After the successful detection of a gravitational-wave (GW) signal and its associated electromagnetic (EM) counterparts from GW170817, neutron star–black hole (NSBH) mergers have been highly expected to be the next type of multimessenger source. However, despite the detection of several NSBH merger candidates during the GW third observation run, no confirmed EM counterparts from these sources have been identified. The most plausible explanation is that these NSBH merger candidates were plunging events mainly because the primary black holes (BHs) had near-zero projected aligned spins based on GW observations. In view of the fact that neutron stars (NSs) can be easily tidally disrupted by BHs with high projected aligned spins, we study an evolution channel to form NSBH binaries with fast-spinning BHs, the properties of BH mass and spin, and their associated tidal disruption probability. We find that if the NSs are born first, the companion helium stars would be tidally spun up efficiently, and would thus finally form fast-spinning BHs. If BHs do not receive significant natal kicks at birth, these NSBH binaries that can merge within Hubble time would have BHs with projected aligned spins χ z ≳ 0.8 and, hence, can certainly allow tidal disruption to happen. Even if significant BH kicks are considered for a small fraction of NSBH binaries, the projected aligned spins of BHs are χ z ≳ 0.2. These systems can still be disrupted events unless the NSs are very massive. Thus, NS-first-born NSBH mergers would be promising multimessenger sources. We discuss various potential EM counterparts associated with these systems and their detectability in the upcoming fourth observation run.

Gamma-ray bursts (GRBs) have been phenomenologically divided into long- and short-duration populations, generally corresponding to collapsar and compact merger origins, respectively. Here, we collect three unique bursts, GRBs 060614, 211211A, and 211227A, all of which are characterized by a long-duration main emission (ME) phase and a rebrightening extended emission (EE) phase, to study their observed properties and their potential origins as neutron star–black hole (NSBH) mergers. NS-first-born (BH-first-born) NSBH mergers tend to contain fast-spinning (nonspinning) BHs that more easily (hardly) allow tidal disruption to occur, while (without) forming electromagnetic signals. We find that NS-first-born NSBH mergers can well interpret the origins of these three GRBs, supported by the following. (1) Their X-ray MEs and EEs show unambiguous fallback accretion signatures, decreasing as ∝ t −5/3, which might account for their long durations. The EEs could result from the fallback accretion of r-process heating materials, which is predicted to occur after NSBH mergers. (2) The beaming-corrected local event-rate density for these types of merger-origin long-duration GRBs is  0 ∼ 2.4 − 1.3 + 2.3 Gpc − 3 yr − 1 , consistent with that of NS-first-born NSBH mergers. (3) Our detailed analysis of the EE, afterglow, and kilonova of the recent high-impact event GRB 211211A reveals that it could be a merger between a ∼ 1.23 − 0.07 + 0.06 M ⊙ NS and a ∼ 8.21 − 0.75 + 0.77 M ⊙ BH, with an aligned spin of χ BH ∼ 0.62 − 0.07 + 0.06 , supporting an NS-first-born NSBH formation channel. A long-duration burst, with a rebrightening fallback accretion signature after the ME, and a bright kilonova, might be commonly observed features for on-axis NSBH mergers. We estimate the multimessenger detection rate between gravitational waves, GRBs, and kilonova emissions from NSBH mergers in O4 (O5) to be ∼0.1 yr−1 (∼1 yr−1).

