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Diffuse Hot Gas under X-ray Absorption line Spectroscopy

This study presents a systematic analysis of Chandra grating spectra for X-ray binaries (XBs) and active galactic nuclei (AGNs) to investigate the absorption lines of OVII, OVIII, and Ne IX. Collaboratively led by Daniel Wang and Yangsen Yao from UMass, in conjunction with experts from Berkeley, Purdue, CfA, and Carnegie Mellon, the research highlights the complexities of measuring the column density and mass of hot gas in the Galactic disk. The findings suggest a significant concentration of X-ray absorption within a few kiloparsecs, revealing critical parameters such as temperature, density, and metal abundance.

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Diffuse Hot Gas under X-ray Absorption line Spectroscopy

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  1. Diffuse Hot Gas under X-ray Absorption line Spectroscopy Daniel Wang and Yangsen Yao (UMass) in collaboration with Taotao Fang (Berkeley), Wei Cui (Purdue), Fabrizio Nicastro (CfA), & Rupert Croft (Carnegie Mellon)

  2. Sd Sc Sb Sa Starburst Why interesting?

  3. Why absorption lines? • Add the velocity and/or depth • Measure the column density, thus the mass • Direct line diagnostics • Independent of cool gas absorption

  4. AGN X-ray binary No detection Lines of sight

  5. Instrument Properties FWHM ~ 5x102 km/s

  6. Absorption line model I()=Ic() exp[-()] ()NHfafi(T)flu(,0,b) b=(2kT/mi+2)1/2 b =100 km/s is actually used!

  7. OVII OVIII Ne IX Ne VIII OVI Ne IX Ionization fraction Assuming CIE and solar abundances

  8. PKS 2155 3C 273 Mkn 421 LETG/ACIS

  9. LMC X-3

  10. Results from extragalactic sources

  11. Galactic source selection and observations • LMXBs with |b| > 2 • S/N > 7 per bin at ~0.6 keV • Excluding sources with intrinsic emission/absorption features • Ten LMXBs with 17 observations (6 with the LETG)

  12. Ne IX absorption lines in LMXB spectra Mean T ~ 106.3+/-0.2 K nH~3.4(-1.4,+1.7)x10-3 cm-3

  13. Exponential distribution models Sphere model nH = 6.1(-3.0,+3.6)x10-2 cm-3 exp[-R/2.7(-0.4,+0.8) kpc] ~3 x 10-3 cm-3 at the Sun NH~6.1 x1019 cm-2 MH~7.5(2.5-16)x108 Msun Disk model nH = 5.0(-1.8,+2.6)x10-3 cm-3 exp[-|z|/1.1(-0.5,+0.7) kpc] NH~1.6 x1019 cm-2

  14. Comparisons with other measurements • OVI absorption (e.g., Savage et al. 2003) • Comparable to the scale height and velocity dispersion of OVI-absorbing gas • The predicted NOVI is ~1/4 of the observed (assuming CIE and cosmic abundances) • X-ray emission • dI/d=4.8(+-0.8) ph/(s cm2 sr) in OVII triple, averaged over a large field (McCammon et al. 2002) • Marginally consistent with the expected from our inferred exponential distribution, assuming T = 106.3+-0.2 K • Correlation with diffuse ¾-keV band intensity

  15. Comparison Cont. • Pulsar dispersion measures (e.g., Gomez et al. 2001) • Ne=6x1019cm-2 (Galactic only) • Ne=2x1020cm-2 (including MC pulsars) • lower limit to the metal abundance of hot gas NH/Ne ~ 0.06 – 0.3 solar • Excluding the HII contribution, the abundance is likely to be solar. • Or a substantial fraction of the pulsar DM is due to hot gas.

  16. Summary A systematic analysis of Chandra grating spectra of XBs and AGNs to study the OVII, OVIII, and Ne IX absorption, accounting for line saturation and multiple line detections • No evidence for significant X-ray absorption beyond the LMC (probably ~< 1019 cm-2, assuming the solar abundance) • X-ray absorption primarily around the Galactic disk within a few kpc • Vertical scale height ~1-2 kpc, similar to the values for OVI absorbers and free electrons from pulsar dispersion measures • Metal abundance probably ~ solar • Total absorbing gas mass ~ (0.2-1.6)x109 Msun • Mean temperature ~ 106.3+-0.2 K, density ~ 3 x 10-3 cm-3 • Predicts ~0.4 x10-8 cm-2 of OVI, ~¼ of the observed.

  17. Chandra grating observations

  18. High Energy Transmission Grating Spectrometer

  19. AGN X-ray binary No detection Lines of sight

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