Our Milky Way Galaxy has a secretive heart of darkness, enshrouded in mystery, and well-hidden from our sight. Within this strange region, there resides a powerful gravitational beast–a supermassive black hole named Sagittarius A*–or Sgr A* (pronounced saj-a-star) for short, that weighs millions of times more than our Sun. Although Sgr A* has kept its many secrets hidden from the prying eyes of curious astronomers, it is now finally beginning to tell its story–and what a story it is! In May 2018, astronomers using NASA’s Chandra X-ray Observatory, announced that they have discovered evidence of the existence of thousands of relatively small stellar-mass black holes, performing an exotic ballet near the dark heart of our Galaxy, where Sgr A* dwells. Black holes of stellar mass typically weigh-in at between 5 to 30 times solar-mass, and this newly discovered treasure trove filled with these “smaller” objects lies within three light years of where Sgr A* reigns in secret, sinister splendor–a bewitching heart of darkness that keeps things to itself.
Three light-years is a very short distance on cosmic scales. Theoretical studies of the dynamics of stars inhabiting galaxies have suggested that a significant population of stellar-mass black holes–perhaps as many as 20,000–could wander inward as time goes by, ultimately gathering around Sgr A*. This recent study using data obtained from Chandra provides the first observational evidence for the existence of such a bevy of bewitching black holes in the heart of our Milky Way.
Stellar-mass objects are born as the result of the gravitational collapse of an especially massive star. This bizarre birth is usually heralded by a brilliant display of celestial fireworks called a supernova. Supernovae are the most powerful stellar explosions known, and they are so bright that they can frequently be observed all the way out to the very edge of the observable Universe–and they can actually outshine their entire galactic host for a brief blink of the eye on cosmic time scales. Stellar-mass black holes are frequently termed collapsars.
A stellar-mass black hole, that is tightly locked in a close orbit with a star, will steal gas from its unfortunate companion. Astronomers term these systems X-ray binaries. The stolen stellar material tumbles into a disk that heats up to millions of degrees and emits X-rays before vanishing into the hungry maw of the gravitational beast. Some of these X-ray binaries appear as point-like sources in the Chandra image.
Therefore, the black hole is observable in X-rays. In contrast, the victimized companion star can be observed by astronomers using optical telescopes. The energy release for both black holes and neutron stars are of the same order of magnitude and, for this reason, astronomers frequently find it difficult to distinguish between the two objects.
Neutron stars are the very dense, city-sized remains of a massive star that has perished in the blazing fireworks of a supernova explosion. Indeed, neutron stars are so dense that a teaspoon full of neutron star-stuff can weigh as much as a school of whales. Nevertheless, the massive stars that are the progenitors of neutron stars are not as massive as the stars that collapse to become stellar-mass black holes.
The good news is that neutron stars sport some identifying attributes. Neutron stars display differential rotation, and can possess both a magnetic field and localized explosions–termed thermonuclear bursts. Whenever these tattle-tale properties are observed by astronomers, the compact object inhabiting the binary system reveals itself to be a neutron star–rather than a stellar-mass black hole.
The derived masses come from observations of compact X-ray sources that combine optical data with X-ray data. All of the neutron stars that have been identified so far show masses below 2.0 solar-masses. None of the compact systems with masses above 2.0 solar-masses, that have been observed, display the properties of a neutron star. Therefore, the combination of these properties makes it increasingly more probable that the class of compact stars sporting masses above 2.0 solar-masses are truly stellar-mass black holes.
Our Galaxy hosts several stellar-mass black hole candidates, which reside closer to Earth than Sgr A*. Most of these candidates are members of X-ray binary systems in which the compact member of the duo steals stellar material from its partner by way of the accretion disk.
A black hole–of any size–can be described as having only three properties. According to certain so-called “no hair” theories, a black hole has three fundamental properties: mass, electric charge and spin (angular momentum). Scientists generally believe that all black holes are born in nature with a spin. However, no definite observation of this spin has been recorded–at least, not yet. The spin of a black hole of stellar-mass is due to the conservation of the angular momentum of the massive progenitor star that produced it.
