skip to main content

Title: Aligning nuclear cluster orbits with an active galactic nucleus accretion disc
ABSTRACT Active galactic nuclei (AGN) are powered by the accretion of discs of gas on to supermassive black holes (SMBHs). Stars and stellar remnants orbiting the SMBH in the nuclear star cluster (NSC) will interact with the AGN disc. Orbiters plunging through the disc experience a drag force and, through repeated passage, can have their orbits captured by the disc. A population of embedded objects in AGN discs may be a significant source of binary black hole mergers, supernovae, tidal disruption events, and embedded gamma-ray bursts. For two representative AGN disc models, we use geometric drag and Bondi–Hoyle–Littleton drag to determine the time to capture for stars and stellar remnants. We assume a range of initial inclination angles and semimajor axes for circular Keplerian prograde orbiters. Capture time strongly depends on the density and aspect ratio of the chosen disc model, the relative velocity of the stellar object with respect to the disc, and the AGN lifetime. We expect that for an AGN disc density $\rho \gtrsim 10^{-11}{\rm g\, cm^{-3}}$ and disc lifetime ≥1 Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.
Authors:
; ; ; ; ;
Award ID(s):
1831412
Publication Date:
NSF-PAR ID:
10292774
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
499
Issue:
2
Page Range or eLocation-ID:
2608 to 2616
ISSN:
0035-8711
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT Accretion discs around supermassive black holes are promising sites for stellar mass black hole mergers detectable with LIGO. Here we present the results of Monte Carlo simulations of black hole mergers within 1-d AGN disc models. For the spin distribution in the disc bulk, key findings are: (1) The distribution of χeff is naturally centred around $\tilde{\chi }_{\rm eff} \approx 0.0$, (2) the width of the χeff distribution is narrow for low natal spins. For the mass distribution in the disc bulk, key findings are: (3) mass ratios $\tilde{q} \sim 0.5\!-\!0.7$, (4) the maximum merger mass in the bulk is $\sim 100\!-\!200\, \mathrm{M}_{\odot }$, (5) $\sim 1{{\ \rm per\ cent}}$ of bulk mergers involve BH $\gt 50\, \mathrm{M}_{\odot }$ with (6) $\simeq 80{{\ \rm per\ cent}}$ of bulk mergers are pairs of first generation BH. Additionally, mergers at a migration trap grow an IMBH with typical merger mass ratios $\tilde{q}\sim 0.1$. Ongoing LIGO non-detections of black holes $\gt 10^{2}\, \mathrm{M}_{\odot }$ puts strong limits on the presence of migration traps in AGN discs (and therefore AGN disc density and structure) as well as median AGN disc lifetime. The highest merger rate occurs for this channel if AGN discs aremore »relatively short-lived (≤1 Myr) so multiple AGN episodes can happen per Galactic nucleus in a Hubble time.« less
  2. ABSTRACT

    The discs of active galactic nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long gamma-ray bursts (GRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star–black hole binaries, leading to short GRBs upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN discs is key to their proper identification and to better constrain the star and compact object population in AGN discs. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion, since they are generated within optically thick regions of the accretion discs. Here, we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black hole masses and disc locations. We find that the radiation from radio to ultraviolet and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disc. In addition, photon diffusion can significantly delay themore »emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broad-band correlated AGN variability with a dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN discs and the disc structure.

    « less
  3. Abstract Stars are likely embedded in the gas disks of active galactic nuclei (AGN). Theoretical models predict that in the inner regions of the disk, these stars accrete rapidly, with fresh gas replenishing hydrogen in their cores faster than it is burned into helium, effectively stalling their evolution at hydrogen burning. We produce order-of-magnitude estimates of the number of such stars in a fiducial AGN disk. We find numbers of order 10 2–4 , confined to the inner r cap ∼ 3000 r s ∼ 0.03 pc. These stars can profoundly alter the chemistry of AGN disks, enriching them in helium and depleting them in hydrogen, both by order-unity amounts. We further consider mergers between these stars and other disk objects, suggesting that star–star mergers result in rapid mass loss from the remnant to restore an equilibrium mass, while star–compact object mergers may result in exotic outcomes and even host binary black hole mergers within themselves. Finally, we examine how these stars react as the disk dissipates toward the end of its life, and find that they may return mass to the disk fast enough to extend its lifetime by a factor of several and/or may drive powerful outflows frommore »the disk. Post-AGN, these stars rapidly lose mass and form a population of stellar mass black holes around 10 M ⊙ . Due to the complex and uncertain interactions between embedded stars and the disk, their plausible ubiquity, and their order-unity impact on disk structure and evolution, they must be included in realistic disk models.« less
  4. ABSTRACT Advanced LIGO and Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) discs of gas around supermasssive BHs. Here, we investigate the relative number of compact object (CO) mergers in AGN disc models, including BH, neutron stars (NS), and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disc grows ∝ t1/3 which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS–BH mergers in AGN discs are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) M−1(− 2). If a fraction fAGN of the observed rate of BH–BH mergers (RBH–BH) come from AGN, the rate of NS–BH (NS–NS) mergers in the AGN channel is ${R}_{\mathrm{ BH}\!-\!\mathrm{ NS}} \sim f_{\mathrm{ AGN}}[10,300]\, \rm {Gpc}^{-3}\, \rm {yr}^{-1},({\mathit{ R}}_{NS\!-\!NS} \le \mathit{ f}_{AGN}400\, \rm {Gpc}^{-3}\, \rm {yr}^{-1}$). Given the ratio of NS–NS/BH–BH LIGO search volumes, from preliminary O3 results the AGN channel is not the dominant contribution to observed NS–NS mergers. The number of lower mass gap events expected is a strong function of the nuclear MF and mass segregation efficiency. CO merger ratiosmore »derived from LIGO can restrict models of MF, mass segregation, and populations embedded in AGN discs. The expected number of electromagnetic (EM) counterparts to NS–BH mergers in AGN discs at z < 1 is $\sim [30,900]\, {\rm {yr}}^{-1}(f_{\mathrm{ AGN}}/0.1)$. EM searches for flaring events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them.« less
  5. ABSTRACT

    As active galactic nuclei (AGN) ‘turn on’, some stars end up embedded in accretion discs around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation, and orbital evolution on short time-scales. The loss of orbital angular momentum in the first ∼0.1 Myr of an AGN leads to a heavy rain of stars (‘starfall’) into the inner disc and on to the SMBH. A large AGN loss cone (θAGN, lc) can result from binary scatterings in the inner disc and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disc. If the SMBH mass is $M_{\rm SMBH} \gtrsim 10^{8}\, \mathrm{M}_{\odot }$, flares truncate abruptly and the star is swallowed. If $M_{\rm SMBH}\lt 10^{8}\, \mathrm{M}_{\odot }$, and if the infalling orbit lies within θAGN, lc, the flare is followed by a TDE that can be prograde or retrograde relative to the AGN inner disc. Retrograde AGN TDEs are overluminous and short-lived as in-plane ejecta collide with the inner disc and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disc gas and so start off looking like regular TDEs butmore »are followed by an AGN high state. Searches for such flare signatures test models of AGN ‘turn on’, SMBH mass, as well as disc properties and the embedded population.

    « less