Extreme stripped-envelope supernovae (SESNe), including Type Ic superluminous supernovae (SLSNe), broad-line Type Ic SNe (SNe Ic-BL), and fast blue optical transients (FBOTs), are widely believed to harbor a newborn fast-spinning highly-magnetized neutron star ("magnetar"), which can lose its rotational energy via spin-down processes to accelerate and heat the ejecta. The progenitor(s) of these magnetar-driven SESNe, and the origin of considerable angular momentum (AM) in the cores of massive stars to finally produce such fast-spinning magnetars upon core-collapse are still under debate. Popular proposed scenarios in the literature cannot simultaneously explain their event rate density, SN and magnetar parameters, and the observed metallicity. Here, we perform a detailed binary evolution simulation that demonstrates that tidal spin-up helium stars with efficient AM transport mechanism in close binaries can form fast-spinning magnetars at the end of stars' life to naturally reproduce the universal energy-mass correlation of these magnetar-driven SESNe. Our models are consistent with the event rate densities, host environments, ejecta masses, and energetics of these different kinds of magnetar-driven SESNe, supporting that the isolated common-envelope formation channel could be a major common origin of magnetar-driven SESNe. The remnant compact binary systems of magnetar-driven SESNe are progenitors of some galactic systems and gravitational-wave transients.

The LIGO, Virgo, and KAGRA (LVK) Collaboration has announced 90 coalescing binary black holes (BBHs) with p astro > 50% to date; however, the origin of their formation channels is still an open scientific question. Given various properties of BBHs (BH component masses and individual spins) inferred using the default priors by the LVK, independent groups have been trying to explain the formation of the BBHs with different formation channels. Of all formation scenarios, the chemically homogeneous evolution (CHE) channel has stood out with distinguishing features, namely, nearly equal component masses and preferentially high individual spins aligned with the orbital angular momentum. We perform Bayesian inference on the BBH events officially reported in GWTC-3 with astrophysically predicted priors representing different formation channels of the isolated binary evolution (common-envelope evolution channel, CEE; CHE; stable mass transfer, SMT). Given assumed models, we report strong evidence for GW190517_055101 being most likely to have formed through the CHE channel. Assuming the BBH events in the subsample are all formed through one of the isolated binary evolution channels, we obtain the lower limits on the local merger rate density of these channels at 11.45 Gpc−3 yr−1 (CEE), 0.18 Gpc−3 yr−1 (CHE), and 0.63 Gpc−3 yr−1 (SMT) at 90% credible level.

Context. To date, various formation channels of merging events have been heavily explored with the detection of nearly 100 double black hole (BH) merger events reported by the LIGO-Virgo-KAGRA (LVK) Collaboration. We here systematically investigate an alternative formation scenario, i.e., binary BHs (BBHs) formed through double helium stars (hereafter double-core evolution channel). In this scenario, the two helium stars (He-rich stars) could be the outcome of the classical isolated binary evolution scenario involving with and without common-envelope phase (i.e., CE channel and stable mass transfer channel), or alternatively of massive close binaries evolving chemically homogeneously (i.e., CHE channel). Aims. We study the properties (i.e., the chirp masses and the effective spins) of binary BHs (BBHs) formed through the doublecore evolution, and investigate the impact of different efficiencies of angular momentum transport within massive He-rich stars on double-core evolution. Methods. We perform detailed stellar structure and binary evolution calculations that take into account internal differential rotation and mass loss of He-rich stars, as well as tidal interactions in binaries. We systematically study the parameter space of initial binary He-rich stars, including initial mass and metallicity of He-rich stars, as well as initial orbital periods. Apart from direct core collapse with mass and angular momentum conserved, we also follow the framework in Batta & Ramirez-Ruiz (2019) to estimate the mass and spin of the resulting BHs. Results. We show that the radii of massive He-rich stars decrease as a function of time, which comes mainly from mass loss and mixing in high metallicity and from mixing in low metallicity. For double He-rich stars with equal masses in binaries, we find that tides start to be at work on the Zero Age Helium Main Sequence (ZAHeMS: the time when a He-rich star starts to burn helium in the core, which is analogous to ZAMS for core hydrogen burning) for initial orbital periods not longer than 1.0 day, depending on the initial metallicities. Besides the stellar mass loss rate and tidal interactions in binaries, we find that the role of the angular momentum transport efficiency in determining the resulting BH spins, becomes stronger when considering BH progenitors originated from a higher metal-metallicity environment. We highlight that double-core evolution scenario does not always produce fast-spinning BBHs and compare the properties of the BBHs reported from the LVK with our modeling. Conclusions. After detailed binary calculations of double-core evolution, we have confirmed that the spin of the BH is not only determined by the interplay of the binary's different initial conditions (metallicity, mass and orbital period), but also dependent on the angular momentum transport efficiency within its progenitor. We predict that, with the sensitivity improvements to the LVK's next observing run (O4), the sample of merging BBHs will contain more sources with positive but moderate (even high) χ ef...

