Search

We study the observable spectral and temporal properties of kilonova remnants analytically, and showcase quantitative differences with respect to supernova remnants. We provide detection prospects of kilonova remnants in the context of ongoing radio surveys. We find that there is a good chance to expect 10s of such objects in future surveys with a flux threshold of $\sim 0.1$ mJy. Kilonova remnants from a postulated population of long lived supramassive neutron star remnants of neutron star mergers are even more likely to be detected as they are extremely bright and peak earlier. For ongoing survey with threshold of $\sim$ mJy, we expect to find 10-100s of such objects if they are a significant fraction of total kilonova population. Considering that there are no promising such kilonovae candidates in current surveys, we constrain the fraction of such extreme kilonova to be no more than 30 percent of the overall kilonovae rate, depending on the details of ejecta mass and external density distribution., Comment: Comments welcome

We study observational signatures of the hadron-quark crossover in binary-neutron-star mergers by systematic numerical-relativity simulations. We employ two equations of state (EoSs) for matter consistent with inference from the observational data. In the crossover scenario the EoS is softened in a density realized in binary-neutron-star mergers and is smoothly continued to quark matter. In the phase transition scenario without crossover, the EoS remains stiff and a first-order phase transition takes place in a density out of reach of mergers. A GW170817-like system forms a remnant massive neutron star in both scenarios, and only the crossover scenario makes it collapse into a black hole due to the softening while gravitational-wave emission is strong. This difference is clearly reflected in the sudden shutdown of gravitational waves. For a given EoS, the lifetime of the merger remnant is determined primarily by the total mass of the system. These features pave the way for clarifying details of hadron-quark transition by compiling a wide variety of events observed with future gravitational-wave detectors. The mass of the accretion disk surrounding the remnant black hole is affected not only by the lifetime of the remnant but also by the mass ratio of the system. Electromagnetic emission associated with the disk outflow will also be useful for detailed investigation of the hadron-quark transition., Comment: 25 pages, 17 figures

Radiation-driven outflows play a crucial role in extracting mass and angular momentum from binary systems undergoing rapid mass transfer at super-Eddington rates. To study the mass transfer process from a massive donor star to a stellar-mass black hole (BH), we perform multi-dimensional radiation-hydrodynamical simulations that follow accretion flows from the first Lagrange point down to about a hundred times the Schwarzschild radius of the accreting BH. Our simulations reveal that rapid mass transfer occurring at over a thousand times the Eddington rate leads to significant mass loss from the accretion disk via radiation-driven outflows. Consequently, the inflow rates at the innermost radius are regulated by two orders of magnitude smaller than the transfer rates. We find that convective motions within the accretion disk drive outward energy and momentum transport, enhancing the radiation pressure in the outskirts of the disk and ultimately generating large-scale outflows with sufficient energy to leave the binary. Furthermore, we observe strong anisotropy in the outflows, which occur preferentially toward both the closest and furthest points from the donor star. However, when averaged over all directions, the specific angular momentum of the outflows is nearly comparable to the value predicted in the isotropic emission case. Based on our simulation results, we propose a formula that quantifies the mass growth rates on BHs and the mass loss rates from binaries due to radiation-driven outflows. This formula provides important implications for the binary evolution and the formation of merging binary BHs., Comment: 16 pages, 10 figures, 2 tables, submitted to MNRAS, comments welcome

H-poor superluminous supernovae (SLSNe-I) are characterized by O II lines around 4,000 - 4,500 A in pre-/near-maximum spectra, so-called W-shaped O II lines. As these lines are from relatively high excitation levels, they have been considered a sign of non-thermal processes, which may give a hint of power sources of SLSNe-I. However, the conditions for these lines to appear have not been understood well. In this work, we systematically calculate synthetic spectra to reproduce observed spectra of eight SLSNe-I, parameterizing departure coefficients from the nebular approximation in the SN ejecta (expressed as b_neb). We find that most of the observed spectra can be reproduced well with b_neb ~< 10, which means that no strong departure is necessary for the formation of the W-shaped O II lines. We also show that the appearance of the W-shaped O II lines is sensitive to temperature; only spectra with temperatures T ~ 14,000 - 16,000 K can produce the W-shaped O II lines without large departures. Based on this, we constrain the non-thermal ionization rate near the photosphere. Our results suggest that spectral features of SLSNe-I can give independent constraints on the power source through the non-thermal ionization rates., Comment: 19 pages, 11 figures, accepted for publication in ApJ

Extensions to General Relativity (GR) allow the polarization of gravitational waves (GW) from astrophysical sources to suffer from amplitude and velocity birefringence, which respectively induce changes in the ellipticity and orientation of the polarization tensor. We introduce a multi-messenger approach to test this polarization behavior of GWs during their cosmological propagation using binary sources, for which the initial polarization is determined by the inclination and orientation angles of the orbital angular momentum vector with respect to the line of sight. In particular, we use spatially-resolved radio imaging of the jet from a binary neutron star (BNS) merger to constrain the orientation angle and hence the emitted polarization orientation of the GW signal at the site of the merger, and compare to that observed on Earth by GW detectors. For GW170817 we constrain the deviation from GR due to amplitude birefringence to $\kappa_A = -0.12^{+0.60}_{-0.61}$, while the velocity birefringence parameter $\kappa_V$ remains unconstrained. The inability to constrain $\kappa_V$ is due to the fact that Virgo did not detect GW170817, and measurements of the polarization orientation require information from a combination of multiple detectors with different alignments. For this reason, we also mock future BNS mergers with resolved afterglow proper motion and project that $\kappa_V$ could be constrained to a precision of $5\,$rad (corresponding to an angular shift of the GW polarization of $\delta\phi_V\approx 0.2\,$rad for a BNS at $100\,$Mpc) by a future network of third-generation ground-based GW detectors such as Cosmic Explorer and the radio High Sensitivity Array. Crucially, this velocity birefringence effect cannot be constrained with dark binary mergers as it requires polarization information at the emission time, which can be provided only by electromagnetic emission.

