[Paper Review] ASTRO-H White Paper - Plasma Diagnostic and Dynamics of the Galactic Center Region

K. Koyama, J. Kataoka+17 more|arXiv (Cornell University)|Dec 3, 2014

Atomic and Molecular Physics3 citations

TL;DR

This white paper proposes using the ASTRO-H X-ray observatory’s high-resolution spectrometer (SXS) to study plasma diagnostics and dynamics in the Galactic Center, focusing on fluorescent Fe Kα lines from X-ray recombination nebulae (XRNe) to trace past flaring activity of Sgr A∗ and measure Galactic rotation curves. The high spectral resolution enables precise line energy and width measurements, offering insights into plasma kinematics, dark matter candidates via sterile neutrino decay, and the interplay between X-ray sources and molecular clouds.

ABSTRACT

The most characteristic high-energy phenomena in the Galactic center (GC) region is the presence of strong K-shell emission lines from highly ionized Si, S, Ar, Ca, Fe and Ni, which form the Galactic Center X-ray Emission (GCXE). These multiple lines suggest that the GCXE is composed of at least two plasmas with temperatures of ~1 and ~7 keV. The GCXE also exhibits the K-shell lines from neutral Si, S, Ar, Ca, Fe and Ni atoms. A debatable issue is the origin of the GCXE plasma; whether it is a diffuse plasma or integrated emission of many unresolved point sources such as cataclysmic variables and active binaries. Detailed spectroscopy for these lines may provide a reliable picture of the GCXE plasma. The origin of the K-shell lines from neutral atoms is most likely the fluorescence by X-rays from a putative past flare of Sgr A*. Therefore ASTRO-H may provide unprecedented data for the past light curve of Sgr A*. All these lines may provide key information for the dynamics of the GCXE, using possible Doppler shift and/or line broadening. This paper overviews these line features and the previous interpretation of their origin. We propose extended or revised science with the ASTRO-H observations of some select objects in the GC region.

Motivation & Objective

• Investigate the past flaring activity of Sgr A∗ by measuring the time history of X-ray recombination nebulae (XRNe) through the 6.4 keV Fe Kα line.
• Map the Galactic Center's plasma dynamics by measuring Doppler shifts and line broadening in Fe Kα lines to derive rotation curves and outflow/inflow motions.
• Search for sterile neutrino dark matter signals in the 1–30 keV range using high-statistics X-ray spectra from the SXS and HXI instruments.
• Establish physical correlations between XRNe and molecular clouds (MCs) by comparing X-ray line kinematics with radio molecular line observations.
• Obtain high-resolution spectra to detect unidentified line features and test new physics beyond the standard model.

Proposed method

• Utilize ASTRO-H's X-ray Spectrometer (SXS) to achieve sub-eV energy resolution (≤1 eV) for precise measurement of Fe Kα line centers.
• Perform long-duration pointed observations of selected regions in the Galactic Center (e.g., Radio Arc, Sgr B, C, D, E) to accumulate high signal-to-noise spectra.
• Combine SXS data with SXI and HXI instruments to extend energy coverage up to ∼80 keV, enhancing sensitivity to potential sterile neutrino decay lines.
• Analyze Doppler shifts in the 6.4 keV line to reconstruct Galactic rotation curves up to ∼100 pc from the center.
• Measure line widths to determine expansion or inflow velocities of plasma, distinguishing between rotation, outflow, and infall motions.
• Stack spectra from multiple pointing observations across the Galactic Center and Bulge regions to improve statistics for detecting faint, narrow line features.

Experimental results

Research questions

• RQ1 What is the time history of past X-ray flares from Sgr A∗, as recorded in the X-ray recombination nebulae (XRNe) via the 6.4 keV Fe Kα line?
• RQ2 How do the kinematics of hot plasma in the Galactic Center—measured via line shifts and widths—compare with the rotation curves derived from radio and infrared observations?
• RQ3 Can the high-resolution X-ray spectra from ASTRO-H detect a narrow emission line at ∼8.7 keV or other features indicative of sterile neutrino decay?
• RQ4 What is the physical relationship between X-ray-emitting plasma (XRNe) and molecular clouds (MCs), as inferred from spatial and kinematic correlations?
• RQ5 Are there any unidentified line-like features in the 1–30 keV range that could signal new physics beyond the standard model?

Key findings

• The SXS instrument will achieve energy resolution of ≤1 eV, enabling velocity measurements with precision of ≤50 km s⁻¹, sufficient to resolve Galactic rotation curves from Fe Kα line shifts.
• The 6.4 keV line center energy measurements in XRNe will allow reconstruction of the Galactic rotation curve out to ≥100 pc from the Galactic Center.
• Line broadening measurements will provide direct constraints on plasma expansion or inflow velocities, distinguishing between outflowing, in-falling, or rotating plasma.
• Stacked high-statistics spectra from ASTRO-H observations in the Galactic Center and Bulge regions will enable a search for faint, narrow lines at 8.7 keV and other energies, potentially indicating sterile neutrino decay.
• The combination of SXS, SXI, and HXI data will provide unprecedented spectral coverage up to ∼80 keV, significantly improving sensitivity to dark matter decay signals compared to Suzaku.
• The paper predicts that the SXS will detect Compton shoulders and Doppler broadening in the 6.4 keV line, allowing detailed diagnostics of plasma conditions in XRNe.