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Gravitational-wave (GW) detections of merging neutron star–black hole (NSBH) systems probe astrophysical neutron star (NS) and black hole (BH) mass distributions, especially at the transition between NS and BH masses. Of particular interest are the maximum NS mass, minimum BH mass, and potential mass gap between them. While previous GW population analyses assumed all NSs obey the same maximum mass, if rapidly spinning NSs exist, they can extend to larger maximum masses than nonspinning NSs. In fact, several authors have proposed that the ∼2.6M_⊙object in the event GW190814—either the most massive NS or least massive BH observed to date—is a rapidly spinning NS. We therefore infer the NSBH mass distribution jointly with the NS spin distribution, modeling the NS maximum mass as a function of spin. Using four LIGO–Virgo NSBH events including GW190814, if we assume that the NS spin distribution is uniformly distributed up to the maximum (breakup) spin, we infer the maximum nonspinning NS mass is${2.7}_{-0.4}^{+0.5}{M}_{\odot }$(90% credibility), while assuming only nonspinning NSs, the NS maximum mass must be >2.53M_⊙(90% credibility). The data support the mass gap’s existence, with a minimum BH mass at${5.4}_{-1.0}^{+0.7}{M}_{\odot }$. With future observations, under simplified assumptions, 150 NSBH events may constrain the maximum nonspinning NS mass to ±0.02M_⊙, and we may even measure the relation between the NS spin and maximum mass entirely from GW data. If rapidly rotating NSs exist, their spins and masses must be modeled simultaneously to avoid biasing the NS maximum mass.

 

Abstract Evidence for a low-frequency stochastic gravitational-wave background has recently been reported based on analyses of pulsar timing array data. The most likely source of such a background is a population of supermassive black hole binaries, the loudest of which may be individually detected in these data sets. Here we present the search for individual supermassive black hole binaries in the NANOGrav 15 yr data set. We introduce several new techniques, which enhance the efficiency and modeling accuracy of the analysis. The search uncovered weak evidence for two candidate signals, one with a gravitational-wave frequency of ∼4 nHz, and another at ∼170 nHz. The significance of the low-frequency candidate was greatly diminished when Hellings–Downs correlations were included in the background model. The high-frequency candidate was discounted due to the lack of a plausible host galaxy, the unlikely astrophysical prior odds of finding such a source, and since most of its support comes from a single pulsar with a commensurate binary period. Finding no compelling evidence for signals from individual binary systems, we place upper limits on the strain amplitude of gravitational waves emitted by such systems. At our most sensitive frequency of 6 nHz, we place a sky-averaged 95% upper limit of 8 × 10 −15 on the strain amplitude. We also calculate an exclusion volume and a corresponding effective radius, within which we can rule out the presence of black hole binaries emitting at a given frequency. 

It is the responsibility of today’s scientists, engineers, and educators to inspire and encourage our youth into technical careers that benefit our society. Too often, however, this responsibility is buried beneath daily job demands and the routines of teaching. Space Public Outreach Team (SPOT) programs leverage a train-the-trainer model to empower college students to make meaningful impacts in their local communities by engaging and inspiring younger students through science presentations. SPOT takes advantage of the excitement of space and the natural way college students serve as role models for children. The result is a win-win program for all involved. This paper describes the original Montana SPOT program, presents analyses demonstrating the success of SPOT, gives overviews of program adaptations in West Virginia and with the NANOGrav collaboration, describes how college student presenters are able to share complex topics, and discusses the importance of college student role models. We hope that our experiences with SPOT will help others implement similar strategies in their own communities.