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The binary-driven hypernova (BdHN) model proposes long gamma-ray bursts (GRBs) originate in binaries composed of a carbon-oxygen (CO) star and a neutron star (NS) companion. The CO core collapse generates a newborn NS and a supernova that triggers the GRB by accreting onto the NSs, rapidly transferring mass and angular momentum to them. This article aims to determine the conditions under which a black hole (BH) forms from NS collapse induced by the accretion and the impact on the GRB observational properties and taxonomy. We perform three-dimensional, smoothed-particle-hydrodynamics simulations of BdHNe using up-to-date NS nuclear equations of state (EOS), with and without hyperons, and calculate the structure evolution in full general relativity. We assess the binary parameters leading either NS in the binary to the critical mass for gravitational collapse into a BH and its occurrence time, $t_{\rm col}$. We include a non-zero angular momentum of the NSs and find that $t_{\rm col}$ ranges from a few tens of seconds to hours for decreasing NS initial angular momentum values. BdHNe I are the most compact (about five minutes orbital period), promptly form a BH and release $\gtrsim 10^{52}$ erg. They form NS-BH binaries with tens of kyr merger timescale by gravitational-wave emission. BdHNe II and III do not form BHs, release $\sim 10^{50}$-$10^{52}$ erg and $\lesssim 10^{50}$ erg. They form NS-NS binaries with a range of merger timescales larger than for NS-BH binaries. In some compact BdHNe II, either NS can become supramassive, i.e., above the critical mass of a non-rotating NS. Magnetic braking by a $10^{13}$ G field can delay BH formation, leading to BH-BH or NS-BH of tens of kyr merger timescale., Comment: Version accepted for publication in The Astrophysical Journal

We present JWST NIRCam (F356W and F444W filters) and MIRI (F770W) images and NIRSpec- IFU spectroscopy of the young supernova remnant Cassiopeia A (Cas A). We obtained the data as part of a JWST survey of Cas A. The NIRCam and MIRI images map the spatial distributions of synchrotron radiation, Ar-rich ejecta, and CO on both large and small scales, revealing remarkably complex structures. The CO emission is stronger at the outer layers than the Ar ejecta, which indicates the reformation of CO molecules behind the reverse shock. NIRSpec-IFU spectra (3 - 5.5 microns) were obtained toward two representative knots in the NE and S fields. Both regions are dominated by the bright fundamental rovibrational band of CO in the two R and P branches, with strong [Ar VI] and relatively weaker, variable strength ejecta lines of [Si IX], [Ca IV], [Ca V] and [Mg IV]. The NIRSpec-IFU data resolve individual ejecta knots and filaments spatially and in velocity space. The fundamental CO band in the JWST spectra reveals unique shapes of CO, showing a few tens of sinusoidal patterns of rovibrational lines with pseudo-continuum underneath, which is attributed to the high-velocity widths of CO lines. The CO also shows high J lines at different vibrational transitions. Our results with LTE modeling of CO emission indicate a temperature of 1080 K and provide unique insight into the correlations between dust, molecules, and highly ionized ejecta in supernovae, and have strong ramifications for modeling dust formation that is led by CO cooling in the early Universe., Comment: accepted for the ApJ letter (17 pages and 10 figures)

We present smoothed-particle-hydrodynamics (SPH) simulations of the binary-driven hypernova (BdHN) scenario of long gamma-ray bursts (GRBs), focusing on the binary stability during the supernova (SN) explosion. The BdHN progenitor is a binary comprised of a carbon-oxygen (CO) star and a neutron star (NS) companion. The core collapse of the CO leads to an SN explosion and a newborn NS ($\nu$NS) at its center. Ejected material accretes onto the NS and the $\nu$NS. BdHNe of type I have compact orbits of a few minutes, the NS reaches the critical mass, forming a black hole (BH), and the energy release is $\gtrsim 10^{52}$ erg. BdHNe II have longer periods of tens of minutes to hours; the NS becomes more massive, remains stable, and the system releases $\sim 10^{50}$-$10^{52}$ erg. BdHN III have longer periods, even days, where the accretion is negligible, and the energy released is $\lesssim 10^{50}$ erg. We assess whether the system remains gravitationally bound after the SN explosion, leading to an NS-BH in BdHN I, an NS-NS in BdHN II and III, or if the SN explosion disrupts the system. The existence of bound systems predicts an evolutionary connection between the long and short GRB populations. We determine the binary parameters for which the binary remains bound after the BdHN event. For these binaries, we derive fitting formulas of the numerical results for the main parameters, e.g., the mass loss, the SN explosion energy, orbital period, eccentricity, center-of-mass velocity, and the relation between the initial and final binary parameters, which are useful for outlined astrophysical applications., Comment: Submitted

The binary-driven hypernova (BdHN) model explains long gamma-ray bursts (GRBs) associated with supernovae (SNe) Ic through physical episodes that occur in a binary composed of a carbon-oxygen (CO) star and a neutron star (NS) companion in close orbit. The CO core collapse triggers the cataclysmic event, originating the SN and a newborn NS (hereafter $\nu$NS) at its center. The $\nu$NS and the NS accrete SN matter. BdHNe are classified based on the NS companion fate and the GRB energetics, mainly determined by the orbital period. In BdHNe I, the orbital period is of a few minutes, so the accretion causes the NS to collapse into a Kerr black hole (BH), explaining GRBs of energies $>10^{52}$ erg. BdHN II, with longer periods of tens of minutes, yields a more massive but stable NS, accounting for GRBs of $10^{50}$--$10^{52}$ erg. BdHNe III have still longer orbital periods (e.g., hours), so the NS companion has a negligible role, which explains GRBs with a lower energy release of $<10^{50}$ erg. BdHN I and II might remain bound after the SN, so they could form NS-BH and binary NS (BNS), respectively. In BdHN III, the SN likely disrupts the system. We perform numerical simulations of BdHN II to compute the characteristic parameters of the BNS left by them, their mergers, and the associated short GRBs. We obtain the mass of the central remnant, whether it is likely to be a massive NS or a BH, the conditions for disk formation and its mass, and the event's energy release. The role of the NS nuclear equation of state is outlined., Comment: Published in Universe in the Special Issue Remo Ruffini Festschrift

