cover the 18,000 deg2 footprint with a semi-regular cadence, with repeat
observations every three to five nights. This strategy will return high-
quality lightcurves with dense sampling, ideal for periodicity detection. In
addition, observations will alternate among six narrow-band optical filters
providing invaluable color information (though making the cadence in a single
color lower than once per three-five nights). Witt et al. (2022) demonstrated
that the quality of the LSST lightcurves is truly a game changer in
periodicity searches returning a minimal contamination with false detections;
the false positive rate is significantly reduced compared to CRTS (see Figure
12).

The High Latitude Time Domain Survey of the planned Roman Space Telescope will
cover a smaller area of the sky ($\sim 19\deg^{2}$) but will go much deeper
than LSST, reaching $m=28$ in a single epoch, or $m=30$ co-added. Haiman et
al. (2023) predict that a four month survey would detect $\sim 100$ periodic
quasars. Requiring observations of 10 cycles, this allows maximum periods of
$\sim 12$ days, while rarity of shorter period SMBHBs and survey cadence limit
to $\sim 5$ days for the shortest detectable periods. The depth of a Roman
survey would uniquely allow discovery of systems powered by SMBHBs in the
$10^{4-6}{M_{\odot}}$ range, which will merge in the LISA band after
$10^{3-5}$ yrs (Haiman et al., 2023).

We also note that an X-ray Survey mission (the proposed Star-X mission, e.g.,
Zhang et al., 2022) would benefit periodicity searches, especially for self-
lensing signatures where emission coming from a small region near the SMBH can
be the most magnified.

###  4.2 Direct Orbital Tracking with High-angular Resolution Experiments

As discussed in §3.6, present and next-generation VLBI experiments and the
GRAVITY+ optical interferometer will be able to access $\mu$as scales
subtended by sub-parsec separation SMBHBs, via imaging and astrometric photo-
center tracking. While difficult, the astrometric or resolved tracking of an
SMBHB orbit over the course of a period offers what many other methods do not,
an unambiguous electromagnetic identification of a sub-parsec separation
SMBHB. This definitive detection could help us to build a detection ladder by
studying the properties of the system on all scales and wavelengths and to
possibly use this information to devise new search methods. With enough
definitively identified SMBHB systems, we may be able to start characterizing
how SMBHB-harboring galaxies differ in their properties, if at all, from the
“normal quasars.”

###  4.3 Low-frequency Gravitational Wave Experiments & Multi-messenger
Promise

As mentioned above, PTAs have already reached a milestone by finding evidence
for the low-frequency GW background in the nanohertz band, the significance of
which is expected to grow in the upcoming years leading to a definitive
detection. Upper limits on the GW background had already provided significant
constraints onto the population of binary candidates detected in time-domain
surveys (Sesana et al., 2018; Holgado et al., 2018). These multi-messenger
consistency checks on close-separation SMBHB candidates will become even more
powerful in vetting the candidate population, as the PTA sensitivity increases
and the background is measured across the PTA frequency range. Several factors
contribute to the expected PTA sensitivity increase, such as: (1) the pulsars
are monitored for longer baselines enabling us to probe lower frequencies of
the GW spectrum, (2) technological advances in the radio receivers reduce the
noise of observations, (3) more advanced statistical techniques are employed
for the analysis of the data, (4) more and higher quality pulsars are
discovered and are systematically monitored. The combination of multiple
datasets, e.g., from individual PTAs into a common International PTA dataset
can also boost the sensitivity. In addition, existing radio telescopes like
the Canadian Hydrogen Intensity Mapping Experiment (CHIME; CHIME/Pulsar
Collaboration et al. 2021) and the Five-hundred-meter Aperture Spherical
Telescope (FAST; Hobbs et al. 2019), along with future radio observatories,
like ngVLA (Burke-Spolaor et al., 2018), the Deep Synoptic Array (DSA-2000;
Hallinan et al. 2019), and the Square Kilometer Array (SKA; Stappers et al.
2018) are expected to vastly increase the number of detected and monitored
pulsars, greatly improving the sensitivity of PTAs.

Figure 13: _Left_ : Binary parameter space for which multi-messenger
observations with time-domain surveys and PTAs are possible. The blue shaded
regions show EM detectable binaries with peak-to-peak Doppler boost
variability $\geq 10\%$ for different inclinations and fixed mass ratio of
$q=1/4$. The purple shaded regions delineate the GW detectability of binaries
of the same mass ratio with current and projected PTA sensitivity. From
Charisi et al. (2022) _Right_ : Comparison of Bayes factors for GW detection
between completely uninformed all-sky searches and searches with some prior
information about the host galaxy or the binary. Adapted from Liu & Vigeland
(2021).

Given the discovery of evidence for the GW background, PTAs are also expected
to detect GWs from individual SMBHBs that produce signals strong enough to
stand above the background (Rosado et al., 2015; Mingarelli et al., 2017;
Kelley et al., 2018; Taylor et al., 2020). Since SMBHBs are expected to emit
both bright electromagnetic signals and strong GWs, they make exceptional
targets for multi-messenger observations (Sesana et al., 2012; Kelley et al.,
2019a; Baker et al., 2019; Charisi et al., 2022). Multi-messenger observations
will offer significant advantages in constraining the dynamical interaction of
binaries with their environment, unveiling the physics of accretion in the
presence of binaries, and probing the co-evolution of SMBHs with their host
galaxies, and more (Kelley et al., 2019a). PTAs and current electromagnetic
time-domain surveys probe the overlapping populations of SMBHBs, i.e. binaries
with similar periods and masses (see Figure 1, and Charisi et al. 2022). In
Figure 13, we show the parameter space of SMBHBs, where multi-messenger
observations are possible given current and future PTA capabilities. Combining
electromagnetic and PTA data has great advantages; incorporating binary
orbital constraints from electromagnetic observations (e.g., period/mass of a
candidate) can improve GW upper limits by a factor of two (Arzoumanian et al.,
2020).

Similarly, including priors from electromagnetic data in PTA searches can
boost the detectability of binaries and improve parameter estimation (Liu &
Vigeland, 2021; Liu et al., 2023; Charisi et al., 2023). In the right panel of
Figure 13, we show the Bayes factor for GW detection for simulated binaries
injected in a PTA that resembles the NANOGrav 11yr dataset. The detectability
of the same binary increases when prior information e.g., known from an
electromagnetic candidate is included in the search. In the case of the
Doppler boost (and self-lensing) variability, the PTA signal can be directly
linked to the periodicity of a quasar candidate for a multi-messenger data
stream (Charisi et al., 2022), potentially allowing for further boost in the
detectability of the source and its parameter estimation.

