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Register a new userKilonovae: nUV/Optical/IR Counterparts of Neutron Star Binary Mergers with TSO
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With the epochal first detection of gravitational waves from a binary neutron star (NS) merger with the GW170817 event, and its direct confirmation that NS-NS mergers are significant sources of the of the r-process nucleosynthesis of heavy elements, an immense new arena for prompt EM (X-rays through IR and radio) studies of fundamental physics has been opened. Over the next decade, GW observatories will expand in scale and sensitivity so the need for facilities that can provide prompt, high sensitivity, broad-band EM followup becomes more urgent. NS-NS or NS-black hole (BH) mergers will be instantly recognized (and announced) by the LIGO-international collaboration. LSST will be a prime resource for rapid tiling of what will usually be large (~10-100 degree squared) error boxes. X-ray through IR Telescopes in space with (nearly) full-sky access that can rapidly image and tile are crucial for providing the earliest imaging and spectroscopic studies of the kilonova emission immediately following NS-NS mergers. The Time-domain Spectroscopic Observatory (TSO) is a proposed Probe-class 1.3 m telescope at L2, with imaging and spectroscopy (R = 200, 1800) in 4 bands (0.3 - 5 micron) and rapid slew capability to 90% of sky. TSO nUV-mid-IR spectra will enable new constraints on NS structure and nucleosynthesis.
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We predict linear polarization for a radioactively-powered kilonova following the merger of a black hole and a neutron star. Specifically, we perform 3-D Monte Carlo radiative transfer simulations for two different models, both featuring a lanthanide-rich dynamical ejecta component from numerical-relativity simulations while only one including an additional lanthanide-free disk wind component. We calculate polarization spectra for nine different orientations at 1.5, 2.5 and 3.5 d after the merger and in the $0.1-2,mu$m wavelength range. We find that both models are polarized at a detectable level 1.5 d after the merger while show negligible levels thereafter. The polarization spectra of the two models are significantly different. The model lacking a disk wind shows no polarization in the optical, while a signal increasing at longer wavelengths and reaching $sim1%-6%$ at $2,mu$m depending on the orientation. The model with a disk-wind component, instead, features a characteristic double-peak polarization spectrum with one peak in the optical and the other in the infrared. Polarimetric observations of future events will shed light on the debated neutron richness of the disk-wind component. The detection of optical polarization would unambiguously reveal the presence of a lanthanide-free disk-wind component, while polarization increasing from zero in the optical to a peak in the infrared would suggest a lanthanide-rich composition for the whole ejecta. Future polarimetric campaigns should prioritize observations in the first $sim48$ hours and in the $0.5-2,mu$m range, where polarization is strongest, but also explore shorter wavelengths/later times where no signal is expected from the kilonova and the interstellar polarization can be safely estimated.
Neutron star binary mergers are strong sources of gravitational waves (GWs). Promising electromagnetic counterparts are short gamma-ray bursts (GRBs) but the emission is highly collimated. We propose that the scattering of the long-lasting plateau emission in short GRBs by the merger ejecta produces nearly isotropic emission for $sim 10^4$ s with flux $10^{-13}-10^{-10}$ erg cm$^{-2}$ s$^{-1}$ at 100 Mpc in X-ray. This is detectable by Swift XRT and wide field X-ray detectors such as ISS-Lobster, Einstein Probe, eROSITA and WF-MAXI, which are desired by the infrared and optical follow-ups to localize and measure the distance to the host galaxy. The scattered X-rays obtain linear polarization, which correlates with the jet direction, X-ray luminosity and GW polarizations. The activity of plateau emission is also a natural energy source of a macronova (or kilonova) detected in short GRB 130603B without the $r$-process radioactivity.
