eROSITA unveils asymmetries in temperature and shape of our Local Hot Bubble

3D model of the solar neighbourhood. The colour bar represents the temperature of the LHB as coloured on the LHB surface. The direction of the Galactic Centre (GC) and Galactic North (N) is shown in the bottom right. The link to the interactive version can be found at the bottom of the page. © Michael Yeung / MPE

Our Solar System dwells in a low-density environment called the Local Hot Bubble (LHB), filled by a tenuous, million-degree hot gas emitting dominantly in soft X-rays. A team led by scientists at the Max Planck Institute for Extraterrestrial Physics (MPE) used the eROSITA All-Sky Survey data and found a large-scale temperature gradient in this bubble, possibly linked with past supernova explosions that expanded and reheated the bubble. The wealth of the eROSITA data also allowed the team to create a new 3D model of the hot gas in the solar neighbourhood. The highlight of this work features the discovery of a new interstellar tunnel towards the constellation Centaurus, potentially joining our LHB with a neighbouring superbubble.

The idea of the Local Hot Bubble has been around for about half a century, first developed to explain the ubiquitous X-ray background below 0.2 keV. Photons of such energies cannot travel very far in the interstellar medium before they are absorbed. In conjunction with the observation that there is almost no interstellar dust in our immediate environment, the scenario where a soft X-ray emitting plasma displaces the neutral materials in the solar neighbourhood, forming the ‘Local Hot Bubble’, was put forth.

This understanding of our immediate environment was not without its challenges, especially after the discovery of the solar wind charge exchange process in 1996 — an interaction between the solar wind ions and neutral atoms within the Earth’s geocorona and the heliosphere that emits X-rays at similar energies as the LHB. After years of analysis, the consensus now is that both contribute to the soft X-ray background, and the LHB must exist to explain the observations.

The eROSITA telescope is the first X-ray observatory to observe the sky from an orbit completely external to the Earth’s geocorona, avoiding the latter’s contamination. Also, the timing of the first eROSITA All-Sky Survey (eRASS1) coincided with the solar minimum, significantly reducing the heliospheric solar wind charge exchange contamination. ‘In other words, the eRASS1 data released to the public this year provides the cleanest view of the X-ray sky to date, making it the perfect instrument for studying the LHB, ‘says Michael Yeung from MPE, the lead author of this work.

3D structure of the LHB with colours indicating its temperature. The two surfaces indicate the measurement uncertainty of the LHB extent: the most probable extent most likely lies between the two. The location of the Sun and a sphere of 100 parsec radius are marked for comparison. © Michael Yeung / MPE

eROSITA’s Unparalleled X-ray Observations

The team divided the western Galactic hemisphere into about 2000 regions, and extracted and analysed the spectra from each one. They also leveraged data from ROSAT, the predecessor of eROSITA built also by MPE, which complements the eROSITA spectra at energies lower than 0.2 keV. They found a clear temperature dichotomy in the LHB, with the Galactic South (0.12 keV; 1.4 MK) slightly hotter than the Galactic North (0.10 keV; 1.2 MK). This feature could be explained by the latest numerical simulations of the LHB caused by supernova explosions in the last few million years.

Diffuse X-ray background spectra inform scientists not just of the temperature but also of the 3D structure of the hot gas. Previous work by the same team has established that the density of the LHB is relatively uniform, calibrating the density of the hot gas with sight lines to giant molecular clouds located on the surface of the LHB. Relying on this assumption, they generated a new 3D model of the LHB from the measured intensity of the LHB emission in each sight line. They found the LHB has a larger extent towards the Galactic poles as expected, as the hot gas prefers to expand towards directions of the least resistance, away from the Galactic disc.

‘This is not surprising, as was already found by the ROSAT survey’, pointed out by Michael Freyberg, a core author of this work and was a part of the pioneering work in the ROSAT era three decades ago. ‘What we didn’t know was the existence of an interstellar tunnel towards Centaurus, which carves a gap in the cooler interstellar medium (ISM). This region stands out in stark relief thanks to the much-improved sensitivity of eROSITA and a vastly different surveying strategy compared to ROSAT,’ added Freyberg. The authors of this work suggest the Centaurus tunnel may just be a local example of a wider hot ISM network sustained by stellar feedback across the Galaxy — a popular idea proposed in the 70s that remains difficult to prove.