In the LISA-era of the next decade, electromagnetic counterparts due to a
plethora of sources have been and continue to be explored, including
circumbinary disk and mini-disk interactions, Doppler boosting and jet
launching in the moments before and through merger (Palenzuela et al., 2010;
Noble et al., 2012; Moesta et al., 2012; Gold et al., 2014a, b; Tang et al.,
2018; Farris et al., 2015a; Kelly et al., 2017; Haiman, 2017; Khan et al.,
2018; d’Ascoli et al., 2018; Kelly et al., 2021; Gutiérrez et al., 2022;
Bright & Paschalidis, 2023; Wang et al., 2023c) as well as post-merger GW
recoil induced shocks (Lippai et al., 2008; Corrales et al., 2010; Rossi et
al., 2010; Meliani et al., 2017; Krauth et al., 2023). If one of these
electromagnetic signatures arises, we can only observe it if we know in
advance which direction on the sky to point telescopes. Mangiagli et al.
(2020) considers LISA’s ability to localize a GW source on the sky in time to
follow up with electromagnetic observations and follow up work (Mangiagli et
al., 2022) considers optical, radio, and X-ray counterparts to SMBHB mergers
and determines detectability with joint LSST plus LISA, the Square Kilometer
Array (SKA Dewdney et al., 2009) plus the European Extremely Large Telescope
(E-ELT, 2023) plus LISA, or Athena (Nandra et al., 2013) plus E-ELT plus LISA
configurations. They determine that a few such electromagnetic counterparts to
SMBHB mergers may be detectable during a four year LISA mission. Further
possibilities with X-ray and GW joint detections with a LISA and Athena are
detailed in Piro et al. (2023).

##  5 Summary of observational searches and candidates

Below we summarize the observational efforts to uncover SMBHB candidates based
on the signatures described in this Chapter.

###  5.1 Searches and candidates from spectroscopic signatures

####  5.1.1 Systematic Searches for Broad Emission Line Shifts in Optical
Spectra from SDSS

  * •

Tsalmantza et al. (2011): Search for broad lines offset with respect to narrow
lines in $\sim$59,000 quasars with $0.1<z<1.5$ revealed 34 sources with
peculiar spectra, of which 9 are binary candidates (4 new, and 5 previously
known).

  * •

Eracleous et al. (2012): Search for displaced broad $H\beta$ lines in 15,900
quasars with $z<0.7$ found 88 candidates (some previously known). Of those, 68
candidates were followed-up revealing significant velocity shifts in 14.

  * •

Ju et al. (2013): Search for velocity shifts in broad Mg II lines in $>1500$
quasars with $0.36<z<2$ and multiple spectra detected 7 (64) strong (possible)
candidates. They inferred that $\leq 18\%$ of quasars can host binaries.

  * •

Shen et al. (2013): Search for broad $H\beta$ line shifts in 700 quasars with
$z<0.7$ and multiple spectra revealed 28 sources, of which 7 are top binary
candidates. If variability is due to binaries and not intrinsic, most quasars
should harbor SMBHBs.

  * •

Liu et al. (2014b): Search for displaced $H\beta$ broad lines in $\sim$ 21,000
quasars found 399 candidates, of which 50 were followed-up revealing 24
sources with significant radial accelerations and 9 were considered top binary
candidates. Unlike previous studies e.g., Eracleous et al. (2012), moderate
velocity offsets ($<$1000km/s) were also considered.

####  5.1.2 Spectroscopic Follow-up of Candidates Identified in Systematic
Searches

  * •

Decarli et al. (2013): Follow-up of the Tsalmantza et al. (2011) sample found
significant line variations in 11 sources. It is unclear whether the
variability is coherent as expected for binaries or transient and longer
monitoring is required.

  * •

Runnoe et al. (2015, 2017): Follow-up of the Eracleous et al. (2012) sample
over 12 years with 3-4 observations per source. Radial velocity curves can be
constructed for 29 sources with 3 candidates showing monotonic velocity
changes. Limits on binary parameters cannot exclude any of the candidates, but
this will change with longer baselines.

  * •

Wang et al. (2017): Follow-up of 21 candidates of the Ju et al. (2013) sample
found that none is consistent with the binary scenario. The search was
repeated in a sample $\sim$1400 quasars with multi-epoch spectra spanning 10
years from BOSS and revealed one candidate. They concluded that $\leq 1\%$ of
the quasars host binaries with separation of order $\sim 0.1$ pc.

  * •

Guo et al. (2019): Follow-up of 52 candidates in Shen et al. (2013); Liu et
al. (2014b) showed that 5 sources are viable candidates based on their radial
velocity curves, concluding that $\sim$13% of quasars are associated with sub-
parsec binaries.

####  5.1.3 Individual Candidates Primarily from Early SDSS Studies

  * •

J0927+2943: Highly blue-shifted Balmer lines could indicate a recoiling SMBH
(Komossa et al., 2008) or an SMBHB (Dotti et al., 2009; Bogdanović et al.,
2009), but both hypotheses were later disfavored by Decarli et al. (2014).

  * •

J1536+0441: Candidate identified by atypical spectrum with two broad emission
line systems (Boroson & Lauer, 2009).

  * •

J1050+3456: Candidate with blue-shifted Balmer lines, but could be a double-
peaked emitter (Shields et al., 2009).

  * •

J1000+2233: A binary was invoked to explain extreme velocity offsets in
$H\alpha$ and $H\beta$ lines (Decarli et al., 2010).

  * •

J0932+0318: Similar to J1050+3456, double-peaked emission is a possible
alternative to binarity (Barrows et al., 2011).

  * •

NGC4151: Lightcurve and radial velocity curve could be explained by an
eccentric binary candidate (Bon et al., 2012).

  * •

Mrk 231: The observed flux deficit in the UV/optical spectrum was associated
with a cavity in a circumbinary disk (Yan et al., 2015), but dust reddening
can better explain the entire spectrum (Leighly et al., 2016).

####  5.1.4 Candidates with Double-peaked Spectra and Follow-ups

  * •

Quasars with double-peaked broad emission lines, like QSO OX 169, Arp 102B, 3C
390.3, 3C 332 were proposed as binary candidates (Gaskell, 1983; Stockton &
Farnham, 1991; Gaskell, 1996).

  * •

Subsequent follow-up monitoring of the above and many other similar sources
disfavored the SMBHB interpretation for this class of quasars, known as
double-peaked emitters. The line variability is inconsistent with expectations
for binaries and they can be more naturally explained by disks around single-
SMBHs (Halpern & Filippenko, 1988; Eracleous & Halpern, 1994; Eracleous et
al., 1997; Eracleous & Halpern, 2003; Eracleous et al., 2009; Liu et al.,
2016; Doan et al., 2020).

###  5.2 Searches and Candidates from Photometric Signatures

####  5.2.1 Systematic Searches in Time-domain Surveys

  * •

Graham et al. (2015b): Search in a sample of 243,500 spectroscopically
confirmed in CRTS revealed 111 candidates with periods of 650-2500 d. The
lightcurves of the candidates were extended with data from LINEAR when
available.

  * •

Charisi et al. (2016): Search in a sample of 35,383 quasars in PTF selected 50
candidates with periods of 130-900 d, 33 of which remain statistically
significant after the re-analysis of extended lightcurves with data from CRTS
and iPTF.

  * •

Liu et al. (2019): Search in a sample of 9000 quasars from Pan-STARRS MDS
selected 26 candidates with periods of 300-1000 d, but follow-up monitoring
with the Discovery Channel Telescope and LCO excluded all but one candidate.

  * •

Chen et al. (2020): Search in a sample of 625 quasars in overlapping DES+SDSS
stripe 82 fields with long baseline lightcurves (20yr) detected 5 candidates
with periods of 3-5 yr.

