How black holes and neutron stars wreak havoc on the interstellar medium (with actual images)
The animation below shows how an extremely energetic neutron star has blown up a hole in the interstellar medium around 400 times our Solar System in size. This system is located in a galaxy at more than 13 million light years from us and has been puzzling astronomers since its discovery.
Astronomers discovered these huge cavities at the end of the 20th century, when searching for supernova remnants in external galaxies. Supernovae are powerful explosions stars around 8 times more massive than our Sun undergo when they end their lives. The blast wave of the explosion — around 10³⁰ times more powerful than the atomic bomb dropped in Hiroshima during World War II — will sweep up and ionize the diffuse interstellar gas, leaving behind a supernova remnant, whereas in its center the core of the star will have collapsed into a compact object — a black hole or a neutron star. The most famous example was witnessed from Earth before mankind even knew about their existence. Here’s the story.
In 1731 British astronomer Jon Bevis reported the discovery of a faint diffuse nebula that was later termed the “Crab Nebula”, because of its shape. Around 200 years later astronomers discovered that the nebula was actually expanding, and estimated that the explosion must have occurred around 900 years ago. Funnily enough, historical records revealed that in 1054 Chinese astronomers witnessed the appearance of a luminous star in that same position in the sky. The “star” became visible during daytime for about 23 days, becoming the brightest object in the night sky after the Sun and the Moon. What the Chinese astronomers did not know was that what they had witnessed was actually the first ever recorded supernova explosion. Today at its center we find a neutron star whose pulses can be observed across the entire electromagnetic spectrum.
Astronomers initially catalogued these large cavities as supernova remnants like the Crab, because they glowed precisely in all the characteristic atomic transitions expected to be excited by the blast wave of a supernova explosion. What the animation above shows is precisely this: as we hit the transition energy (or wavelength) of a well-known transition of hydrogen, the most abundant element in the Universe, the gas around this neutron star lights up. The images were taken with the Multi-Unit Spectroscopic Explorer (MUSE), an instrument mounted at the Very Large Telescope in the Atacama Desert (Chile). This advanced instrument takes several images in narrow wavelengths simultaneously, allowing to precisely isolate the emission from given atomic elements, revealing the excited gas around this neutron star in unprecedented level of detail.
Yet these same astronomers also noted something unusual about them: these cavities were abnormally large when compared to know remnants (the size of the largest known supernova remnant is depicted in the animation for comparison).
Not a typical supernova
Based on the size and the expansion velocity of the cavity, astronomers can gauge how much energy is released in the initial explosion. Curiously enough, one can use the same equations physicist Sir Geoffrey Ingram Taylor derived in 1947 to estimate the explosion energy of the first atomic bombs. The result? The explosion that formed the cavity must have released orders of magnitude more energy than typical supernovae, something that seemed implausible. Some suggested that perhaps these cavities were the result of multiple supernovae exploding close to each other, but this seemed to require some fine-tuning and was not supported by observations. A key piece of evidence in the quest for understanding these cavities would come from X-ray observations, when X-ray imaging capabilities allowed for the first time to take high-resolution images of external galaxies.
X-rays: the signature of a black hole
Ever wondered how astronomers detect black holes if they do not emit light? Light from black holes can only be detected when they are in a binary system with a regular star like our Sun. When the star orbits too close to the black hole (or to a neutron star), the extreme gravitational pull of the compact object will suck the outer layers of the regular star, causing the infalling gas to slowly spiral onto the compact object and heat up to extreme temperatures, forming an accretion disk. The heat will make the accretion disk glow thermal radiation, much like our Sun or very hot iron, but due to the temperatures involved most of the emission is produced in the X-rays — high-energy photons that we cannot perceive with our eyes, but that we can capture using special telescopes. It is this X-ray emission that allows astronomers to detect black holes, measure their masses, spins and study relativistic effects around them.
Those first X-ray images of the external galaxies revealed something odd: along with the X-ray emission from binary systems in other galaxies, many hosted some abnormally bright X-ray sources, far brighter than anything seen before in our Galaxy.
The most powerful compact objects
While it was agreed that this bright X-ray and variable emission should come from accreting compact objects, an explanation for their bright luminosity was lacking. Given their bright X-ray luminosity and unknown nature, astronomers dubbed these systems Ultraluminous X-ray sources (ULXs) — yes, astrophysics is plagued with cool-sounding names. It was hypothesized that perhaps these simply harbored larger black holes than those commonly seen in our galaxy — all roughly below twenty solar masses — or that these were simply an extreme version of regular X-ray binaries, where the star could be feeding the compact object at such fast rates that the accretion energy was liberated in much more violent way.
