SUZAKU OBSERVATIONS OF THE DIFFUSE X-RAY EMISSION ACROSS THE FERMI BUBBLES' EDGES
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We present Suzaku X-ray observations along two edge regions of the Fermi Bubbles, with eight ≃ 20 ks pointings across the northern part of the North Polar Spur (NPS) surrounding the north bubble and six across the southernmost edge of the south bubble. After removing compact X-ray features, diffuse X-ray emission is clearly detected and is well reproduced by a three-component spectral model consisting of unabsorbed thermal emission (temperature kT ≃ 0.1 keV) from the Local Bubble, absorbed kT ≃ 0.3 keV thermal emission related to the NPS and/or Galactic halo (GH), and a power-law component at a level consistent with the cosmic X-ray background. The emission measure (EM) of the 0.3 keV plasma decreases by ≃ 50% toward the inner regions of the northeast bubble, with no accompanying temperature change. However, such a jump in the EM is not clearly seen in the south bubble data. While it is unclear whether the NPS originates from a nearby supernova remnant or is related to previous activity within or around the Galactic center, our Suzaku observations provide evidence that suggests the latter scenario. In the latter framework, the presence of a large amount of neutral matter absorbing the X-ray emission as well as the existence of the kT ≃ 0.3 keV gas can be naturally interpreted as a weak shock driven by the bubbles' expansion in the surrounding medium, with velocity v exp ∼ 300 km s −1 (corresponding to shock Mach number ), compressing the GH gas to form the NPS feature. We also derived an upper limit for any non-thermal X-ray emission component associated with the bubbles and demonstrate that, in agreement with the aforementioned findings, the non-thermal pressure and energy estimated from a one-zone leptonic model of its broadband spectrum, are in rough equilibrium with that of the surrounding thermal plasma.
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The energy density in the eROSITA bubbles is ε ≃2×10^-12 erg cm^-3.
“Therefore, the energy density and the total energy in the eROSITA bubbles are, ϵ = nskBTs / ( γ -1) ≃ 2 × 10 -12 erg cm -3 and E th ≃ 10 56 erg, respectively (Kataoka et al. 2013, 2015; Predehl et al. 2020).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.3 (page not provided) estimates a thermal pressure of about 2 × 10^-12 dyn cm^-2 for NPS gas surrounding the Fermi bubbles. It does not report an energy density of 2 × 10^-12 erg cm^-3 for the eROSITA bubbles; the cited number is a pressure estimate for a different component. (LLM, conf=0.86)
Experimental — accuracy not yet measuredThe total energy in the eROSITA bubbles is Eth ≃10^56 erg.
“Therefore, the energy density and the total energy in the eROSITA bubbles are, ϵ = nskBTs / ( γ -1) ≃ 2 × 10 -12 erg cm -3 and E th ≃ 10 56 erg, respectively (Kataoka et al. 2013, 2015; Predehl et al. 2020).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.3 estimates E_th ≃ 10^56 erg for a thermal X-ray envelope or shell surrounding the Fermi bubbles. It does not identify that shell as the eROSITA bubbles, so the numerical thermal-energy estimate is supported but its attribution to the eROSITA bubbles is not established by this paper. (LLM, conf=0.86)
Experimental — accuracy not yet measuredX-ray observations toward the NPS imply an absorbing Hydrogen column density of about 3 × 10^20 cm^-2, close to the Galactic values in those directions.
“Xray observations toward the NPS find that the spectra imply an absorbing Hydrogen column density of ∼ 3 × 10 20 cm -2 (Willingale et al. 2003; Kataoka et al. 2013), close to the Galactic values in those directions.”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.2 reports that X-ray spectra toward the NPS imply absorbing hydrogen columns that are substantial fractions of the Galactic values in those directions; Section 3.2.2 says most fitted values are consistent with the full Galactic column. But the paper does not report ∼3 × 10^20 cm^-2 as a measured NPS absorbing column: its use of that figure in Section 4.2 is an upper estimate for gas accumulated within 100 pc by a hypothetical expanding shock. (LLM, conf=0.85)
Experimental — accuracy not yet measuredX-ray spectra indicate a metallicity of 0.2-0.7 Z⊙, consistent with the metallicity of the CGM (≃0.5 Z⊙) rather than the ISM (≃Z⊙).
