Using Observations & Deductions To Infer The Identity Of The Nearest Black Hole

Astronomers deduced the closest black hole to Earth in the constellation Telescopium.

Supermassive black holes—millions or billions of times more massive than the Sun—anchor the centers of most galaxies. We examined how the history of such monsters has been documented in a previous post, based on a presentation at the 236th AAS meeting, i.e.

• A New Perspective On The History Of Supermassive B...

But we know that smaller  (stellar) black holes,  i.e. at just a few solar masses, exist and should theoretically be scattered throughout galaxies. A few hundred candidates have been found in the Milky Way, including Cygnus X-1,. Now, as reported in Eos: Space Science Journal,  we learn  that Thomas Rivinius, an astronomer at the European Southern Observatory in Santiago, Chile, and his team have recently identified another one of these stellar mass black holes.  Further, it holds a special importance given  it’s the closest black hole to Earth yet discovered. The ESO team's findings also shed light on the dynamics of supernova explosions that create black holes, the team suggested.

Let's be clear it isn't easy to find any of the many black holes that aren’t consuming matter—since they don’t produce X-rays like Cygnus X-1. Sometimes serendipity strikes, however, and the universe aligns itself just right to reveal these "wallflower" black holes. That’s what Rivinius and his collaborators found when they examined HR 6819, a seemingly ordinary pair of stars about 1,000 light-years away in the constellation Telescopium.

In 2004, Rivinius and his colleagues trained a 2.2-meter telescope in La Silla, Chile, on HR 6819.  According to Rivinius:

“We thought it was only two stars,”

But to their surprise, they discovered that one of the stars was wobbling in a circle, or "being flung around" in the parlance of Rivinius.  That’s the telltale sign of a companion star, a nearby object that’s tugging gravitationally on the observed celestial object. So HR 6819 wasn’t just a pair of stars—it was three objects: one star on a relatively wide orbit and one star paired with something unseen, the team concluded.

The astronomers calculated that the mysterious third object in HR 6819 had to be at least about 4 times the mass of the Sun. Given a star of that mass we know it would generate light to be visible even if it belonged to the dimmest class of stars (spectral class M),  Rivinius and his collaborators ruled out fainter objects like white dwarfs and neutron stars because they’re typically of much lower mass. That left only one logical conclusion: The unseen object was a black hole.

That hypothesis based on this deduction languished for several years, however, after tragedy struck unexpectedly: A team member died in a car accident in June 2014.    In Rivinius' words: “The study stalled,”.  But last year, new results spurred Rivinius and his colleagues to revisit their findings. Another team of researchers had reported finding a black hole using the same method. Rivinius then recalled seeing a press release and thinking, “Wait a second—I have something in the drawer that looks exactly the same.”

Rivinius and his collaborators estimated that the black hole in HR 6819 was about 1,000 light-years from Earth, making it the closest known black hole. Its proximity implied that systems like this one are common.  In Rivinius' words (ibid.):

“Our neighborhood is nothing special.  If it’s here, it must be everywhere.”

These results were published this month in Astronomy and Astrophysics.

As for the relevance to the dynamics of supernova explosions,  the existence of HR 6819 sheds light on how these create black holes.  It’s long been believed that such explosions are antisymmetric, meaning they send matter flying preferentially in one direction, with the result that the black hole is launched in the other direction. However,  finding a black hole gravitationally bound to a star implies that in some cases, black holes aren’t flung from their birthplaces. That means supernova explosions are sometimes symmetric.

Determining what fraction of supernovas are symmetric versus antisymmetric clearly requires a larger sample size for these less massive black holes. According to  Todd Thompson, a theoretical  astrophysicist at the Ohio State University in Columbus not involved in the research, that is entirely possible.  In his words (ibid.):

“There are probably a million black holes in the galaxy that have binary companions that are stars. That’s a very big sample that we should get busy trying to understand.”