  * •

Chen et al. (2022b): Search in 143,700 quasars from ZTF detected 127
candidates with periods of 500-1000 d.

  * •

Serafinelli et al. (2020); Liu et al. (2020) Two independent searches in
Swift/BAT 105-month survey. The former analyzed 553 quasars, found a new
candidate, and validated a known one (MCG+11-11-032). The latter analyzed 941
quasars and found no evidence for periodicity including the previously known
candidate.

  * •

Sandrinelli et al. (2018); Holgado et al. (2018): These two studies (and
references therein) summarize the binary candidates identified from their
quasi-periodic behaviour in Fermi gamma-ray lightcurves.

####  5.2.2 Periodic Candidates Identified Individually

  * •

Optical:

    * –

OJ 287: This bright blazar has a lightcurve spanning a century (albeit sparse
initially) with quasi-periodicity of 11.86yr and twin flares 1yr apart. The
periodicity is explained by an eccentric and unequal mass binary; the flares
are produced every time the secondary plunges through the primary’s accretion
disk on a relativistically precessing orbit (Valtonen et al., 2008; Komossa et
al., 2021). The most recently expected flare, however, was not confirmed by
observations (Komossa et al., 2023a; Valtonen et al., 2023).

    * –

PG 1302-102: The first candidate found in the recent systematic searches
(Graham et al., 2015b). Because of its brightness it allowed for extensive
follow-up studies, which potentially reveal additional evidence for its binary
nature.

    * –

SDSS J0159+0105: This candidate was detected in CRTS, although it was not
selected by Graham et al. (2015b). It has two periods (741 and 1500 days) and
was interpreted as having the predicted by simulations 2:1 period ratio. The
broad H$\beta$ line exhibits variability and it has unusually bright UV
spectrum (Zheng et al., 2016).

    * –

Q J0158-4325: Periodicity of $\sim 176$ d was detected in this microlensed
quasar (Millon et al., 2022).

    * –

J025214.67−002813.7: One of the 5 candidates detected in DES+SDSS search with
a period of $1607$ d was associated with accretion variability models (Liao et
al., 2021).

    * –

Spikey: This candidate was identified from its symmetric flare (Hu et al.,
2020). The variability is well fit by an eccentric self-lensing model, but its
periodicity has not been confirmed yet.

  * •

Radio:

    * –

PKS 2131−021: This candidate has a periodicity of $1729$ d in the 14.5-15.5
GHz lightcurves, and was first put forth by Ren et al. (2021). The periodicity
disappears and appears almost in phase at a later epoch and was interpreted
with a jet-Doppler model by O’Neill et al. (2022).

  * •

Gamma Rays:

    * –

PG 1553+113: The first gamma-ray variable blazar with a detected periodicity
of $2.18$ y in rest frame ($z=1.285$) (Ackermann et al., 2015; Sandrinelli et
al., 2018) has prompted many multi-wavelength follow-up studies.

    * –

PKS 2247−131: A 34.5 period was attributed to jet helicity due to an orbiting
SMBHB (Zhou et al., 2018).

    * –

PKS 2155−304: A periodicity of $620$ d was detected in this candidate
(Sandrinelli et al., 2018).

    * –

BL Lac: Candidate identified from periodicity of $1.86$ yr Sandrinelli et al.
(2018).

###  5.3 Candidates from Jet Signatures

####  5.3.1 Individual Candidates with Helical or Periodic Jet Structures

  * •

S5 1928+738: The helical structure and periodic components of the jet can be
explained by the orbital motion of the jet-launching SMBH in a binary system
(Roos et al., 1993; Kun et al., 2014).

  * •

Mkn 501: A binary model could explain the complex structure of the curved
helical jet (Villata & Raiteri, 1999).

  * •

3C 273: The rapid jet precession may be due to precession of in the inner
accretion disk induced by a second SMBH in a non-co-planar orbit (Abraham &
Romero, 1999; Romero et al., 2000).

  * •

PKS 0420-014: A binary model can explain the motion of jet components on
helical trajectories (Britzen et al., 2001).

  * •

BL Lacertae (2200+420): A straight jet precessing with a short period could be
caused by an SMBHB (Stirling et al., 2003), in which the jet is associated
with the secondary (less massive) SMBH (Caproni et al., 2013).

  * •

3C 120: A binary was invoked to explain the helical jet structure and optical
periodicity (Caproni & Abraham, 2004a).

  * •

AGN 1156+295: The oscillatory pattern in the jet could be explained by a
binary candidate (Hong et al., 2004), but follow-up observations are more
consistent with a Kelvin Helmholtz instability (Zhao et al., 2011).

  * •

3C 345: The position and velocity of multiple jet components suggest a
wobbling jet with a short precession period consistent with an SMBHB (Caproni
& Abraham, 2004b; Lobanov & Roland, 2005; Qian et al., 2009).

  * •

PKS 1830-211: The helical jet may be caused by rapid precession due to an
SMBHB (Nair et al., 2005).

  * •

S5 1803+784: A binary was proposed to explain the evolution of the VLBI
coordinates of a single jet component (Roland et al., 2008), but none of the
existing models can explain all the jet properties (Britzen et al., 2010).

  * •

B1308+326: The kinematics of multiple jet components can be explained by jet
precession due to an SMBHB, but other explanations (e.g., spin precession of a
single SMBH) are also possible (Qian et al., 2017).

  * •

3C 279: The complex jet structure of this candidate is consistent with two
precessing jets with the same period but different orientations (Qian et al.,
2019), but detailed observations by EHT challenge this model (Kim et al.,
2020).

  * •

3C 454.3: The kinematic behavior of multiple jet components suggests the
existence of two groups of knots produced by a double-jet in a binary system
(Qian et al., 2021).

  * •

4C31.61 (2201+315): The jet structure can be explained by a system of three
distinct jets, associated with three SMBHs separated by $\sim 1-2$ pc (Roland
et al., 2020).

  * •

Britzen et al. (2023) apply jet precession models to OJ 287 as well as 11
other quasars listed in their Table 6 (3C 279, 3C 273, PKS 0735+178, 2200+420,
PG 1553+113, 3C 345, 3C 120, 1308+326, TXS 0506+056, 3C 84, PKS 1502+106).

####  5.3.2 Follow-up of Candidates Selected with Other Methods

  * •

Spikey: VLBI images show a wiggly jet structure consistent with the binary
scenario (Kun et al., 2020).

  * •

PG1302: The morphology of the jet at kilo-parsec scale along with parsec scale
kinematics support the binary scenario for this candidate (Kun et al., 2015;
Mohan et al., 2016).

  * •

OJ287: The highly variable jet of this source can be fit by a helical model
produced by the binary orbital motion Valtonen & Pihajoki (2013).

  * •

PG1553: The kinematics of several jet components suggest a precessing jet
consistent with the periodicity detected in the gamma-ray lightcurve (Caproni
et al., 2017).

####  5.3.3 X and Z-shaped Jets

  * •

Yang et al. (2019) present a catalog of 290 “winged” or X-shaped radio
galaxies.

