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Binary Neutron Star Mergers

Binary Neutron Star Mergers

In parallel to efforts in full GR, there has also been great progress in numerical simulations that include approximate relativistic treatments but a more detailed approach to microphysical issues. The first simulations to use a realistic EOS for NS-NS mergers were performed by Ruffert, Janka, and collaborators [ 253 , 139 , 254 ], who assumed the Lattimer–Swesty EOS for their Newtonian PPM-based Eulerian calculations. They were able to determine a physically meaningful tem- perature for NS-NS merger remnants of 30 – 50 MeV, an overall neutrino luminosity of roughly 10 53 erg/s for tens of milliseconds, and a corresponding annihilation rate of 2 – 5 Ö 10 50 erg/s given the computed annihilation efficiencies of a few parts in a thousand. This resulted in an energy loss of 2 – 4 Ö 10 49 erg over the lifetime of the remnant [ 139 ], a value later confirmed in multigrid simulations that replaced the newly formed HMNS by a Newtonian or quasi-relativistic BH surrounded by the bound material making up a disk [ 251 ]. The temperatures in the result- ing neutron-rich (𝑌 𝑒 ≈ 0.05 – 0.2) remnant were thought to be encouraging for the production of r-process elements [ 254 ], although numerical resolution of the low-density ejecta limited the ability to make quantitatively accurate estimates of its exact chemical distribution. Further calculations, some of which involved unequal-mass binaries, indicated that the temperatures and electron frac- tions in the ejecta were likely not sufficient to produce solar abundances of r-process elements [ 252 ], with electron fractions in particular smaller than those set by hand in the r-process production model that appears in [ 111 , 245 ]. More recently, it was suggested [ 123 ] that the decompression of matter originally located in the inner crust of a NS and ejected during a merger has a nearly solar elemental distribution for heavy r-process elements (𝐴 > 140). This indicates that NS-NS mergers may be the source of the observed cosmic r-process elements should there be sufficient mass loss per merger event, 𝑀 𝑒𝑗 ∼ 3 – 5 × 10 −5 𝑀 ⊙ , although these amounts have yet to be observed in full GR simulations which have often admittedly been performed using cruder microphysical treatments.
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Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

Localization of binary neutron star mergers with second and third generation gravitational-wave detectors

The observation of gravitational wave signals from binary black hole and binary neutron star mergers has established the field of gravitational wave astronomy. It is expected that future networks of gravitational wave detectors will possess great potential in probing various aspects of astronomy. An important consideration for successive improvement of current detectors or establishment on new sites is knowledge of the minimum number of detectors required to perform precision astronomy. We attempt to answer this question by assessing the ability of future detector networks to detect and localize binary neutron stars mergers on the sky. Good localization ability is crucial for many of the scientific goals of gravitational wave astronomy, such as electromagnetic follow-up, measuring the properties of compact binaries throughout cosmic history, and cosmology. We find that although two detectors at improved sensitivity are sufficient to get a substantial increase in the number of observed signals, at least three detectors of comparable sensitivity are required to localize majority of the signals, typically to within around 10 deg 2 —adequate for follow-up with most wide field of view optical telescopes.
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Binary neutron star mergers and third generation detectors: localization and early warning

Binary neutron star mergers and third generation detectors: localization and early warning

For third generation gravitational wave detectors, such as the Einstein Telescope, gravitational wave signals from binary neutron stars can last up to a few days before the neutron stars merge. To estimate the measurement uncertainties of key signal parameters, we develop a Fisher matrix approach which accounts for effects on such long duration signals of the time-dependent detector response and the Earth ’s rotation. We use this approach to characterize the sky localization uncertainty for gravitational waves from binary neutron stars at 40, 200, 400, 800, and 1600 Mpc, for the Einstein Telescope and Cosmic Explorer individually and operating as a network. We find that the Einstein Telescope alone can localize the majority of detectable binary neutron stars at a distance of ≤200 Mpc to within 100 deg 2 with 90% confidence. A network consisting of the Einstein Telescope and Cosmic Explorer can enhance the sky localization performance significantly —with the 90% credible region of Oð1Þ deg 2 for most sources at ≤200 Mpc and ≤100 deg 2 for most sources at ≤1600 Mpc. We also investigate the prospects for third generation detectors identifying the presence of a signal prior to merger. To do this, we require a signal to have a network signal- to-noise ratio of ≥12 and ≥5.5 for at least two interferometers, and to have a 90% credible region for the sky localization that is no larger than 100 deg 2 . We find that the Einstein Telescope can send out such “early- warning ” detection alerts 1–20 hours before merger for 100% of detectable binary neutron stars at 40 Mpc and for ∼58% of sources at 200 Mpc. For sources at a distance of 400 Mpc, a network of the Einstein telescope and Cosmic Explorer can produce detection alerts up to ∼3 hours prior to merger for 98% of detectable binary neutron stars.
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The Needle in the 100 deg² Haystack: The Hunt for Binary Neutron Star Mergers with LIGO and Palomar Transient Factory