We present 4,987 white dwarf (WD) candidates selected from matched stars between the multi-band imaging datasets of Subaru Hyper Suprime-Cam (HSC) Survey and SDSS in the Stripe82 region covering about 165 deg$^2$. We first select WD candidates from the "reduced proper motion" diagram that is obtained by combining the apparent magnitude in the range $i=19$ -- 24 and the proper motion measured by comparing the astrometric positions of each object between the two datasets over about 14 yr time baseline. We refine the WD candidates by fitting blackbody and template WD atmosphere models to HSC photometries for each candidate, enabling the estimation of photometric distance and tangential velocity ($v_{\rm t}$) with respect to the Sun. The deep HSC data allows us to identify low-temperature ($<4000$ K) and faint WD candidates down to absolute magnitude, $M_{\rm bol}\simeq 17$. We evaluate the selection function of our WD candidates using the mock catalogue for spatial and kinematic distributions of WDs in the (thin and thick) disc and halo regions based on the standard Milky Way model. We construct the samples of disc and halo WD candidates by selecting WDs with the cuts of tangential velocity, $40

We explore the joint detection prospects of short gamma-ray bursts (sGRBs) and their gravitational wave (GW) counterparts by the current and upcoming high-energy GRB and GW facilities from binary neutron star (BNS) mergers. We consider two GW detector networks: (1) A four-detector network comprising LIGO Hanford, Livingston, Virgo, and Kagra, (IGWN4) and (2) a future five-detector network including the same four detectors and LIGO India (IGWN5). For the sGRB detection, we consider existing satellites Fermi and Swift and the proposed all-sky satellite Daksha. Most of the events for the joint detection will be off-axis, hence, we consider a broad range of sGRB jet models predicting the off-axis emission. Also, to test the effect of the assumed sGRB luminosity function, we consider two different functions for one of the emission models. We find that for the different jet models, the joint sGRB and GW detection rates for Fermi and Swift with IGWN4 (IGWN5) lie within 0.07-0.62$\mathrm{\ yr^{-1}}$ (0.8-4.0$\mathrm{\ yr^{-1}}$) and 0.02-0.14$\mathrm{\ yr^{-1}}$ (0.15-1.0$\mathrm{\ yr^{-1}}$), respectively, when the BNS merger rate is taken to be 320$\mathrm{\ Gpc^{-3}~yr^{-1}}$. With Daksha, the rates increase to 0.2-1.3$\mathrm{\ yr^{-1}}$ (1.3-8.3$\mathrm{\ yr^{-1}}$), which is 2-9 times higher than the existing satellites. We show that such a mission with higher sensitivity will be ideal for detecting a higher number of fainter events observed off-axis or at a larger distance. Thus, Daksha will boost the joint detections of sGRB and GW, especially for the off-axis events. Finally, we find that our detection rates with optimal SNRs are conservative, and noise in GW detectors can increase the rates further., Comment: 9 pages, 6 figures, 2 tables; accepted for publication in MNRAS. The definitive version will be available on the journal page

HEX-P will launch at a time when the sky is being routinely scanned for transient gravitational wave, electromagnetic and neutrino phenomena that will require the capabilities of a sensitive, broadband X-ray telescope for follow up studies. These include the merger of compact objects such as neutron stars and black holes, stellar explosions, and the birth of new compact objects. \hexp\ will probe the accretion and ejecta from these transient phenomena through the study of relativistic outflows and reprocessed emission, provide unique capabilities for understanding jet physics, and potentially revealing the nature of the central engine., Comment: Submitted to Frontiers in Astronomy and Space Sciences

Elementary particles such as quarks and gluons are expected to be fundamental degrees of freedom at ultra high temperatures or densities, while natural phenomena in our daily lives are described in terms of hadronic degrees of freedom. Massive neutron stars and remnants of binary neutron star mergers may contain quark matter, but it is not known how the transition from hadron matter to quark matter occurs. Different transition scenarios predict different gravitational waveforms emitted from binary neutron star mergers. If the difference between the equations of state occurs at sufficiently high density, it is expected that the difference between waveforms mainly appears in the merger or the post-merger phase rather than in the inspiral phase. The typical frequency of gravitational waves after the coalescence is higher than 2 kHz, which is difficult to observe using current detectors. In this study, we performed Bayesian model selection for two representative scenarios and investigated whether observations with future detectors will allow us to identify the correct model. We assume that the relatively low density equation of state around the nuclear saturation density is completely known from accumulated observations. Under this assumption, we find that it is reasonable to expect to be able to identify the correct transition scenario with third-generation detectors or specialized detectors with high sensitivity at high frequencies designed for post-merger signal observation, e.g., NEMO.

Kilonova spectra provide us with information of r-process nucleosynthesis in neutron star mergers. However, it is still challenging to identify individual elements in the spectra mainly due to lack of experimentally accurate atomic data for heavy elements in the near-infrared wavelengths. Recently, Domoto et al. (2022) proposed the absorption features around 14500 A in the observed spectra of GW170817/AT2017gfo as Ce III lines. But they used theoretical transition probabilities (gf-values) whose accuracy is uncertain. In this paper, we derive the astrophysical gf-values of the three Ce III lines, aiming at verification of this identification. We model high resolution H-band spectra of four F-type supergiants showing the clear Ce III absorption features by assuming stellar parameters derived from optical spectra in literatures. We also test the validity of the derived astrophysical gf-values by estimating Ce III abundances in Ap stars. We find that the derived astrophysical gf-values of the Ce III lines are systematically lower by about 0.25 dex than those used in previous work of kilonovae, while they are still compatible within the uncertainty range. By performing radiative transfer simulations of kilonovae with the derived gf-values, we find that the identification of Ce III as a source of the absorption features in the observed kilonova spectra still stands, even considering the uncertainties in the astrophysical gf-values. This supports identification of Ce in the spectra of GW170817/AT2017gfo., Comment: 15 pages, 7 figures, 6 tables, accepted for publication in ApJ

The late-time spectra of the kilonova AT 2017gfo associated with GW170817 exhibit a strong emission line feature at $2.1\,{\rm \mu m}$. The line structure develops with time and there is no apparent blue-shifted absorption feature in the spectra, suggesting that this emission line feature is produced by electron collision excitation. We attribute the emission line to a fine structure line of Tellurium (Te) III, which is one of the most abundant elements in the second r-process peak. By using a synthetic spectral modeling including fine structure emission lines with the solar r-process abundance pattern beyond the first r-process peak, i.e., atomic mass numbers $A\gtrsim 88$, we demonstrate that [Te III] $2.10\,\rm \mu m$ is indeed expected to be the strongest emission line in the near infrared region. We estimate that the required mass of Te III is $\sim 10^{-3}M_{\odot}$, corresponding to the merger ejecta of $0.05M_{\odot}$, which is in agreement with the mass estimated from the kilonova light curve., Comment: 4 pages, 2 figures, submitted to MNRAS

Observations of the kilonova from a neutron star merger event GW170817 opened a way to directly study r-process nucleosynthesis by neutron star mergers. It is, however, challenging to identify the individual elements in the kilonova spectra due to lack of complete atomic data, in particular, at near-infrared wavelengths. In this paper, we demonstrate that spectra of chemically peculiar stars with enhanced heavy element abundances can provide us with an excellent astrophysical laboratory for kilonova spectra. We show that the photosphere of a late B-type chemically peculiar star HR 465 has similar lanthanide abundances and ionization degrees with those in the line forming region in a kilonova at $\sim 2.5$ days after the merger. The near-infrared spectrum of HR 465 taken with Subaru/IRD indicates that Ce III lines give the strongest absorption features around 16,000 A and there are no other comparably strong transitions around these lines. The Ce III lines nicely match with the broad absorption features at 14,500 A observed in GW170817 with a blueshift of v=0.1c, which supports recent identification of this feature as Ce III by Domoto et al. (2022)., Comment: 12 pages, 6 figures, accepted for publication in ApJ