The mergers of binary compact objects such as neutron stars and black holes are of central interest to several areas of astrophysics, including as the progenitors of gamma-ray bursts (GRBs), sources of high-frequency gravitational waves and likely production sites for heavy element nucleosynthesis via rapid neutron capture (the r-process). These heavy elements include some of great geophysical, biological and cultural importance, such as thorium, iodine and gold. Here we present observations of the exceptionally bright gamma-ray burst GRB 230307A. We show that GRB 230307A belongs to the class of long-duration gamma-ray bursts associated with compact object mergers, and contains a kilonova similar to AT2017gfo, associated with the gravitational-wave merger GW170817. We obtained James Webb Space Telescope mid-infrared (mid-IR) imaging and spectroscopy 29 and 61 days after the burst. The spectroscopy shows an emission line at 2.15 microns which we interpret as tellurium (atomic mass A=130), and a very red source, emitting most of its light in the mid-IR due to the production of lanthanides. These observations demonstrate that nucleosynthesis in GRBs can create r-process elements across a broad atomic mass range and play a central role in heavy element nucleosynthesis across the Universe., Comment: Submitted. Comments welcome! Nature (2023)

We report the discovery of GRB 221009A, the highest flux gamma-ray burst ever observed by the Fermi Gamma-ray Burst Monitor (GBM). This GRB has continuous prompt emission lasting more than 600 seconds which smoothly transitions to afterglow visible in the GBM energy range (8 keV--40 MeV), and total energetics higher than any other burst in the GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the GBM trigger time (t0; 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock-breakout, with significant emission up to $\sim$15 MeV. We characterize the onset of external shock at t0+600 s and find evidence of a plateau region in the early-afterglow phase which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the GBM sample and find that this GRB has the highest total isotropic-equivalent energy ($\textrm{E}_{\gamma,\textrm{iso}}=1.0\times10^{55}$ erg) and second highest isotropic-equivalent luminosity ($\textrm{L}_{\gamma,\textrm{iso}}=9.9\times10^{53}$ erg/s) based on redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of GBM from the beginning of the prompt emission phase through the onset of early afterglow., Comment: 26 pages 7 figures - accepted for publication in ApJL

We extend previous ab initio calculations of lanthanide opacities (Fontes et al., 2020, MNRAS, 493, 4143) to include a complete set of actinide opacities for use in the modeling of kilonova light curves and spectra. Detailed, fine-structure line features are generated using the configuration-interaction approach. These actinide opacities display similar trends to those observed for lanthanide opacities, such as the lighter actinides producing higher opacity than the heavier ones for relevant conditions in the dynamical ejecta. A line-binned treatment is employed to pre-compute opacity tables for 14 actinide elements $(89 \le Z \le 102)$ over a grid of relevant temperatures and densities. These tabular opacities will be made publicly available for general usage in kilonova modeling. We demonstrate the usefulness of these opacities in kilonova simulations by exploring the sensitivity of light curves and spectra to different actinide abundance distributions that are predicted by different nuclear theories, as well as to different choices of ejecta mass and velocity. We find very little sensitivity to the two considered distributions, indicating that opacities for actinides with $Z \ge 99$ do not contribute strongly. On the other hand, a single actinide element, protactinium, is found to produce faint spectral features in the far infrared at late times (5-7 days post merger). More generally, we find that the choice of ejecta mass and velocity have the most significant effect on KN emission for this study., Comment: 18 pages, 14 figures

Gamma-ray bursts (GRBs) are flashes of high-energy radiation arising from energetic cosmic explosions. Bursts of long (>2 s) duration are produced by the core-collapse of massive stars, those of short (< 2 s) duration by the merger of two neutron stars (NSs). A third class of events with hybrid high-energy properties was identified, but never conclusively linked to a stellar progenitor. The lack of bright supernovae rules out typical core-collapse explosions, but their distance scales prevent sensitive searches for direct signatures of a progenitor system. Only tentative evidence for a kilonova has been presented. Here we report observations of the exceptionally bright GRB211211A that classify it as a hybrid event and constrain its distance scale to only 346 Mpc. Our measurements indicate that its lower-energy (from ultraviolet to near-infrared) counterpart is powered by a luminous (~1E42 erg/s) kilonova possibly formed in the ejecta of a compact binary merger., Comment: final version, including supplementary methods

GRB 171205A is a low-luminosity, long-duration gamma-ray burst (GRB) associated with SN 2017iuk, a broad-line type Ic supernova (SN). It is consistent with being formed in the core-collapse of a widely separated binary, which we have called the binary-driven hypernova (BdHN) of type III. The core-collapse of the CO star forms a newborn NS ($\nu$NS) and the SN explosion. Fallback accretion transfers mass and angular momentum to the $\nu$NS, here assumed to be born non-rotating. The accretion energy injected into the expanding stellar layers powers the prompt emission. The multiwavelength power-law afterglow is explained by the synchrotron radiation of electrons in the SN ejecta, powered by energy injected by the spinning $\nu$NS. We calculate the amount of mass and angular momentum gained by the $\nu$NS, as well as the $\nu$NS rotational evolution. The $\nu$NS spins up to a period of $47$ ms, then releases its rotational energy powering the synchrotron emission of the afterglow. The paucity of the $\nu$NS spin explains the low-luminosity characteristic and that the optical emission of the SN from the nickel radioactive decay outshines the optical emission from the synchrotron radiation. From the $\nu$NS evolution, we infer that the SN explosion had to occur at most $7.36$ h before the GRB trigger. Therefore, for the first time, the analysis of the GRB data leads to the time of occurrence of the CO core-collapse leading to the SN explosion and the electromagnetic emission of the GRB event., Comment: 19 pages, 5 figures, accepted for publication in ApJ

