A New Playing Field for Multi-(Messi)nger Black Holes Kicks

Title: Optical Emission Model for Binary Black Hole Merger Remnants Travelling through Discs of Active Galactic Nucleus

Authors: J. C. Rodríguez-Ramírez, C. R. Bom, B. Fraga, R. Nemmen

First Authors’ Institution: Centro Brasileiro de Pesquisas Fisicas (CBPF), Rua Dr Xavier Sigaud 150, CEP 22290-180 Rio de Janeiro RJ, Brazil

Status: Submitted to MNRAS

In astronomy, information about the busy universe – collisions, flares, explosions – is carried through space by various “messengers“, different modes of receiving information about the target source. These universal messengers, together made up of electromagnetic (EM) light, neutrinos, cosmic rays, and gravitational waves (GWs), allow us to peek through different windows into the cosmos, altogether adding up to more than just the sum of their parts.

A famous example of a multi-messenger event is the 2017 merger of two neutron stars, otherwise called a kilonova. This was detected at the LIGOVirgo detectors with gravitational waves, and emitted a gamma-ray burst 1.7ms later, which was detected by the Fermi Gamma-ray Space Telescope and INTEGRAL. This detection was proof that gravitational waves travel at the same speed as photons (or at least very, very close) and allowed us to identify the host galaxy of the two neutron stars, leading to interesting and important cosmological constraints!

While scientists have been keenly preparing to chase more kilonovae with EM telescopes in tandem with their GW counterparts, many other GW and EM multi-messenger transients could be detected over the coming years. Today’s authors consider how binary black holes (BBHs), which usually emit GWs but not EM radiation on their own, could emit both GWs and EM light when they merge near the gaseous accretion disks around supermassive black holes (SMBHs). 

Stellar-mass black holes are predicted to gather in the inner regions of SMBH accretion disks, an ideal watering hole for them to collide and form binaries while they orbit around the central SMBH. If these binaries merge to create a secondary remnant black hole, the remnant will receive a recoil kick of hundreds of kilometres per second, which could throw it out of the disk.  This remnant could merge again with another black hole, in what’s called a hierarchical merger event.

This paper considers what happens if this second merger leads to the next remnant getting kicked back through the accretion disk, erupting the disk material with powerful jets. These jets would form due to the accretion of matter onto the remnant black hole, in turn inflating cocoons of gas in the disk. When these cocoons reach the edge of the disk, they will punch through the edge of the disk material, as illustrated in Figure 1. This initial material breaking through the edge of the disk could cause a brief EM flare. However, this paper focuses on the remaining gas cloud pushed out of the edge of the disk, which the authors name the outflow cloud. Once the cloud cools, the thermal photons can escape, creating another bright EM transient. Specifically, the authors look at how long it takes for the outflow to emit light after the original black holes merge, the EM frequencies of the transient, and how the flux of the emission varies with time.

Figure 1: The proposed scenario for an EM counterpart to a BBH GW event near an accretion disk of an SMBH (middle band across the image). At (A), two black holes merge within the disk and kick the remnant out of the disk. At (B), this remnant merges again, releasing GWs and the second remnant gets kicked back through the disk, erupting it with powerful jets. At (C) these jets break through the disk boundary. At (D), some time later, the material ejected from the disk cools and releases an EM counterpart. Adapted by Storm Colloms from Figure 1 in the paper.

They analytically model the underlying processes of the remnant travelling through the disk and the thermal processes of the outflow cloud, calculating the total delay to the EM emission. Considering the sum of all the steps in Figure 1, the EM emission would appear 40-80 days after the GWs are emitted from the second merger event. All of these processes depend on the strength of the remnant kick, assuming an initial distance and entry angle of the remnant. They find that for slower kicked remnants, it takes longer to produce the jets, but for faster remnants it takes longer for the outflow to cool, finding the fastest time to the EM transient as a sweet spot in the middle.

Secondly, they calculate the spectral energy distribution (SED) of the emission – how bright the emission is over a range of wavelengths. At this stage, they also compare the SED to that of a model of an accreting SMBH to figure out at what wavelengths the transient will be able to outshine the normal emission from the active galactic nuclei (AGN). This is shown in Figure 2, for two masses of SMBH, and two rates of accretion. For higher masses and rates of accretion, the AGN will be brighter, and so the outflow transient will be outshone. However for low accretion rates the outflow emission can outshine the AGN background in near infrared (NIR), optical, and extreme ultraviolet (EUV) wavelengths.

Figure 2: The SED of the outflow cloud created by a remnant black hole travelling through the AGN disk at 400 km/s (blue curves) and that of the overall emission of the AGN (grey lines). The four plots show how the SEDs compare for SMBH masses of 5×10^8 and 1×10^9 solar masses, as well as accretion rates onto the SMBH of 0.01 and 0.1 times the maximum Eddington luminosity allowed by accretion. The thicknesses of the blue lines correspond to the strength of the emission for different numbers of days after the GW merger. For the smallest mass SMBH and the lowest accretion, the thickest blue curve surpasses the grey line and so when the cloud first emits after cooling it can outshine the AGN. This could be detected at NIR, optical, and EUV wavelengths, corresponding to the brown, green, and cyan vertical lines in the plots respectively. Figure 4 in the paper.

However, there is a caveat to this.  The accretion disk of a SMBH is a busy place, and things like instabilities in the magnetic field of the disk or changes in the accretion rate can change the brightness of the AGN. To be able to distinguish this from other AGN variability, the authors find how the flux of the EM counterpart from the outflow cloud changes with time for different recoil kick strengths. For progressively larger kicks they find that the maximum cloud brightness increases, attains a maximum and then decreases, and that the time taken for the counterpart to fade increases. Furthermore, remnant black holes with larger masses produce flares with shorter timescales, but for larger accretion rates of the SMBH, the flares have larger timescales.

For particular configurations of remnant black hole and SMBH masses, kick velocities, and accretion rates, the outflow cloud could outshine the AGN by 50-200%. However, this is just one situation with a lot of assumptions, and more detailed simulations are necessary to figure out how plausible this scenario is and how often events like these could occur around SMBHs. These further studies could help us prepare for a brand new kind of multi-messenger goal to cheer on from the stands.

Astrobite edited by Archana Aravindan

Featured image credit: ESA/Hubble, adapted by Storm Colloms

About Storm Colloms

Storm is a postgraduate researcher at the University of Glasgow, Scotland. They work on understanding populations of binary black holes and neutron stars from the gravitational wave signals emitted when they merge, and what that tells us about the lives and deaths of massive stars. Outwith astrophysics they spend their time taking digital and film photos, and making fun doodles of their research.

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