Kilonova spectra imprint valuable information about the elements synthesized in neutron star mergers. In AT2017gfo, the kilonova associated with GW170817, the spectroscopic feature centered around 8000 angstroms has been interpreted as the P-Cygni profile arising from singly ionized Strontium. Recently, Perego et al. (2022) suggested that Helium 10833 line can be an alternative explanation of the feature. Here, we study the line features under non-local thermodynamic equilibrium. We find that the ionization of ejecta by the stopping of radioactive decay product can significantly enhance the ionization states around the line forming region. We compute the kilonova spectrum under the assumption of spherical symmetry and uniform elemental fraction in the line-forming region. We find that 0.2\% (in mass) of Helium in the ejecta can reproduce the P-Cygni feature in the observed spectrum at $1.43$ -- $4.40$ days. Strontium with a mass fraction of $1\%$ is also able to make the absorption feature at $\sim 1.5\,$days, but it gets weaker with time due to ionization by radioactive decay products. The strength of the He line signature depends sensitively on UV strength for the first two epochs. Further modeling of UV line blanketing by $r$-process elements and the optical properties of light $r$-process elements would be crucial to distinguish between Helium and Strontium features. The mass fraction of He is a good indicator for ejecta entropy that allows us to probe the mass ejection mechanism., Comment: 16 pages, 9 figures

We present the science case for the proposed Daksha high energy transients mission. Daksha will comprise of two satellites covering the entire sky from 1~keV to $>1$~MeV. The primary objectives of the mission are to discover and characterize electromagnetic counterparts to gravitational wave source; and to study Gamma Ray Bursts (GRBs). Daksha is a versatile all-sky monitor that can address a wide variety of science cases. With its broadband spectral response, high sensitivity, and continuous all-sky coverage, it will discover fainter and rarer sources than any other existing or proposed mission. Daksha can make key strides in GRB research with polarization studies, prompt soft spectroscopy, and fine time-resolved spectral studies. Daksha will provide continuous monitoring of X-ray pulsars. It will detect magnetar outbursts and high energy counterparts to Fast Radio Bursts. Using Earth occultation to measure source fluxes, the two satellites together will obtain daily flux measurements of bright hard X-ray sources including active galactic nuclei, X-ray binaries, and slow transients like Novae. Correlation studies between the two satellites can be used to probe primordial black holes through lensing. Daksha will have a set of detectors continuously pointing towards the Sun, providing excellent hard X-ray monitoring data. Closer to home, the high sensitivity and time resolution of Daksha can be leveraged for the characterization of Terrestrial Gamma-ray Flashes., Comment: 19 pages, 7 figures. Accepted in Experimental Astronomy. More details about the mission at https://www.dakshasat.in/

The detection of gravitational waves from binary neuron star merger GW170817 and electromagnetic counterparts GRB170817 and AT2017gfo kick-started the field of gravitational wave multimessenger astronomy. The optically red to near infra-red emission (`red' component) of AT2017gfo was readily explained as produced by the decay of newly created nuclei produced by rapid neutron capture (a kilonova). However, the ultra-violet to optically blue emission (`blue' component) that was dominant at early times (up to 1.5 days) received no consensus regarding its driving physics. Among many explanations, two leading contenders are kilonova radiation from a lanthanide-poor ejecta component or shock interaction (cocoon emission). In this work, we simulate AT2017gfo-like light curves and perform a Bayesian analysis to study whether an ultra-violet satellite capable of rapid gravitational wave follow-up, could distinguish between physical processes driving the early `blue' component. We find that a Dorado-like ultra-violet satellite, with a 50 sq. deg. field of view and a limiting magnitude (AB) of 20.5 for a 10 minute exposure is able to distinguish radiation components up to at least 160 Mpc if data collection starts within 3.2 or 5.2 hours for two possible AT2017gfo-like light curve scenarios. We also study the degree to which parameters can be constrained with the obtained photometry. We find that, while ultra-violet data alone constrains parameters governing the outer ejecta properties, the combination of both ground-based optical and space-based ultra-violet data allows for tight constraints for all but one parameter of the kilonova model up to 160 Mpc. These results imply that an ultra-violet mission like Dorado would provide unique insights into the early evolution of the post-merger system and its driving emission physics., Comment: 13 pages, 2 tables, 7 figures. Comments welcome. To be submitted to ApJ

The observations of GW170817/AT2017gfo have provided us with evidence that binary neutron star mergers are sites of $r$-process nucleosynthesis. However, the observed signatures in the spectra of GW170817/AT2017gfo have not been fully decoded especially in the near-infrared (NIR) wavelengths. In this paper, we investigate the kilonova spectra over the entire wavelength range with the aim of elemental identification. We systematically calculate the strength of bound-bound transitions by constructing a hybrid line list that is accurate for important strong transitions and complete for weak transitions. We find that the elements on the left side of the periodic table, such as Ca, Sr, Y, Zr, Ba, La, and Ce, tend to produce prominent absorption lines in the spectra. This is because such elements have a small number of valence electrons and low-lying energy levels, resulting in strong transitions. By performing self-consistent radiative transfer simulations for the entire ejecta, we find that La III and Ce III appear in the NIR spectra, which can explain the absorption features at $\lambda\sim 12000$-14000 A in the spectra of GW170817/AT2017gfo. The mass fractions of La and Ce are estimated to be $>2\times 10^{-6}$ and $\sim$ (1-100)$\times 10^{-5}$, respectively. An actinide element Th can also be a source of absorption as the atomic structure is analogous to that of Ce. However, we show that Th III features are less prominent in the spectra because of the denser energy levels of actinides compared to those of lanthanides., Comment: 23 pages, 19 figures, 5 tables, accepted for publication in ApJ

GW170817 is the first binary neutron star (NS) merger detected in gravitational waves (GWs) and photons, and so far remains the only GW event of its class with a definitive electromagnetic (EM) counterpart. Radio emission from the structured jet associated with GW170817 has faded below the sensitivity achievable via deep radio observations with the most sensitive radio arrays currently in operation. Hence, we now have the opportunity to probe the radio re-brightening that some models predict, should emerge at late times from the interaction of the dynamically-stripped merger ejecta with the interstellar medium. Here we present the latest results from our deep radio observations of the GW170817 field with the Karl G. Jansky Very Large Array (VLA), 4.5 years after the merger. Our new data at $3\,$GHz do not show any compelling evidence for emission in excess to the tail of the jet afterglow ($<3.3\,\mu$Jy), confirming our previous results. We thus set new constraints on the dynamical ejecta afterglow models. These constraints favor single-speed ejecta with energy $\lesssim 10^{50}\,$erg (for an ejecta speed of $\beta_0=0.5$), or steeper energy-speed distributions of the kilonova ejecta. Our results also suggest larger values of the cold, non-rotating maximum NS mass in equal mass scenarios. However, without a detection of the dynamical ejecta afterglow, obtaining precise constraints on the NS equation of state remains challenging., Comment: 6 pages, 5 figures. Published in ApJ

