New LIGO Events Demolish The Idea Of A ‘Mass Gap’ Between Neutron Stars And Black Holes
“For decades, we knew only of neutron stars that existed below about twice the Sun’s mass, and black holes that existed at or above about five times the Sun’s mass. Beginning in 2017, we started to see neutron stars merging together to form black holes that fell into that empty range, but those events were relatively infrequent. However, this latest discovery — of two low-mass black holes merging together to form a heavier black hole — should close off the “mass gap” range for good.
What was once a region of unknowns should now be filled in by black holes. Although there’s still a lot of science left to do to determine how rare or common black holes of different masses are, particularly in the realm of population statistics, it would now be very surprising if there were a gap in masses between neutron stars and black holes. LIGO’s latest data has demolished that idea. Despite cries of, “NOT NOW LIGO,” the Universe continues to send data our way, and our scientific discoveries go on.”
For decades, we’ve known that supernovae make both neutron stars and black holes. But until LIGO started detecting gravitational waves, we’d never seen a neutron star over 2 solar masses, and we’d never seen a black hole of less than 5 solar masses. Although LIGO saw two neutron stars merge to form a black hole a couple of times, leading to a black hole in that “mass gap” range in both instances, it had never seen two “mass gap” black holes merging before.
Well, we’re almost a year into LIGO’s third (and upgraded) data run, and already it’s seen four, including a new one this past Monday. The mass gap should be gone, and LIGO’s the observatory that demolished it.
Did LIGO Just Discover Two Fundamentally Different Types Of Neutron Star Mergers?
“The first neutron star-neutron star merger ever directly observed was seen in both gravitational waves and in various forms of light, giving us a window into the nature of short gamma ray bursts, kilonovae, and the origin of the heaviest elements of all. The second one, however, had no robustly confirmed electromagnetic counterpart at all. The only major physical differences were the combined mass (2.74 vs. 3.4 solar masses), the initial object formed (neutron star vs. black hole), and the distance to the event (130 vs. 518 million light-years).
It’s possible that there really was an electromagnetic counterpart, and we simply weren’t able to see it. However, it’s also possible that binary neutron star mergers that directly lead to a black hole don’t produce electromagnetic signatures or enriched, heavy elements at all. It’s possible that this binary neutron star system, the most massive one ever discovered to date, represents a fundamentally different class of objects than have ever been seen before. This incredible idea should get put to the test over the next few years, as gravitational wave detectors continue to find more and more of these mergers. If there are two different classes of neutron star mergers, LIGO and Virgo will lead us to that conclusion, but we have to wait for the scientific data to know for sure.”
Neutron stars, when they merge, can produce gamma ray bursts, ejecta, the heaviest elements of all, and electromagnetic afterglows that cover nearly the full spectrum of where light can exist. We saw this with a gravitational wave + gamma ray signal that occurred on August 17, 2017, leading to a revolutionary understanding of kilonovae. But our second merging binary neutron star system, which was seen in gravitational waves on April 25, 2019, displayed no such signal at all.
It might be a failure of detection, or it might be due to some fundamental differences between these different events. My bet’s on the latter; here’s what we think might be going on!
NASA’s NICER Mission Reveals An Unexpected Neutron Star Surprise
“Even with our most powerful telescopes in all wavelengths of light, neutron stars only appear as points. NASA’s NICER mission, installed aboard the ISS in 2017, sought to change all that. The low-energy X-ray observatory measures timing signals down to 300 nanoseconds and at unprecedented sensitivities. NICER enables measurements of neutron stars’ sizes, masses, cooling times, stabilities, and internal structures.”
By looking at one particular pulsar located 1,100 light-years away, J0030+0451, two teams of scientists were able to measure the mass and size of a neutron star to unprecedented accuracy, but then did something even better. Based on the properties of the X-rays they saw, they were able to reconstruct the first-ever surface map of a neutron star. Whereas everyone expected that the neutron star would display a picture akin to one many of us have seen before, of a dipole magnet within a spinning stellar corpse, we instead found multiple ‘hot spots,’ all of which lie in the neutron star’s southern hemisphere.
This unexpected neutron star surprise really is forcing us to change what we thought we new about these objects! Come get the latest in the ongoing story of pulsars, one new data point at a time.
Yes, Virtual Particles Can Have Real, Observable Effects
“Now that the effect of vacuum birefringence has been observed — and by association, the physical impact of the virtual particles in the quantum vacuum — we can attempt to confirm it even further with more precise quantitative measurements. The way to do that is to measure RX J1856.5-3754 in the X-rays, and measuring the polarization of X-ray light.
