This just in: Multi-messenger merger made meticulously on massive supercomputer!

Title: Jet from binary neutron star merger with prompt black hole formation 

Authors: Kota Hayashi, Kenta Kiuchi, Koutarou Kyutoku, Yuichiro Sekiguchi, and Masaru Shibata

First Author’s Institution: Max Planck Institute for Gravitational Physics 

Status: Published on Arxiv [open access]

Multi-messenger astrophysics: all the cool kids are doing it! If you’re not in the know, multi-messenger astrophysics is when we observe the same event through multiple observational channels. One of these is typically light, since that’s what we’ve been using since we learned to tilt our heads back 45 degrees to look at the sky. The other can be a bit more out there: cosmic rays, neutrinos,  gravitational waves, what have you. With the first observation of gravitational waves in 2015, they have been the hot new thing to use for multi-messenger astronomy. However, we have a grand total of one successful example of this, GW170817, where we observed the gravitational wave signal of two merging neutron stars and a gamma ray burst 1.7 seconds later, which lasted for about 2 seconds.

Figure 1: An artist’s impression of 2 merging neutron stars. Image Credit: Wikimedia Commons, University of Warwick/Mark Garlick

We’d really love to be able to do this again sometime, as the observation of neutron star collisions could be extremely useful in measuring cosmological distances (as standard sirens) or to better understand physics of materials at extreme densities (by probing neutron star equation of state). We still don’t have a great understanding of neutron stars and what happens when they merge, since there’s a lot of difficult physics going on. With that in mind, today’s authors sought to remedy this by performing the longest simulation of a binary neutron star merger, lasting a total of 1.5 seconds (though it took 130 million CPU hours to compute, so that’s nothing to sneeze at). What can this simulation teach us about these violent events? Today’s astrobite seeks to find out!

The merger in general

A binary neutron star merger (BNS) has three important phases: the inspiral, merger, and post-merger or ringdown. The inspiral is the phase when the neutron stars are orbiting each other and getting closer and closer due to the loss of orbital energy to gravitational wave emission. In this simulation, the two neutron stars, which are 1.25 and 1.65 solar masses respectively, orbit each other five times before merging. At the merger, there are a few possible options for what happens to these two NSs; either the two NSs can rapidly collapse to form a black hole, or if the resulting remnant is spinning fast enough, the centrifugal force can prevent it from collapsing. These hypermassive neutron stars  would survive less than half a second, but spin hundreds of times in that brief existence. However, this simulation is modeling a prompt collapse with no hypermassive neutron star formation. Once collapsed, the resulting black hole has a mass of 2.77 solar masses and is spinning very rapidly. Those of you with a calculator handy might notice that 1.25 + 1.65 is not 2.77, where did the missing mass go? Well, some of it is ejected from the system entirely and some of it forms a disk of matter surrounding the resulting black hole, called an accretion disk. This disk is about to be very important in how the system gives off light, so keep it in mind. So far, 20 milliseconds have passed.

Powering up a gamma ray burst!

Figure 2. A schematic for the magnetorotational instability. When two masses experience a disturbance while rotating in the disk, the magnetic force between them causes them to quickly spiral apart in a feedback loop process. This eventually drives turbulence in the disk. Image Credits: Understanding Relativistic Jets: Marek Abramowicz, Accretion disks.

Now we have a spinning disk of particles surrounding our resulting black hole, and whenever charged particles get moving really fast, you know crazy magnetic fields aren’t far behind. In particular, disks like this are subject to what is called a magnetorotational instability. With a seventeen letter word like “magnetorotational” you know the process is quite complicated, but for the purposes of this bite all you need to know is that within a rotating system, magnetic forces hold particles close together as though they were connected by a spring. Take a look at Figure 2. The two particles start out orbiting together, but say they undergo some small disturbance which causes one particle to take a more inner orbit (mi) and the other to take a more outer orbit (mo). Thanks to conservation of angular momentum, the inner particle tries to speed up. But the “spring” (i.e. the magnetic forces between the particles) pulls back on mi and pulls forward on m_o. But mi getting slowed down means it just falls into a lower orbit and mo getting sped up pushes it into a further out orbit, and now the exact process repeats again, except stronger since the “spring” has stretched even further! You can see why this is called an instability: a small little nudge causes the particles to undergo a feedback loop of spiraling apart. 

This runaway effect causes the disk to become turbulent, and the disk becomes what is called a dynamo. A dynamo is when the motion of an electrically charged fluid causes a magnetic field. For an example a little closer to home, it is believed that the reason that Earth has a magnetic field is because the motion of liquid iron in the outer core forms a dynamo. In our disk, this dynamo drives a very strong magnetic field which begins to launch the neutron star guts out of the system. The pressure of the gasses from the disk forces the material into a narrow beam or funnel, see Figure 3. Specifically, the material is launched out of the “north pole” and “south pole” of the system, away from the plane of  the disk. Along with this material comes an extremely bright outpouring of light as the strong magnetic field forms electromagnetic waves that can be launched out of the system. This light is extremely bright, carrying 1042 joules per second! (!!)

Figure 3: A snapshot of the simulation as the outflow of material forms. The arrows represent the strong magnetic field formed by the dynamo in the disk. You can find a movable 3D version of this figure here! Figure 1 in the paper

That’s about 100,000 times brighter than every star in our Milky Way galaxy combined. It is this extremely bright flash that is believed to be what we observe as gamma ray bursts (GRBs). After about a second, enough matter has been ejected from the system that that gas pressure  can’t sustain the funnel shape and the magnetic fields spread the beam open, decreasing the luminosity. Once most of the disk has been ejected, the system gets even dimmer. This prediction of the simulation, that the GRB begins to stop after about a second, matches quite well with real observed GRBs! However, despite being insanely bright, these simulations actually still underpredict the amount of light we actually observe, so there is still more to learn about what drives these impressive bursts of energy.

Other cool signals

What else might we expect to see in BNS mergers? The merger of two massive objects results in a gravitational wave signal that increases in frequency and strength as the merger proceeds. If you’ve read any other gravitational wave astrobites, you’ve almost certainly seen a figure that looks like Figure 4 before. 

Figure 4: The gravitational waveform seen in the simulate merger. The frequency and amplitude increase during the inspiral until the merger, then a smaller more jumbled signal as the black hole settles during ringdown. Figure 4 in the paper.

What is maybe a little more unexpected is the little wiggle right after the merger (which occurs at 0 on the plot). This comes from the resulting black hole as it warps and bends from the violence of the merger – the black hole rings like a bell. It quickly settles into its final spherical shape, but we can see the effects of this shaking in the gravitational wave signal; this is called the “ringdown” phase of the merger. 

The final observable we might expect to see from a BNS merger are neutrinos! These extremely tiny fundamental particles are produced in lots of particle reactions that involve the weak force. There are several different types of neutrino, each associated with a specific lepton (like an electron) or anti-lepton (anti-electron). The simulation is able to capture how many of each type of neutrino is produced in the disk during the event (see Figure 5). The small peak seen at 0.1 seconds is thanks to the magnetorotational instability in the disk we mentioned before hitting its maximum strength.

Figure 5: The different types of neutrinos produced in the merger as a function of time. Figure 4 in the Paper.

Afterwards, the production falls off as the disk begins to lose gas pressure, the temperature drops and the interactions that produce neutrinos become less frequent. At this point, more particles begin to fall into the black hole and fewer escape. While this effect does not last long, the energy flux is so huge we might be able to detect these neutrinos here on Earth.

Sitting back to watch the whole show

So in total, what did we see? While pre-merger physics drives a strong gravitational signal, the intricate magnetohydrodynamic processes in the disk post-merger also drive some crazy energetic phenomena, making the BNS an excellent candidate for multi-messenger observations. It seems that our understanding of these events is relatively good, as the GRB produced is the correct length of time to match observations, if  a bit on the dim side. These gamma ray bursts need to be understood quite well if they are to be used in cosmology or understanding the physics of neutron stars, so the authors end by proposing another simulation that will better capture the physics of the magnetic field. Now that you’ve reached the end of this astrobite, I highly recommend you watch the amazingly cool video of the entire simulation here.  Simulations and experiments like this really highlight the interconnectedness of all of physics, the interplay of fluid mechanics, particle physics, electrodynamics, and gravity all come together to create some of the most spectacular fireworks shows in the universe!

Astrobite Edited By: Megan Masterson

Featured Image Credit: Wikimedia Commons, NSF/LIGO/Sonoma State University/A. Simonnet

Author

  • Cole Meldorf

    I am a PhD student at the University of Pennsylvania studying Astrophysics, specifically observational and theoretical cosmology. I also do some research with the Dark Energy Survey on galaxy evolution and supernova cosmology. When I’m not dying under the crushing weight of finals, I play the violin, do a little theater, and like to cook!

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