The cores of neutron stars (NSs) near the maximum mass realize the most highly compressed matter in the universe where quark degrees of freedom may be liberated. Such a state of dense matter is hypothesized as quark matter (QM) and its presence has awaited to be confirmed for decades in nuclear physics. Gravitational waves from binary NS mergers are expected to convey useful information called the equation of state (EOS). However, the signature for QM with realistic EOS is not yet established. Here, we show that the gravitational wave in the post-merger stage can distinguish the theory scenarios with and without a transition to QM. Instead of adopting specific EOSs as studied previously, we compile reliable EOS constraints from the ab initio approaches. We demonstrate that early collapse to a black hole after NS merger signifies softening of the EOS associated with the onset of QM in accord with ab initio constraints. Nature of hadron-quark phase transition can be further constrained by the condition that electromagnetic counterparts need to be energized by the material left outside the remnant black hole., Comment: 20 pages, 5 figures

Infrared emission lines arising from transitions between fine structure levels of heavy elements are expected to produce kilonova nebular emission. For the kilonova in GW170817, strong emission at 4.5 ${\rm \mu m}$ at late times was detected by the Spitzer Space Telescope but no source was detected at 3.6 ${\rm \mu m}$. This peculiar spectrum indicates that there exist strong line emitters around 4.5 ${\rm \mu m}$ and the absence of strong lines around 3.6 ${\rm \mu m}$. To model the spectrum we prepare a line list based on the selection rules in LS coupling from the experimentally calibrated energy levels in the NIST database. This method enables to generate the synthetic spectra with accurate line wavelengths. We find that the spectrum is sensitive to the abundance pattern whether or not the first r-process peak elements are included. In both cases, the synthetic spectra can match the observed data, leading to two possible interpretations. If the first peak elements are abundant a Se III line dominates the flux. If otherwise, W III with Os III, Rh III, and Ce IV can be the main sources. Observing nebular spectra for the future kilonovae in a wider wavelength range can provide more conclusive elemental identification., Comment: 5 pages, 3 figures, submitted to MNRAS

We explore the electromagnetic counterparts that will associate with binary neutron star mergers for the case that remnant massive neutron stars survive for $\gtrsim 0.5\,$s after the merger. For this study, we employ the outflow profiles obtained by long-term general-relativistic neutrino-radiation magneto-hydrodynamics simulations with a mean field dynamo effect. We show that a synchrotron afterglow with high luminosity can be associated with the merger event if the magnetic fields of the remnant neutron stars are significantly amplified by the dynamo effect. We also perform a radiative transfer calculation for kilonovae and find that for the highly amplified magnetic field cases, the kilonovae can be bright in the early epoch, while it shows the optical emission rapid declining in a few days and the long-lasting ($\sim 10\,{\rm d}$) emission very bright in the near-infrared wavelength. All these features have not been found in GW170817, indicating that the merger remnant neutron star formed in GW170817 might have collapsed to a black hole within several hundreds ms or magnetic-field amplification might be a minor effect., Comment: 25 pages, 14 figures, published in ApJ

We discuss the prospects for identifying nearest isolated black holes (IBHs) in our Galaxy. IBHs accreting gas from the interstellar medium (ISM) likely form magnetically arrested disks (MADs). We show that thermal electrons in the MADs emit optical signals through the thermal synchrotron process while non-thermal electrons accelerated via magnetic reconnections emit a flat-spectrum synchrotron radiation in the X-ray to MeV gamma-ray ranges. The Gaia catalog will include at most a thousand of IBHs within $\lesssim 1$ kpc that are distributed on and around the cooling sequence of white dwarfs (WDs) in the Hertzsprung-Russell diagram. These IBH candidates should also be detected by eROSITA, with which they can be distinguished from isolated WDs and neutron stars. Followup observations with hard X-ray and MeV gamma-ray satellites will be useful to unambiguously identify IBHs., Comment: 10 pages, 4 figures, 1 table, accepted for publications in ApJL, discussions are added, conclusions are unchanged, a few typos are corrected

A core-collapse supernova occurs when exothermic fusion ceases in the core of a massive star, typically due to exhaustion of nuclear fuel. Theory predicts that fusion could be interrupted earlier, by merging of the star with a compact binary companion. We report a luminous radio transient, VT J121001+495647, found in the Very Large Array Sky Survey. The radio emission is consistent with supernova ejecta colliding with a dense shell of material, potentially ejected by binary interaction in the centuries prior to explosion. We associate the supernova with an archival X-ray transient, which implies a relativistic jet was launched during the explosion. The combination of an early relativistic jet and late-time dense interaction is consistent with expectations for a merger-driven explosion., Comment: 33 pages, 4 figures. v2: Updated fig 1 to include panel D

High-velocity neutron stars (HVNSs) that were kicked out from their birth location can be potentially identified with their large proper motions, and possibly with large parallax, when they come across the solar neighborhood. In this paper, we study the feasibility of hunting isolated HVNSs in wide-area optical surveys by modeling the evolution of NS luminosity taking into account spin-down and thermal radiation. Assuming the upcoming 10-year VRO LSST observation, our model calculations predict that about 10 HVNSs mainly consisting of pulsars with ages of $10^4$--$10^5$ yr and thermally emitting NSs with $10^5$--$10^6$ yr are detectable. We find that a few NSs with effective temperature $< 5 \times 10^5$ K, which are likely missed in the current and future X-ray surveys, are also detectable. In addition to the standard neutron star cooling models, we consider a dark matter heating model. If such a strong heating exists we find that the detectable HVNSs would be significantly cooler, i.e., $\lesssim 5\times 10^5$ K. Thus, the future optical observation will give an unique NS sample, which can provide essential constraints on the NS cooling and heating mechanisms. Moreover, we suggest that providing HVNS samples with optical surveys is helpful for understanding the intrinsic kick-velocity distribution of NSs., Comment: 8 pages, 6 figures, submitted to MNRAS

The detection of gravitational waves from a neutron star merger, GW170817, marked the dawn of a new era in time-domain astronomy. Monitoring of the radio emission produced by the merger, including high-resolution radio imaging, enabled measurements of merger properties including the energetics and inclination angle. In this work we compare the capabilities of current and future gravitational wave facilities to the sensitivity of radio facilities to quantify the prospects for detecting the radio afterglows of gravitational wave events. We consider three observing strategies to identify future mergers -- widefield follow-up, targeting galaxies within the merger localisation and deep monitoring of known counterparts. We find that while planned radio facilities like the Square Kilometre Array will be capable of detecting mergers at gigaparsec distances, no facilities are sufficiently sensitive to detect mergers at the range of proposed third-generation gravitational wave detectors that would operate starting in the 2030s.