## Acknowledgments

The authors would like to thank Jessie Runnoe, Aaron Stemo for comments on an
early version of the manuscript. D.J.D. received funding from the European
Union’s Horizon 2020 research and innovation programme under Marie Sklodowska-
Curie grant agreement No. 101029157, and from the Danish Independent Research
Fund through Sapere Aude Starting Grant No. 121587. MC acknowledges support
from NSF award AST-2007993.

## References

  * Abraham & Romero (1999) Abraham, Z., & Romero, G. E. 1999, A&A, 344, 61
  * Ackermann et al. (2015) Ackermann, M., Ajello, M., Albert, A., et al. 2015, ApJL, 813, L41
  * Agazie et al. (2023a) Agazie, G., Anumarlapudi, A., Archibald, A. M., et al. 2023a, ApJL, 951, L50
  * Agazie et al. (2023b) —. 2023b, ApJL, 952, L37
  * Agazie et al. (2023c) —. 2023c, ApJL, 951, L8
  * Agazie et al. (2023d) —. 2023d, ApJL, 956, L3
  * Amaro-Seoane et al. (2017) Amaro-Seoane, P., Audley, H., Babak, S., et al. 2017, arXiv e-prints, arXiv:1702.00786
  * Amaro-Seoane et al. (2023) Amaro-Seoane, P., Andrews, J., Arca Sedda, M., et al. 2023, Living Reviews in Relativity, 26, 2
  * Antoniadis et al. (2023a) Antoniadis, J., Arumugam, P., Arumugam, S., et al. 2023a, arXiv e-prints, arXiv:2306.16226
  * Antoniadis et al. (2023b) —. 2023b, arXiv e-prints, arXiv:2306.16227
  * Arcavi et al. (2014) Arcavi, I., Gal-Yam, A., Sullivan, M., et al. 2014, ApJ, 793, 38
  * Armano et al. (2018) Armano, M., Audley, H., Baird, J., et al. 2018, Phys. Rev. Lett., 120, 061101
  * Armstrong (2006) Armstrong, J. W. 2006, Living Reviews in Relativity, 9, 1
  * Arzoumanian et al. (2020) Arzoumanian, Z., Baker, P. T., Brazier, A., et al. 2020, ApJ, 900, 102
  * Arzoumanian et al. (2021) —. 2021, ApJ, 914, 121
  * Arzoumanian et al. (2023) Arzoumanian, Z., Baker, P. T., Blecha, L., et al. 2023, ApJL, 951, L28
  * Atwood et al. (2009) Atwood, W. B., Abdo, A. A., Ackermann, M., et al. 2009, ApJ, 697, 1071
  * Azadi et al. (2017) Azadi, M., Coil, A. L., Aird, J., et al. 2017, ApJ, 835, 27
  * Bailes et al. (2020) Bailes, M., Jameson, A., Abbate, F., et al. 2020, PASA, 37, e028
  * Baker et al. (2019) Baker, J., Haiman, Z., Rossi, E. M., et al. 2019, BAAS, 51, 123
  * Bansal et al. (2017) Bansal, K., Taylor, G. B., Peck, A. B., Zavala, R. T., & Romani, R. W. 2017, ApJ, 843, 14
  * Barausse et al. (2020) Barausse, E., Dvorkin, I., Tremmel, M., Volonteri, M., & Bonetti, M. 2020, ApJ, 904, 16
  * Barnes (2002) Barnes, J. E. 2002, MNRAS, 333, 481
  * Barnes & Hernquist (1996) Barnes, J. E., & Hernquist, L. 1996, ApJ, 471, 115
  * Barrows et al. (2011) Barrows, R. S., Lacy, C. H. S., Kennefick, D., Kennefick, J., & Seigar, M. S. 2011, New A, 16, 122
  * Begelman et al. (1980) Begelman, M. C., Blandford, R. D., & Rees, M. J. 1980, Nature, 287, 307
  * Blundell et al. (2018) Blundell, K. M., Laing, R., Lee, S., & Richards, A. 2018, ApJL, 867, L25
  * Bogdanović et al. (2009) Bogdanović, T., Eracleous, M., & Sigurdsson, S. 2009, ApJ, 697, 288
  * Bogdanović et al. (2022) Bogdanović, T., Miller, M. C., & Blecha, L. 2022, Living Reviews in Relativity, 25, 3
  * Bogdanović et al. (2008) Bogdanović, T., Smith, B. D., Sigurdsson, S., & Eracleous, M. 2008, ApJS, 174, 455
  * Bon et al. (2012) Bon, E., Jovanović, P., Marziani, P., et al. 2012, ApJ, 759, 118
  * Bonetti et al. (2019) Bonetti, M., Sesana, A., Haardt, F., Barausse, E., & Colpi, M. 2019, MNRAS, 486, 4044
  * Boroson & Lauer (2009) Boroson, T. A., & Lauer, T. R. 2009, Nature, 458, 53
  * Bortolas et al. (2021) Bortolas, E., Franchini, A., Bonetti, M., & Sesana, A. 2021, ApJL, 918, L15
  * Bortolas et al. (2018) Bortolas, E., Mapelli, M., & Spera, M. 2018, MNRAS, 474, 1054
  * Bright & Paschalidis (2023) Bright, J. C., & Paschalidis, V. 2023, MNRAS, 520, 392
  * Britzen et al. (2001) Britzen, S., Roland, J., Laskar, J., et al. 2001, A&A, 374, 784
  * Britzen et al. (2023) Britzen, S., Zajaček, M., Gopal-Krishna, et al. 2023, ApJ, 951, 106
  * Britzen et al. (2010) Britzen, S., Kudryavtseva, N. A., Witzel, A., et al. 2010, A&A, 511, A57
  * Burke-Spolaor et al. (2018) Burke-Spolaor, S., Blecha, L., Bogdanović, T., et al. 2018, in Astronomical Society of the Pacific Conference Series, Vol. 517, Science with a Next Generation Very Large Array, ed. E. Murphy, 677
  * Burke-Spolaor et al. (2019) Burke-Spolaor, S., Taylor, S. R., Charisi, M., et al. 2019, A&A Rev., 27, 5
  * Caproni & Abraham (2004a) Caproni, A., & Abraham, Z. 2004a, MNRAS, 349, 1218
  * Caproni & Abraham (2004b) —. 2004b, ApJ, 602, 625
  * Caproni et al. (2013) Caproni, A., Abraham, Z., & Monteiro, H. 2013, MNRAS, 428, 280
  * Caproni et al. (2017) Caproni, A., Abraham, Z., Motter, J. C., & Monteiro, H. 2017, ApJL, 851, L39
  * Casey-Clyde et al. (2022) Casey-Clyde, J. A., Mingarelli, C. M. F., Greene, J. E., et al. 2022, ApJ, 924, 93
  * Charisi et al. (2016) Charisi, M., Bartos, I., Haiman, Z., et al. 2016, MNRAS, 463, 2145