The Needle in the 100 deg² Haystack: The Hunt for Binary Neutron Star Mergers with LIGO and Palomar Transient Factory

signal would remain in band for Initial L I G O for about 25 s, it would be detectable by Advanced L I G O for as long as 1000 s (see Equation (3.1) in Chapter 3). A second consequence of longer signals is that the signal can accumulate more power and a larger total phase shift while in band, improving the ability to measure the mass of the binary but dramatically increasing the number of G W templates required to adequately tile the parameter space. A third problem is that we cannot assume that the detector and the instrument noise are in a stationary state for the durations of these long signals; we must adaptively condition or whiten the data as the noise level rises or falls, and we must be able to carry on integrating the signal over gaps or glitches. These are all formidable problems for traditional fast Fourier transform (F F T)-based matched filter pipelines, which have inflexible data handling, whose latency grows with the length of the signal, and whose computational requirements increase with both the length and number of template signals. To effectively search for these signals in real time we need a detection pipeline whose latency and computational demands do not scale much with the duration of the G W signal.
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Analysis of Gravitational-Wave Signals from Binary Neutron Star Mergers Using Machine Learning

Analysis of Gravitational-Wave Signals from Binary Neutron Star Mergers Using Machine Learning

One of the possible contenders to aid matched filtering in the first data analysis stage is machine learning. It is a field of computer science with the goal to create computer programs which adapt to a problem without direct human interference, i.e. learning from a set of experiences. Most of today’s state of the art machine learning algorithms are implementations of neural networks (NNs). They have application in many fields, like computer vision [ 12 ], sound generation [ 13 ] or natural language processing [ 14 ]. The advantages of NNs are manifold, the most important one in the context of this thesis being computational efficiency once the network is optimized. This efficiency and their general success in almost any area make NNs a promising tool for GW data-analysis. Daniel George and E.A. Huerta were the first to apply a deep NN to whitened time series strain data to try to recover GW signals. They were able to reach performances comparable to those of matched filtering at a fraction of the computational cost [ 15 ]. Their network, however, was only optimized for signals from binary black hole (BBH) mergers, thus not covering the cases of BNS signals where quick notifications are most valuable.
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Gravitational waves and mass ejecta from binary neutron star mergers: Effect of large eccentricities

Gravitational waves and mass ejecta from binary neutron star mergers: Effect of large eccentricities

two initially mutually unbound neutron stars in a dense stellar system such as a nuclear star cluster via the emis- sion of gravitational radiation during a close non-merging encounter [ 31 ]. Reference [ 31 ] reports an estimate 1 of the volume rate of such encounters of ∼ 0.003–6 Gpc −3 yr −1 . To obtain a possible detection rate, we need to incor- porate the sensitive volume of ET. We assume two dif- ferent scenarios to estimate the sensitive volume, which should bracket the sensitive volumes appropriate for re- alistic data analysis techniques (see, e.g., [ 32 ] for initial work on such techniques). Specifically, we assume (i) the use of an unmodelled search for the kHz GW radiation emitted during the merger, for which one obtains a range of ∼ 20 Mpc from Fig. 21 in [ 33 ], 2 and (ii) that it is pos- sible to construct a template bank of sufficiently accurate waveforms to enable a matched-filter search for these sys- tems, giving a range of ∼ 8 Gpc, where we estimate the range for highly eccentric binaries to be about half that for quasicircular binaries (obtained from Fig. 18 in [ 33 ]), since the matched filtering range for Advanced LIGO for highly eccentric binaries shown in Fig. 16 of [ 32 ] is (on the larger side) about half that for quasicircular binaries quoted in [ 13 ].
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General relativistic magnetohydrodynamic simulations of binary neutron star mergers