We present new radio observations of the binary neutron star merger GW170817 carried out with the Karl G. Jansky Very large Array (VLA) more than 3\,yrs after the merger. Our combined dataset is derived by co-adding more than $\approx32$\,hours of VLA time on-source, and as such provides the deepest combined observation (rms sensitivity $\approx 0.99\,\mu$Jy) of the GW170817 field obtained to date at 3\,GHz. We find no evidence for a late-time radio re-brightening at a mean epoch of $t\approx 1200$\,d since merger, in contrast to a $\approx 2.1\,\sigma$ excess observed at X-ray wavelengths at the same mean epoch. Our measurements agree with expectations from the post-peak decay of the radio afterglow of the GW170817 structured jet. Using these results, we constrain the parameter space of models that predict a late-time radio re-brightening possibly arising from the high-velocity tail of the GW170817 kilonova ejecta, which would dominate the radio and X-ray emission years after the merger (once the structured jet afterglow fades below detection level). Our results point to a steep energy-speed distribution of the kilonova ejecta (with energy-velocity power law index $\alpha \gtrsim 5$). We suggest possible implications of our radio analysis, when combined with the recent tentative evidence for a late-time re-brightening in the X-rays, and highlight the need for continued radio-to-X-ray monitoring to test different scenarios., Comment: 6 pages, 2 figures. Version published in ApJL (includes minor updates to figures, data and refs)

In recent years, there have been significant advances in multi-messenger astronomy due to the discovery of the first, and so far only confirmed, gravitational wave event with a simultaneous electromagnetic (EM) counterpart, as well as improvements in numerical simulations, gravitational wave (GW) detectors, and transient astronomy. This has led to the exciting possibility of performing joint analyses of the GW and EM data, providing additional constraints on fundamental properties of the binary progenitor and merger remnant. Here, we present a new Bayesian framework that allows inference of these properties, while taking into account the systematic modeling uncertainties that arise when mapping from GW binary progenitor properties to photometric light curves. We extend the relative binning method presented in Zackay et al. (2018) to include extrinsic GW parameters for fast analysis of the GW signal. The focus of our EM framework is on light curves arising from r-process nucleosynthesis in the ejected material during and after merger, the so called kilonova, and particularly on black hole - neutron star systems. As a case study, we examine the recent detection of GW190425, where the primary object is consistent with being either a black hole (BH) or a neutron star (NS). We show quantitatively how improved mapping between binary progenitor and outflow properties, and/or an increase in EM data quantity and quality are required in order to break degeneracies in the fundamental source parameters., Comment: 22 pages, 12 figures, comments are welcome

The nebular phase of lanthanide-rich ejecta of a neutron star merger (NSM) is studied by using a one-zone model, in which the atomic properties are represented by a single species, neodymium (Nd). Under the assumption that beta-decay of r-process nuclei is the heat and ionization source, we solve the ionization and thermal balance of the ejecta under non-local thermodynamic equilibrium. The atomic data including energy levels, radiative transition rates, collision strengths, and recombination rate coefficients, are obtained by using atomic structure codes, GRASP2K and HULLAC. We find that both permitted and forbidden lines roughly equally contribute to the cooling rate of Nd II and Nd III at the nebular temperatures. We show that the kinetic temperature and ionization degree increase with time in the early stage of the nebular phase while these quantities become approximately independent of time after the thermalization break of the heating rate because the processes relevant to the ionization and thermalization balance are attributed to two-body collision between electrons and ions at later times. As a result, in spite of the rapid decline of the luminosity, the shape of the emergent spectrum does not change significantly with time after the break. We show that the emission-line nebular spectrum of the pure Nd ejecta consists of a broad structure from $0.5\,\mu m$ to $20\,\mu m$ with two distinct peaks around $1\,\mu m$ and $10\,\mu m$., Comment: 20 pages, 18 figures, submitted to MNRAS

The abundances of r-process elements of very metal-poor stars capture the history of the r-process enrichment in the early stage of star formation in a galaxy. Currently, various types of astrophysical sites including neutron star mergers, magneto-rotational supernovae, and collapsars, are suggested as the origin of r-process elements. The time delay between the star formation and the production of r-process elements is the key to distinguish these scenarios with the caveat that the diffusion of r-process elements in the interstellar medium may induce the delay in r-process enrichment because r-process events are rare. Here we study the observed Ba abundance data of very metal-poor stars as the tracer of the early enrichment history of r-process elements. We find that the gradual increase of [Ba/Mg] with [Fe/H], which is remarkably similar among the Milky Way and classical dwarfs, requires a significant time delay (100 Myr -- 1 Gyr) of r-process events from star formation rather than the diffusion-induced delay. We stress that this conclusion is robust to the assumption regarding s-process contamination in the Ba abundances because the sources with no delay would overproduce Ba at very low metallicities even without the contribution from the s-process. Therefore we conclude that sources with a delay, possibly neutron star mergers, are the origins of r-process elements., Comment: 10 pages, 6 figures, ApJ Letters in press

The dominant site of production of $r$-process elements remains unclear despite recent observations of a neutron star merger. Observational constraints on the properties of the sites can be obtained by comparing $r$-process abundances in different environments. The recent Gaia data releases and large samples from high-resolution optical spectroscopic surveys are enabling us to compare $r$-process element abundances between stars formed in an accreted dwarf galaxy, Gaia-Enceladus, and those formed in the Milky Way. We aim to understand the origin of $r$-process elements in Gaia-Enceladus. We first construct a sample of stars to study Eu abundances without being affected by the detection limit. We then kinematically select 71 Gaia-Enceladus stars and 93 in-situ stars from the Galactic Archaeology with HERMES (GALAH) DR3, of which 50 and 75 stars can be used to study Eu reliably. Gaia-Enceladus stars clearly show higher ratios of [{Eu}/{Mg}] than in-situ stars. High [{Eu}/{Mg}] along with low [{Mg}/{Fe}] are also seen in relatively massive satellite galaxies such as the LMC, Fornax, and Sagittarius dwarfs. On the other hand, unlike these galaxies, Gaia-Enceladus does not show enhanced [{Ba}/{Eu}] or [{La}/{Eu}] ratios suggesting a lack of significant $s$-process contribution. From comparisons with simple chemical evolution models, we show that the high [{Eu}/{Mg}] of Gaia-Enceladus can naturally be explained by considering $r$-process enrichment by neutron-star mergers with delay time distribution that follows a similar power-law as type~Ia supernovae but with a shorter minimum delay time., Comment: accepted to A\&A

We present a comprehensive study of neutrino shock acceleration in core-collapse supernova (CCSN). The leading players are heavy leptonic neutrinos, $\nu_{\mu}$ and $\nu_{\tau}$; the former and latter potentially gain the energy up to $\sim 100$ MeV and $\sim 200$ MeV, respectively, through the shock acceleration. Demonstrating the neutrino shock acceleration by Monte Carlo neutrino transport, we make a statement that it commonly occurs in the early post bounce phase ($\lesssim 50$ ms after bounce) for all massive stellar collapse experiencing nuclear bounce and would reoccur in the late phase ($\gtrsim 100$ ms) for failed CCSNe. This opens up a new possibility to detect high energy neutrinos by terrestrial detectors from Galactic CCSNe; hence, we estimate the event counts for Hyper(Super)-Kamiokande, DUNE, and JUNO. We find that the event count with the energy of $\gtrsim 80$ MeV is a few orders of magnitude higher than that of the thermal neutrinos regardless of the detectors, and muon production may also happen in these detectors by $\nu_{\mu}$ with the energy of $\gtrsim 100$ MeV. The neutrino signals provide a precious information on deciphering the inner dynamics of CCSN and placing a constraint on the physics of neutrino oscillation; indeed, the detection of the high energy neutrinos through charged current reaction channels will be a smoking gun evidence of neutrino flavor conversion., Comment: Accepted to MNRAS