While we don’t have a space telescope capable of measuring X-ray polarization right now, one of them is in the works: the ESA’s Athena mission. Unlike the ~15% polarization observed by the VLT in the wavelengths it probes, X-rays should be fully polarized, displaying right around an 100% effect. Athena is currently slated for launch in 2028, and could deliver this confirmation for not just one but many neutron stars. It’s another victory for the unintuitive, but undeniably fascinating, quantum Universe.”
If you think about empty space at a quantum level, you’ll find that it isn’t so empty, after all. Due to the inherent effects of quantum uncertainty, particle/antiparticle pairs pop into and out of existence continuously, including electrically charged particles. If you look at the quantum vacuum in the presence of a strong enough external magnetic field, the positive and negative particles, even though they’re only virtual particles, will move differently, and therefore will affect the real particles that pass through them differently than if there were no magnetic field. This leads to a real, observable signal that can be seen in space: around neutron stars!
Heisenberg first predicted this in 1936, and today, we know it’s true. Get the story of the first observable effect of vacuum birefringence today.
Merging Neutron Stars Made An Unstoppable Jet, And It Moves At Nearly The Speed Of Light
“How can you make a jet like this? We’ve only ever seen them from one other source: from black holes that are feeding on matter. That must be the clue that solves the puzzle! It isn’t that the merger itself created a jet, but that the completed merger produced a black hole, and this spinning black hole accelerated the matter around it, producing the jets that we saw afterwards. It explains why there was a dimming followed by a second round of brightening, and it explains the collimated structure and the fantastically large energies and speeds. Without a central black hole, there’s no known way to do it.
In science, sometimes the best results are the ones you weren’t expecting. We may have anticipated that merging neutron stars would create the heaviest elements of all, but no one saw a structured jet emerging from a black hole afterwards as something that should occur. Yet here we are, reaping the gifts of the Universe. It’s a reminder from the cosmos to us: the day we stop our scientific inquiries, we stop uncovering the mysteries that underlie our existence.”
On 2017, we saw two neutron stars inspiral and merge together, marking the first time we saw such a thing happen in both gravitational waves and in traditional light signals. But then we kept looking, and noticed something funny: while the light faded away after the explosion, it all of a sudden spiked again in the X-ray and radio parts of the spectrum. It took combining 207 days of data from 32 telescopes across five continents to figure out why, but the culprit is now clear.
There’s a black hole that formed, and it’s powering a jet that all the matter thrown off during the merger cannot stop. Come get the full story today.
What Was It Like When The First Stars Died?
“It’s theorized that this is the origin of the seeds of the supermassive black holes that occupy the centers of galaxies today: the deaths of the most massive stars, which create black holes hundreds or thousands of times the mass of the Sun. Over time, mergers and gravitational growth will lead to the most massive black holes known in the Universe, black holes that are millions or even billions of times the mass of the Sun by today.
It took perhaps 100 million years to form the very first stars in the Universe, but just another million or two after that for the most massive among them to die, creating black holes and spreading heavy, processed elements into the interstellar medium. As time goes on, the Universe, at long last, will begin to resemble what we actually see today.”
Our Universe, shortly after the Big Bang, proceeded in a number of momentous steps. The first atomic nuclei formed just minutes after the Big Bang, while neutral atoms took hundreds of thousands of years. It took another 50 to 100 million years for the very first star to be created, but only, perhaps, a million or two years for the most massive among the first stars to die. They may have been short-lived, but the first stars were truly spectacular, and their deaths set up the first steps in a changing Universe that would take us from a pristine set of materials to, eventually, the Universe as we know it today.
Take a major step in the cosmic journey of how we got to today by looking at what it was like when the first stars died!
What Happens When Planets, Stars, And Black Holes Collide?
“Brown dwarf collisions. Want to make a star, but you didn’t accumulate enough mass to get there when the gas cloud that created you first collapsed? There’s a second chance available to you! Brown dwarfs are like very massive gas giants, more than a dozen times as massive as Jupiter, that experience strong enough temperatures (about 1,000,000 K) and pressures at their centers to ignite deuterium fusion, but not hydrogen fusion. They produce their own light, they remain relatively cool, and they aren’t quite true stars. Ranging in mass from about 1% to 7.5% of the Sun’s mass, they are the failed stars of the Universe.
But if you have two in a binary system, or two in disparate systems that collide by chance, all of that can change in a flash.”
