Shaping the diffuse X-ray sky: Structure, Variability and Visibility

Abstract

The Local Bubble (LB) is a hot, low-density cavity in the solar neighborhood, inside which the Solar System is currently located. The X-ray emission from such bubbles is strongly governed by the gas density, temperature, and the effects of line-of-sight column density. Yet the physical processes that control the formation and evolution of this emission remain incompletely understood. We analyze a LB analogue identified within a magnetohydrodynamical simulation to investigate the key physical factors that shape its X-ray properties. In post-processing, we examine the spatial distribution, variability, and observational constraints of the X-ray emission. Our study reveals three main results: (1) Shortly after a supernova (SN), the bulk of the X-ray emission arises from a small fraction of the bubble's volume, concentrated in hot regions around recent SN sites. Approximately 95% of the X-ray luminosity originates from less than 1% of the total bubble volume. During quiescent phases without recent SNe, the emission morphology changes substantially, with X-ray-bright regions becoming more volume-filling. (2) Column density effects strongly modulate the observable X-ray signal. Gas with column densities exceeding $N_\mathrm{H} \gtrsim 10^{20} \,\mathrm{cm}^{-2}$ efficiently absorbs soft X-ray photons, limiting the depth to which observations can probe. This absorption causes a significant fraction of the sky to be obscured from external soft X-rays. Differences between active and quiescent phases further influence how much of the total bubble emission is visible from within. (3) The X-ray flux shows pronounced temporal variability on Myr timescales, with SN events producing rapid, transient luminosity enhancements, followed by steep declines due to adiabatic cooling. The total flux varies by several orders of magnitude, with SN-driven peaks fading within $10^5$ years.

Figures (23)

• Figure 1: Number density in a cut through the disk midplane of the total simulation box with the selected bubble in the center.
• Figure 2: Distribution of emission as a function of temperature and density. Only the hot gas with $T>10^6\,\mathrm{K}$ emits X-rays at relevant intensities. The hot dense gas dominates the emission.
• Figure 3: Time evolution of the bubble shown in cuts through the center of the bubble. From top to bottom, we show the different times from $8$ to $13\,\mathrm{Myr}$. The columns correspond to the three different orientations, $x$--$y$ (in the midplane), $y$--$z$, and $x$--$z$ (vertical structure). We note that the bubble is more confined by dense gas at early times (bubble is "closed") and opens up asymmetrically first towards the bottom and later also at the top.
• Figure 4: Same as Fig. \ref{['fig:bubble-time-evol']} but for the gas dynamics in the simulations. Color-coded is the radial velocity with respect to the center of the bubble, which is the center of the map. The hatched area between the contour lines shows densities $n\gtrsim1\,\mathrm{cm}^{-3}$. We note that the interior of the bubble shows patches of low-density gas moving in different directions with short temporal correlation. The outflow region in the south indicates fast outflowing gas.
• Figure 5: Time evolution of the physical state of the bubble within a 100 pc radius sphere centered on its origin. From top to bottom the panels show: (1) the volume-weighted temperature, (2) the volume filling fraction of hot gas with $T > 5 \times 10^8$ K, (3) the mean hydrogen number density and the number density of free electrons in the ionized interior (total and from gas with $T > 8 \times 10^5$,K), (4) the total X-ray luminosity, and (5) the cumulative volume fraction responsible for a given fraction of the X-ray luminosity. Vertical dashed lines mark SNe occurring within 100 pc of the center; dotted lines indicate SNe between 100 and 120 pc.