Joint multi-messenger observations with gravitational waves and electromagnetic data offer new insights into the astrophysical studies of compact objects. The third Advanced LIGO and Advanced Virgo observing run began on April 1, 2019; during the eleven months of observation, there have been 14 compact binary systems candidates for which at least one component is potentially a neutron star. Although intensive follow-up campaigns involving tens of ground and space-based observatories searched for counterparts, no electromagnetic counterpart has been detected. Following on a previous study of the first six months of the campaign, we present in this paper the next five months of the campaign from October 2019 to March 2020. We highlight two neutron star - black hole candidates (S191205ah, S200105ae), two binary neutron star candidates (S191213g and S200213t) and a binary merger with a possible neutron star and a "MassGap" component, S200115j. Assuming that the gravitational-wave candidates are of astrophysical origin and their location was covered by optical telescopes, we derive possible constraints on the matter ejected during the events based on the non-detection of counterparts. We find that the follow-up observations during the second half of the third observing run did not meet the necessary sensitivity to constrain the source properties of the potential gravitational-wave candidate. Consequently, we suggest that different strategies have to be used to allow a better usage of the available telescope time. We examine different choices for follow-up surveys to optimize sky localization coverage vs.\ observational depth to understand the likelihood of counterpart detection.

Theory and observations suggest that single-star evolution is not able to produce black holes (BHs) with masses in the range $3-5M_{\odot}$ and above $\sim 45M_{\odot}$, referred to as the lower mass gap (LMG) and the upper mas gap (UMG), respectively. However, it is possible to form BHs in these gaps through merger of compact objects in dense clusters, e.g. the LMG and the UMG can be populated through binary neutron star- and BBH mergers, respectively. This implies that if binary mergers are observed in gravitational waves (GWs) with at least one mass gap object, then either clusters are effective in assembling binary mergers, or our single-star models have to be revised. Understanding how effective clusters are at populating both mass gaps have therefore major implications for both stellar- and GW astrophysics. In this paper we present a systematic study on how efficient stellar clusters are at populating both mass gaps through in-cluster GW mergers. For this, we derive a set of closed form relations for describing the evolution of compact object binaries undergoing dynamical interactions and GW merger inside their cluster. By considering both static and time evolving populations, we find in particular that globular clusters are clearly inefficient at populating the LMG in contrast to the UMG. We further describe how these results relate to the characteristic mass, time, and length scales associated with the problem., Comment: 19 pages. 7 figures. comments welcome

We present the full panchromatic afterglow light curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data, and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups, and attempted to mitigate these differences to provide a more uniform dataset. Simple power-law fits to the uniform afterglow light curve indicate a $t^{0.86\pm0.04}$ rise, a $t^{-1.92\pm0.12}$ decline, and a peak occurring at $155\pm4$ days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index ($-0.584\pm0.002$) across all epochs. This gives a precise and updated estimate of the electron power-law index, $p=2.168\pm0.004$. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized ISM density, $<$0.01 cm$^{-3}$, consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having magnetic field strength between 10$^{10.4}$ G and 10$^{16}$ G. Our fits to the afterglow data using an analytical model that includes VLBI proper motion from Mooley et al. (2018), and a structured jet model that ignores the proper motion, indicates that the proper motion measurement needs to be considered while seeking an accurate estimate of the viewing angle., Comment: Accepted by ApJ. Revised X-ray and up-to-date afterglow measurements are available in ASCII format on GitHub (https://github.com/kmooley/GW170817/) and mirrored on tauceti.caltech.edu (http://www.tauceti.caltech.edu/kunal/gw170817/). The online dataset will be continuously updated as new measurements become available

The LIGO/Virgo Scientific Collaboration recently announced the detection of a compact object binary merger, GW190412, as the first asymmetric binary black hole (BBH) merger with mass ratio $q\approx0.25$. Other than the mass ratio, this BBH has shown to have a positive effective spin of around $\chi_{\rm eff}\approx0.28$. Assuming a field formation channel, associating this effective spin to either the primary or the secondary BH each has its implications: If the spin of the BBH comes form the primary BH, it has consequences for the efficiency of angular momentum transport in the formation of the BH. If, on the other hand, the spin is due to the secondary BH through tidal spin-up processes, one has to note that (i) such processes have very short delay-times, and (ii) subsequently, their local merger rate is determined by local star formation rate at assumed formation metallicity of the BBH. We show that the predicted merger rate density from this channel is $\lesssim 0.3~\rm Gpc^{-3} yr^{-1}$ and in tension with the rather high local merger rate of such systems which we estimate from this single event to be $\sim 1.7^{+2.5}_{-1.4}~\rm Gpc^{-3} yr^{-1}$ (90\% confidence interval, and assuming 50 days of observing time). Large natal kicks ($v\gtrsim 500\,{\rm km/s}$) would be required to get such BBHs with an in-plane spin component to account for the marginal detection of precession in GW190412. However, this would only exacerbate the tension as the estimated local merger rate would be further decreased. Similarly, the formation of such systems through the dynamical assembly is exceedingly rare, leaving this system a dilemma hard to account for with the currently accepted paradigms of BBH formation., Comment: Matches the published version

We study turbulent gas diffusion affects on $r$-process abundances in Milky Way stars, by a combination of an analytical approach and a Monte Carlo simulation. Higher $r$-process event rates and faster diffusion, lead to more efficient mixing corresponding to a reduced scatter of $r$-process abundances and causing $r$-process enriched stars to start appearing at lower metallicities. We use three independent observations to constrain the model parameters: (i) the scatter of radioactively stable $r$-process element abundances, (ii) the largest $r$-process enrichment values observed in any solar neighborhood stars and (iii) the isotope abundance ratios of different radioactive $r$-process elements ($^{244}$Pu/$^{238}$U and $^{247}$Cm/$^{238}$U) at the early solar system as compared to their formation. Our results indicate that the Galactic $r$-process rate and the diffusion coefficient are respectively $r<4\times 10^{-5}\mbox{ yr}^{-1}, D>0.1 \mbox{ kpc}^2\mbox{Gyr}^{-1}$ ($r<4\times 10^{-6}\mbox{ yr}^{-1}, D>0.5 \mbox{ kpc}^2\mbox{Gyr}^{-1}$ for collapsars or similarly prolific $r$-process sources) with allowed values satisfying an approximate anti-correlation such that $D\approx r^{-2/3}$, implying that the time between two $r$-process events that enrich the same location in the Galaxy, is $\tau_{\rm mix}\approx 100-200\mbox{ Myr}$. This suggests that a fraction of $\sim 0.8$ ($\sim 0.5$) of the observed $^{247}$Cm ($^{244}$Pu) abundance is dominated by one $r$-process event in the early solar system. Radioactively stable element abundances are dominated by contributions from $\sim 10$ different events in the early solar system. For metal poor stars (with [Fe/H]$\lesssim -2$), their $r$-process abundances are dominated by either a single or several events, depending on the star formation history., Comment: 11 pages, 9 figures, accepted for publication in MNRAS