Nothing in the Universe exists in total isolation. Planets and stars all have a common origin inside of star clusters; galaxies clump and cluster together and are the homes for the smaller masses in the Universe. In an environment such as this, collisions between objects are all but inevitable. We think of space as being extremely sparse, but gravity is always attractive and the Universe sticks around for a long time. Eventually, collisions will occur between planets, stars, stellar remnants, and black holes.
What happens when they run into one another? Unbelievably, we not only know, we have the evidence to back it up!
6 Facts You Never Imagined About The Nearest Stars To Earth
“4.) There are no neutron stars or black holes within 10 parsecs. And, to be honest, you have to go out way further than 10 parsecs to find either of these! In 2007, scientists discovered the X-ray object 1RXS J141256.0+792204, nicknamed “Calvera,” and identified it as a neutron star. This object is a magnificent 617 light years away, making it the closest neutron star known. To arrive at the closest known black hole, you have to go all the way out to V616 Monocerotis, which is over 3,000 light years away. Of all the 316 star systems identified within 10 parsecs, we can definitively state that there are none of them with black hole or neutron star companions. At least where we are in the galaxy, these objects are rare.”
In the mid-1990s, astronomy was a very different place. We had not yet discovered brown dwarfs; exoplanet science was in its infancy; and we had discovered 191 star systems within 10 parsecs (32.6 light years) of Earth. Of course, low-mass stars have been discovered in great abundance now, exoplanet science has thousands of identified planets, and owing to projects like the RECONS collaboration, we’ve now discovered a total of 316 star systems within 10 parsecs of Earth. This has huge implications for what the Universe is actually made of, which we can learn just by looking in our own backyard. From how common faint stars are to planets, lifetimes, multi-star systems and more, there’s a huge amount of information to be gained, and the RECONS collaboration just put out their latest, most comprehensive results ever.
We’ve now confidently identified over 90% of the stars that are closest to us, and here’s what we’ve learned so far. Come get some incredible facts today!
LIGO’s Greatest Discovery Almost Didn’t Happen
“If all we had done was look at the automated signals, we would have gotten just one “single-detector alert,” in the Hanford detector, while the other two detectors would have registered no event. We would have thrown it away, all because the orientation was such that there was no significant signal in Virgo, and a glitch caused the Livingston signal to be vetoed. If we left the signal-finding solely to algorithms and theoretical decisions, a 1-in-10,000 coincidence would have stopped us from finding this first-of-its-kind event. But we had scientists on the job: real, live, human scientists, and now we’ve confidently seen a multi-messenger signal, in gravitational waves and electromagnetic light, for the very first time.”
Imagine the scene: it’s mid-August, 2017, and the Virgo detector has just joined the twin LIGO detectors barely two weeks ago. Amazingly, on August 14th, you’ve seen a gravitational wave signal in all three detectors; another black hole-black hole merger. Then, all of a sudden, even though the LIGO detectors are set to shut down later in the month, an extraordinarily significant signal goes off… but only in one detector. The LIGO Hanford detector sees a signal with a false-alarm probability of just one part in 300 billion; a slam dunk. Yet both LIGO Livingston and Virgo see nothing. A non-coincident signal should automatically be rejected, but somehow, one of the young researchers working on the project thought to check the Livingston data by hand… and that was where the secret lay.
LIGO’s greatest discovery, of two merging neutron stars, almost was overlooked. Thankfully, the hands-on nature of the scientists working on gravitational waves were able to turn this into the discovery of the century! (So far!)
Ask Ethan: How Does Spinning Affect The Shape Of Pulsars?
“[S]ome pulsars have incredible spin rates. How much does this distort the object, and does it shed material this way or is gravity still able to bind all of the material to the object?”
If you spin too quickly, the matter on the outskirts of your surface will fly off. If you’re in hydrostatic equilibrium, your shape will simply distort until your equatorial bulge and your polar flattening result in the most stable, lowest-energy configuration. For our Earth, this means the best place to launch a rocket is near the equator, and our planet’s polar diameter is a little more than 20 km shorter than its equatorial diameter. But what about for the fastest-rotating natural object we know of: a neutron star. While most neutron stars rotate a few times a second, the fastest one makes 766 rotations in that span, meaning that a neutron on the surface moves at about 16% the speed of light. Much faster, and could it escape? Or, perhaps, is the pulsar’s shape highly distorted, either due to that rotation or to the incredibly strong magnetic fields inside? Neutron star matter is very different from anything we’re used to, so don’t bet on any of those.
Other than the first few fractions-of-a-second, changes to neutron stars are slow and mostly inconsequential. Come find out how bad it is on this edition of Ask Ethan!