The gravitational collapse of a massive star is a natural process. It is inevitable that when a massive star at last reaches the end of that long stellar road–in which all of its stellar energy sources are depleted–it collapses under the relentless pull of its own gravity, and then blows itself to smithereens in the fiery grand finale of a supernova blast. If the mass of the collapsing portion of the progenitor star is below the limit for neutron-degenerate matter (Tolman-Oppenheimer-Volkoff–TOV–limit), the end result is a compact star. The compact star can be either a white dwarf or a neutron star–but it may also be a still-hypothetical stellar object called a quark star. However, if the progenitor star, that is in the process of collapsing, sports a mass greater than the TOV limit, the intense squeeze of its own gravity will go on and on and on until zero volume is reached and a new stellar-mass black hole is born around that point in space.
According to Albert Einstein’s Theory of General Relativity (1915), a black hole of any mass can exist in nature. The lower the mass, the higher the density of matter has to be in order to give rise to a black hole. No known process can form a black hole with a mass less than a few times solar-mass. If there are black holes that small, existing anywhere in the Universe, they are probably primordial black holes.
Over the past twenty years, astronomers have managed to gather sufficient evidence in support of the idea that our Milky Way does indeed host a supermassive beast in its secretive heart of darkness. There, hidden in our Galaxy’s center, it waits for its dinner–a shredded star, perhaps, or a cloud of doomed gas. Because this mysterious object lurks relatively close to our own planet, it provides valuable information to astronomers about the bewitching, bothersome, and bewildering way extreme gravity behaves. For this reason, Sgr A* also sheds fascinating new light on General Relativity. Because black holes are so completely black, astronomers try to understand their exotic properties by observing the light that is emitted from the searing-hot, glaring gas immediately surrounding them (accretion disk).
Treasure-Trove Of Stellar-Mass Black Holes
A team of astronomers, led by Dr. Charles Hailey of Columbia University in New York, used data derived from Chandra to hunt for X-ray binaries that contain black holes residing close to Sgr A*. The scientists studied the X-ray spectra (the amount of X-rays observed at differing energies) of sources residing within approximately 12 light-years of our Milky Way’s supermassive dark heart.
The team then went on to select sources displaying X-ray spectra similar to those of known X-ray binaries that showed relatively large amounts of low-energy X-rays. Using this technique, the scientists were able to spot fourteen X-ray binaries located within about three light-years of Sgr A*. A duo of the X-ray sources are thought to contain neutron stars. This probability is based on the detection of characteristic neutron star outbursts seen in earlier studies. For this reason, the two sources were eliminated from the analysis.
Dr. Hailey and his team concluded that most of the remaining X-ray binaries probably contain stellar-mass black holes. The amount of variability that they have displayed over the passage of years is different from that predicted for X-ray binaries hosting neutron stars.
Only the most brilliant X-ray binaries containing black holes are detectable at the distance our Galaxy’s resident supermassive black hole is from Earth. For this reason, the detections included in this research suggest that a considerably larger population of undetected, fainter X-ray binaries (at least 300 to up to a thousand) host stellar-mass black holes in the general neighborhood surrounding Sgr A*.
This population of black holes, that possess a stellar companion close to Sgr A*, could shed new light on the mysterious formation of X-ray binaries resulting from close passages between stars and stellar-mass black holes. This discovery could also help future gravitational wave studies. That is because knowing the number of black holes, lurking in the heart of a typical galaxy, can help astronomers better predict how many gravitational wave events may be associated with them. Gravitational waves are ripples in the fabric of Spacetime itself, and they provide astronomers with a new way to study the Universe.
An even larger population of black holes of stellar mass, that are solitary, with no companion star to call their own, should also be dancing around near Sgr A*. According to theoretical follow-up research conducted by Dr. Aleksey Generozov (Columbia University) and his colleagues, there should be more than approximately 10,000 stellar-mass black holes haunting the hidden dark heart of our Milky Way Galaxy.
While Dr. Hailey and his colleagues favor the stellar-mass black hole scenario for their findings, they are not ruling out the possibility that as many as 50% of the observed sources are really from a population of millisecond pulsars. A Millisecond pulsar is a rapidly, regularly spinning newborn neutron star, fresh from the funeral pyre of its progenitor star that perished in the brilliant blast of a supernova. Millisecond pulsars possess very powerful magnetic fields.
A paper describing these results appears in the April 5, 2018 issue of the journal Nature.
The Chandra X-ray Observatory is a space observatory launched by NASA on July 23, 1999.