  * Charisi et al. (2015) Charisi, M., Bartos, I., Haiman, Z., Price-Whelan, A. M., & Márka, S. 2015, MNRAS, 454, L21
  * Charisi et al. (2018) Charisi, M., Haiman, Z., Schiminovich, D., & D’Orazio, D. J. 2018, MNRAS, 476, 4617
  * Charisi et al. (2022) Charisi, M., Taylor, S. R., Runnoe, J., Bogdanovic, T., & Trump, J. R. 2022, MNRAS, 510, 5929
  * Charisi et al. (2023) Charisi, M., et al. 2023, in prep, doi:in prep
  * Chen et al. (2011) Chen, X., Sesana, A., Madau, P., & Liu, F. K. 2011, ApJ, 729, 13
  * Chen et al. (2022a) Chen, Y.-C., Hwang, H.-C., Shen, Y., et al. 2022a, ApJ, 925, 162
  * Chen et al. (2020) Chen, Y.-C., Liu, X., Liao, W.-T., et al. 2020, MNRAS, 499, 2245
  * Chen et al. (2022b) Chen, Y.-J., Zhai, S., Liu, J.-R., et al. 2022b, arXiv e-prints, arXiv:2206.11497
  * CHIME/Pulsar Collaboration et al. (2021) CHIME/Pulsar Collaboration, Amiri, M., Bandura, K. M., et al. 2021, ApJS, 255, 5
  * Christian & Loeb (2017) Christian, P., & Loeb, A. 2017, MNRAS, 469, 930
  * Colpi (2014) Colpi, M. 2014, SSR, 183, 189
  * Comerford et al. (2013) Comerford, J. M., Schluns, K., Greene, J. E., & Cool, R. J. 2013, ApJ, 777, 64
  * Corbin & Cornish (2010) Corbin, V., & Cornish, N. J. 2010, arXiv e-prints, arXiv:1008.1782
  * Corrales et al. (2010) Corrales, L. R., Haiman, Z., & MacFadyen, A. 2010, MNRAS, 404, 947
  * Coughlin & Armitage (2017) Coughlin, E. R., & Armitage, P. J. 2017, MNRAS, 471, L115
  * Coughlin & Armitage (2018) —. 2018, MNRAS, 474, 3857
  * Coughlin et al. (2019) Coughlin, E. R., Armitage, P. J., Lodato, G., & Nixon, C. J. 2019, SSR, 215, 45
  * Coughlin et al. (2017) Coughlin, E. R., Armitage, P. J., Nixon, C., & Begelman, M. C. 2017, MNRAS, 465, 3840
  * Coughlin et al. (2018) Coughlin, E. R., Darbha, S., Kasen, D., & Quataert, E. 2018, ApJL, 863, L24
  * Darbha et al. (2018) Darbha, S., Coughlin, E. R., Kasen, D., & Quataert, E. 2018, MNRAS, 477, 4009
  * d’Ascoli et al. (2018) d’Ascoli, S., Noble, S. C., Bowen, D. B., et al. 2018, ApJ, 865, 140
  * Davelaar & Haiman (2022a) Davelaar, J., & Haiman, Z. 2022a, PRD, 105, 103010
  * Davelaar & Haiman (2022b) —. 2022b, Phys. Rev. Lett., 128, 191101
  * Dayal et al. (2019) Dayal, P., Rossi, E. M., Shiralilou, B., et al. 2019, MNRAS, 486, 2336
  * De Rosa et al. (2019) De Rosa, A., Vignali, C., Bogdanović, T., et al. 2019, New A Rev., 86, 101525
  * Decarli et al. (2013) Decarli, R., Dotti, M., Fumagalli, M., et al. 2013, MNRAS, 433, 1492
  * Decarli et al. (2014) Decarli, R., Dotti, M., Mazzucchelli, C., Montuori, C., & Volonteri, M. 2014, MNRAS, 445, 1558
  * Decarli et al. (2010) Decarli, R., Dotti, M., Montuori, C., Liimets, T., & Ederoclite, A. 2010, ApJL, 720, L93
  * Dewdney et al. (2009) Dewdney, P. E., Hall, P. J., Schilizzi, R. T., & Lazio, T. J. L. W. 2009, IEEE Proceedings, 97, 1482
  * Dexter et al. (2020) Dexter, J., Lutz, D., Shimizu, T. T., et al. 2020, ApJ, 905, 33
  * Dittmann & Ryan (2022) Dittmann, A. J., & Ryan, G. 2022, MNRAS, 513, 6158
  * Dittmann & Ryan (2023) —. 2023, arXiv e-prints, arXiv:2310.07758
  * Djorgovski et al. (2011) Djorgovski, S. G., et al. 2011, ArXiv e-prints, arXiv:1102.5004
  * Doan et al. (2020) Doan, A., Eracleous, M., Runnoe, J. C., et al. 2020, MNRAS, 491, 1104
  * D’Orazio & Di Stefano (2018) D’Orazio, D. J., & Di Stefano, R. 2018, MNRAS, 474, 2975
  * D’Orazio & Di Stefano (2020) —. 2020, MNRAS, 491, 1506
  * D’Orazio & Haiman (2017) D’Orazio, D. J., & Haiman, Z. 2017, MNRAS, 470, 1198
  * D’Orazio et al. (2015a) D’Orazio, D. J., Haiman, Z., Duffell, P., Farris, B. D., & MacFadyen, A. I. 2015a, MNRAS, 452, 2540
  * D’Orazio et al. (2016) D’Orazio, D. J., Haiman, Z., Duffell, P., MacFadyen, A., & Farris, B. 2016, Monthly Notices of the Royal Astronomical Society, 459, 2379
  * D’Orazio et al. (2013) D’Orazio, D. J., Haiman, Z., & MacFadyen, A. 2013, MNRAS, 436, 2997
  * D’Orazio et al. (2015b) D’Orazio, D. J., Haiman, Z., & Schiminovich, D. 2015b, Nature, 525, 351
  * D’Orazio & Loeb (2018) D’Orazio, D. J., & Loeb, A. 2018, ApJ, 863, 185
  * D’Orazio & Loeb (2019) —. 2019, PRD, 100, 103016
  * D’Orazio & Loeb (2020) —. 2020, PRD, 101, 083031
  * D’Orazio & Loeb (2021) —. 2021, PRD, 104, 063015
  * Dotti et al. (2022) Dotti, M., Bonetti, M., D’Orazio, D. J., Haiman, Z., & Ho, L. C. 2022, MNRAS, 509, 212
  * Dotti et al. (2009) Dotti, M., Montuori, C., Decarli, R., et al. 2009, MNRAS, 398, L73
  * Doğan et al. (2015) Doğan, S., Nixon, C., King, A., & Price, D. J. 2015, MNRAS, 449, 1251
  * Duffell et al. (2020) Duffell, P. C., D’Orazio, D., Derdzinski, A., et al. 2020, ApJ, 901, 25
  * Dunhill et al. (2015) Dunhill, A. C., Cuadra, J., & Dougados, C. 2015, MNRAS, 448, 3545
  * E-ELT (2023) E-ELT. 2023, https://www.eso.org/sci/facilities/eelt/, accessed: 2023-04-06
  * Ebisuzaki et al. (1991) Ebisuzaki, T., Makino, J., & Okumura, S. K. 1991, Nature, 354, 212
