Neutrinos and Natal Kicks in the Inert Black Hole Binary VFTS 243

Title: Constraints on neutrino natal kicks from black-hole binary VFTS 243

Authors: Alejandro Vigna-Gómez, Reinhold Willcox, Irene Tamborra, Ilya Mandel, Mathieu Renzo, Tom Wagg, Hans-Thomas Janka, Daniel Kresse, Julia Bodensteiner, Tomer Shenar, Thomas M. Tauris

First Author’s Institution: Max-Planck-Institut für Astrophysik, Garching, Germany

Status: Published in PRL [open access]

While it may seem morbid to some, astronomers love to fixate on death. One type of death that astronomers find particularly fascinating is that of a massive star, referred to as a core-collapse supernova. Astronomers obsess over core-collapse supernovae because their precise mechanisms remain cloaked in uncertainty In the standard picture of stellar death, massive stars with masses greater than 8 times the mass of the Sun collapse, forming either a neutron star or black hole (BH) and ejecting a large fraction of their stellar material. This mass ejection is typically asymmetric, and thus in order to conserve momentum, the newly birthed neutron stars or BHs are accelerated in a process known as a natal kick

Artist’s impression of the inert BHB VFTS 243, the subject of today’s autopsy (bite). Image credit: Shenar et al. (2022). Nature.

However, it is also possible that very little stellar mass is ejected, giving rise to a phenomenon known as the complete or direct collapse into a BH. Since only a small amount of stellar material is ejected, the process is less asymmetric than standard core-collapse supernovae, meaning that the BH natal kick velocities are typically low. Instead, energy and momentum are conserved via the emission of gravitational waves (learn more about them here and here) and neutrinos, incredibly tiny and abundant fundamental particles. 

To understand the natal kicks imparted to BH upon stellar collapse, astronomers often turn to observations of high-mass X-ray binaries. These are two-body systems consisting of a compact object, for example a BH, which accretes mass from a hot, massive, companion star. However, if no mass transfer occurs, these objects are X-ray quiet and are referred to simply as inert BH binaries (BHBs). Compared to high-mass X-ray binaries, inert BHBs are characterised by larger and less eccentric or elliptical orbits, higher BH masses, and lower overall velocities. Thus, if we observe a BHB with such features, it is likely to be the result of a star’s complete collapse to a BH, and can serve as a direct probe of natal kicks given that little mass transfer has occurred. 

Meet VFTS 243, an inert BHB in the Large Magellanic Cloud with a BH mass of ~10 times the mass of the Sun and a low eccentricity of 0.017. Together, these properties suggest that the system is likely the result of the direct collapse of a BH. 

To test this, the authors of today’s paper applied a model to calculate the probability that a given pre-collapse binary that received a natal kick during BH formation could produce the orbital configuration observed today. Their analysis assumed the initial binary was circular, the initial orbital period was between 5 and 15 days, and that the natal kick was unidirectional. 

The results of their analysis are shown in Figure 1, which gives the amount of mass ejected, dM, on the x-axis and the natal kick velocity, vkick, on the y-axis. In the plot, the darker the colour, the more likely the combination of vkick and dM are to produce the observed properties of VFTS 243. Evidently, values of vkick around 4 km/s and dM around 0.3 times the mass of the Sun are most probable. 

Figure 1. A plot of the most likely ejected masses (dM) and kick velocities, v_kick, that can reproduce the orbital configuration of VFTS 243 observed today. The darker regions indicate the more probable parameter values. The vertical blue region, derived from supernova simulations, indicates the range of ejected masses that can result from the emission of neutrinos alone. Figure 1 in the paper.

The vertical blue band in Figure 1 represents the estimated range of ejected masses from stellar models that may be attributed to the emission of neutrinos alone rather than stellar material. Given that the most probable values of vkick and dM for the model of VFTS 243 fall within this range, it is likely that VFTS 243 is indeed the result of the direct collapse of a star to a BH with the majority of the mass lost in the form of neutrinos.

Previously, astronomers only knew of a handful of inert BHBs in the Milky Way and the Large Magellanic Cloud. Therefore, the likely confirmation of VFTS 243 as the result of the direct collapse of a massive star to a BH and that its observed kick velocity is due to the ejection of neutrinos alone is particularly exciting. With this result, it is becoming apparent that very massive stars with core masses greater than 10 times the mass of the Sun can indeed end their lives via the direct collapse mechanism. In the future, the detection of new inert BHBs should make things clearer, and provide additional material for those astronomers who love to ruminate on stellar death.

Astrobite edited by William Smith

Featured image credit: Shenar et al. (2022). Nature.

About Sonja Panjkov

I'm a second-year PhD student at the University of Melbourne. My research focuses on the high-energy emission from the supernova remnants in the Magellanic Clouds. In my spare time, I enjoy hanging out with my cats and going to see live music.

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