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Register a new userA Search for Neutron Star-Black Hole Binary Mergers in the Short Gamma-Ray Burst Population
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Short gamma-ray bursts (SGRBs) are now known to be the product of the merger of two compact objects. However, two possible formation channels exist: neutron star -- neutron star (NS -- NS) or NS -- black hole (BH). The landmark SGRB 170817A provided evidence for the NS -- NS channel, thanks to analysis of its gravitational wave signal. We investigate the complete population of SGRBs with an associated redshift (39 events), and search for any divisions that may indicate that a NS -- BH formation channel also contributes. Though no conclusive dichotomy is found, we find several lines of evidence that tentatively support the hypothesis that SGRBs with extended emission (EE; 7 events) constitute the missing merger population: they are unique in the large energy band-sensitivity of their durations, and have statistically distinct energies and host galaxy offsets when compared to regular (non-EE) SGRBs. If this is borne out via future gravitational wave detections it will conclusively disprove the magnetar model for SGRBs. Furthermore, we identify the first statistically significant anti-correlation between the offsets of SGRBs from their host galaxies and their prompt emission energies.
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The central engine of short gamma-ray bursts (sGRBs) is hidden from direct view, operating at a scale much smaller than that probed by the emitted radiation. Thus we must infer its origin not only with respect to the formation of the {it trigger} - the actual astrophysical configuration that is capable of powering a sGRB - but also from the consequences that follow from the various evolutionary pathways that may be involved in producing it. Considering binary neutron star mergers we critically evaluate, analytically and through numerical simulations, whether the neutrino-driven wind produced by the newly formed hyper-massive neutron star can allow the collimated relativistic outflow that follows its collapse to actually produce a sGRB or not. Upon comparison with the observed sGRB duration distribution, we find that collapse cannot be significantly delayed (<=100 ms) before the outflow is choked, thus limiting the possibility that long-lived hyper-massive remnants can account for these events. In the case of successful breakthrough of the jet through the neutrino-driven wind, the energy stored in the cocoon could contribute to the precursor and extended emission observed in sGRBs.
Short duration Gamma-Ray Bursts are thought to originate from the coalescence of neutron stars in binary systems. They are detected as a brief ($<$ 2s), intense flash of gamma-ray radiation followed by a weaker, rapidly decreasing afterglow. They are expected to be detected by Advanced LIGO and Virgo when their sensitivity will be low enough. In a recent study we identified a population of short Gamma-Ray Bursts that are intrinsically faint and nearby. Here we provide evidence in favor of the existence of this new population that can hardly be reproduced with a model of field neutron star binary coalescences. We propose that these systems may be produced dynamically in globular clusters, and may result from the merger of a black hole and a neutron star. The advanced LIGO and Virgo observation of a high rate of NSBH mergers compatible with the dynamical formation in globular clusters would be a confirmation of this hypothesis and would enable for the derivation of the mass function of black holes inside globular clusters, as well as the luminosity function of faint short GRBs.
The most popular model for short gamma-ray bursts (sGRBs) involves the coalescence of binary neutron stars. Because the progenitor is actually hidden from view, we must consider under which circumstances such merging systems are capable of producing a successful sGRB. Soon after coalescence, winds are launched from the merger remnant. In this paper, we use realistic wind profiles derived from global merger simulations in order to investigate the interaction of sGRB jets with these winds using numerical simulations. We analyze the conditions for which these axisymmetric winds permit relativistic jets to breakout and produce a sGRB. We find that jets with luminosities comparable to those observed in sGRBs are only successful when their half-opening angles are below ~20{deg}. This jet collimation mechanism leads to a simple physical interpretation of the luminosities and opening angles inferred for sGRBs. If wide, low luminosity jets are observed, they might be indicative of a different progenitor avenue such as the merger of a neutron star with a black hole. We also use the observed durations of sGRB to place constraints on the lifetime of the wind phase, which is determined by the time it takes the jet to breakout. In all cases we find that the derived limits argue against completely stable remnants for binary neutron star mergers that produce sGRBs.
Synoptic searches for the optical counterpart to a binary neutron star (BNS) or neutron star-black hole (NSBH) merger can pose significant challenges towards the discovery of kilonovae and performing multi-messenger science. In this work, we describe the advantage of a global multi-telescope network towards this end, with a particular focus on the key and complementary role the Dark Energy Camera (DECam) plays in multi-facility follow-up. We describe the Global Relay of Observatories Watching Transients Happen (GROWTH) Target-of-Opportunity (ToO) Marshal, a common web application we built to ingest events, plan observations, search for transient candidates, and retrieve performance summary statistics for all of the telescopes in our network. Our infrastructure enabled us to conduct observations of two events during O3a, S190426c and S190510g. Furthermore, our analysis of deep DECam observations of S190814bv conducted by the DESGW team, and access to a variety of global follow-up facilities allowed us to place meaningful constraints on the parameters of the kilonova and the merging binary. We emphasize the importance of a global telescope network in conjunction with a power telescope like DECam in performing searches for the counterparts to gravitational-wave sources.
Coalescing neutron star (NS)-black hole (BH) binaries are promising sources of gravitational-waves (GWs) to be detected within the next few years by current GW observatories. If the NS is tidally disrupted outside the BH innermost stable circular orbit, an accretion torus may form, and this could eventually power a short gamma-ray burst (SGRB). The observation of an SGRB in coincidence with gravitational radiation from an NS-BH coalescence would confirm the association between the two phenomena and also give us new insights on NS physics. We present here a new method to measure NS radii and thus constrain the NS equation of state using joint SGRB and GW observations of NS-BH mergers. We show that in the event of a joint detection with realistic GW signal-to-noise ratio (S/N) of 10, the NS radius can be constrained to $lesssim,$20% accuracy at 90% confidence.
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