Kilonovae, one source of electromagnetic emission associated with neutron star mergers, are powered by the decay of radioactive isotopes in the neutron-rich merger ejecta. Models for kilonova emission consistent with the electromagnetic counterpart to GW170817 predict characteristic abundance patterns, determined by the relative balance of different types of material in the outflow. Assuming the observed source is prototypical, this inferred abundance pattern in turn must match $r$-process abundances deduced by other means, such as what is observed in the solar system. We report on analysis comparing the input mass-weighted elemental compositions adopted in our radiative transfer simulations to the mass fractions of elements in the Sun, as a practical prototype for the potentially universal abundance signature from neutron-star mergers. We characterize the extent to which our parameter inference results depend on our assumed composition for the dynamical and wind ejecta and examine how the new results compare to previous work. We find that a dynamical ejecta composition calculated using the FRDM2012 nuclear mass and FRLDM fission models with extremely neutron-rich ejecta ($Y_{\rm{e}} = 0.035$) along with moderately neutron-rich ($Y_{\rm e} = 0.27$) wind ejecta composition yields a wind-to-dynamical mass ratio of $M_{\rm{w}}/M_{\rm{d}}$ = 0.47 which best matches the observed AT2017gfo kilonova light curves while also producing the best-matching abundance of neutron-capture elements in the solar system., Comment: 16 pages, 9 figures, submitted to PRD

Nuclear Astrophysics is a field at the intersection of nuclear physics and astrophysics, which seeks to understand the nuclear engines of astronomical objects and the origin of the chemical elements. This white paper summarizes progress and status of the field, the new open questions that have emerged, and the tremendous scientific opportunities that have opened up with major advances in capabilities across an ever growing number of disciplines and subfields that need to be integrated. We take a holistic view of the field discussing the unique challenges and opportunities in nuclear astrophysics in regards to science, diversity, education, and the interdisciplinarity and breadth of the field. Clearly nuclear astrophysics is a dynamic field with a bright future that is entering a new era of discovery opportunities., Comment: 96 pages. Submitted to Journal of Physics G

The Gamow Explorer will use Gamma Ray Bursts (GRBs) to: 1) probe the high redshift universe (z > 6) when the first stars were born, galaxies formed and Hydrogen was reionized; and 2) enable multi-messenger astrophysics by rapidly identifying Electro-Magnetic (IR/Optical/X-ray) counterparts to Gravitational Wave (GW) events. GRBs have been detected out to z ~ 9 and their afterglows are a bright beacon lasting a few days that can be used to observe the spectral fingerprints of the host galaxy and intergalactic medium to map the period of reionization and early metal enrichment. Gamow Explorer is optimized to quickly identify high-z events to trigger follow-up observations with JWST and large ground-based telescopes. A wide field of view Lobster Eye X-ray Telescope (LEXT) will search for GRBs and locate them with arc-minute precision. When a GRB is detected, the rapidly slewing spacecraft will point the 5 photometric channel Photo-z Infra-Red Telescope (PIRT) to identify high redshift (z > 6) long GRBs within 100s and send an alert within 1000s of the GRB trigger. An L2 orbit provides > 95% observing efficiency with pointing optimized for follow up by the James Webb Space Telescope (JWST) and ground observatories. The predicted Gamow Explorer high-z rate is >10 times that of the Neil Gehrels Swift Observatory. The instrument and mission capabilities also enable rapid identification of short GRBs and their afterglows associated with GW events. The Gamow Explorer will be proposed to the 2021 NASA MIDEX call and if approved, launched in 2028., Comment: 14 pages, 8 Figures

In this paper we explore the scientific synergies between Athena and some of the key multi-messenger facilities that should be operative concurrently with Athena. These facilities include LIGO A+, Advanced Virgo+ and future detectors for ground-based observation of gravitational waves (GW), LISA for space-based observations of GW, IceCube and KM3NeT for neutrino observations, and CTA for very high energy observations. These science themes encompass pressing issues in astrophysics, cosmology and fundamental physics such as: the central engine and jet physics in compact binary mergers, accretion processes and jet physics in Super-Massive Binary Black Holes (SMBBHs) and in compact stellar binaries, the equation of state of neutron stars, cosmic accelerators and the origin of Cosmic Rays (CRs), the origin of intermediate and high-Z elements in the Universe, the Cosmic distance scale and tests of General Relativity and the Standard Model. Observational strategies for implementing the identified science topics are also discussed. A significant part of the sources targeted by multi-messenger facilities is of transient nature. We have thus also discussed the synergy of \textsl{Athena} with wide-field high-energy facilities, taking THESEUS as a case study for transient discovery. This discussion covers all the Athena science goals that rely on follow-up observations of high-energy transients identified by external observatories, and includes also topics that are not based on multi-messenger observations, such as the search for missing baryons or the observation of early star populations and metal enrichment at the cosmic dawn with Gamma-Ray Bursts (GRBs)., Comment: White paper of the multi-messenger Athena synergy excersise

Depending upon the properties of their compact remnants and the physics included in the models, simulations of neutron star mergers can produce a broad range of ejecta properties. The characteristics of this ejecta, in turn, define the kilonova emission. To explore the effect of ejecta properties, we present a grid of 2-component 2D axisymmetric kilonova simulations that vary mass, velocity, morphology, and composition. The masses and velocities of each component vary, respectively, from 0.001 to 0.1 M$_{\odot}$ and 0.05 to 0.3$c$, covering much of the range of results from the neutron star merger literature. The set of 900 models is constrained to have a toroidal low electron fraction ($Y_e$) ejecta with a robust r-process composition and either a spherical or lobed high-$Y_e$ ejecta with two possible compositions. We simulate these models with the Monte Carlo radiative transfer code SuperNu using a full suite of lanthanide and 4th row element opacities. We examine the trends of these models with parameter variation, show how it can be used with statistical tools, and compare the model light curves and spectra to those of AT2017gfo, the electromagnetic counterpart of GW170817., Comment: 19 pages, 11 figures, submitted to ApJ

