potentially lowering the required luminosity of the primary source. Although significantly deeper observations with IXPE would be required to unequivocally distinguish between the scenarios, a combination of high-resolution imaging and micro-calorimetric spectroscopy offers an additional promising path forward.

Figures (13)

• Figure 1: Sketch illustrating the polarization of the X-ray emission arising due to scattering of an intrinsically unpolarized short flare. The coordinate system is chosen so that the primary source, Sgr A*, is located at the origin, $z$-axis points from the observer toward it, and the $xz$-plane contains the center of the reflecting molecular complex (gray sphere). The line of sight and image plane positions of the cloud are connected by the parabolic relation (Eq. \ref{['eq:parabola']}). For an intrinsically unpolarized primary source, the electric field vector of the reflected emission is perpendicular to the scattering (i.e., $xz$) plane, and the polarization degree (color coded) is set by the cosine of the scattering angle $\theta$. The inset shows the definition of the Stokes parameters used throughout the paper (with $+Q$ and $+U$ pointing toward the celestial north and northeast, respectively) and the predicted direction of electric field vector (red arrow).
• Figure 2: Chandra images of the Sgr A complex in the 3--8 keV band taken at different epochs starting from 2013 to 2023. The images have been smoothed with a $\sigma=5"$ Gaussian to highlight the morphology of the diffuse emission (${\rm ct~s^{-1}~pixel^{-1}}$, linear scale, Galactic coordinates). The epochs marked as IXPE-1 and IXPE-2 most closely correspond to the two epochs of IXPE observations in 2022 and 2023. The large circle has a 4.5$\arcmin$ radius and was used in Paper I for the extraction of the polarization signal after masking the region around the pulsar and related nonthermal nebula G0.13-0.11 (marked with "nt" and a dashed circle of 1$\arcmin$ radius). The brightest region of the Sgr A complex is marked with a small solid circle ("$r=1\arcmin$," also 1$\arcmin$ radius). The location of the Arches cluster is also marked in the top part of the image. Clear time variations of the diffuse emission are visible, indicating regions likely dominated by the reflected emission (the data from 2013 and 2023 have lower exposure, resulting in enhanced photon-counting noise fluctuations). Sgr A* lies outside the region shown, at a few arc minutes from the right boundary.
• Figure 3: Epoch-to-epoch variations in the 3--8 keV surface brightness (normalized per $0.5\arcsec \times 0.5\arcsec$ pixels after subtraction of the particle background) for the regions depicted in Fig. \ref{['fig:chandra_epochs']}. Namely, this figure shows the light curves for a region encompassing the Arches cluster (green), the central brightest region of the Sgr A complex (red), the nonthermal (NT) nebula G0.13-0.11 (blue), and the full Sgr A complex excluding the last two subregions (black) in comparison to a nearby background region (gray). The full time span of the observations is more than 20 years, and the time windows encompassing IXPE observations are shown as shaded boxes. For most of the data points, the statistical uncertainty of the flux measurement is smaller than the symbol size.
• Figure 4: Same as Fig. \ref{['fig:chandra_epochs']} but showing a zoom-in of the interior part of the Sgr A complex and focusing on the difference between two epochs, 2004 (top) and 2022 (bottom), to highlight the small-scale variations in the morphology of the diffuse emission. Big circular regions are 1$\arcmin$ in radius and are used for the differential spectral analysis (after masking the brightest point sources marked with small crossed circles). These regions correspond to the brightest reflection spot (marked "re"), a region dominated by nonthermal emission of the PWN G0.13-0.11 (marked "nt"), and a representative subregion of the Sgr A complex (marked "bg").
• Figure 5: Epoch-to-epoch variations in the spectra of several regions within the Sgr A complex shown in Fig. \ref{['fig:chandra_zoom']}. Top panel: Brightest (in 2022) reflection region, "Sgr A:re". Middle panel: Region around G0.13-0.11, "Sgr A:nt". Bottom panel: Relatively faint region within the Sgr A complex, "Sgr A:bg". The spectra taken from Chandra observations in 2004 (black) and 2022 (red) are fit with the same model with all parameters tied except for normalizations of the reflected continuum and fluorescent lines. The composite reflection models for each observation are shown with dashed lines. The expected contribution of the spatially smooth Galactic bulge emission is shown in blue for comparison. The data were binned only for visualization purposes.