The jet opening angle and inclination of GW170817 -- the first detected binary neutron star merger -- were vital to understand its energetics, relation to short gamma-ray bursts, and refinement of the standard siren-based determination of the Hubble constant, $H_0$. These basic quantities were determined through a combination of the radio lightcurve and Very Long Baseline Interferometry (VLBI) measurements of proper motion. In this paper we discuss and quantify the prospects for the use of radio VLBI observations and observations of scintillation-induced variability to measure the source size and proper motion of merger afterglows, and thereby infer properties of the merger including inclination angle, opening angle and energetics. We show that these techniques are complementary as they probe different parts of the circum-merger density/inclination angle parameter space and different periods of the temporal evolution of the afterglow. We also find that while VLBI observations will be limited to the very closest events it will be possible to detect scintillation for a large fraction of events beyond the range of current gravitational wave detectors. Scintillation will also be detectable with next generation telescopes such as the Square Kilometre Array, 2000 antenna Deep Synoptic Array and the next generation Very Large Array, for a large fraction of events detected with third generation gravitational wave detectors. Finally, we discuss prospects for the measurement of the $H_0$ with VLBI observations of neutron star mergers and compare this technique to other standard siren methods.

On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron star--black hole (NSBH) merger, or potentially a low-mass binary black hole merger. Due to the low false alarm rate and the precise localization (23 deg$^2$ at 90\%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to a NSBH merger. To search for potential counterparts, the GROWTH collaboration performed real-time image subtraction on 6 nights of public Dark Energy Camera (DECam) images acquired in the three weeks following the merger, covering $>$98\% of the localization probability. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and we placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be $M_\mathrm{ej} < 0.04$~$M_{\odot}$ at polar viewing angles, or $M_\mathrm{ej} < 0.03$~$M_{\odot}$ if the opacity is $\kappa < 2$~cm$^2$g$^{-1}$. Assuming a tidal deformability for the neutron star at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be $ \chi < 0.7$ for mass ratios $Q < 6$, with weaker constraints for more compact neutron stars. We publicly release the photometry from this campaign at http://www.astro.caltech.edu/~danny/static/s190814bv., Comment: Accepted to ApJ with no major changes

We present results from a search for a radio transient associated with the LIGO/Virgo source S190814bv, a likely neutron star-black hole (NSBH) merger, with the Australian Square Kilometre Array Pathfinder. We imaged a $30\,{\rm deg}^2$ field at $\Delta T$=2, 9 and 33 days post-merger at a frequency of 944\,MHz, comparing them to reference images from the Rapid ASKAP Continuum Survey observed 110 days prior to the event. Each epoch of our observations covers $89\%$ of the LIGO/Virgo localisation region. We conducted an untargeted search for radio transients in this field, resulting in 21 candidates. For one of these, \object[AT2019osy]{AT2019osy}, we performed multi-wavelength follow-up and ultimately ruled out the association with S190814bv. All other candidates are likely unrelated variables, but we cannot conclusively rule them out. We discuss our results in the context of model predictions for radio emission from neutron star-black hole mergers and place constrains on the circum-merger density and inclination angle of the merger. This survey is simultaneously the first large-scale radio follow-up of an NSBH merger, and the most sensitive widefield radio transients search to-date.

GW170817 showed that neutron star mergers not only emit gravitational waves but also can release electromagnetic signatures in multiple wavelengths. Within the first half of the third observing run of the Advanced LIGO and Virgo detectors, there have been a number of gravitational wave candidates of compact binary systems for which at least one component is potentially a neutron star. In this article, we look at the candidates S190425z, S190426c, S190510g, S190901ap, and S190910h, predicted to have potentially a non-zero remnant mass, in more detail. All these triggers have been followed up with extensive campaigns by the astronomical community doing electromagnetic searches for their optical counterparts; however, according to the released classification, there is a high probability that some of these events might not be of extraterrestrial origin. Assuming that the triggers are caused by a compact binary coalescence and that the individual source locations have been covered during the EM follow-up campaigns, we employ three different kilonova models and apply them to derive possible constraints on the matter ejection consistent with the publicly available gravitational-wave trigger information and the lack of a kilonova detection. These upper bounds on the ejecta mass can be related to limits on the maximum mass of the binary neutron star candidate S190425z and to constraints on the mass-ratio, spin, and NS compactness for the potential black hole-neutron star candidate S190426c. Our results show that deeper electromagnetic observations for future gravitational wave events near the horizon limit of the advanced detectors are essential.

Gravitational wave (GW) sources are an excellent probe of the luminosity distance and offer a novel measure of the Hubble constant, $H_0$. This estimation of $H_0$ from standard sirens requires an accurate estimation of the cosmological redshift of the host galaxy of the GW source, after correcting for its peculiar velocity. Absence of an accurate peculiar velocity correction affects both the precision and accuracy of the measurement of $H_0$, particularly for nearby sources. We propose a framework to incorporate such a peculiar velocity correction for GW sources. A first implementation of our method to the event GW170817 combined with the Very Large Baseline Interferometry (VLBI) observation leads to a revised value of $H_0= 68.3^{+ 4.6}_{-4.5}$ km/s/Mpc. While this revision is minor, it demonstrates that our method makes it possible for obtaining an unbiased and accurate measurements of $H_0$ at the precision required for the standard siren cosmology., Comment: 12 pages, 10 figures. Matches the version accepted for publication in Astronomy & Astrophysics

We study the heating rate of r-process nuclei and thermalization of decay products in neutron star merger ejecta and macronova (kilonova) light curves. Thermalization of charged decay products, i.e., electrons, $\alpha$-particles, and fission fragments is calculated according to their injection energy. The $\gamma$-ray thermalization processes are also properly calculated by taking the $\gamma$-ray spectrum of each decay into account. We show that the $\beta$-decay heating rate at later times approaches a power-law decline as $\propto t^{-2.8}$, which agrees with the result of Waxman et al. (2019). We present a new analytic model to calculate macronova light curves, in which the density structure of the ejecta is accounted for. We demonstrate that the observed bolometric light curve and temperature evolution of the macronova associated with GW170817 are reproduced well by the $\beta$-decay heating rate with the solar r-process abundance pattern. We interpret the break in the observed bolometric light curve around a week as a result of the diffusion wave crossing a significant part of the ejecta rather than a thermalization break. We also show that the time-weighted integral of the bolometric light curve (Katz integral) is useful to provide an estimate of the total r-process mass from the observed data, which is independent of the highly uncertain radiative transfer. For the macronova in GW170817, the ejecta mass is robustly estimated as $\approx 0.05M_{\odot}$ for $A_{\rm min}\leq 72$ and $85\leq A_{\rm min}\leq 130$ with the solar r-process abundance pattern. The code for computation of the heating rate and light curve for given initial nuclear abundances is publicly available., Comment: 16 pages, 11 figures, code available at https://github.com/hotokezaka/HeatingRate

