95 research outputs found
Simulating X-ray Reverberation in the UV-Emitting Regions of Active Galactic Nuclei Accretion Disks with 3D Multi-Frequency Magnetohydrodynamic Simulations
Active galactic nuclei (AGN) light curves observed with different wavebands
show that the variability in longer wavelength bands lags the variability in
shorter wavelength bands. Measuring these lags, or reverberation mapping, is
used to measure the radial temperature profile and extent of AGN disks,
typically with a reprocessing model that assumes X-rays are the main driver of
the variability in other wavelength bands. To demonstrate how this reprocessing
works with realistic accretion disk structures, we use 3D local shearing box
multi-frequency radiation magnetohydrodynamic (MHD) simulations to model the
UV-emitting region of an AGN disk, which is unstable to the magnetorotational
instability (MRI) and convection. At the same time, we inject hard X-rays
(~keV) into the simulation box to study the effects of X-ray irradiation on
the local properties of the turbulence and the resulting variability of the
emitted UV light curve. We find that disk turbulence is sufficient to drive
intrinsic variability in emitted UV light curves and that a damped random walk
(DRW) model is a good fit to this UV light curve for timescales ~days.
Meanwhile, the injected X-rays have almost no impact on the power spectrum of
the emitted UV light curve. In addition, the injected X-ray and emitted UV
light curves are only correlated if there is X-ray variability on timescales
~day, in which case we find a correlation coefficient . These
results suggest that hard X-rays with scattering dominated opacity are likely
not the main driver of the reverberation signals.Comment: 9 pages, 3 figures, submitted to ApJ
Negative Lags on the Viscous Timescale in Quasar Photometry and Prospects for Detecting More with LSST
The variability of quasar light curves can be used to study the structure of
quasar accretion disks. For example, continuum reverberation mapping uses
delays between variability in short and long wavelength bands ("short" lags) to
measure the radial extent and temperature profile of the disk. Recently, a
potential reverse lag, where variations in shorter wavelength bands lag the
longer wavelength bands at the much longer viscous timescale, was detected for
Fairall 9. Inspired by this detection, we derive a timescale for these "long"
negative lags from fluctuation propagation models and recent simulations. We
use this timescale to forecast our ability to detect long lags using the Vera
Rubin Legacy Survey of Space and Time (LSST). After exploring several methods,
including the interpolated cross-correlation function, a Von-Neumann estimator,
javelin, and a maximum-likelihood Fourier method, we find that our two main
methods, javelin and the maximum-likelihood method, can together detect long
lags of up to several hundred days in mock LSST light curves. Our methods work
best on proposed LSST cadences with long season lengths, but can also work for
the current baseline LSST cadence, especially if we add observations from other
optical telescopes during seasonal gaps. We find that LSST has the potential to
detect dozens to hundreds of additional long lags. Detecting these long lags
can teach us about the vertical structure of quasar disks and how it scales
with different quasar properties.Comment: 40 pages, 36 figures, submitted to Ap
A Negative Long Lag from the Optical to the UV Continuum in Fairall 9
We report the detection of a long-timescale negative lag, where the blue
bands lag the red bands, in the nearby Seyfert 1 galaxy Fairall 9. Active
Galactic Nuclei (AGN) light curves show variability over a wide range of
timescales. By measuring time lags between different wavelengths, the otherwise
inaccessible structure and kinematics of the accretion disk can be studied. One
common approach, reverberation mapping, quantifies the continuum and line lags
moving outwards through the disk at the light-travel time, revealing the size
and temperature profile of the disk. Inspired by numerical simulations, we
expect longer lags to exist in AGN light curves that travel inward on longer
timescales, tracing the accretion process itself. By analyzing AGN light curves
in both temporal and frequency space, we report the detection of long-timescale
lags ( days) in Fairall 9 which propagate in the opposite direction
to the reverberation lag. The short continuum lag ( days) is also detected
and is consistent with reverberation lags reported in the literature. When
fitting the longer lag as a function of frequency with a model motivated by the
thin disk model, we find that the disk scale height likely increases outward in
the disk. This detection raises the exciting prospect of mapping accretion disk
structures across a wide range of AGN parameters.Comment: 24 pages, 14 figures, submitted to Ap
Aligning Retrograde Nuclear Cluster Orbits with an Active Galactic Nucleus Accretion Disc
Stars and stellar remnants orbiting a supermassive black hole (SMBH) can
interact with an active galactic nucleus (AGN) disc. Over time, prograde
orbiters (inclination ) decrease inclination, as well as
semi-major axis () and eccentricity () until orbital alignment with the
gas disc ("disc capture"). Captured stellar-origin black holes (sBH) add to the
embedded AGN population which drives sBH-sBH mergers detectable in
gravitational waves using LIGO-Virgo-KAGRA (LVK) or sBH-SMBH mergers detectable
with LISA (Laser Interferometer Space Antenna). Captured stars can be tidally
disrupted by sBH or the SMBH or rapidly grow into massive 'immortal' stars.
Here, we investigate the behaviour of polar and retrograde orbiters () interacting with the disc. We show that retrograde stars are
captured faster than prograde stars, flip to prograde () during
capture and decrease dramatically towards the SMBH. For sBH, we find a
critical angle , below which retrograde sBH decay
towards embedded prograde orbits (), while for sBH decay towards embedded retrograde orbits (). sBH near polar orbits () and stars on nearly
embedded retrograde orbits () show the greatest decreases
in . Whether a star is captured by the disc within an AGN lifetime depends
primarily on disc density, and secondarily on stellar type and initial . For
sBH, disc capture time is longest for polar orbits, low mass sBH and lower
density discs. Larger mass sBH should typically spend more time in AGN discs,
with implications for the embedded sBH spin distribution.Comment: 10 pages, 7 figures, 1 table; submitted to MNRA
Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes. II. Spins and Incoming Objects
Ram-pressure stripping of a kicked Hill sphere:Prompt electromagnetic emission from the merger of stellar mass black holes in an AGN accretion disk
Accretion disks around supermassive black holes (SMBHs) are promising sites for stellar mass black hole (BH) mergers due to mass segregation and merger acceleration by disk gas torques. Here we show that a gravitational-wave (GW) kick at BH merger causes ram-pressure stripping of gas within the BH Hill sphere. If R_H ≥ H, the disk height, an off-center UV flare at a_(BH) ~ 10³ r_g, emerges within t_(UV) ~ O(2 days)(a_(BH)/10³ r_g)(M_(SMBH)/10⁸ M_⊙)(v_(kick)/10² km s⁻¹) postmerger and lasts O(R_H/v_(kick)) ~ O(5t_(UV)). The flare emerges with luminosity O(10⁴² erg s⁻¹(t_(UV)/2 days)⁻¹(M_(Hill)/1M_⊙)(v_(kick)/10² km s⁻¹)². Active galactic nucleus optical/UV photometry is altered and asymmetric broad emission line profiles can develop after weeks. If R_H 50M_⊙
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