Temperature map of the LHB in the western Galactic hemisphere in zenithal equal-area projection. The high-latitude region in the northern and southern hemispheres exhibits a clear temperature dichotomy. © Michael Yeung / MPE

A 3D Model of the Solar Neighbourhood

In addition to the 3D LHB model, the team compiled a list of known supernova remnants, superbubbles, and 3D dust information from the literature and created an interactive 3D model of the solar neighbourhood. Some features of the LHB could be easily appreciated from such representation, for instance, the well-known Canis Majoris tunnel on the Galactic disc, possibly connecting the LHB to the Gum nebula or another superbubble (called GSH238+00+09), as well as dense molecular clouds (in orange) lying close to the surface of the LHB in the direction of the Galactic Centre (GC). Recent works found that these clouds possess velocities in the radial direction (away from us). The location and the velocity of the clouds could be explained if they were formed from the condensation of swept-up materials during the early stage of the LHB formation. ‘Another interesting fact is that the Sun must have entered the LHB a few million years ago, a short time compared to the age of the Sun, remarked Gabriele Ponti, a co-author of this work. ‘It is purely coincidental that the Sun seems to occupy a relatively central position in the LHB as we continuously move through the Milky Way.’

3D interactive view of the LHB and the solar neighbourhood

Source: Max Planck Institute for Extraterrestrial Physics (MPE)/News ’

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Contacts:

Michael Yeung
PhD Student Highenergy-Group
tel:+49 89 30000-3899
mjf@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

Dr. Michael Freyberg
Scientist Highenergy Group
tel:+49 89 30000-3849
myeung@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

Dr. Gabriele Ponti
Visiting Scientist Highenergy Group
tel:+49 89 30000-3572
ponti@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

Dr. Andrea Merloni
Senior Scientist Highenergy Group; PI eROSITA
tel:+49 89 30000-3893
am@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

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Original Publication

M. C. H. Yeung, G. Ponti, M. J. Freyberg et al.
The SRG/eROSITA diffuse soft X-ray background. I. The local hot bubble in the western Galactic hemisphere
A&A, 690, A399

Source

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Further Information

eROSITA website of the MPE

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December 14, 2023

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Massive black holes in low-mass galaxies: what happened to the X-ray Corona?

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The X-ray sky opens to the world

January 31, 2024
First eROSITA sky-survey data release makes public the largest ever catalogue of high-energy cosmic sources

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NASA-funded X-ray Instrument Settles Interstellar Debate

Colors indicate the density of interstellar helium near Earth and its enhancement in a downstream cone as the neutral atoms respond to the sun's gravity (blue is low density, red is high). Also shown are the observing angles for DXL and ROSAT. Image Credit: NASA's Goddard Space Flight Center. Hi-Res Image

New findings from a NASA-funded instrument have resolved a decades-old puzzle about a fog of low-energy X-rays observed over the entire sky. Thanks to refurbished detectors first flown on a NASA sounding rocket in the 1970s, astronomers have now confirmed the long-held suspicion that much of this glow stems from a region of million-degree interstellar plasma known as the local hot bubble, or LHB.

At the same time, the study also establishes upper limits on the amount of low-energy, or soft, X-rays produced within our planetary system by the solar wind, a gusty outflow of charged particles emanating from the sun.

"Interactions between the solar wind and neutral atoms in comets, the outer atmospheres of planets, and even interstellar gas produce soft X-rays," explained team member Steve Snowden, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We need to account for these processes because the X-rays they produce complicate our observations of the wider universe."

Decades of mapping the sky in X-rays with energies around 250 electron volts -- about 100 times the energy of visible light -- revealed strong emission precisely where it shouldn't be. This glow, known as the soft X-ray diffuse background, is surprisingly bright in the gas-rich central plane of our galaxy, where it should be strongly absorbed. This suggested the background was a local phenomenon, arising from a bubble of hot gas extending out a few hundred light-years from the solar system in all directions. Improved measurements also made it increasingly clear that the sun resides in a region where interstellar gas is unusually sparse. Taken together, the evidence suggests our solar system is moving through a region that may have been blasted clear by one or more supernova explosions during the past 20 million years.