General relativistic magnetohydrodynamic simulations of binary neutron star mergers

mergers and short gamma-ray bursts by studying the possible production of relativistic jets. Since the funnel that is typically formed along the BH spin-axis has a very low density, we set the atmosphere as low as possible in order to reduce possible contamination due to the interaction between the low-density funnel and the artificial atmosphere. We also used the adaptive mesh refinement driver Carpet with a total of six refinement levels. The finest grids cover each of the NSs during the inspiral and, after merger, they are merged into a larger one that covers the resulting hypermassive NS (HMNS). We adopted a resolution on the finest grids of ≈ 222 m in the runs using an ideal-fluid EOS and of ≈ 186 m in the runs using the H4 EOS. This choice has been made so that the NSs are covered by approximately the same number of points in both cases. The external boundary is located at a distance of ≈ 1400 km in the ideal-fluid case and ≈ 1200 km in the H4 case. All the simulations employed reflection symmetry across the equatorial plane to reduce computational costs.
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Binary neutron star mergers in the gravitational wave era

Binary neutron star mergers in the gravitational wave era

In Chapter 3 we formally derived the equations for a gravitational wave propa- gating in space-time, starting from the linearized field equations of general relativity, and below we solved analytically the case of two objects in circular orbit around the system’s centre of mass, and we found the waveform for the inspiral phase. However, as we pointed out, this is an approximation, which is valid only when the objects are far enough so that the weak field and the slow motion conditions hold on. In fact, the spherical symmetry of the neutron star is an approximation too. While there is a chance to solve analytically the inspiralling system, no chances are given for the merger phase. When the stars start to be close enough that the weak field approx- imation doesn’t hold on anymore, we cannot apply the linearized field equations. Thus, we need full general relativistic simulations to compute the signal expected. In addition, since neutron star magnetic fields are very strong, magnetohydrodynamics comes into play and makes things more complicated.
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An r−process macronova/kilonova in GRB 060614: evidence for the merger of a neutron star-black hole binary

An r−process macronova/kilonova in GRB 060614: evidence for the merger of a neutron star-black hole binary

of r−process material from a black hole-neutron star merger, as recently found in some numerical simulations, can produce it. If this interpretation is correct, it represents the first time that a multi-epoch / band lightcurve of a Li-Paczynski macronova (also known as kilonova) has been obtained and black hole-neutron star mergers are sites of significant production of r− process elements.

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Multi-messenger Observations of a Binary Neutron Star Merger

Multi-messenger Observations of a Binary Neutron Star Merger

In 2005, the fi eld of short gamma-ray burst ( sGRB ) studies experienced a breakthrough ( for reviews see Nakar 2007; Berger 2014 ) with the identi fi cation of the fi rst host galaxies of sGRBs and multi-wavelength observation ( from X-ray to optical and radio ) of their afterglows ( Berger et al. 2005; Fox et al. 2005; Gehrels et al. 2005; Hjorth et al. 2005b; Villasenor et al. 2005 ) . These observations provided strong hints that sGRBs might be associated with mergers of neutron stars with other neutron stars or with black holes. These hints included: ( i ) their association with both elliptical and star-forming galaxies ( Barthelmy et al. 2005; Prochaska et al. 2006; Berger et al. 2007; Ofek et al. 2007; Troja et al. 2008; D ’ Avanzo et al. 2009; Fong et al. 2013 ) , due to a very wide range of delay times, as predicted theoretically ( Bagot et al. 1998; Fryer et al. 1999; Belczynski et al. 2002 ) ; ( ii ) a broad distribution of spatial offsets from host-galaxy centers ( Berger 2010; Fong & Berger 2013; Tunnicliffe et al. 2014 ) , which was predicted to arise from supernova kicks ( Narayan et al. 1992; Bloom et al. 1999 ) ; and ( iii ) the absence of associated supernovae ( Fox et al. 2005; Hjorth et al. 2005c, 2005a; Soderberg et al. 2006; Kocevski et al. 2010; Berger et al. 2013a ) . Despite these strong hints, proof that sGRBs were powered by neutron star mergers remained elusive, and interest intensi fi ed in following up gravitational-wave detections electro- magnetically ( Metzger & Berger 2012; Nissanke et al. 2013 ) .
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Analysis methods for gravitational wave from binary neutron star coalescences: investigation on the post-merger phase