A revolutionary step in our understanding of these ULXs and these unusual supernova remnants was made around the early 2000s, when it was observed that many ULXs were found to reside at the center of them. In fact, the neutron star lying at the center of the cavity shown earlier is around 50–500 times brighter in X-rays than neutron stars found in our Galaxy.
It is believed that this fast and violent accretion process around ULXs can, not only explain their extreme X-ray brightness, but also the presence of these large cavities around them. The violent release of energy that occurs at fast accretion rates is expected to produce relativistic ejections of gas and jets from the accretion disk. We believe that these supersonic ejections of gas are slowly carving the interstellar medium, forming these so-called bubbles. Effectively, instead of being created due to a single explosion, these have been inflated over time.
With the MUSE instrument, we have been able to uncover another bubble around a ULX, for which the nature of the compact object remains still a mystery. The image illustrates how with MUSE we are able to precisely isolate the diffuse gas around the ULX, which would be otherwise unnoticeable with the continuum filters used by the Hubble Space Telescope (left panel). The MUSE image is a composite of three filters precisely isolating lines of sulfur, hydrogen (the same as shown earlier) and oxygen. But there’s more: these atomic lines tell us valuable information about the mechanism exciting the gas. Gas excited due to shock-waves emits strongly in sulfur lines, something that can be readily appreciated from the red-ish colours of the bubble. Instead, gas excited by X-ray radiation will emit strongly in oxygen lines, as can be seen by the blue-ish colour south from the ULX position. This is an indication that the X-ray radiation from the accretion disk is ionizing the nearby gas. In essence, this image is a perfect illustration of the astounding power a single one of these sources can have on the interstellar medium.
To explain such cavity by a single supernova explosion, we have estimated that the energy required would be more than 100 times greater than that of typical supernovae. Instead, the large cavity could be the result of a powerful ejection of gas shocking the interstellar medium, providing more evidence in favor of this particular ULX as an extreme version of regular X-ray binaries.
But it gets even more interesting. As you can see there are two bright red-ish blobs north of the ULX towards the left-upper corner of the image. The strong sulfur emission there tells us that these regions are also excited by shocks. However, while at first glance it may appear to belong to the cavity created by the ULX, we have used high-resolution imaging from the Hubble Space Telescope to show instead that these are two supernova remnants coincidentally close to the bubble, almost as if trying to fool us! Our study illustrates perfectly why these ULX bubbles were initially identified as supernova remnants: the blast-wave from a supernova explosion will make the gas glow in similar atomic lines as those excited by the supersonic ejections from the ULX. At the same time, it is also obvious from the image that the size of these supernova remnants is dwarfed by the ULX bubble.
The seashell nebula: a distorted supernova remnant by the jets of SS433
While ULXs were discovered when observing external galaxies, you may be wondering whether there is something similar in our Galaxy. It turns out that there is a fascinating binary in our Galaxy termed SS433, which, due to its relative proximity, has allowed astronomers to learn a great deal about it. The compact object (whose nature is yet unknown) is in a binary system with a supergiant star which orbit each other every 13 days. The system is thought to be engulfed in its own dense outflowing gas as a result of the intense accretion process, similar to ULXs. More surprisingly is the fact that SS433 launches two bipolar and relativistic jets from the accretion disk, at 0.26 times the speed of light! After decades of observations, astronomers have discovered that, yet for unclear reasons, the accretion disk wobbles or precesses — like a spinning top— with a period of 162 days, causing the relativistic jets to trace a corkscrew pattern (as shown in the video). And yet more interestingly, SS433 sits at the center of an enormous faint nebula: it is thought that the jets have drilled the supernova remnant that gave birth to the compact object, creating the beautiful seashell-shaped nebula around it.
SS433 could be an analogous nearby version of the extragalactic ULXs, offering an accessible template to understand their nature. It may be therefore possible that the bubbles found around ULXs were also created as a result of the powerful ejections, distorting the pre-existing supernova remnant which formed the compact object. Whatever the exact formation mechanism, the presence of these large bubbles of ionized gas indicates that, notwithstanding their tiny sizes, these mighty sources might even play an important role in shaping their host galaxies and therefore, having an impact on their formation and evolution.
Link to the scientific article.
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