The Local Hot Bubble was initially proposed nearly 50 years ago to explain the soft X-ray background observed below 0.2 keV. Since X-ray photons at this energy level are quickly absorbed within the interstellar medium, it was postulated that a bubble filled with hot, X-ray-emitting plasma exists, displacing neutral materials within the Solar System's vicinity. However, the theory faced challenges, particularly following the 1996 discovery of X-ray emissions arising from solar wind interactions within the Earth's geocorona. Years of analysis have since led to a consensus that both solar wind interactions and the LHB contribute to the soft X-ray background observed in our region of space.

eROSITA, positioned outside Earth's geocorona, is the first X-ray observatory to capture images without contamination from solar wind interactions. This telescope's first all-sky survey coincided with a solar minimum, further minimizing interference and providing an exceptionally clear view of the X-ray sky. "The eRASS1 data released this year provides the cleanest view of the X-ray sky to date, making it the perfect instrument for studying the LHB," explained Michael Yeung from MPE, lead author of the study.

New Temperature and Structural Insights
The team examined around 2,000 regions within the western Galactic hemisphere, analyzing spectra from each region. Using eROSITA data and additional information from the ROSAT mission, they identified a distinct temperature gradient in the LHB: the Galactic South registers slightly warmer at 0.12 keV (1.4 MK) compared to the Galactic North at 0.10 keV (1.2 MK). This temperature disparity aligns with recent simulations suggesting that supernova explosions over the last few million years played a role in shaping the LHB.

In addition to temperature data, the X-ray spectra have revealed the 3D structural composition of the LHB. Past studies by the MPE team indicated that the LHB density remains fairly uniform, using sightlines to large molecular clouds for calibration. Based on this information, the team generated a new 3D model, which shows the bubble's larger extent toward the Galactic poles - a result of hot gas expanding in directions offering the least resistance.

"This isn't surprising, as ROSAT discovered similar structures," remarked Michael Freyberg, an MPE scientist and co-author involved in previous ROSAT studies. "What's new is the presence of an interstellar tunnel towards Centaurus, which creates a distinct gap in the cooler interstellar medium." Freyberg further explained that this discovery points to a potential network of hot interstellar medium sustained by stellar activity, an idea first suggested in the 1970s.

Interactive 3D Model of the Solar Neighborhood
The MPE team also compiled supernova remnants, superbubble data, and 3D dust maps from existing literature to create an interactive 3D model of the solar neighborhood. This model highlights notable features such as the Canis Majoris tunnel, which may connect the LHB to the Gum nebula or another superbubble known as GSH238+00+09. Dense molecular clouds lying near the LHB surface toward the Galactic Center, recently observed to have outward radial velocities, might have condensed from materials swept up during the LHB's early expansion.

"It's interesting to note that the Sun only entered the LHB a few million years ago, a relatively short time compared to the Sun's age," commented Gabriele Ponti, a co-author of the study. "It's purely coincidental that the Sun currently occupies a central position in the LHB as it moves through the Milky Way."

Research Report:The SRG/eROSITA diffuse soft X-ray background. I. The local hot bubble in the western Galactic hemisphere

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Max Planck Institute for Extraterrestrial Physics
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