In the 1990s, a six-month all-sky survey by the German X-ray observatory ROSAT provided improved maps of the diffuse background, but it also revealed that comets were an unexpected source of soft X-rays. As scientists began to understand this process, called solar wind charge exchange, they realized it could occur anywhere neutral atoms interacted with solar wind ions.

This animation illustrates solar wind charge exchange in action. An atom of interstellar helium (blue) collides with a solar wind ion (red), losing one of its electrons (yellow) to the other particle. As it settles into a lower-energy state, the electron emits a soft X-ray. Image Credit: NASA's Goddard Space Flight Center. Download this video in HD formats from NASA Goddard's Scientific Visualization Studio

Within the last decade, some scientists have been challenging the LHB interpretation, suggesting that much of the soft X-ray diffuse background is a result of charge exchange," said F. Scott Porter, a Goddard astrophysicist also participating in the study. "The only way to check is to design an instrument and make measurements."

Led by Massimiliano Galeazzi, a professor of physics at the University of Miami in Coral Gables, Florida, an international collaboration developed a mission to do just that. The team includes scientists from NASA, the University of Wisconsin -- Madison, the University of Michigan at Ann Arbor, the University of Kansas at Lawrence, Johns Hopkins University in Baltimore, Maryland, the French National Center for Scientific Research (CNRS), headquartered in Paris, and other institutions.

Galeazzi and his colleagues rebuilt, tested, calibrated, and adapted X-ray detectors originally designed by the University of Wisconsin and flown on sounding rockets in the 1970s. Components from another instrument flown on space shuttle Endeavour in 1993 also were given new life. The mission was named DXL, for Diffuse X-ray emission from the Local Galaxy. 

On Dec. 12, 2012, DXL launched from White Sands Missile Range in New Mexico atop a NASA Black Brant IX sounding rocket, reaching a peak altitude of 160 miles (258 km) and spending five minutes above Earth's atmosphere. The mission design allowed the instrument to observe a worst-case scenario involving charge exchange with interstellar gas.

The solar system is currently passing through a small cloud of cold interstellar gas as it moves through the galaxy. The cloud’s neutral hydrogen and helium atoms stream through the planetary system at about 56,000 mph (90,000 km/h). While hydrogen atoms quickly ionize and respond to numerous forces, the helium atoms travel paths largely governed by the sun's gravity. This creates a "helium focusing cone" downstream from the sun that crosses Earth's orbit and is located high in the sky near midnight in early December.

"This helium focusing creates a region with a much greater density of neutral atoms and a correspondingly enhanced charge exchange rate," Snowden said.

The solar wind is accelerated in the sun's corona, the hottest part of its atmosphere, so its atoms have been ionized -- stripped of many of their electrons. When a neutral atom collides with a solar wind ion, one of its electrons often jumps to the charged particle. Once captured by the ion, the electron briefly remains in an excited state, then emits a soft X-ray and settles down at a lower energy. This is solar wind charge exchange in action.

To establish a baseline for the soft X-ray background, the researchers used data captured by the ROSAT mission in September 1990 in a direction looking along, rather than into, the helium focusing cone. The results, published online in the journal Nature on July 27, indicate that only about 40 percent of the soft X-ray background originates within the solar system.

"We now know that the emission comes from both sources but is dominated by the local hot bubble,” said Galeazzi. "This is a significant discovery. Specifically, the existence or nonexistence of the local bubble affects our understanding of the area of the galaxy close to the sun, and can, therefore, be used as a foundation for future models of the galaxy structure."

Galeazzi and his collaborators are already planning the next flight of DXL, which will include additional instruments to better characterize the emission. The launch is currently planned for December 2015.

"The DXL team is an extraordinary example of cross-disciplinary science, bringing together astrophysicists, planetary scientists, and heliophysicists," added Porter. "It’s unusual but very rewarding when scientists with such diverse interests come together to produce such groundbreaking results."

Related Links:

• Download HD video and additional images from NASA Goddard's Scientific Visualization Studio

• Paper: "The Origin of the Local 1/4-keV X-ray Flux in Both Charge Exchange and a Hot Bubble"

• Paper: "Pressure Equilibrium between the Local Interstellar Clouds and the Local Hot Bubble"

• DXL Launches Successfully (12.13.2012)

• Imagine the Universe! The Soft X-ray Diffuse Background

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland