Title: Mixed origins: strong natal kicks for some black holes and none for others
Authors: Pranav Nagarajan, Kareem El-Badry
First Author’s Institution: Department of Astronomy, California Institute of Technology
Status: Accepted to PASP
Massive stars die in brilliant, fiery explosions called supernovae. In these cosmic fireworks, the outer layers of a star bloom into pretty structures like the Crab Nebula, while the inner core collapses in on itself to produce a neutron star or black hole.
Unfortunately, these baby compact objects don’t get any sweet lullabies — some supernovae ‘kick’ their newborn neutron stars or black holes instead. These are called natal kicks, and they’re caused by asymmetries in the mass loss and the emission of neutrinos during the supernova. Studying these natal kicks can tell us more about the formation and evolution of neutron stars and black holes. While neutron star natal kicks have been studied in great detail, we don’t yet understand black hole kicks nearly as well. Today’s authors help to fix this gap in our knowledge.
Isolated black holes are hard to study
One part of the problem is that isolated black holes are very difficult to observe. While neutron stars can often be seen by radio telescopes as bright pulsars (rapidly rotating neutron stars that burp out pulses of intense radiation), isolated black holes can only be detected through microlensing, where light from a distant object bends due to the passing of a black hole or another dense object along our line of sight. However, this alignment is pretty rare, and we’ve only confirmed the discovery of one microlensed black hole so far.
Thankfully, there’s another option: we can study binary systems composed of a black hole and a star instead. Looking at the bright star’s motion allows us to deduce information on its companion black hole. Today’s authors look at twelve of these binary systems in order to investigate possible natal kicks.
The velocities of black holes can help reveal if they were kicked at birth
In order to investigate whether any of these black holes were kicked, the authors created a comparison sample of local stars. If the black holes were kicked hard enough at birth, their velocities should be significantly different from the general stellar population around them.
The authors found that for six of their twelve systems, the black hole’s velocity showed possible evidence for a natal kick. Their velocities were different from over 68% of the comparison sample of local stars, with a few found to be different from over 95% of the comparison sample — wow! The velocity distributions for two of the black hole systems — one that shows evidence of a kick, and one that doesn’t — are shown in Figure 1.
(Adapted from Figures 2 and 3 in today’s paper.)
Looking at orbits can help, too!
For black holes that haven’t been kicked, we expect their galactic orbits to be similar to our Sun: they should be more-or-less circular and shouldn’t travel far from the midplane of the galaxy. A natal kick could disturb a black hole’s orbit and make it puffier (meaning it travels further from the midplane) and more elliptical. An important caveat here is that the galactic orbit of a black hole can be affected by more than just natal kicks, so this isn’t solid evidence on its own.
In order to investigate this in their sample of twelve black holes, the authors used the Python package galpy to model their orbits back in time over one billion years. For nearly all of the black holes that didn’t show evidence of kicks in their velocities, the authors found that their orbits did look like the Sun’s! In contrast, the black holes that had some evidence of natal kicks had puffier and more elliptical orbits. This supports the idea that those black holes could have been kicked at birth. Modeled orbits for a kicked and un-kicked black hole are shown in Figure 2, along with the Sun’s orbit for comparison.
So, what can we say about black holes in general?
The authors concluded that half of their binary black hole systems have at least some evidence for a natal kick, and a couple of them have strong evidence for a natal kick. A caveat is that the strongest natal kicks can unbind binary systems (and the black hole would be ejected away from its companion star), so any study that only looks at binary systems — like this one — will be biased towards finding gentler kicks.
Regardless, the authors find that there’s compelling evidence that some black holes in the universe receive significant natal kicks and others do not. Future observations will need to study more binary systems in order to tighten the constraints on black hole natal kicks and help us better understand our super-dense newborn neighbors.
Astrobite edited by Ryan White and Megan Masterson
Featured image credit: ESO/L. Calçada