Detailed radiative transfer simulations of kilonovae are difficult to apply directly to observations; they only sparsely cover simulation parameters, such as the mass, velocity, morphology, and composition of the ejecta. On the other hand, semianalytic models for kilonovae can be evaluated continuously over model parameters, but neglect important physical details which are not incorporated in the simulations, thus introducing systematic bias. Starting with a grid of 2D anisotropic simulations of kilonova light curves covering a wide range of ejecta properties, we apply adaptive-learning techniques to iteratively choose new simulations and produce high-fidelity surrogate models for those simulations. These surrogate models allow for continuous evaluation across model parameters while retaining the microphysical details about the ejecta. Using a new code for multimessenger inference, we demonstrate how to use our interpolated models to infer kilonova parameters. Comparing to inferences using simplified analytic models, we recover different ejecta properties. We discuss the implications of this analysis which is qualitatively consistent with similar previous work using detailed ejecta opacity calculations and which illustrates systematic challenges for kilonova modeling. An associated data and code release provides our interpolated light-curve models, interpolation implementation which can be applied to reproduce our work or extend to new models, and our multimessenger parameter inference engine., Comment: 19 pages, 15 figures

We present a detailed multi-wavelength analysis of two short Gamma-Ray Bursts (sGRBs) detected by the Neil Gehrels Swift Observatory: GRB 160624A at $z=0.483$ and GRB 200522A at $z=0.554$. These sGRBs demonstrate very different properties in their observed emission and environment. GRB 160624A is associated to a late-type galaxy with an old stellar population ($\approx$3 Gyr) and moderate on-going star formation ($\approx$1 $M_{\odot}$ yr$^{-1}$). Hubble and Gemini limits on optical/nIR emission from GRB 160624A are among the most stringent for sGRBs, leading to tight constraints on the allowed kilonova properties. In particular, we rule out any kilonova brighter than AT2017gfo, disfavoring large masses of wind ejecta ($\lesssim$0.03 $M_\odot$). In contrast, observations of GRB 200522A uncovered a luminous ($L_\textrm{F125W}\approx 10^{42}$ erg s$^{-1}$ at 2.3~d) and red ($r-H\approx 1.3$ mag) counterpart. The red color can be explained either by bright kilonova emission powered by the radioactive decay of a large amount of wind ejecta (0.03 $M_\odot$ $\lesssim$ $M$ $\lesssim$ 0.1 $M_\odot$) or moderate extinction, $E(B-V)\approx0.1-0.2$ mag, along the line of sight. The location of this sGRB in the inner regions of a young ($\approx$0.1 Gyr) star-forming ($\approx$2-6 $M_{\odot}$ yr$^{-1}$) galaxy and the limited sampling of its counterpart do not allow us to rule out dust effects as contributing, at least in part, to the red color., Comment: MNRAS, In Press

Observations of X-ray binaries indicate a dearth of compact objects in the mass range from $\sim 2-5 M_{\odot}$. The existence of this (first mass) gap has been used to discriminate between proposed engines behind core-collapse supernovae. From LIGO/Virgo observations of binary compact remnant masses, several candidate first mass gap objects (either neutron stars (NSs) or black holes (BHs)) were identified during the O3 science run. Motivated by these new observations, we study the formation of BH-NS mergers in the framework of isolated classical binary evolution, using population synthesis methods to evolve large populations of binary stars (Population I and II) across cosmic time. 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, typically BH-NS systems form with moderate mass ratios $q=0.1-0.2$. If we adopt a delayed supernova engine, we conclude that $\sim 30\%$ of BH-NS mergers may host at least one compact object in the first mass gap (FMG$^\circ$). Even allowing for uncertainties in the processes behind compact object formation, we expect the fraction of BH-NS systems ejecting mass during the merger to be small (from $\sim 0.6-9\%$). In our reference model, we assume: (i) the formation of compact objects within the FMG, (ii) natal NS/BH kicks decreased by fallback, (iii) low BH spins due to Tayler-Spruit angular momentum transport in massive stars. We find that $\lesssim 1\%$ of BH-NS mergers will have any mass ejection and about the same percentage will produce kilonova bright enough to have a chance of being detected with a large (Subaru-class) $8$m telescope. Interestingly, all these mergers will have both a BH and an NS in the FMG., Comment: 15 pages, 12 figures

We present the discovery of the radio afterglow and near-infrared (NIR) counterpart of the Swift short GRB 200522A, located at a small projected offset of $\approx 1$ kpc from the center of a young, star-forming host galaxy at $z=0.5536$. The radio and X-ray luminosities of the afterglow are consistent with those of on-axis cosmological short GRBs. The NIR counterpart, revealed by our HST observations at a rest-frame time of $\approx2.3$ days, has a luminosity of $\approx (1.3-1.7) \times 10^{42}$ erg s$^{-1}$. This is substantially lower than on-axis short GRB afterglow detections, but is a factor of $\approx 8$-$17$ more luminous than the kilonova of GW170817, and significantly more luminous than any kilonova candidate for which comparable observations exist. The combination of the counterpart's color ($i-y = -0.08\pm 0.21$; rest-frame) and luminosity cannot be explained by standard radioactive heating alone. We present two scenarios to interpret the broad-band behavior of GRB 200522A: a synchrotron forward shock with a luminous kilonova (potentially boosted by magnetar energy deposition), or forward and reverse shocks from a $\approx14^{\circ}$, relativistic ($\Gamma_0 \gtrsim 80$) jet. Models which include a combination of enhanced radioactive heating rates, low-lanthanide mass fractions, or additional sources of heating from late-time central engine activity may provide viable alternate explanations. If a stable magnetar was indeed produced in GRB 200522A, we predict that late-time radio emission will be detectable starting $\approx 0.3$-$6$ years after the burst for a deposited energy of $\approx 10^{53}$ erg. Counterparts of similar luminosity to GRB 200522A associated with gravitational wave events will be detectable with current optical searches to $\approx\!250$ Mpc., Comment: 33 pages, 13 figures, 5 tables. Submitted to AAS Journals

We report on our observing campaign of the compact binary merger GW190814, detected by the Advanced LIGO and Advanced Virgo detectors on August 14th, 2019. This signal has the best localisation of any observed gravitational wave (GW) source, with a 90% probability area of 18.5 deg$^2$, and an estimated distance of ~ 240 Mpc. We obtained wide-field observations with the Deca-Degree Optical Transient Imager (DDOTI) covering 88% of the probability area down to a limiting magnitude of $w$ = 19.9 AB. Nearby galaxies within the high probability region were targeted with the Lowell Discovery Telescope (LDT), whereas promising candidate counterparts were characterized through multi-colour photometry with the Reionization and Transients InfraRed (RATIR) and spectroscopy with the Gran Telescopio de Canarias (GTC). We use our optical and near-infrared limits in conjunction with the upper limits obtained by the community to constrain the possible electromagnetic counterparts associated with the merger. A gamma-ray burst seen along its jet's axis is disfavoured by the multi-wavelength dataset, whereas the presence of a burst seen at larger viewing angles is not well constrained. Although our observations are not sensitive to a kilonova similar to AT2017gfo, we can rule out high-mass (> 0.1 M$_{\odot}$) fast-moving (mean velocity >= 0.3c) wind ejecta for a possible kilonova associated with this merger., Comment: 17 pages, 11 figures, 5 tables; updated acknowledgement section. Accepted for publication in MNRAS (10 September 2020)

We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2-0.3 and its central pulsar PSR J1811-1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index Gamma = 2.1 +/- 0.1, which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with Integral, we find that the pulsar spectrum can be fit by a power-law with Gamma=1.32 +/- 0.07 up to 300 keV without evidence of curvature.

LOX is a lunar-orbiting astrophysics mission that will probe the cosmos at MeV energies. It is guided by open questions regarding thermonuclear, or Type-Ia, supernovae (SNeIa) and will characterize these inherently radioactive objects by enabling a systematic survey of SNeIa at gamma-ray energies for the first time. Astronomical investigations from lunar orbit afford new opportunities to advance our understanding of the cosmos. The foundation of LOX is an observational approach well suited to the all-sky monitoring demands of supernova investigations and time-domain astronomy. Its inherently wide field-of-view and continuous all-sky monitoring provides an innovative way of addressing decadal survey questions at MeV energies (0.1-10 MeV). The LOX approach achieves high sensitivity with a simple, high-heritage instrument design that eliminates the need for complex, position-sensitive detectors, kinematic event reconstruction, masks, or other insensitive detector mass, while also mitigating technology development, implementation complexity, and their associated costs. LOX can be realized within existing programs, like Explorer., Comment: Astro2020 APC White Paper, Submitted to the National Academy of Sciences Decadal Survey on Astronomy and Astrophysics (Astro2020); 15 pages

The electromagnetic observations of GW170817 were able to dramatically increase our understanding of neutron star mergers beyond what we learned from gravitational waves alone. These observations provided insight on all aspects of the merger from the nature of the gamma-ray burst to the characteristics of the ejected material. The ejecta of neutron star mergers are expected to produce such electromagnetic transients, called kilonovae or macronovae. Characteristics of the ejecta include large velocity gradients, relative to supernovae, and the presence of heavy $r$-process elements, which pose significant challenges to the accurate calculation of radiative opacities and radiation transport. For example, these opacities include a dense forest of bound-bound features arising from near-neutral lanthanide and actinide elements. Here we investigate the use of fine-structure, line-binned opacities that preserve the integral of the opacity over frequency. Advantages of this area-preserving approach over the traditional expansion-opacity formalism include the ability to pre-calculate opacity tables that are independent of the type of hydrodynamic expansion and that eliminate the computational expense of calculating opacities within radiation-transport simulations. Tabular opacities are generated for all 14 lanthanides as well as a representative actinide element, uranium. We demonstrate that spectral simulations produced with the line-binned opacities agree well with results produced with the more accurate continuous Monte Carlo Sobolev approach, as well as with the commonly used expansion-opacity formalism. Additional investigations illustrate the convergence of opacity with respect to the number of included lines, and elucidate sensitivities to different atomic physics approximations, such as fully and semi-relativistic approaches., Comment: 27 pages, 22 figures. arXiv admin note: text overlap with arXiv:1702.02990

As of today, we have directly detected exactly one source in both gravitational waves (GWs) and electromagnetic (EM) radiation, the binary neutron star merger GW170817, its associated gamma-ray burst GRB170817A, and the subsequent kilonova SSS17a/AT 2017gfo. Within ten years, we will detect hundreds of events, including new classes of events such as neutron-star-black-hole mergers, core-collapse supernovae, and almost certainly something completely unexpected. As we build this sample, we will explore exotic astrophysical topics ranging from nucleosynthesis, stellar evolution, general relativity, high-energy astrophysics, nuclear matter, to cosmology. The discovery potential is extraordinary, and investments in this area will yield major scientific breakthroughs. Here we outline some of the most exciting scientific questions that can be answered by combining GW and EM observations., Comment: submitted to Astro2020

The first multimessenger observation of a neutron star merger was independently detected in gamma-rays by Fermi-GBM and INTEGRAL SPI-ACS and gravitational waves by Advanced LIGO and Advanced Virgo. Gravitational waves are emitted from systems with accelerating quadrupole moments, and detectable sources are expected to be compact objects. Nearly all distant astrophysical gamma-ray sources are compact objects. Therefore, serendipitous observations of these two messengers will continue to uncover the sources of gravitational waves and gamma-rays, and enable multimessenger science across the Astro2020 thematic areas. This requires upgrades to the ground-based gravitational wave network and ~keV-MeV gamma-ray coverage for observations of neutron star mergers, and broadband coverage in both gravitational waves and gamma-rays to monitor other expected joint sources., Comment: Astro2020 White Paper for the Thematic Area Multimessenger Astronomy and Astrophysics

Electromagnetic observations of the sky have been the basis for our study of the Universe for millennia, cosmic ray studies are now entering their second century, the first neutrinos from an astrophysical source were identified three decades ago, and gravitational waves were directly detected only four years ago. Detections of these messengers are now common. Astrophysics will undergo a revolution in the 2020s as multimessenger detections become routine. The 8th Astro2020 Thematic Area is Multimessenger Astronomy and Astrophysics, which includes the identification of the sources of gravitational waves, astrophysical and cosmogenic neutrinos, cosmic rays, and gamma-rays, and the coordinated multimessenger and multiwavelength follow-ups. Identifying and characterizing multimessenger sources enables science throughout and beyond astrophysics. Success in the multimessenger era requires: (i) sensitive coverage of the non-electromagnetic messengers, (ii) full coverage of the electromagnetic spectrum, with either fast-response observations or broad and deep high-cadence surveys, and (iii) improved collaboration, communication, and notification platforms., Comment: Astro2020 White Paper for the 8th Thematic Area of Multimessenger Astronomy and Astrophysics

(Abridged) The explosion mechanism of electron-capture supernovae (ECSNe) remains equivocal. We attempt to constrain the explosion mechanism (neutron-star-forming implosion or thermonuclear explosion) and the frequency of occurrence of ECSNe using nucleosynthesis simulations of the latter scenario, population synthesis, the solar abundance distribution, pre-solar meteoritic oxide grain isotopic ratio measurements and the white dwarf mass-radius relation. Tracer particles from 3d hydrodynamic simulations were post-processed with a large nuclear reaction network in order to determine the complete compositional state of the bound ONeFe remnant and the ejecta, and population synthesis simulations were performed in order to estimate the ECSN rate with respect to the CCSN rate. The 3d deflagration simulations drastically overproduce the neutron-rich isotopes $^{48}$Ca, $^{50}$Ti, $^{54}$Cr, $^{60}$Fe and several of the Zn isotopes relative to their solar abundances. Using the solar abundance distribution as our constraint, we place an upper limit on the frequency of thermonuclear ECSNe as 1$-$3~\% the frequency at which core-collapse supernovae (FeCCSNe) occur. This is on par with or 1~dex lower than the estimates for ECSNe from single stars. The upper limit from the yields is also in relatively good agreement with the predictions from our population synthesis simulations. The $^{54}$Cr/$^{52}$Cr and $^{50}$Ti/$^{48}$Ti isotopic ratios in the ejecta are a near-perfect match with recent measurements of extreme pre-solar meteoritc oxide grains, and $^{53}$Cr/$^{52}$Cr can also be matched if the ejecta condenses before mixing with the interstellar medium. Theoretical mass-radius relations for the bound ONeFe WD remnants of these explosions are apparently consistent with several observational WD candidates., Comment: 23 pages, 13 figures, 7 tables; accepted for publication in Astronomy & Astrophysics

Binary black hole (BBH) mergers found by the LIGO and Virgo detectors are of immense scientific interest to the astrophysics community, but are considered unlikely to be sources of electromagnetic emission. To test whether they have rapidly fading optical counterparts, we used the Dark Energy Camera to perform an $i$-band search for the BBH merger GW170814, the first gravitational wave detected by three interferometers. The 87-deg$^2$ localization region (at 90\% confidence) centered in the Dark Energy Survey (DES) footprint enabled us to image 86\% of the probable sky area to a depth of $i\sim 23$ mag and provide the most comprehensive dataset to search for EM emission from BBH mergers. To identify candidates, we perform difference imaging with our search images and with templates from pre-existing DES images. The analysis strategy and selection requirements were designed to remove supernovae and to identify transients that decline in the first two epochs. We find two candidates, each of which is spatially coincident with a star or a high-redshift galaxy in the DES catalogs, and they are thus unlikely to be associated with GW170814. Our search finds no candidates associated with GW170814, disfavoring rapidly declining optical emission from BBH mergers brighter than $i\sim 23$ mag ($L_{\rm optical} \sim 5\times10^{41}$ erg/s) 1-2 days after coalescence. In terms of GW sky map coverage, this is the most complete search for optical counterparts to BBH mergers to date, Comment: 11 pages, 3 figures

Neutron star mergers offer unique conditions for the creation of the heavy elements and additionally provide a testbed for our understanding of this synthesis known as the $r$-process. We have performed dynamical nucleosynthesis calculations and identified a single isotope, $^{254}$Cf, which has a particularly high impact on the brightness of electromagnetic transients associated with mergers on the order of 15 to 250 days. This is due to the anomalously long half-life of this isotope and the efficiency of fission thermalization compared to other nuclear channels. We estimate the fission fragment yield of this nucleus and outline the astrophysical conditions under which $^{254}$Cf has the greatest impact to the light curve. Future observations in the middle-IR which are bright during this regime could indicate the production of actinide nucleosynthesis., Comment: 9 pages, 5 figures

We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky core falls in to the core of the target. However, for higher angular momentum impacts, significant amounts become embedded anisotropically as lumps in the ice layer. Furthermore, most of the impactor's ice and energy is deposited in a hot, high-entropy shell at a radius of ~3 Earth radii. This could explain Uranus' observed lack of heat flow from the interior and be relevant for understanding its asymmetric magnetic field. We verify the results from the single previous study of lower resolution simulations that an impactor with a mass of at least 2 Earth masses can produce sufficiently rapid rotation in the post-impact Uranus for a range of angular momenta. At least 90% of the atmosphere remains bound to the final planet after the collision, but over half can be ejected beyond the Roche radius by a 2 or 3 Earth mass impactor. This atmospheric erosion peaks for intermediate impactor angular momenta (~3*10^36 kg m^2 s^-1). Rock is more efficiently placed into orbit and made available for satellite formation by 2 Earth mass impactors than 3 Earth mass ones, because it requires tidal disruption that is suppressed by the more massive impactors., Comment: Published in ApJ. 14 pages, 11 figures

