Studying the Interstellar Medium and the Inner Region of NPS/Loop 1 With Shadow Observations Toward Mbm36

Abstract

We analyzed data from a shadow observation of the high density molecular cloud MBM36 (l∼4°, b∼35°) with Suzaku. MBM36 is located in a region that emits relatively weakly in the 3/4 keV band compared to the surrounding North Polar Spur (NPS)/Loop 1 structure and the Galactic Bulge (GB). The contrast between high and low density targets in the MBM36 area allows one to separate the local and distant contributors to the soft diffuse X-ray background, providing a much better characterization of the individual components compared to single pointing observations. We identify two non-local thermal components, one at kT≈0.12 keV and one at kT≈0.29 keV. The colder component matches well with models of emission from the higher latitude region of the GB. The emission of the warmer component is in agreement with models predicting that the NPS is due to a hypershell from the center of the Milky Way. Geometrical and pressure calculations rule out a nearby bubble as responsible for the emission associated with the NPS. Any Galactic Halo/circumgalactic halo emission, if present, is outshined by the other components. We also report an excess emission around 0.9 keV, likely due to an overabundance of Ne IX.

Spatial Distribution of the Milky Way Hot Gaseous Halo Constrained by Suzaku X-Ray Observations

“…The surface brightness of the local emission component spans 1.3-6.8×10 −12 erg cm −2 s −1 deg −2 in the 0.4-1.0 keV band. These parameter ranges roughly agree with those obtained by previous observations with Suzaku and XMM-Newton (Smith et al 2007;Galeazzi et al 2007;Henley & Shelton 2015;Ursino et al 2016)…”

An Analysis of the North Polar Spur Using HaloSat

“…Our absorption fitting yielded inconsistent results from field to field. Similar to other reported results (Kataoka et al 2013;Ursino et al 2015;Akita et al 2018), we found that without placing upper bounds our best-fit absorbing column densities were larger than the determined full Galactic column for 10 of 14 fields. For X-ray absorption, we fixed our column densities at a value of 0.6 of the Galactic absorption for that field.…”

“…Following the procedure in Akita et al (2018), we can estimate the range of path lengths » h EM n e 2 for the NPS hot component assuming constant electron densities. If there is no large variation in magnetic field between the LHB and a local interpretation of the NPS, Ursino et al (2015) estimate =´-n 1.6 10 cm e 3 3 , and thus we might expect NPS hot path lengths between 14.8 and 51.6 kpc. Adopting instead the upper bound of n e =10×10 −3 cm −3 from Sofue et al (2016) for the shock region of the Galactic center hypershell, we might expect NPS hot path lengths between 0.4-1.3 kpc.…”

“…The measurement of pressure we find for a local interpretation is significantly higher than the magnetic field expected in the local ISM. Even for a larger estimate of the magnetic field of 4.9 μG adopted by Ursino et al (2015), the local interpretation is still inconsistent.…”

Detection of large-scale X-ray bubbles in the Milky Way halo

“…This velocity implies a characteristic expansion time to the present size of around 20 Myr, which translates to an energy-release rate of roughly (1-3) × 10 41 erg s −1 . The cooling time 38 of such tenuous hot gas (density n = 0.002 cm −3 log(T) = 6.5) is approximately 1.9 × 10 8 yr, much longer than the estimated age of the bubbles. Therefore, once heated, the interior of the bubbles will be visible in X-rays for a long time, even after the energy release has ceased.…”