Observations of X-ray binaries indicate a dearth of compact objects in the mass range from $sim 2-5$ $M_{odot}$ and the existence of this (first mass) gap has been used to advance our understanding of the engines behind core-collapse supernovae. LIGO/Virgo observations provide an independent measure of binary compact remnant masses and several candidate first mass gap objects (either NS or BH) were observed in the O3 science run. We study the formation of BH-NS mergers in the framework of isolated classical binary evolution. We use population synthesis method to evolve binary stars (Population I and II) across cosmic time. The predicted BH-NS mergers from the isolated classical binary evolution are sufficiently abundant ($sim 0.4-10$ $Gpc^{-3} yr^{-1}$) in the local Universe ($zapprox0$) to produce the observed LIGO/Virgo candidates. We present results on the NS to BH mass ratios ($q=M_{rm NS}/M_{rm BH}$ ) in merging systems, showing that although systems with a mass ratio as low as $q=0.02$ can exist, only a small fraction ($sim 0.05%-5%$) of LIGO/Virgo detectable BH-NS mergers have mass ratios below $q=0.05$. We find that with appropriate constraints on the (delayed) supernova engine $sim 30-40%$ of LIGO/Virgo BH-NS mergers may host at least one compact object in the gap. The uncertainties in the processes behind compact object formation imply that the fraction of BH-NS systems ejecting mass during the merger is $sim 0-9%$. In our reference we find that only $sim 0.2%$ of BH-NS mergers will have any mass ejection, and about the same percentage would produce kilonova bright enough to have a chance to be detected even with a large (Subaru-class) $8$m telescope. Interestingly, all these mergers will have both BH and NS in the first mass gap.
Detections of gravitational waves (GWs) may soon uncover the signal from the coalescence of a black hole - neutron star (BHNS) binary, that is expected to be accompanied by an electromagnetic (EM) signal. In this paper, we present a composite semi-analytical model to predict the properties of the expected EM counterpart from BHNS mergers, focusing on the kilonova emission and on the gamma-ray burst afterglow. Four main parameters rule the properties of the EM emission: the NS mass $M_mathrm{NS}$, its tidal deformability $Lambda_mathrm{NS}$, the BH mass and spin. Only for certain combinations of these parameters an EM counterpart is produced. Here we explore the parameter space, and construct light curves, analysing the dependence of the EM emission on the NS mass and tidal deformability. Exploring the NS parameter space limiting to $M_mathrm{NS}-Lambda_mathrm{NS}$ pairs described by a physically motivated equations of state (EoS), we find that the brightest EM counterparts are produced in binaries with low mass NSs (fixing the BH properties and the EoS). Using constraints on the NS EoS from GW170817, our modeling shows that the emission falls in a narrow range of absolute magnitudes. Within the range of explored parameters, light curves and peak times are not dissimilar to those from NSNS mergers, except in the B band. The lack of an hyper/supra-massive NS in BHNS coalescences causes a dimming of the blue kilonova emission in absence of the neutrino interaction with the ejecta.
Following merger, a neutron star (NS) binary can produce roughly one of three different outcomes: (1) a stable NS, (2) a black hole (BH), or (3) a supra-massive, rotationally-supported NS, which then collapses to a BH following angular momentum losses. Which of these fates occur and in what proportion has important implications for the electromagnetic transient associated with the mergers and the expected gravitational wave signatures, which in turn depend on the high density equation of state (EOS). Here we combine relativistic calculations of NS masses using realistic EOSs with Monte Carlo population synthesis based on the mass distribution of NS binaries in our Galaxy to predict the distribution of fates expected. For many EOSs, a significant fraction of the remnants are NSs or supra-massive NSs. This lends support to scenarios where a quickly spinning, highly magnetized NS may be powering an electromagnetic transient. This also indicates that it will be important for future gravitational wave (GW) observatories to focus on high frequencies to study the post merger GW emission. Even in cases where individual GW events are too low in signal to noise to study the post merger signature in detail, the statistics of how many mergers produce NSs versus BHs can be compared with our work to constrain the EOS. To match short gamma-ray burst (SGRB) X-ray afterglow statistics, we find that the stiffest EOSs are ruled out. Furthermore, many popular EOSs require ~60-70% of SGRBs to be from NS-BH mergers rather than just binary NSs.
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