We present the first three-dimensional (3D) smoothed-particle-hydrodynamics (SPH) simulations of the induced gravitational collapse (IGC) scenario of long-duration gamma-ray bursts (GRBs) associated with supernovae (SNe). We simulate the SN explosion of a carbon-oxygen core (CO$_{\rm core}$) forming a binary system with a neutron star (NS) companion. We follow the evolution of the SN ejecta, including their morphological structure, subjected to the gravitational field of both the new NS ($\nu$NS) formed at the center of the SN, and the one of the NS companion. We compute the accretion rate of the SN ejecta onto the NS companion as well as onto the $\nu$NS from SN matter fallback. We determine the fate of the binary system for a wide parameter space including different CO$_{\rm core}$ and NS companion masses, orbital periods and SN explosion geometry and energies. We identify, for selected NS nuclear equations-of-state, the binary parameters leading the NS companion, by hypercritical accretion, either to the mass-shedding limit, or to the secular axisymmetric instability for gravitational collapse to a black hole (BH), or to a more massive, fast rotating, stable NS. We also assess whether the binary remains or not gravitationally bound after the SN explosion, hence exploring the space of binary and SN explosion parameters leading to $\nu$NS-NS and $\nu$NS-BH binaries. The consequences of our results for the modeling of long GRBs, i.e. X-ray flashes and binary-driven hypernovae, are discussed., Comment: Accepted for publication in The Astrophysical Journal

The detection of GW170817 in both gravitational waves and electromagnetic waves heralds the age of gravitational-wave multi-messenger astronomy. On 17 August 2017 the Advanced LIGO and Virgo detectors observed GW170817, a strong signal from the merger of a binary neutron-star system. Less than 2 seconds after the merger, a gamma-ray burst (GRB 170817A) was detected within a region of the sky consistent with the LIGO-Virgo-derived location of the gravitational-wave source. This sky region was subsequently observed by optical astronomy facilities, resulting in the identification of an optical transient signal within $\sim 10$ arcsec of the galaxy NGC 4993. These multi-messenger observations allow us to use GW170817 as a standard siren, the gravitational-wave analog of an astronomical standard candle, to measure the Hubble constant. This quantity, which represents the local expansion rate of the Universe, sets the overall scale of the Universe and is of fundamental importance to cosmology. Our measurement combines the distance to the source inferred purely from the gravitational-wave signal with the recession velocity inferred from measurements of the redshift using electromagnetic data. This approach does not require any form of cosmic "distance ladder;" the gravitational wave analysis can be used to estimate the luminosity distance out to cosmological scales directly, without the use of intermediate astronomical distance measurements. We determine the Hubble constant to be $70.0^{+12.0}_{-8.0} \, \mathrm{km} \, \mathrm{s}^{-1} \, \mathrm{Mpc}^{-1}$ (maximum a posteriori and 68% credible interval). This is consistent with existing measurements, while being completely independent of them. Additional standard-siren measurements from future gravitational-wave sources will provide precision constraints of this important cosmological parameter., Comment: 26 pages, 5 figures, Nature in press. For more information see https://dcc.ligo.org/LIGO-P1700296/public

We present UV, optical, and NIR photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at $0.47$ days to $18.5$ days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy distribution (SED) inferred from this photometry at $0.6$ days is well described by a blackbody model with $T\approx 8300$ K, a radius of $R\approx 4.5\times 10^{14}$ cm (corresponding to an expansion velocity of $v\approx 0.3c$), and a bolometric luminosity of $L_{\rm bol}\approx 5\times10^{41}$ erg s$^{-1}$. At $1.5$ days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set we find that models with heating from radioactive decay of $^{56}$Ni, or those with only a single component of opacity from $r$-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data, the resulting "blue" component has $M_\mathrm{ej}^\mathrm{blue}\approx 0.01$ M$_\odot$ and $v_\mathrm{ej}^\mathrm{blue}\approx 0.3$c, and the "red" component has $M_\mathrm{ej}^\mathrm{red}\approx 0.04$ M$_\odot$ and $v_\mathrm{ej}^\mathrm{red}\approx 0.1$c. These ejecta masses are broadly consistent with the estimated $r$-process production rate required to explain the Milky Way $r$-process abundances, providing the first evidence that BNS mergers can be a dominant site of $r$-process enrichment., Comment: 13 Pages, 3 Figures, 2 Tables. ApJL, In Press. Keywords: GW170817, LVC

We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced LIGO/Virgo (GW170817) and as a short gamma-ray burst by Fermi/GBM and Integral/SPI-ACS (GRB170817A). The evolution of the transient light is consistent with predictions for the behaviour of a "kilonova/macronova", powered by the radioactive decay of massive neutron-rich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide dominated ejecta, and the much slower evolution in the near-infrared Ks-band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the 3rd r-process peak (atomic masses A~195). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major - if not the dominant - site of rapid neutron capture nucleosynthesis in the universe.

We present the Dark Energy Camera (DECam) discovery of the optical counterpart of the first binary neutron star merger detected through gravitational wave emission, GW170817. Our observations commenced 10.5 hours post-merger, as soon as the localization region became accessible from Chile. We imaged 70 deg$^2$ in the $i$ and $z$ bands, covering 93\% of the initial integrated localization probability, to a depth necessary to identify likely optical counterparts (e.g., a kilonova). At 11.4 hours post-merger we detected a bright optical transient located $10.6''$ from the nucleus of NGC\,4993 at redshift $z=0.0098$, consistent (for $H_0 = 70$\, km s$^{-1}$ Mpc$^{-1}$) with the distance of $40 \pm 8$\, Mpc reported by the LIGO Scientific Collaboration and the Virgo Collaboration (LVC). At detection the transient had magnitudes $i\approx 17.30$ and $z\approx 17.45$, and thus an absolute magnitude of $M_i = -15.7$, in the luminosity range expected for a kilonova. We identified 1,500 potential transient candidates. Applying simple selection criteria aimed at rejecting background events such as supernovae, we find the transient associated with NGC\,4993 as the only remaining plausible counterpart, and reject chance coincidence at the 99.5\% confidence level. We therefore conclude that the optical counterpart we have identified near NGC\,4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves, and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources., Comment: 9 Pages, 3 Figures, 2 Tables. ApJL, In Press. Keywords: GW170817, LVC