Analysis methods for gravitational wave from binary neutron star coalescences: investigation on the post-merger phase

• In Ref.[59], Stergioulas et al., studied nonaxisymmetric oscillation modes in mergers of compact object binaries and the associated gravitational wave emissions. The outcome of compact object mergers can lead to either prompt collapse to black hole (BH) or hypermassive compact object (hypermassive neutron star, HMNS) which collapse to BH after few milliseconds. In case of HMNS, a non nonaxisymmetric object is generated with transient deformations such as a bar-like shape, spiral arms and quasiradial and nonaxisymmetric oscillations of the matter. Considering HMNS as an isolated gravitating fluid, they studied its oscillation modes through the Fourier transforms of the evolved variables and reveal that the oscillations identified in the fluid are in direct correspondence with peaks in the GW spectrum (obtained through the quadrupole formula). From the comparison of analysis of the dynamics of the fluid and GW spectrum of different EoSs, focusing on dominant oscillation mode m = 2 of post-merger remnant, they associated specific frequency peaks in GW spectrum with the nonlinear components (see Figure 3 - Figure 8 in Ref.[59]). In particular, the dominant oscillation mode m= 2 of post-merger remnant (fluid) forms a triplet whose side bands are due to nonlinear coupling to the fundamental quasiradial m=0. A corresponding triplet of frequencies is identified in it so that the highest frequency peak f 2 coincides with l=m=2 mode
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r Process Nucleosynthesis in Neutron Star Mergers with the New Nuclear Reaction Network SkyNet

r Process Nucleosynthesis in Neutron Star Mergers with the New Nuclear Reaction Network SkyNet

et al., 2009) were formed within the first 100 Myr of star formation in our galaxy. Furthermore, neutron star mergers are rare events that release r-process material and that material has to mix with the interstellar medium in the galaxy before it can be incorporated into new stars. Thus one might expect a significant scatter in the r-process abundances in different parts of the galaxy, depending on whether there was a neutron star merger nearby. The observed scatter in the r-process abundances might be lower than what one would expect from mergers (e.g., Argast et al., 2004). However, even though the average delay time for compact object mergers is 0.1 − 1 Gyr, population synthesis models (e.g., De Donder and Vanbeveren, 2004; Do- minik et al., 2012) predict that there are a few percent of binary neutron star systems that have delay times as short as a few Myr. These tight binaries can be created by unstable mass transfer due to Roche lobe overflow. The exact distribution of delay times and especially the minimum delay time depend strongly on the treatment of the common envelope phase of binary stellar evolution (e.g., Dominik et al., 2012). Us- ing advanced population synthesis models and inhomogeneous mixing into account, several authors have found that neutron star mergers could be the dominant source of the r-process in the Milky Way, possibly with some early magnetorotationally- driven CCSNe, and can also account for the observed scatter of heavy elements (e.g., Ishimaru et al., 2015; Cescutti et al., 2015; Wehmeyer et al., 2015; van de Voort et al., 2015). Figure 1.14 shows a computation by van de Voort et al. (2015, their Figure 1) of galactic r-process enrichment in a cosmological simulation. They find that neutron star mergers alone can account for the observed r-process-to-iron ratios as a function of [Fe/H].
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Breaking the Viewing Angle-Distance Degeneracy for Binary Neutron Star Events Using Optical Measurements

Breaking the Viewing Angle-Distance Degeneracy for Binary Neutron Star Events Using Optical Measurements

GW170817 also confirmed the long-held belief that BNS mergers could be the source of extra-galactic gramma-ray bursts. As the stars merge, their magnetic fields align and are pointed towards the poles. A small amount of mass is then launched along the magnetic field lines, creating a jet, which has a narrow opening angle (≈ 10 ◦ ). The ejected mildly relativistic mass can produce photons through synchrotron radiation, which occurs when photons are emitted when charged particles are accelerated in a curved path. The charged particles can also further increase the energy of already present photons by transferring part of their energy to the photons through inverse Compton scattering. [15].
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Compact radio emission indicates a structured jet was produced by a binary neutron star merger

Compact radio emission indicates a structured jet was produced by a binary neutron star merger

