Impact of Natal Kicks on Merger Rates and Spin–Orbit Misalignments of Black Hole–Neutron Star Mergers

Abstract

Neutron stars are born out of core-collapse supernovae, and they are imparted natal kicks at birth as a consequence of asymmetric ejection of matter and possibly neutrinos. Unless the force resulting from the kicks is exerted exactly at their center, it will also cause the neutron star to rotate. In this paper, we discuss the possibility that neutron stars may receive off-center natal kicks at birth, which imprint a natal rotation. In this scenario, the observed pulsar spin and transverse velocity in the Galaxy are expected to correlate. We develop a model of the natal rotation imparted to neutron stars and constrain it by the observed population of pulsars in our Galaxy. At 90% confidence, we find that the location of the off-center kick is R kick = 1.12 +4.79 −0.97 km. Our result is robust when considering pulsars with different observed periods, transverse velocities, and ages. Our constraint can be used as a guide for core-collapse simulations of massive stars.1. INTRODUCTION Since the discovery of the first pulsar (Hewish et al. 1968), the number of known neutron stars has grown to more than three thousands, including a rich ensemble of binary and millisecond pulsars (Manchester et al. 2005) 1 . The study of pulsars combines a wide range of physical processes, which shed light on the fundamental laws of nature and the structure of our Galaxy (e.g., Lorimer & Kramer 2004;Lorimer et al. 2019). Timing of pulsars in binary systems has not only allowed exquisite tests of strong gravity, but has also given insights into the equation of state that describes them and the nature of the binary systems they inhabit (e.g., Hulse

Modelling the formation of the first two neutron star-black hole mergers, GW200105 and GW200115: metallicity, chirp masses and merger remnant spins

“…The observational data for GW200115 shows possible evidence for significant spin-orbit misalignment (Abbott et al 2021). Several authors have argued that high natal kicks would be necessary to mis-align the spin to such a high degree (Fragione et al 2021;Gompertz et al 2021;Zhu 2021). However, Broekgaarden & Berger (2021) showed smaller supernova kicks provide a better match with observations once the rates of all compact object mergers are taken into account.…”

Black hole-neutron star mergers are unlikely multi-messenger sources

“…We also self-consistently keep track of the spin-orbit misalignment (I BH−NS ) produced as a result of natal kicks. For details, see Fragione et al (2021).…”

“…In this case, we fix σ = 260 km s −1 and α CM = 1 in our population synthesis models; other combinations of σ and α CM lead to qualitatively similar results. Note that we self-consistently account for the cosmological distribution of inclination angles, I BH−NS , (see Fragione et al 2021), which is critical to determine whether a NS directly plunges onto or is tidally disrupted by the BH. We also report in Figure 3 the median mass for the LVK candidate event GW190426, the two confirmed LVK BH-NS mergers GW201005 and GW20015, and the re-analysis by Mandel & Smith (2021) of GW200115.…”

“…For these systems only, a post-merger phase is expected, during which NS matter debris can be ejected or accreted onto the BH producing a luminous event. BH-NS mergers that produce an EM counterpart can provide constraints on the BH spin and accretion process and unique information on the nuclear equation of state (EOS) (e.g., Pannarale et al 2011;Tsang et al 2012;Foucart et al 2018;Ascenzi et al 2019;Hinderer et al 2019;Fragione & Loeb 2021).…”

Neutron star black hole binaries in LIGO/Virgo O3b run were formed from Population I/II binaries

“…Using metallicity-specific star formation rate density prescriptions (Broekgaarden et al 2021b) in an isolated binary evolution scenario, Broekgaarden & Berger (2021) concluded that the observed two NS-BHs can be explained from this formation channel, and gave some constraint on the common envelope efficiency and the natal kick velocities of supernovae by treating BH-BHs and NS-NSs simultaneously. Taking the spin direction of BH of GW200115 into account, Fragione et al (2021) discussed this NS-BH in the evolution of isolated massive binary stars and effects 2021) have predicted that 16-40% events of short-duration gamma-ray burst will be hostless. Antoniadis et al (2021) focused on highly unequal mass components of BH-BHs and NS-BHs, and showed that the landscape of core-collapse supernovae would cause the unequalness.…”