A long-standing paradigm in astrophysics is that collisions- or mergers- of two neutron stars (NSs) form highly relativistic and collimated outflows (jets) powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the observational support for this model is only indirect. A hitherto outstanding prediction is that gravitational wave (GW) events from such mergers should be associated with GRBs, and that a majority of these GRBs should be off-axis, that is, they should point away from the Earth. Here we report the discovery of the X-ray counterpart associated with the GW event GW170817. While the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow from freshly synthesized r-process material in the merger ejecta, known as kilonova, observations at X-ray and, later, radio frequencies exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short GRBs and GWs from NS mergers, and gives independent confirmation of the collimated nature of the GRB emission., Comment: 38 pages, 10 figures, Nature, in press

We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced LIGO/Virgo. Our dataset comprises seven epochs of J+H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 microns. Our fiducial model with 0.04 M_sun of ejecta, an ejection velocity of v=0.1c, and a lanthanide concentration of X_lan=1e-2 provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 d after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements., Comment: 9 pages, 4 figures, 1 table, ApJL, in press

With the first direct detection of merging black holes in 2015, the era of gravitational wave (GW) astrophysics began. A complete picture of compact object mergers, however, requires the detection of an electromagnetic (EM) counterpart. We report ultraviolet (UV) and X-ray observations by Swift and the Nuclear Spectroscopic Telescope ARray (NuSTAR) of the EM counterpart of the binary neutron star merger GW170817. The bright, rapidly fading ultraviolet emission indicates a high mass ($\approx0.03$ solar masses) wind-driven outflow with moderate electron fraction ($Y_{e}\approx0.27$). Combined with the X-ray limits, we favor an observer viewing angle of $\approx 30^{\circ}$ away from the orbital rotation axis, which avoids both obscuration from the heaviest elements in the orbital plane and a direct view of any ultra-relativistic, highly collimated ejecta (a gamma-ray burst afterglow)., Comment: Science, in press; 56 pages, 12 figures

We provide here a significant extension of the NuGrid Set 1 models in mass coverage and toward lower metallicity, adopting the same physics assumptions. The combined data set now includes the initial masses M/Msun = 1, 1.65, 2, 3, 4, 5, 6, 7, 12, 15, 20, 25 for Z = 0.02, 0.01, 0.006, 0.001, 0.0001 with alpha-enhanced composition for the lowest three metallicities. These models are computed with the MESA stellar evolution code and are evolved up to the AGB, the white dwarf stage, or until core collapse. The nucleosynthesis was calculated for all isotopes in post-processing with the NuGrid mppnp code. Explosive nucleosynthesis is based on semi-analytic 1D shock models. Metallicity-dependent mass loss, convective boundary mixing in low- and intermediate mass models and H and He core burning massive star models is included. Convective O-C shell mergers in some stellar models lead to the strong production of odd-Z elements P, Cl, K and Sc. In AGB models with hot dredge-up the convective boundary mixing efficiency is reduced to accommodate for its energetic feedback. In both low-mass and massive star models at the lowest metallicity H-ingestion events are observed and lead to i-process nucleosynthesis and substantial N-15 production. Complete yield data tables, derived data products and online analytic data access are provided., Comment: Published. Detailed online data available at Canadian Astronomical Data Center, DOI:10.11570/18.0002 (http://www.canfar.phys.uvic.ca/vospace/nodes/AstroDataCitationDOI/CISTI.CANFAR/18.0002/18.0002.html?view=data). See Appendix for details

All ten LIGO/Virgo binary black hole (BH-BH) coalescences reported from the O1/O2 runs have near zero effective spins. There are only three potential explanations of this fact. If the BH spin magnitudes are large then (i) either both BH spin vectors must be nearly in the orbital plane or (ii) the spin angular momenta of the BHs must be oppositely directed and similar in magnitude. Or, (iii) the BH spin magnitudes are small. We test the third hypothesis within the framework of the classical isolated binary evolution scenario of the BH-BH merger formation. We test three models of angular momentum transport in massive stars: a mildly efficient transport by meridional currents (as employed in the Geneva code), an efficient transport by the Tayler-Spruit magnetic dynamo (as implemented in the MESA code), and a very-efficient transport (as proposed by Fuller et al.) to calculate natal BH spins. We allow for binary evolution to increase the BH spins through accretion and account for the potential spin-up of stars through tidal interactions. Additionally, we update the calculations of the stellar-origin BH masses, include revisions to the history of star formation and to the chemical evolution across cosmic time. We find that we can match simultaneously the observed BH-BH merger rate density, BH masses, and effective spins. Models with efficient angular momentum transport are favored. The updated stellar-mass weighted gas-phase metallicity evolution now used in our models appears to be a key in better reproducing the LIGO/Virgo merger rate estimate. Mass losses during the pair-instability pulsation supernova phase are likely overestimated if the merger GW170729 hosts a BH more massive than 50 Msun. We also estimate rate of BH-NS mergers from recent LIGO/Virgo observations. Our updated models of BH-BH, BH-NS and NS-NS mergers are now publicly available at www.syntheticuniverse.org., Comment: A&A accepted: revised comparison with LIGO/Virgo (41 pages)

We report on the first results from a large-scale observing campaign aiming to use astrometric microlensing to detect and place limits on the mass of single objects, including stellar remnants. We used the Hubble Space Telescope to monitor stars near the Galactic Center for 3 years, and we measured the brightness and positions of $\sim$2 million stars at each observing epoch. In addition to this, we monitored the same pointings using the VIMOS imager on the Very Large Telescope. The stars we monitored include several bright microlensing events observed from the ground by the OGLE collaboration. In this paper, we present the analysis of our photometric and astrometric measurements for 6 of these events, and derive mass constraints for the lens in each of these. Although these constraints are limited by the photometric precision of ground-based data, and our ability to determine the lens distance, we were able to constrain the size of the Einstein ring radius thanks to our precise astrometric measurements, the first routine measurements of this type from a large-scale observing program. This demonstrates the power of astrometric microlensing as a tool to constrain the masses of stars, stellar remnants, and, in the future, of extrasolar planets, using precise ground- and space-based observations., Comment: ApJ, in press; 16 pages, 10 figures