White Dwarfs, Neutron Stars, and Fast Radio Bursts

White Dwarfs, Neutron stars, and fast radio bursts

Having ejected nearly all their black holess, the centers of core-collapsed globular clusters are dominated by the next most massive stellar remnant populations: neutron stars and massive white dwarfs (see Figure 1). In Kremer et al. (2021b), we showed the presence of white dwarfs in globular clusters leads naturally to the dynamical formation of merging white dwarf+white dwarf binaries that are observable as gravitational wave sources at milli- and decihertz frequencies as they inspiral. Notably, we showed that over 90% of these mergers have a total mass greater than the Chandrasekhar limit as a result of mass segregation in the cluster. Thus, if the merger remnants are not destroyed completely in an explosive transient, they likely collapse and create a population of young pulsars/magnetars.

Figure 1: Adapted from Kremer et al. 2021b. Enclosed mass versus radius for various stellar populations for our best-fit model for the cluster NGC 6397. The vertical dashed black line denotes the cluster-centric distance within which Vitral & Mamon 2021 constrained a diffuse subsystem of roughly 1000 solar masses in NGC 6397. We argue that, in NGC 6397, white dwarfs are the most plausible explanation for this massive dark population.

A recent result connected to this topic is the repeating fast radio burst observed in a globular cluster in the nearby galaxy M81 (Bhardwaj et al. 2021; Kirsten et al. 2021). The origin of fast radio bursts remains unclear, however the Galactic fast radio burst found coincident with a known magnetar (e.g., Bochenek et al. 2020) provides clear evidence that some fast radio bursts are magnetar-powered. Magnetars are traditionally expected to form in association with massive stellar evolution in part due to the association of a number of Galactic magnetars with supernova remnants (e.g., Kaspi & Beloborodov 2017). However, in old (t>10 Gyr) globular clusters, neutron stars formed through massive star evolution have been inactive for billions of years. Thus, alternative models are necessary to explain the M81 fast radio burst. In Kremer et al. 2021c, I explored dynamical formation scenarios for fast radio bursts in old globular clusters and showed that young highly-magnetized neutron stars formed through white dwarf mergers are the most natural mechanism. This channel operates at a rate consistent with the fast radio burst rate inferred from the M81 detection. Also, properties of neutron stars expected to form through WD mergers (e.g., magnetic fields of roughly 10^11 G and spin periods of 10 ms; Schwab 2021) produce burst energetics consistent with the M81 burst.

Figure 2: In black we show all observed Galactic radio pulsars (circles; Manchester et al. 2005) and Galactic magnetars (stars; Olausen & Kaspi 2014). In blue we show the four apparently young pulsars in the Milky Way globular clusters (Boyles et al. 2011). In green, we show estimates for birth properties of neutron stars formed via white dwarf mergers computed from the observed features of isolated white dwarfs in Ferrario et al. (2015) and (the green triangle) Caiazzo et al. (2021). As the neutron stars represented as green points evolve along tracks of constant magnetic field (dashed contour lines) as they spin down via magnetic dipole radiation, they pass through the region occupied by the four young pulsars, suggesting a connection between these two populations.

Of further relevance is a population of six slow-spinning radio pulsars (spin periods roughly 0.3-3 seconds) in the Milky Way globular clusters (Boyles et al. 2011, Zhou et al. 2024). With magnetic fields of roughly 10^12 G and characteristic ages less than 100 Myr, these apparently young pulsars are also difficult to reconcile with standard neutron star formation scenarios involving massive stars. Invoking observed properties of magnetic white dwarfs, I showed in Kremer et al. 2022 that these neutron stars can also be accounted for by massive white dwarf mergers. All six of these young pulsars are found in dense clusters that have (or have nearly) undergone core collapse, consistent with prediction for white dwarf merger origin. Fascinatingly, these young pulsars are consistent with being descendants of neutron stars initially capable of powering FRBs analogous to the source observed in M81.

Based on the presence of the M81 FRB source, in Kremer et al. 2022, I estimate the number of similar FRB sources detectable in the globular cluster systems of nearby galaxies (e.g., Harris et al. 2013). The large elliptical galaxy M87 – known to host in excess of 10,000 globular clusters – likely contains the most active FRB sources of all nearby galaxies at present day, up to of order 10 sources. By scaling to the detected burst rate of the M81 FRB source and incorporating the uncertain burst energy distribution of such sources, I also estimated the detectable burst rate in these globular cluster systems. A dedicated radio survey of M87 of duration roughly 10 hr by FAST has a 90% chance of detecting at least one globular cluster FRB, even for the most pessimistic assumed burst energy distribution. The detection of even a small number of additional FRBs like the M81 source in nearby galaxies that could be localized to specific globular clusters would be pivotal to the FRB field. My work suggest such detections are not only possible, but likely, motivating targeted radio surveys of the globular cluster systems of local galaxies.

Figure 3: From Kremer et al. 2022. Distribution of white dwarf merger masses for all mergers occurring at late times in an M87 globular cluster sample. Merger outcomes are adapted from Shen (2015). Solid black lines denote boundaries separating the different white dwarf compositions (also shown as different colored scatter points): helium white dwarfs (M<0.5M⊙), carbon-oxygen white dwarfs (0.5M⊙<M<1.2M⊙), and oxygen-neon white dwarfs (M>1.2M⊙). The gray shaded region in the bottom-right denotes where an accretion disk forms and stable mass transfer is expected (e.g., Marsh et al. 2004). As shown, roughly 90% of mergers have mass in excess of the Chandrasekhar mass (dashed line) and roughly 70% have properties consistent with a neutron star formation outcome.

In addition to my dynamics work, I have studied mass transfer in close double white dwarf binaries. In Kremer et al. 2015, I examined the stability of such systems as they undergo direct-impact accretion and in Kremer et al. 2017, I explored the implications of these accreting white dwarf binaries for LISA, predicting a unique class of gravitational wave sources: mass-transferring white dwarf binaries that exhibit reverse gravitational chirps, i.e., systems that evolve toward lower gravitational wave frequencies as a consequence of mass-transfer.