The first binary neutron star merger, detected through both electromagnetic radiation and gravitational waves on the 17 th of August 2017, raised the question whether a narrow relativistic jet or a more isotropic outflow was launched as a consequence of the merger. High resolution measurements of the source size and position can provide the answer. Very Long Baseline Interferometry observations, performed 207.4 days after the binary merger through a global network of 32 radio telescopes spread over five continents, constrain the apparent source size to be smaller than 2 milliarcseconds at the 90% confidence level. This excludes the possibility that a nearly isotropic, mildly relativistic outflow is responsible for the emission, as in this case its apparent size, after more than six months of expansion, should have been significantly larger and resolved by the VLBI observation. Our size measurement proves that in at least 10% of neutron star mergers a structured relativistic jet should be produced.
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A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

A magnetar-powered X-ray transient as the aftermath of a binary neutron-star merger

Other mechanisms to produce cosmological X-ray transients are disfavored by the ob- servational data of CDF-S XT2 (see Methods). Low-luminosity LGRBs or massive-star shock breakout events are typically associated with active star formation. This is inconsistent with the host-galaxy type of CDF-S XT2 and its large offset with respect to the host-galaxy center. GRB orphan afterglows and tidal disruption events typically have much longer durations and very different light-curve shapes. Comparison of CDF-S XT2 with future multi-messenger observations of X-ray transients directly detectable by gravitational wave detectors can help verify its NS-NS merger origin, and help provide unprecedented insight into the physics of NS-NS mergers.
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On Discovering Electromagnetic Emission from Neutron Star Mergers: The Early Years of Two Gravitational Wave Detectors

On Discovering Electromagnetic Emission from Neutron Star Mergers: The Early Years of Two Gravitational Wave Detectors

The advent of advanced ground-based interferometers this decade is expected to usher in the era of routine grav- itational wave (GW) detection (Barish & Weiss 1999; LIGO Scientific Collaboration 2008; Accadia et al. 2011; Somiya 2012). Binary neutron star (NS) mergers are an- ticipated to be amongst the most numerous and strongest GW sources (Abadie et al. 2010). NS mergers are pre- dicted to produce neutron-rich outflows and emit electro- magnetic (EM) radiation across many wavelengths and timescales as the ejected debris interacts with its environ- ment — gamma (e.g., Eichler et al. 1989; Paczynski 1991; Narayan et al. 1992), optical (e.g., Li & Paczy´ nski 1998; Kulkarni 2005; Metzger et al. 2010; Roberts et al. 2011; Piran et al. 2012; Rosswog 2013), infrared (e.g., Barnes & Kasen 2013; Kasen et al. 2013; Tanaka & Hotokezaka 2013; Grossman et al. 2013) and radio (e.g., Hansen & Lyutikov 2001; Pshirkov & Postnov 2010; Nakar & Piran 2011).
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Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A

The joint observation of GW170817 and GRB  170817A con fi rms the association of SGRBs with BNS mergers. With just one joint event, we have set stringent limits on fundamental physics and probed the central engine of SGRBs in ways that have not been possible with EM data alone, demonstrating the importance of multi-messenger astronomy. The small time offset and independent localizations, though coarse, allowed an unambiguous association of these two events. Because GRB  170817A occurred nearby, an autono- mous trigger on-board GBM alerted follow-up observers to the presence of a counterpart to GW170817. At design sensitivity, however, Advanced LIGO and Virgo could in principle detect GW170817 beyond the distance that any active gamma-ray observatory would trigger on a burst like GRB  170817A. Subthreshold searches for SGRBs can extend the gamma-ray horizon and the detection of GRB  170817A provides motivation for further subthreshold search development.
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Search for Post-merger Gravitational Waves from the Remnant of the Binary Neutron Star Merger GW170817

Search for Post-merger Gravitational Waves from the Remnant of the Binary Neutron Star Merger GW170817