“Additionally, the X-ray spectra indicate a metallicity of 0 . 2 -0 . 7 Z ⊙ (Kataoka et al. 2013; Gu et al. 2016; Lallement et al. 2016), consistent with the metallicity of the CGM ( ≃ 0 . 5 Z ⊙ ; Miller & Bregman 2015; Faerman et al. 2017, 2020) than the ISM ( ≃ Z ⊙ ; Maciel & Costa 2010; De Cia et al. 2021).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 3.2.2 reports that the diffuse X-ray spectra favor subsolar metallicity, with a median of about 0.2 Z⊙. The paper does not report the cited 0.2–0.7 Z⊙ range, and Appendix B notes that limited photon statistics prevent a precise metallicity determination. The CGM and ISM comparison is separately cited to other works in the citing sentence. (LLM, conf=0.86)
Experimental — accuracy not yet measuredThe energy requirement for the X-shaped region is about 10^55 erg, about an order of magnitude smaller than the required energy of about 10^56 erg for the eROSITA bubbles.
“The energy requirement for the X-shaped region is ∼ 10 55 erg (Bland-Hawthorn & Cohen 2003), about an order of magnitude smaller than the required energy of ∼ 10 56 erg for the eROSITA bubbles (Sofue 2000; Kataoka et al. 2013; Predehl et al. 2021).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.3 (page unavailable) estimates about 10^56 erg for the non-thermal energy of the Fermi bubbles and for thermal gas around them. It does not identify that figure as the required energy of the eROSITA bubbles. The X-shaped region’s 10^55 erg estimate is attributed to a different citation in the citing sentence. (LLM, conf=0.86)
Experimental — accuracy not yet measured
Such strong shocks would imply a shock velocity ≳10^3 km/s and a shock temperature ≳1.2 keV, much higher than the temperature observed in the eROSITA bubbles or inferred from Fermi bubble edges.
“Such strong shocks would imply a shock velocity, vs ≳ 10 3 km s -1 and a shock temperature, Ts ≳ 1 . 2 keV, much higher than the temperature observed in the eROSITA bubbles (Kataoka et al. 2013) or inferred from FB-edges (Keshet & Gurwich 2018, also see section 2.1).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.2 reports a roughly 0.3 keV plasma component at the Fermi bubbles’ edges, well below 1.2 keV. But the paper reports Suzaku observations, not a temperature observed in the eROSITA bubbles; the Introduction describes eROSITA as a future instrument. (LLM, conf=0.87)
Experimental — accuracy not yet measuredThe required energy and dynamical time for the 'Bipolar Hyper-Shell' were found to be ∼3e56 erg and ∼15 Myr, very close to the currently accepted values for the eROSITA bubbles.
“The required energy and dynamical time for the 'Bipolar Hyper-Shell' were found to be ∼ 3 × 10 56 erg and ∼ 15 Myr, very close to the currently accepted values for the eROSITA bubbles (Kataoka et al. 2013; Predehl et al. 2020).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.4 (page unavailable) estimates a formation timescale of about 10 Myr for the Fermi bubbles, while Sections 4.3–4.4 estimate their non-thermal energy at about 10^56 erg. These are broadly similar in scale to the claimed values, but the paper does not give 3 × 10^56 erg and 15 Myr for a ‘Bipolar Hyper-Shell’ or compare them with eROSITA bubbles. (LLM, conf=0.86)
Experimental — accuracy not yet measuredThe problem of the high shock temperature still remains for such blast wave bubbles since they produce a shock temperature (for the NPS) to be ≈0.4 keV compared to the observed ≈0.3 keV.
“The problem of the high shock temperature still remains for such blast wave bubbles since they produce a shock temperature (for the NPS) to be ≈ 0 . 4 keV compared to the observed ≈ 0 . 3 keV (Kataoka et al. 2013; Miller & Bregman 2016).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 4.2 reports an observed NPS temperature of approximately 0.3 keV. Sections 4.3–4.4 describe a low-Mach shock heating halo gas from approximately 0.2 to 0.3 keV; they do not report a predicted 0.4 keV shock temperature or identify a remaining temperature discrepancy for the bubble model. (LLM, conf=0.87)
Experimental — accuracy not yet measuredThe required absorbing column density is much higher than the total neutral gas content within 100 pc of the Sun unless there is a dense shell of gas in between.
