Diffuse Low-Ionization Gas in the Galactic Halo Casts Doubts on $z\simeq 0.03$ WHIM Detections

Abstract

In this Letter we demonstrate that the two claims of $z\simeq 0.03$ OVII K$\hat{I}\pm$ absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H~2356-309 (Buote et al., 2009; Fang et al., 2010) and Mkn~501 (Ren, Fang \& Buote, 2014) are likely misidentifications of the $z=0$ OII K${\hat{I}}^2$ line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's Interstellar and Circum-Galactic mediums. We perform detailed modeling of all the available high signal-to-noise Chandra LETG and XMM-Newton RGS spectra of H 2356-309 and Mkn 501 and demonstrate that the $z\simeq 0.03$ WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the $z\simeq 0.03$ OVII K$\hat{I}\pm$ absorber along the line of sight to H~2356-309. In our model the temperature of the putative $z=0.031$ WHIM filament is T$=3\times 10^5$ K and the OVII column density is N${}_{OVII}\le 4\times 10^{15}$ cm${}^{-2}$, twenty times smaller than the OVII column density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.

6 pages, 2 figures, accepted by MNRAS (Letter)