Session K14: Multimessenger Signals from Gravitational-Wave Sources

Structure and Dynamics of Minidisks in Spinning Black Hole Binaries: Simulations in Full General Relativity

We perform magnetohydrodynamic simulations of accreting, equal-mass binary black holes in full general relativity focusing on the structure and dynamics of minidisks. We build upon previous work which established the presence of minidisks in systems where the Hill-sphere is significantly larger than the effective innermost stable circular orbit. We explore the details of the structure of the minidisks and the impact of minidisk formation and black hole spin on accretion rates and other parameters of the system. We also investigate the impact of minidisk formation on periodic behaviors in the system in accretion rate, mass contained within the minidisks, and Poynting luminosities.   

Circumbinary Disk Dynamics

We will present the results of numerical simulations of a binary black hole surrounded by a thin, massless, locally isothermal, viscous accretion disk. We describe the numerical setup of these time-dependent simulations, and investigate the evolution of the fluid in the presence of the gravitational potential generated by the binary. We have investigated a series of configurations characterized by different mass ratios and inclinations of the disk with respect to the binary's orbit. We analyze the properties of the density distribution, such as the location of the maximum, size of the central gap, etc., as a function of our model parameters. Furthermore, calculating pressure and velocity of the fluid, we can characterize additional properties of the fluid flow, such as the azimuthal propagation of angular momentum, dynamical and viscous torque balance, accretion rate, luminosity, stability, etc. Having in mind the recent active research in the field of electromagnetic searches for supermassive black-hole binaries, and the eventual searches for gravitational waves from these same sources, we present findings which are the most relevant for these two domains of astrophysics.   

Simulating short gamma-ray burst jets with genuine surrounding environments.

GRB 170817A provided clear evidence of an intrinsic connection between short gamma-ray bursts (SGRBs) and binary neutron star (BNS) mergers. The mechanism by which the latter are able to feed the former, however, remains poorly understood, and many theoretical and phenomenological models, combining observational data with results of numerical calculations, are currently under development with the aim of unveiling it. In this talk, I will present results of numerical simulations in which we modelled, for the first time, the three-dimensional evolution of an SGRB jet propagating through the post-merger environment of a fully general-relativistic (GR) hydrodynamic BNS merger simulation, reaching space-time scales relevant for the gamma-ray emission, and providing self-consistent predictions for the jet's final structure and energetics. I will also discuss the ongoing work concerning the inclusion of magnetic fields in our jet simulations, directly importing magnetized initial data from GR magneto-hydrodynamic BNS merger simulations.   

Using Archival VERITAS Data to Search for Counterparts to Sub-Threshold Neutron-Star Merger Candidates

During Advanced LIGO's first observing run, which ran from September 2015 to January 2016, the detector confidently detected three binary black hole mergers in gravitational waves (GWs) and no binary neutron star (BNS) mergers. However, in a post-factum analysis, 103 sub-threshold BNS merger candidates were identified. The first confidently detected BNS merger was not measured until the subsequent observing season, but, notably, this event was also associated with an electromagnetic counterpart extending up to low-energy (LE; 100 keV - 100 MeV) gamma rays. The association of such an event at even higher energies, like those probed by imaging atmospheric Cherenkov telescope (IACT) arrays remains an open question. In this contribution, I will describe a study performed using archival data from the VERITAS IACT array to search for transient events in very-high-energy (VHE; > 100 GeV) gamma rays motivated by the sub-threshold BNS merger candidates from LIGO's first observing run. I will present the results from this study, and further describe the promise of this technique for joint sub-threshold searches with current- and future-generation IACT arrays.   

IceCube search for neutrinos coincident with gravitational wave detections reported for LIGO-Virgo's third observing run

The observation of an astrophysical source capable of emitting gravitational waves and neutrinos jointly remained elusive. Current investigations include searches for high-energy neutrinos coinciding with confirmed gravitational-wave catalog events from LIGO/Virgo's third observing run using IceCube's neutrino triggers. We discuss an unbinned maximum likelihood analysis as well as a Bayesian analysis using astrophsical priors for the joint search.   

Gravitational-wave alerts in the upcoming LIGO-Virgo-KAGRA observing run

During the third observing run (O3) of the advanced LIGO-Virgo gravitational-wave (GW) detectors, low-latency GW alerts were published publicly for the first time, allowing for follow-up of over 50 events. The upcoming fourth observing run (O4), slated to begin not earlier than December 2022, is forecasted to observe up to twice as many events as in O3. Many of these will be promising targets for multi-messenger follow-up in electromagnetic bands and in neutrinos. We will summarize the successes and challenges of the low-latency public alert infrastructure in O3, and describe updates and plans by the LIGO-Virgo-KAGRA collaboration for the system in O4.