  * EPTA Collaboration et al. (2023) EPTA Collaboration, InPTA Collaboration, Antoniadis, J., et al. 2023, A&A, 678, A50
  * Eracleous et al. (2012) Eracleous, M., Boroson, T. A., Halpern, J. P., & Liu, J. 2012, ApJS, 201, 23
  * Eracleous & Halpern (1994) Eracleous, M., & Halpern, J. P. 1994, ApJS, 90, 1
  * Eracleous & Halpern (2003) —. 2003, ApJ, 599, 886
  * Eracleous et al. (1997) Eracleous, M., Halpern, J. P., M. Gilbert, A., Newman, J. A., & Filippenko, A. V. 1997, ApJ, 490, 216
  * Eracleous et al. (2009) Eracleous, M., Lewis, K. T., & Flohic, H. M. L. G. 2009, New A Rev., 53, 133
  * Event Horizon Telescope Collaboration et al. (2019) Event Horizon Telescope Collaboration, Akiyama, K., Alberdi, A., et al. 2019, ApJL, 875, L1
  * Event Horizon Telescope Collaboration et al. (2022) —. 2022, ApJL, 930, L12
  * Fabian et al. (2000) Fabian, A. C., Iwasawa, K., Reynolds, C. S., & Young, A. J. 2000, PASP, 112, 1145
  * Falxa et al. (2023) Falxa, M., Babak, S., Baker, P. T., et al. 2023, MNRAS, arXiv:2303.10767
  * Farris et al. (2014) Farris, B. D., Duffell, P., MacFadyen, A. I., & Haiman, Z. 2014, ApJ, 783, 134
  * Farris et al. (2015a) —. 2015a, MNRAS, 447, L80
  * Farris et al. (2015b) —. 2015b, MNRAS, 446, L36
  * Fendt & Yardimci (2022) Fendt, C., & Yardimci, M. 2022, ApJ, 933, 71
  * Foord et al. (2022) Foord, A., Liu, X., Gültekin, K., et al. 2022, ApJ, 927, 3
  * Foord et al. (2019) Foord, A., Gültekin, K., Reynolds, M. T., et al. 2019, ApJ, 877, 17
  * French et al. (2016) French, K. D., Arcavi, I., & Zabludoff, A. 2016, ApJL, 818, L21
  * Fu et al. (2011) Fu, H., Zhang, Z.-Y., Assef, R. J., et al. 2011, ApJL, 740, L44
  * Gardiner et al. (2023) Gardiner, E. C., Kelley, L. Z., Lemke, A.-M., & Mitridate, A. 2023, arXiv e-prints, arXiv:2309.07227
  * Gardner et al. (2006) Gardner, J. P., Mather, J. C., Clampin, M., et al. 2006, SSR, 123, 485
  * Gaskell (1983) Gaskell, C. M. 1983, in Liege International Astrophysical Colloquia, Vol. 24, Liege International Astrophysical Colloquia, ed. J.-P. Swings, 473–477
  * Gaskell (1996) Gaskell, C. M. 1996, ApJL, 464, L107
  * Generozov & Haiman (2014) Generozov, A., & Haiman, Z. 2014, MNRAS, 443, L64
  * Gold et al. (2014a) Gold, R., Paschalidis, V., Etienne, Z. B., Shapiro, S. L., & Pfeiffer, H. P. 2014a, PRD, 89, 064060
  * Gold et al. (2014b) Gold, R., Paschalidis, V., Ruiz, M., et al. 2014b, PRD, 90, 104030
  * Goldstein et al. (2019) Goldstein, J. M., Sesana, A., Holgado, A. M., & Veitch, J. 2019, MNRAS, 485, 248
  * Goldstein et al. (2018) Goldstein, J. M., Veitch, J., Sesana, A., & Vecchio, A. 2018, MNRAS, 477, 5447
  * Gopal-Krishna et al. (2003) Gopal-Krishna, Biermann, P. L., & Wiita, P. J. 2003, ApJL, 594, L103
  * Goulding et al. (2019) Goulding, A. D., Pardo, K., Greene, J. E., et al. 2019, ApJL, 879, L21
  * Goulding et al. (2018) Goulding, A. D., Greene, J. E., Bezanson, R., et al. 2018, PASJ, 70, S37
  * Gower et al. (1982) Gower, A. C., Gregory, P. C., Unruh, W. G., & Hutchings, J. B. 1982, ApJ, 262, 478
  * Graham et al. (2015a) Graham, M. J., Djorgovski, S. G., Stern, D., et al. 2015a, Nature, 518, 74
  * Graham et al. (2015b) —. 2015b, MNRAS, 453, 1562
  * Gravity+ Collaboration et al. (2022) Gravity+ Collaboration, Abuter, R., Alarcon, P., et al. 2022, The Messenger, 189, 17
  * Grupe et al. (2015) Grupe, D., Komossa, S., & Saxton, R. 2015, ApJL, 803, L28
  * Gültekin & Miller (2012) Gültekin, K., & Miller, J. M. 2012, ApJ, 761, 90
  * Guo et al. (2019) Guo, H., Liu, X., Shen, Y., et al. 2019, MNRAS, 482, 3288
  * Guo et al. (2020) Guo, H., Liu, X., Zafar, T., & Liao, W.-T. 2020, MNRAS, 492, 2910
  * Gutiérrez et al. (2022) Gutiérrez, E. M., Combi, L., Noble, S. C., et al. 2022, ApJ, 928, 137
  * Haiman (2017) Haiman, Z. 2017, PRD, 96, 023004
  * Haiman et al. (2009) Haiman, Z., Kocsis, B., & Menou, K. 2009, ApJ, 700, 1952
  * Haiman et al. (2023) Haiman, Z., Xin, C., Bogdanović, T., et al. 2023, arXiv e-prints, arXiv:2306.14990
  * Hallinan et al. (2019) Hallinan, G., Ravi, V., Weinreb, S., et al. 2019, in Bulletin of the American Astronomical Society, Vol. 51, 255
  * Halpern & Filippenko (1988) Halpern, J. P., & Filippenko, A. V. 1988, Nature, 331, 46
  * Hayasaki & Loeb (2016) Hayasaki, K., & Loeb, A. 2016, Scientific Reports, 6, 35629
  * Hellings & Downs (1983) Hellings, R. W., & Downs, G. S. 1983, ApJL, 265, L39
  * Hobbs (2013) Hobbs, G. 2013, Classical and Quantum Gravity, 30, 224007
  * Hobbs et al. (2019) Hobbs, G., Dai, S., Manchester, R. N., et al. 2019, Research in Astronomy and Astrophysics, 19, 020
  * Hobbs et al. (2010) Hobbs, G., Archibald, A., Arzoumanian, Z., et al. 2010, Classical and Quantum Gravity, 27, 084013
  * Hogg (1999) Hogg, D. W. 1999, ArXiv Astrophysics e-prints, astro-ph/9905116
  * Holgado et al. (2018) Holgado, A. M., Sesana, A., Sandrinelli, A., et al. 2018, MNRAS, 481, L74
  * Hong et al. (2004) Hong, X. Y., Jiang, D. R., Gurvits, L. I., et al. 2004, A&A, 417, 887