“The required absorbing column density for explaining the strong suppression of X-ray emission at b ≲ 10 ◦ is ∼ 5 × 10 21 cm -2 is much higher than the total neutral gas content within 100 pc of the Sun (assuming average ISM density of 1 cm -3 ) unless there is a dense shell of gas in between (Kataoka et al. 2013).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
The absorbing column density required to explain the strong suppression of X-ray emission at b ≲ 10° is about 5 × 10^21 cm^-2.
“The required absorbing column density for explaining the strong suppression of X-ray emission at b ≲ 10 ◦ is ∼ 5 × 10 21 cm -2 is much higher than the total neutral gas content within 100 pc of the Sun (assuming average ISM density of 1 cm -3 ) unless there is a dense shell of gas in between (Kataoka et al. 2013).”
From The Fermi/eROSITA bubbles: a look into the nuclear outflow from the Milky Way (2024) · cites this paper as Kataoka et al. 2013
Section 3.2.2 discusses absorbing columns in the observed fields, and Section 4.2 discusses absorption of the North Polar Spur, but neither gives a column required to explain X-ray suppression at b ≲ 10°. Searches for that low-latitude suppression and for 5 × 10^21 cm^-2 found no passage stating the claimed estimate. Page numbers were not available in our copy. (LLM, conf=0.74)
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References 65 · Cited by 118
Research works printed but not linked: 4
- Giommi, P., Angelini, L., Jacobs, P., & Tagliaferri, G. 1992, in ASP Conf. Ser. 25, Astronomical Data Analysis Software and Systems I, ed. D. M. Worrall, C. Biemesderfer, & J. Barnes (San Francisco, CA: ASP), 100
- Hartman, R. C., Bertsch, D. L., Bloom, S. D., et al. 1999, ApJS, 123, 79 · DOI
- Lacki, B. C. 2013, ApJ, submitted (arXiv:1304.6142) · arXiv
- Merloni, A., Predehl, P., Becker, W., et al. 2012, MPE Document, ed. S. Allen et al. (arXiv:1209.3114) · arXiv
- Authors
- Kataoka, J., Tahara, M., Totani, T., Sofue, Y., Stawarz, Ł., Takahashi, Y., Takeuchi, Y., Tsunemi, H., Kimura, M., Takei, Y., Cheung, C. C., Inoue, Y., & Nakamori, T.
- Affiliations
- Waseda University (JP)
- The University of Tokyo (JP)
- Jagiellonian University (PL)
- The University of Osaka (JP)
- Institute of Space and Astronautical Science (JP)
- United States Naval Research Laboratory (US)
- Kavli Institute for Particle Astrophysics and Cosmology (US)
- Yamagata University (JP)
- Corresponding author
- Kataoka, J.
- ORCID
- Venue
- The Astrophysical Journal
- Publisher
- American Astronomical Society
- Published
- 2013-11-25
- Volume
- 779
- Issue
- 1
- Pages
- 57
- ISSN
- 0004-637X (ISSN-L)
- 1538-4357
- OpenAlex
- W2130101183
- Cited by
- 118
- References
- 65
- Citations by year
- 2026: 3 · 2025: 6 · 2024: 8 · 2023: 10 · 2022: 11 · 2021: 7 · 2020: 11 · 2019: 5 · 2018: 14 · 2017: 10 · 2016: 6 · 2015: 15 · 2014: 11 · 2013: 1
- FWCI
- 4.88
- Type
- journal article
- Language
- en
- Open access
- bronze
- doi.org
- Licence
- Locations
- The Astrophysical Journal (publishedVersion)
- arXiv (Cornell University) (submittedVersion)
- Homo Politicus (Academy of Humanities and Economics in Lodz) (submittedVersion, cc-by-nc-nd)
- arXiv (Cornell University)
- Funders
- Japan Society for the Promotion of Science (23000004)
- Topics
- Physical Sciences › Physics and Astronomy › Nuclear and High Energy Physics › Astrophysics and Cosmic Phenomena
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysical Phenomena and Observations
- Physical Sciences › Physics and Astronomy › Astronomy and Astrophysics › Astrophysics and Star Formation Studies
- Keywords
- Fermi bubbles, North Polar Spur, diffuse X-ray emission, Local Bubble, Galactic halo, cosmic X-ray background, weak shock waves, bubble growth, non-thermal X-ray emission, leptonic model, Suzaku
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