When and where did GW150914 form?

The recent LIGO detection of gravitational waves (GW150914), likely originating from the merger of two ∼ 30M_⊙ black holes suggests progenitor stars of low metallicity ([Z/Z_⊙] ≲ 0.3), constraining when and where the progenitor of GW150914 may have formed. We combine estimates of galaxy properties (star-forming gas metallicity, star formation rate and merger rate) across cosmic time to predict the low redshift black hole – black hole merger rate as a function of present day host galaxy mass, M_(gal), the formation redshift of the progenitor system z_f and different progenitor metallicities Z_p. For Z_p ⩾ 0.1Z_⊙, the signal is dominated by binaries in massive galaxies with z_f ≃ 2 while below Z_p ⩽ 0.1Z_⊙ most mergers come from binaries formed around z_f ≃ 0.5 in dwarf galaxies. Additional gravitational wave detections from merging massive black holes will provide constraints on the mass–metallicity relation and massive star formation at high redshifts

Disks Around Merging Binary Black Holes: From GW150914 to Supermassive Black Holes

We perform magnetohydrodynamic simulations in full general relativity of disk accretion onto nonspinning black hole binaries with mass ratio 36:29. We survey different disk models which differ in their scale height, total size and magnetic field to quantify the robustness of previous simulations on the initial disk model. Scaling our simulations to LIGO GW150914 we find that such systems could explain possible gravitational wave and electromagnetic counterparts such as the Fermi GBM hard X-ray signal reported 0.4s after GW150915 ended. Scaling our simulations to supermassive binary black holes, we find that observable flow properties such as accretion rate periodicities, the emergence of jets throughout inspiral, merger and post-merger, disk temperatures, thermal frequencies, and the time-delay between merger and the boost in jet outflows that we reported in earlier studies display only modest dependence on the initial disk model we consider here.Comment: 14 pages, 6 figures, 5 tables, added discussion and references, matches published versio

The Emergence of Gravitational Wave Science: 100 Years of Development of Mathematical Theory, Detectors, Numerical Algorithms, and Data Analysis Tools

On September 14, 2015, the newly upgraded Laser Interferometer Gravitational-wave Observatory (LIGO) recorded a loud gravitational-wave (GW) signal, emitted a billion light-years away by a coalescing binary of two stellar-mass black holes. The detection was announced in February 2016, in time for the hundredth anniversary of Einstein's prediction of GWs within the theory of general relativity (GR). The signal represents the first direct detection of GWs, the first observation of a black-hole binary, and the first test of GR in its strong-field, high-velocity, nonlinear regime. In the remainder of its first observing run, LIGO observed two more signals from black-hole binaries, one moderately loud, another at the boundary of statistical significance. The detections mark the end of a decades-long quest, and the beginning of GW astronomy: finally, we are able to probe the unseen, electromagnetically dark Universe by listening to it. In this article, we present a short historical overview of GW science: this young discipline combines GR, arguably the crowning achievement of classical physics, with record-setting, ultra-low-noise laser interferometry, and with some of the most powerful developments in the theory of differential geometry, partial differential equations, high-performance computation, numerical analysis, signal processing, statistical inference, and data science. Our emphasis is on the synergy between these disciplines, and how mathematics, broadly understood, has historically played, and continues to play, a crucial role in the development of GW science. We focus on black holes, which are very pure mathematical solutions of Einstein's gravitational-field equations that are nevertheless realized in Nature, and that provided the first observed signals.Comment: 41 pages, 5 figures. To appear in Bulletin of the American Mathematical Societ

The Delay Time of Gravitational Wave — Gamma-Ray Burst Associations

The first gravitational wave (GW) — gamma-ray burst (GRB) association, GW170817/GRB 170817A, had an offset in time, with the GRB trigger time delayed by ∼1.7 s with respect to the merger time of the GW signal. We generally discuss the astrophysical origin of the delay time, Δt, of GW-GRB associations within the context of compact binary coalescence (CBC) — short GRB (sGRB) associations and GW burst — long GRB (lGRB) associations. In general, the delay time should include three terms, the time to launch a clean (relativistic) jet, Δtjet; the time for the jet to break out from the surrounding medium, Δtbo; and the time for the jet to reach the energy dissipation and GRB emission site, ΔtGRB. For CBC-sGRB associations, Δtjet and Δtbo are correlated, and the final delay can be from 10 ms to a few seconds. For GWB-lGRB associations, Δtjet and Δtbo are independent. The latter is at least ∼10 s, so that Δt of these associations is at least this long. For certain jet launching mechanisms of lGRBs, Δt can be minutes or even hours long due to the extended engine waiting time to launch a jet. We discuss the cases of GW170817/GRB 170817A and GW150914/GW150914-GBM within this theoretical framework and suggest that the delay times of future GW/GRB associations will shed light into the jet launching mechanisms of GRBs