Mergers of binary neutron stars and black hole-neutron star binaries are one of the most promising sources for the ground-based gravitational-wave (GW) detectors and also a high-energy astrophysical phenomenon as illustrated by the observations of gravitational waves and electromagnetic (EM) waves in the event of GW170817. Mergers of these neutron-star binaries are also the most promising site for r-process nucleosynthesis. Numerical simulation in full general relativity (numerical relativity) is a unique approach to the theoretical prediction of the merger process, GWs emitted, mass ejection process, and resulting EM emission. We summarize our current understanding for the processes of neutron star mergers and subsequent mass ejection based on the results of the latest numerical-relativity simulations. We emphasize that the predictions of the numerical-relativity simulations agrees broadly with the optical and infrared observations of GW170817., Comment: 26 pages, 7 figures, preprint version of a review to appear in Annual Review of Nuclear and Particle Science, 69 (2019)

We constrain the formation rate of Galactic magnetars directly from observations. Combining spin-down rates, magnetic activity, and association with supernova remnants, we put a 2$\sigma$ limit on their Galactic formation rate at $2.3-20\mbox{kyr}^{-1}$. This leads to a fraction $0.4_{-0.28}^{+0.6}$ of neutron stars being born as magnetars. We study evolutionary channels that can account for this rate as well as for the periods, period derivatives and luminosities of the observed population. We find that their typical magnetic fields at birth are $3\times 10^{14}-10^{15}$G, and that those decay on a time-scale of $\sim 10^4$years, implying a maximal magnetar period of $P_{\rm max}\approx 13$s. A sizable fraction of the magnetars' energy is released in outbursts. Giant Flares with $E\geq 10^{46}$ erg are expected to occur in the Galaxy at a rate of $\sim 5\mbox{kyr}^{-1}$. Outside our Galaxy, such flares remain observable by {\it Swift} up to a distance of $\sim 100$~Mpc, implying a detection rate of $\sim 5\mbox{ yr}^{-1}$. The specific form of magnetic energy decay is shown to be strongly tied to the total number of observable magnetars in the Galaxy. A systematic survey searching for magnetars could determine the former and inform physical models of magnetic field decay., Comment: 13 pages, 10 figures

The field of time-domain astrophysics has entered the era of Multi-messenger Astronomy (MMA). One key science goal for the next decade (and beyond) will be to characterize gravitational wave (GW) and neutrino sources using the next generation of Extremely Large Telescopes (ELTs). These studies will have a broad impact across astrophysics, informing our knowledge of the production and enrichment history of the heaviest chemical elements, constrain the dense matter equation of state, provide independent constraints on cosmology, increase our understanding of particle acceleration in shocks and jets, and study the lives of black holes in the universe. Future GW detectors will greatly improve their sensitivity during the coming decade, as will near-infrared telescopes capable of independently finding kilonovae from neutron star mergers. However, the electromagnetic counterparts to high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus demand ELT capabilities for characterization. ELTs will be important and necessary contributors to an advanced and complete multi-messenger network., Comment: White paper submitted to the Astro2020 Decadal Survey

We report our Spitzer Space Telescope observations and detections of the binary neutron star merger GW170817. At 4.5um, GW170817 is detected at 21.9 mag AB at +43 days and 23.9 mag AB at +74 days after merger. At 3.6um, GW170817 is not detected to a limit of 23.2 mag AB at +43 days and 23.1 mag AB at +74 days. Our detections constitute the latest and reddest constraints on the kilonova/macronova emission and composition of heavy elements. The 4.5um luminosity at this late phase cannot be explained by elements exclusively from the first abundance peak of the r-process. Moreover, the steep decline in the Spitzer band, with a power-law index of 3.4 +/- 0.2, can be explained by a few of the heaviest isotopes in the third abundance peak with half-life around 14 days dominating the luminosity (e.g. 140Ba, 143Pr, 147Nd, 156Eu, 191Os, 223Ra, 225Ra, 233Pa, 234Th) or a model with lower deposition efficiency. This data offers evidence that the heaviest elements in the second and third r-process abundance peak were indeed synthesized. Our conclusion is verified by both analytics and network simulations and robust despite intricacies and uncertainties in the nuclear physics. Future observations with Spitzer and James Webb Space Telescope will further illuminate the relative abundance of the synthesized heavy elements.

General-relativistic simulations of binary neutron star mergers with viscosity reveal a new outflow mechanism operating in unequal mass binaries on dynamical timescales and enabled by turbulent viscosity. These "viscous-dynamical" ejecta are launched during merger due to the thermalization of mass exchange streams between the secondary and the primary neutron star. They are characterized by asymptotic velocities extending up to ${\sim} 0.8\, c$, and have masses that depend on the efficiency of the viscous mechanism. Depending on the unknown strength of the effective viscosity arising from magnetohydrodynamics instabilities operating during merger, the overall mass of the dynamical ejecta could be enhanced by a factor of a few and the mass of the fast tail of the ejecta having asymptotic velocities $\geq 0.6\, c$ by up to four orders of magnitude. The radioactive decay of the expanding viscous-dynamical ejecta could produce bright kilonova transients with signatures of free neutron decay in the first hour and enhanced near infrared flux on a timescale of a few days. The synchrotron remnant produced by the interaction between the ejecta and the interstellar medium could also be significantly enhanced by viscosity. Such remnant could be detected in the case of GW170817 as a rebrightening of the radio signal in the next months to years., Comment: 6 pages, 4 figures, version accepted in ApJL

We present a systematic numerical relativity study of the mass ejection and the associated electromagnetic transients and nucleosynthesis from binary neutron star (NS) mergers. We find that a few $10^{-3}\, M_\odot$ of material are ejected dynamically during the mergers. The amount and the properties of these outflow depend on binary parameters and on the NS equation of state (EOS). A small fraction of these ejecta, typically ${\sim}10^{-6}\, M_\odot$, is accelerated by shocks formed shortly after merger to velocities larger than $0.6\, {\rm c}$ and produces bright radio flares on timescales of weeks, months, or years after merger. Their observation could constrain the strength with which the NSs bounce after merger and, consequently, the EOS of matter at extreme densities. The dynamical ejecta robustly produce second and third $r$-process peak nuclei with relative isotopic abundances close to solar. The production of light $r$-process elements is instead sensitive to the binary mass ratio and the neutrino radiation treatment. Accretion disks of up to ${\sim}0.2\, M_\odot$ are formed after merger, depending on the lifetime of the remnant. In most cases, neutrino- and viscously-driven winds from these disks dominate the overall outflow. Finally, we generate synthetic kilonova light curves and find that kilonovae depend on the merger outcome and could be used to constrain the NS EOS., Comment: 33 pages, 32 figures, 3 tables. Dynamical ejecta data from the simulation is available at https://zenodo.org/record/3235675