Diffuse X-Ray Emission in a Deep Chandra Image of the Galactic Center

By M. Muno, F. Baganoff, M. Bautz, Eric D. Feigelson, G. Garmire, Mark R. Morris, S. Park, G. Ricker, L. K. T. Ucla, Mit and 1 more. 2004

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We examine the spectrum of diffuse emission detected in the 17′ × 17′ field around Sgr A* during 625 ks of Chandra observations. The spectrum exhibits He-like and H-like lines from Si, S, Ar, Ca, and Fe that are consistent with an origin in a two-temperature plasma, as well as a prominent low-ionization Fe Kα line. The cooler, kT ≈ 0.8 keV plasma differs in surface brightness across the image in the range (0.2-1.8) × 10-13 ergs cm-2 s-1 arcmin-2 (observed, 2-8 keV). This soft plasma is probably heated by supernovae, along with a small contribution from the winds of massive Wolf-Rayet (W-R) and O stars. The radiative cooling rate of the soft plasma within the inner 20 pc of the Galaxy could be balanced by 1% of the kinetic energy of one supernova every 3 × 105 yr. The hotter, kT ≈ 8 keV component is more spatially uniform, with a surface brightness of (1.5-2.6) × 10-13 ergs cm-2 s-1 arcmin-2 (observed, 2-8 keV). The intensity of the hard plasma is correlated with that of the soft, but they are probably only indirectly related, because neither supernova remnants nor W-R/O stars are observed to produce thermal plasma hotter than kT ≈ 3 keV. Moreover, a kT ≈ 8 keV plasma would be too hot to be bound to the Galactic center and therefore would form a slow wind or fountain of plasma. The energy required to sustain such a freely expanding plasma within the inner 20 pc of the Galaxy is ~1040 ergs s-1. This corresponds to the entire kinetic energy of one supernova every 3000 yr, which is unreasonably high. However, alternative explanations for the kT ≈ 8 keV diffuse emission are equally unsatisfying. The hard X-rays are unlikely to result from undetected point sources, because no known population of stellar objects is numerous enough to account for the observed surface brightness. Neither is there evidence that nonthermal mechanisms for producing the hard emission are operating, as the expected shifts in the line energies and ratios from their collisional-equilibrium values are not observed. We are left to conclude either that there is a significant shortcoming in our understanding of the mechanisms that heat the interstellar medium or that a population of faint (<1031 ergs s-1) hard X-ray sources that is a factor of 10 more numerous than cataclysmic variables remains to be discovered.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/ac00a68dff3ffcf3a76ff85964aabd61a8068137

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Citations in the corpus

Nuclear Activity in Nearby Galaxies (2008, background): “…& Wilson 2003b; Pellegrini 2005; Rinn, Sambruna & Gliozzi 2005; Cappi et al. 2006; González-Mart́ın et al. 2006; Soria et al. 2006), including our own (Muno et al. 2004) and our close neighbor M31 (Garcia et al. 2005), but quantitative, statistical properties of the gas are now becoming available.”
X-ray luminosity function of faint point sources in the Milky Way (2005, background): “…and XMM-Newton in the Galactic plane and in the Galactic center region resolved only ∼10–30% of the ridge emission (above 2 keV) into point sources (Muno et al. 2004; Ebisawa et al. 2005; Hands et al. 2004), leaving the question open as to what fraction of the unresolved emission is truly…”
Measuring the kinetic power of active galactic nuclei in the radio mode (2007, background): “Such a mechanical power input into the galactic center could play a significant role in the production of the TeVγ-rays recently observed by the HESS (High Energy Stereoscopic System) collaboration (Aharonian et al. 2004; Atoyan & Dermer 2004), as wellas in the heating of the hot (> 8 keV) diffused ”
GIANT GAMMA-RAY BUBBLES FROM FERMI-LAT: ACTIVE GALACTIC NUCLEUS ACTIVITY OR BIPOLAR GALACTIC WIND? (2010, methodology): “Diffuse X-ray Emission— Muno et al. (2004) studied the diffuse X-ray emission within ∼ 20 pc of the GC in detail using Chandra observations.”
A CATALOG OF X-RAY POINT SOURCES FROM TWO MEGASECONDS OF CHANDRA OBSERVATIONS OF THE GALACTIC CENTER (2008, methodology): “We also include sources previously identified in 630 ks of data on the inner 20 pc around Sgr A∗ (Muno et al. 2003a, 2004b); 30 12 ks exposures of the 2◦ × 0.”
Magnetic White Dwarfs (2015, background): “The deep Chandra survey of the galactic centre has revealed a large number of low-luminosity hard X-ray sources attributed to MCVs of the IP type (Muno et al. 2004a; Hong et al. 2012).”