In addition to improving the sensitivity to potential post- merger signals of GW170817, another important program is to improve our ability to detect post-merger GWs from future LIGO-Virgo discoveries of binary NS mergers. At design sensitivity, Advanced LIGO and Advanced Virgo both aim to be approximately a factor of three better in broadband sensitivity than during the second observing run ( Abbott et al. 2016a ) , and next-generation detectors will improve the sensitivities signi fi cantly beyond that. This provides a number of opportunities. Clearly, increased sensitivity in the  kHz range implies improved ability to detect single post-merger signals. Moreover, increased broadband sensitivity implies higher rates of binary NS inspiral and merger detections, and hence might make possible power or coherent stacking of events to increase our sensitivity to post-merger physics ( Bose et al. 2017; Yang et al. 2017 ) .
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Multi-Messenger Observations of a Binary Neutron Star Merger

Multi-Messenger Observations of a Binary Neutron Star Merger

In 2005, the field of short gamma-ray burst (sGRB) studies experienced a breakthrough (for reviews see Nakar 2007 ; Berger 2014 ) with the identification of the first host galaxies of sGRBs and multi-wavelength observation (from X-ray to optical and radio ) of their afterglows (Berger et al. 2005 ; Fox et al. 2005 ; Gehrels et al. 2005 ; Hjorth et al. 2005b ; Villasenor et al. 2005 ). These observations provided strong hints that sGRBs might be associated with mergers of neutron stars with other neutron stars or with black holes. These hints included: (i) their association with both elliptical and star-forming galaxies (Barthelmy et al. 2005 ; Prochaska et al. 2006 ; Berger et al. 2007 ; Ofek et al. 2007 ; Troja et al. 2008 ; D ’Avanzo et al. 2009 ; Fong et al. 2013 ), due to a very wide range of delay times, as predicted theoretically (Bagot et al. 1998 ; Fryer et al. 1999 ; Belczynski et al. 2002 ); (ii) a broad distribution of spatial offsets from host-galaxy centers (Berger 2010 ; Fong & Berger 2013 ; Tunnicliffe et al. 2014 ), which was predicted to arise from supernova kicks (Narayan et al. 1992 ; Bloom et al. 1999 ); and (iii) the absence of associated supernovae (Fox et al. 2005 ; Hjorth et al. 2005c , 2005a ; Soderberg et al. 2006 ; Kocevski et al. 2010 ; Berger et al. 2013a ). Despite these strong hints, proof that sGRBs were powered by neutron star mergers remained elusive, and interest intensi fied in following up gravitational-wave detections electro- magnetically (Metzger & Berger 2012 ; Nissanke et al. 2013 ).
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Upper limits on the rates of binary neutron star and neutron star–black hole mergers from Advanced LIGO's first observing run

Upper limits on the rates of binary neutron star and neutron star–black hole mergers from Advanced LIGO's first observing run

We can compare our upper limits with rate predictions for compact object mergers involving NSs, shown for BNS in Figure 6 and for NSBH in Figure 7 . A wide range of predictions derived from population synthesis and from binary pulsar observations were reviewed in 2010 to produce rate estimates for canonical 1.4 M e NSs and 10 M e BHs(Abadie et al. 2010 ). We additionally include some more recent estimates from population synthesis for both NSBH and BNS (de Mink & Belczynski 2015 ; Dominik et al. 2015 ; Belczynski et al. 2016 ; keeping in mind these calculations do not simultaneously and widely explore all uncertainties in binary evolution, hence underestimating the underlying uncertainties; cf. O’Shaughnessy et al. 2005b , 2008 , 2010 , and references therein) and binary pulsar observations for BNS (Kim et al. 2015 ). Finally, to give a sense of scale to the results shown in Figures 6 and 7 , we note that the core-collapse supernova rate, in these units, is ∼10 5 Gpc −3 yr −1 (Cappellaro et al. 2015 and references therein). We also compare our upper limits for NSBH and BNS systems to beaming-corrected estimates of short GRB rates in the local universe. Short GRBs are considered likely to be produced by the merger of compact binaries that include NSs, i.e., BNS or NSBH systems(Berger 2014 ). The rate of short GRBs can predict the rate of progenitor mergers (Coward et al. 2012 ; Petrillo et al. 2013 ; Siellez et al. 2014 ; Fong et al. 2015 ). For NSBH, systems with small BH masses are considered more likely to be able to produce short GRBs (e.g.Duez 2010 ; Giacomazzo et al. 2013 ; Pannarale et al. 2015 ), so we compare to our 5 M e –1.4 M e NSBH rate constraint. The observation of a kilonova is also considered to be an indicator of a binary merger(Metzger & Berger 2012 ),
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