  * Hopkins et al. (2007) Hopkins, P. F., Richards, G. T., & Hernquist, L. 2007, ApJ, 654, 731
  * Hu et al. (2020) Hu, B. X., D’Orazio, D. J., Haiman, Z., et al. 2020, MNRAS, 495, 4061
  * Hwang et al. (2020) Hwang, H.-C., Shen, Y., Zakamska, N., & Liu, X. 2020, ApJ, 888, 73
  * Ingram et al. (2021) Ingram, A., Motta, S. E., Aigrain, S., & Karastergiou, A. 2021, MNRAS, 503, 1703
  * Ivezić et al. (2019) Ivezić, Ž., Kahn, S. M., Tyson, J. A., et al. 2019, ApJ, 873, 111
  * Izquierdo-Villalba et al. (2020) Izquierdo-Villalba, D., Bonoli, S., Dotti, M., et al. 2020, MNRAS, 495, 4681
  * Ji et al. (2021a) Ji, X., Lu, Y., Ge, J., Yan, C., & Song, Z. 2021a, ApJ, 910, 101
  * Ji et al. (2021b) —. 2021b, ApJ, 910, 101
  * Joshi et al. (2018) Joshi, B. C., Arumugasamy, P., Bagchi, M., et al. 2018, Journal of Astrophysics and Astronomy, 39, 51
  * Ju et al. (2013) Ju, W., Greene, J. E., Rafikov, R. R., Bickerton, S. J., & Badenes, C. 2013, ApJ, 777, 44
  * Jun et al. (2015) Jun, H. D., Stern, D., Graham, M. J., et al. 2015, ApJL, 814, L12
  * Kaiser & McWilliams (2021) Kaiser, A. R., & McWilliams, S. T. 2021, Classical and Quantum Gravity, 38, 055009
  * Katz et al. (2020) Katz, M. L., Kelley, L. Z., Dosopoulou, F., et al. 2020, MNRAS, 491, 2301
  * Kelley et al. (2019a) Kelley, L., Charisi, M., Burke-Spolaor, S., et al. 2019a, BAAS, 51, 490
  * Kelley (2021) Kelley, L. Z. 2021, MNRAS, 500, 4065
  * Kelley et al. (2017a) Kelley, L. Z., Blecha, L., & Hernquist, L. 2017a, MNRAS, 464, 3131
  * Kelley et al. (2017b) Kelley, L. Z., Blecha, L., Hernquist, L., Sesana, A., & Taylor, S. R. 2017b, MNRAS, 471, 4508
  * Kelley et al. (2018) —. 2018, MNRAS, 477, 964
  * Kelley et al. (2021) Kelley, L. Z., D’Orazio, D. J., & Di Stefano, R. 2021, MNRAS, 508, 2524
  * Kelley et al. (2019b) Kelley, L. Z., Haiman, Z., Sesana, A., & Hernquist, L. 2019b, MNRAS, 485, 1579
  * Kelly et al. (2017) Kelly, B. J., Baker, J. G., Etienne, Z. B., Giacomazzo, B., & Schnittman, J. 2017, PRD, 96, 123003
  * Kelly et al. (2021) Kelly, B. J., Etienne, Z. B., Golomb, J., et al. 2021, PRD, 103, 063039
  * Khan et al. (2018) Khan, A., Paschalidis, V., Ruiz, M., & Shapiro, S. L. 2018, PRD, 97, 044036
  * Khan et al. (2011) Khan, F. M., Just, A., & Merritt, D. 2011, ApJ, 732, 89
  * Kim et al. (2020) Kim, J.-Y., Krichbaum, T. P., Broderick, A. E., et al. 2020, A&A, 640, A69
  * King & Pounds (2015) King, A., & Pounds, K. 2015, ARA&A, 53, 115
  * Kollmeier et al. (2017) Kollmeier, J. A., Zasowski, G., Rix, H.-W., et al. 2017, arXiv e-prints, arXiv:1711.03234
  * Komberg (1968) Komberg, B. V. 1968, Sov. Astron., 11, 727
  * Komossa et al. (2003) Komossa, S., Burwitz, V., Hasinger, G., et al. 2003, ApJL, 582, L15
  * Komossa et al. (2008) Komossa, S., Zhou, H., & Lu, H. 2008, ApJL, 678, L81
  * Komossa et al. (2021) Komossa, S., Grupe, D., Kraus, A., et al. 2021, Universe, 7, 261
  * Komossa et al. (2023a) —. 2023a, MNRAS, arXiv:2302.11646
  * Komossa et al. (2023b) Komossa, S., Kraus, A., Grupe, D., et al. 2023b, Astronomische Nachrichten, 344, e20220126
  * Kormendy & Bender (2009) Kormendy, J., & Bender, R. 2009, ApJL, 691, L142
  * Kormendy & Ho (2013) Kormendy, J., & Ho, L. C. 2013, ARA&A, 51, 511
  * Kormendy & Richstone (1995) Kormendy, J., & Richstone, D. 1995, ARA&A, 33, 581
  * Koss et al. (2011) Koss, M., Mushotzky, R., Treister, E., et al. 2011, ApJL, 735, L42
  * Koss et al. (2019) Koss, M., U, V., Hodges-Kluck, E., et al. 2019, Astro2020: Decadal Survey on Astronomy and Astrophysics, 2020, 504
  * Koss et al. (2023) Koss, M. J., Treister, E., Kakkad, D., et al. 2023, ApJL, 942, L24
  * Kozłowski et al. (2010) Kozłowski, S., Kochanek, C. S., Udalski, A., et al. 2010, ApJ, 708, 927
  * Kramer & Champion (2013) Kramer, M., & Champion, D. J. 2013, Classical and Quantum Gravity, 30, 224009
  * Krauth et al. (2023) Krauth, L. M., Davelaar, J., Haiman, Z., et al. 2023, MNRAS, arXiv:2304.02575
  * Kulkarni & Loeb (2016) Kulkarni, G., & Loeb, A. 2016, MNRAS, 456, 3964
  * Kulkarni et al. (2019) Kulkarni, G., Worseck, G., & Hennawi, J. F. 2019, MNRAS, 488, 1035
  * Kun et al. (2020) Kun, E., Frey, S., & Gabányi, K. É. 2020, MNRAS, 496, 3336
  * Kun et al. (2015) Kun, E., Frey, S., Gabányi, K. É., et al. 2015, MNRAS, 454, 1290
  * Kun et al. (2014) Kun, E., Gabányi, K. É., Karouzos, M., Britzen, S., & Gergely, L. Á. 2014, MNRAS, 445, 1370
  * Kurczynski et al. (2022) Kurczynski, P., Johnson, M. D., Doeleman, S. S., et al. 2022, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 12180, Space Telescopes and Instrumentation 2022: Optical, Infrared, and Millimeter Wave, ed. L. E. Coyle, S. Matsuura, & M. D. Perrin, 121800M
  * Lai (2014) Lai, D. 2014, MNRAS, 440, 3532
  * Lalakos et al. (2022) Lalakos, A., Gottlieb, O., Kaaz, N., et al. 2022, ApJL, 936, L5
  * Lauer (1985) Lauer, T. R. 1985, ApJ, 292, 104
  * Leahy & Williams (1984) Leahy, J. P., & Williams, A. G. 1984, MNRAS, 210, 929
