# Introduction

## Introduction

Accreting black holes are central in explaining a range of high-energy astrophysical phenomena that we observe in our Universe, such as X-ray binaries, active galactic nuclei (AGN), and quasars. Recently, substantial theoretical and observational effort has gone into understanding accretion onto binary black holes and the emergent electromagnetic (EM) signatures these systems may generate, because they are anticipated to exist at the centers of distant AGNs and quasars. The bulk of the research so far has focused on supermassive black holes binaries and about 150 candidate accreting supermassive black hole binaries have been identified in quasar surveys, with a number of them in the gravitational-wave driven regime. However, in addition to accreting supermassive binary black holes, there may exist black hole binaries of a few tens of solar masses that could be accreting matter from a circumbinary disk. This scenario has attracted a lot of attention recently because of the direct detection of gravitational waves (GWs).

In particular, on September 14, 2015, the LIGO and Virgo collaborations made the first direct detection of a GW signal $-$ event GW150914. GW150914 was entirely consistent with an inspiraling and merging binary black hole (BHBH) in vacuum as predicted by the theory of general relativity (GR). This detection provided the best evidence yet for the existence of black holes. Meanwhile, the detection of a transient EM signal (event GW150914-GBM) at photon energies > 50 keV that lasted 1 s and appeared 0.4 s after the GW signal was reported using data from the Gamma-ray Burst Monitor (GBM) aboard the Fermi satellite. The Fermi GBM satellite was covering 75% of the probability map associated with the LIGO localization event in the sky, thus this signal could be a chance coincidence. An MeV-scale EM signal lasting for 32 ms and occurring 0.46 s before the third GW detection made by the LIGO detectors (event GW170104) $-$ also consistent with a BHBH $-$ was reported using data from the AGILE mission. While these candidate counterpart EM signals were not confirmed by other satellites operating at the same time, they still excited the interest of the community and several ifdeas about how to generate accretion disks onto "heavy" black holes, such as those that LIGO detected, have been proposed.

Circumbinary disks were proposed because inspiraling and merging BHBHs in vacuum do not generate EM signals, but they may do so in the presence of ambient gas, such as binaries residing in AGNs or those with gas remaining from their stellar progenitors. There are multiple studies that suggest possible connections between LIGO GW150914 and GW150914-GBM. Sources that could generate both the GWs and the hard X-rays reported include binary black hole-neutron stars, binary black hole massive stars, and rapidly rotating massive stars.

In previous work we initiated magnetohydrodynamic (MHD) studies of nonspinning binary black holes accreting from a circumbinary disk in full GR. We modeled both the binary-disk predecoupling and post-decoupling phases. We investigated the effects of magnetic fields and the binary mass ratio, and discovered that these systems launch magnetically driven jets (even though the black holes were nonspinning) whose Poynting luminosity is of order 0.1% of the accretion power. Therefore, these systems could serve both as radio, X-ray and gamma-ray engines for supermassive BHBHs in AGNs and as gamma-ray-burst-like engines for LIGO-observed BHBH masses, if in the latter case the hyperaccreting phase associated with the merger lasts for $O(1s)$. We also discovered that $\sim 2 - 5(M/10^8M_{\odot})(1 + z)$ d [or $\sim 0.1-0.3(M/65M_{\odot})(1 +z)$ s] following merger, there is a significant boost both in the Poynting luminosity and the efficiency for converting accretion power to EM luminosity. The time delay in units of M between merger and the jet luminosity boost, as well as the magnitude of the boost, depends on the binary mass ratio. In particular, more equal-mass binaries exhibit a longer time delay and stronger boost. The origin of this boost is currently unclear. It could be due to the fact that, prior to merger, the black holes in our simulations were nonspinning, while after merger a single, spinning black hole formed. It could also be due to an increase of magnetic flux accreted onto the BHs prior to and after merger, as the binary tidal torques modify the accretion flow $-$ the higher the mass ratio the weaker the tidal torques and the more magnetized matter is present in the vicinity of the binary prior to merger. It has never been explored whether this time delay depends on the initial disk model. For example, one could anticipate some dependence on the disk thickness because the binary tidal torques depend on the disk thickness, as well.

In this simulation we consider a black hole binary with mass ratio 36:29 - motivated by the inferred value of the mass ratio of GW150914 - and consider different initial disk models to explore their impact on the emergence of jets, the outgoing Poynting luminosity, the presence of periodicities in the accretion rate, and the time delay between merger and the boost in the jet luminosity that we discovered before. As in our previous studies we ignore the disk self-gravity and detailed microphysics, which then allows us to scale the binary black hole mass and disk densities to arbitrary values. Thus, our results can be applied to both supermassive black hole binaries and LIGO/Virgo black hole binaries.