The association of GW170817/GRB 170817A/AT2017gfo provides the first direct evidence for neutron star mergers as significant sources of r-process nucleosynthesis. A gamma-ray transient (GRT) would be powered by the radioactive decay of the freshly synthesized r-process elements. By analyzing the composition and gamma-ray opacity of the kilonova ejecta in detail, we calculate the lightcurve and spectrum of the GRT for a range of spherically symmetric merger ejecta models with mass M ej = 0.001 to ∼0.05M ⊙ and expansion velocity v ej = 0.1c to ∼0.4c. It is found that the peak of the GRT lightcurve depends on M ej and v ej as t pk ≈ 0.5 days ( M ej / 0.01 M ⊙ ) 1 / 2 ( v ej / 0.1 c ) − 1 and L pk ≈ 2.0 × 10 41 erg s − 1 ( M ej / 0.01 M ⊙ ) 1 / 2 ( v ej / 0.1 c ) . Most radiating photons are in the 100–3000 keV band and the spectrum peaks at about 800 keV for different nuclear physics inputs. The line features are blurred out by the Doppler broadening effect. Adopting the ejecta parameters reported in the literature, we examine the detection probability of the possible GRT associated with AT2017gfo. We show that the GRT cannot be convincingly detected with either current or proposed missions in the MeV band, such as ETCC and AMEGO. The low gamma-ray flux, together with the extremely low event rate at local universe, makes a discovery of GRTs a great challenge.

Gamma-ray bursts (GRBs) have been phenomenologically divided into long-and short-duration populations, generally corresponding to the collapsar and compact merger origin, respectively. Here we collect three unique bursts, GRBs 060614, 211211A and 211227A, all characterized by a long-duration main emission (ME) phase and a rebrightening extended emission (EE) phase, to study their observed properties and the potential origin as neutron star-black hole (NSBH) mergers. NS-first-born (BHfirst-born) NSBH mergers tend to contain fast-spinning (non-spinning) BHs that more easily (hardly) allow tidal disruption to happen with (without) forming electromagnetic signals. We find that NS-firstborn NSBH mergers can well interpret the origins of these three GRBs, supported by that: (1) Their X-ray MEs and EEs show unambiguous fall-back accretion signatures, decreasing as ∝ t −5/3 , which might account for their long duration. The EEs can result from the fall-back accretion of r-process heating materials, predicted to occur after NSBH mergers. (2) The beaming-corrected local event rate density for this type of merger-origin long-duration GRBs is R 0 ∼ 2.4 +2.3 −1.3 Gpc −3 yr −1 , consistent with that of NS-first-born NSBH mergers. (3) Our detailed analysis on the EE, afterglow and kilonova of the recently high-impact event GRB 211211A reveals it could be a merger between a ∼ 1.23 +0.06 −0.07 M NS and a ∼ 8.21 +0.77 −0.75 M BH with an aligned-spin of χ BH ∼ 0.62 +0.06 −0.07 , supporting an NS-first-born NSBH formation channel. Long-duration burst with rebrightening fall-back accretion signature after ME, and bright kilonova might be commonly observed features for on-axis NSBH mergers. We estimate the multimessenger detection rate between gravitational waves, GRB and kilonova emissions from NSBH mergers in O4 (O5) is ∼ 0.1 yr −1 (∼ 1 yr −1 ).

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