Indeed.  And if black hole astrophysics is to progress it means the dynamic problems associated with the initiating supernova explosion are essential to solve.   This will requires excellent observations in tandem with improved theoretical astrophysics.

See Also:

https://www.nationalgeographic.com/science/2020/05/closest-black-hole-to-earth-found-hiding-in-plain-sight/

Selected Questions-Answers From ALL Experts Astronomy Forum (Dark Matter Planets?)

Question: Scientists know that there must be more mass than what is visible in galaxies as they manage to hold themselves together despite not being enough ordinary matter, so they call it dark matter as it doesn't emit anything but is observable through its gravity. Then if this dark matter can generate gravity similar to ordinary matter then can it not interact with itself and make dark matter clumps that are gravitationally detectable. Like a dark matter planet? If this was true i imagine a dark matter planet would be detectable as it would skew the orbits of ordinary matter planets? - Puzzled

Answer:

The conception of dark matter planets or solar systems for that matter is dependent on how well we can distinguish exotic forms from ordinary forms. In the latter case one might reference Fritz Zwicky's measurements of galaxy clusters which  highlighted a ‘missing mass’. He found that the mass needed to bind a cluster of galaxies together gravitationally was at least ten times the apparent mass visible.  Around the same time there were observations of stellar motions in the galactic plane by Dutch astronomer Jan Oort. He found there had to be at least three times the mass visibly presenting in order for stars not to escape the galaxy and fly off into space.

What exactly constituted this 'missing mass'.? The short answer is we don't know but there a number of candidates. By the late 1970s (with Cygnus X-1), astronomers realized there were other forms of dark matter. Among the most discussed candidates were black holes, marking the end stage of evolution for very massive stars. In the black hole, the gravity is so strong that no light escapes and the mass typically is much greater than that of the Sun.

Currently, we are aware of a super black hole at the center of our galaxy with 9.7 billion times the mass of the Sun.  Can such an entity form planets? Hardly! It would more likely 'devour' them.

Dark matter generally occurs in either baryonic or non-baryonic forms, depending on whether the matter reacts with radiation or not. If it doesn’t, it’s non-baryonic. Baryons include protons and neutrons, while non-baryons include electrons and neutrinos.

Non-baryonic dark matter further breaks down into cold dark matter and hot dark matter. The terms hot and cold are not so much indicative of current temperatures, as the phase of the early universe at which the particular dark matter ‘decoupled’ from the hot radiation background. An early decoupling implies a higher ambient background radiation temperature of the primeval cosmos. A later decoupling correlates to a cooler temperature. Perhaps the most widely studied candidate of hot dark matter is the neutrino.

By contrast, cold dark matter candidates tend to have larger mass and amongst the most likely suspects are: gravitinos, magnetic monopoles, and primordial black holes. However, there are a couple of exceptions to this, which include: WIMPs and Axions.

How any of these entities could leave specific gravity signatures that distinguish any one from their dark counterparts is a matter of continued inquiry.  Until such signatures are identified and clarified there is little chance of even remotely confirming on the existence of  "dark matter planets". Especially if it is proposed that such a planet could orbit an ordinary matter (e.g. hot plasma) star.

It is certainly plausible that a dark matter object (like a small black hole) could "skew the orbit" of an ordinary matter planet - or star. But that a dark matter planet could exist within the confines of an ordinary matter solar system is another matter. And if such a solar system was entirely of dark matter, how would one detect it at all?

Your conjecture of dark matter clumps  spinning off from a much larger dark matter mass (e.g. proto- star)  evokes a takeoff on the nebular hypothesis of origin for our own solar system.  The problem is that while such an entity (dark matter proto-planet) might be possible, there is no evidence whatsoever it exists. For example, all of the hundreds of exoplanets found thus far have all been ordinary matter objects - no 'dark matter'.

This is not to say such planets will never be found, but rather the investment of time, technology an resources may be much more than current cost-benefit analyses allow.