  * Lee (2016) Lee, K. J. 2016, in Astronomical Society of the Pacific Conference Series, Vol. 502, Frontiers in Radio Astronomy and FAST Early Sciences Symposium 2015, ed. L. Qain & D. Li, 19
  * Leighly et al. (2016) Leighly, K. M., Terndrup, D. M., Gallagher, S. C., & Lucy, A. B. 2016, ApJ, 829, 4
  * Leloudas et al. (2016) Leloudas, G., Fraser, M., Stone, N. C., et al. 2016, Nature Astronomy, 1, 0002
  * Lezhnin & Vasiliev (2019) Lezhnin, K., & Vasiliev, E. 2019, MNRAS, 484, 2851
  * Liao et al. (2021) Liao, W.-T., Chen, Y.-C., Liu, X., et al. 2021, MNRAS, 500, 4025
  * Lico et al. (2023) Lico, R., Jorstad, S. G., Marscher, A. P., et al. 2023, Galaxies, 11, 17
  * Lippai et al. (2008) Lippai, Z., Frei, Z., & Haiman, Z. 2008, ApJL, 676, L5
  * Liska et al. (2018) Liska, M., Hesp, C., Tchekhovskoy, A., et al. 2018, MNRAS, 474, L81
  * Liu et al. (2009) Liu, F. K., Li, S., & Chen, X. 2009, ApJL, 706, L133
  * Liu et al. (2014a) Liu, F. K., Li, S., & Komossa, S. 2014a, ApJ, 786, 103
  * Liu et al. (2016) Liu, J., Eracleous, M., & Halpern, J. P. 2016, ApJ, 817, 42
  * Liu et al. (2023) Liu, T., Cohen, T., McGrath, C., Demorest, P. B., & Vigeland, S. J. 2023, ApJ, 945, 78
  * Liu et al. (2018a) Liu, T., Gezari, S., & Miller, M. C. 2018a, ApJL, 859, L12
  * Liu & Vigeland (2021) Liu, T., & Vigeland, S. J. 2021, ApJ, 921, 178
  * Liu et al. (2019) Liu, T., Gezari, S., Ayers, M., et al. 2019, ApJ, 884, 36
  * Liu et al. (2020) Liu, T., Koss, M., Blecha, L., et al. 2020, ApJ, 896, 122
  * Liu et al. (2018b) Liu, X., Guo, H., Shen, Y., Greene, J. E., & Strauss, M. A. 2018b, ApJ, 862, 29
  * Liu et al. (2014b) Liu, X., Shen, Y., Bian, F., Loeb, A., & Tremaine, S. 2014b, ApJ, 789, 140
  * Liu et al. (2010) Liu, X., Shen, Y., Strauss, M. A., & Greene, J. E. 2010, ApJ, 708, 427
  * Lobanov & Roland (2005) Lobanov, A. P., & Roland, J. 2005, A&A, 431, 831
  * Lobanov & Zensus (2001) Lobanov, A. P., & Zensus, J. A. 2001, Science, 294, 128
  * Luo et al. (2016) Luo, J., Chen, L.-S., Duan, H.-Z., et al. 2016, Classical and Quantum Gravity, 33, 035010
  * Lynden-Bell (1969) Lynden-Bell, D. 1969, Nature, 223, 690
  * MacFadyen & Milosavljević (2008) MacFadyen, A. I., & Milosavljević, M. 2008, ApJ, 672, 83
  * MacLeod et al. (2010) MacLeod, C. L., Ivezić, Ž., Kochanek, C. S., et al. 2010, ApJ, 721, 1014
  * Mainzer et al. (2014) Mainzer, A., Bauer, J., Cutri, R. M., et al. 2014, ApJ, 792, 30
  * Mangiagli et al. (2022) Mangiagli, A., Caprini, C., Volonteri, M., et al. 2022, PRD, 106, 103017
  * Mangiagli et al. (2020) Mangiagli, A., Klein, A., Bonetti, M., et al. 2020, PRD, 102, 084056
  * Marchesi et al. (2020) Marchesi, S., Gilli, R., Lanzuisi, G., et al. 2020, A&A, 642, A184
  * Margalit et al. (2018) Margalit, B., Metzger, B. D., Berger, E., et al. 2018, MNRAS, 481, 2407
  * Martin et al. (2005) Martin, D. C., Fanson, J., Schiminovich, D., et al. 2005, ApJL, 619, L1
  * Martini (2004) Martini, P. 2004, in Coevolution of Black Holes and Galaxies, ed. L. C. Ho, 169
  * Martizzi et al. (2013) Martizzi, D., Teyssier, R., & Moore, B. 2013, MNRAS, 432, 1947
  * McGrath & Creighton (2021) McGrath, C., & Creighton, J. 2021, MNRAS, 505, 4531
  * McGrath et al. (2022) McGrath, C., D’Orazio, D. J., & Creighton, J. 2022, MNRAS, 517, 1242
  * McGurk et al. (2015) McGurk, R. C., Max, C. E., Medling, A. M., Shields, G. A., & Comerford, J. M. 2015, ApJ, 811, 14
  * McKernan et al. (2013) McKernan, B., Ford, K. E. S., Kocsis, B., & Haiman, Z. 2013, MNRAS, 432, 1468
  * McLaughlin (2013) McLaughlin, M. A. 2013, Classical and Quantum Gravity, 30, 224008
  * Meliani et al. (2017) Meliani, Z., Mizuno, Y., Olivares, H., et al. 2017, A&A, 598, A38
  * Merritt (2006) Merritt, D. 2006, ApJ, 648, 976
  * Merritt & Ekers (2002) Merritt, D., & Ekers, R. D. 2002, Science, 297, 1310
  * Merritt & Milosavljević (2005) Merritt, D., & Milosavljević, M. 2005, Living Reviews in Relativity, 8
  * Merritt & Poon (2004) Merritt, D., & Poon, M. Y. 2004, ApJ, 606, 788
  * Mignon-Risse et al. (2023) Mignon-Risse, R., Varniere, P., & Casse, F. 2023, MNRAS, arXiv:2301.06566
  * Millon et al. (2022) Millon, M., Dalang, C., Lemon, C., et al. 2022, A&A, 668, A77
  * Milosavljević & Merritt (2003) Milosavljević, M., & Merritt, D. 2003, ApJ, 596, 860
  * Milosavljević et al. (2002) Milosavljević, M., Merritt, D., Rest, A., & van den Bosch, F. C. 2002, MNRAS, 331, L51
  * Mingarelli et al. (2013) Mingarelli, C. M. F., Sidery, T., Mandel, I., & Vecchio, A. 2013, PRD, 88, 062005
  * Mingarelli et al. (2017) Mingarelli, C. M. F., Lazio, T. J. W., Sesana, A., et al. 2017, Nature Astronomy, 1, 886
  * Moesta et al. (2012) Moesta, P., Alic, D., Rezzolla, L., Zanotti, O., & Palenzuela, C. 2012, ApJL, 749, L32
  * Mohan et al. (2016) Mohan, P., An, T., Frey, S., et al. 2016, MNRAS, 463, 1812
  * Moody et al. (2019) Moody, M. S. L., Shi, J.-M., & Stone, J. M. 2019, ApJ, 875, 66
  * Moore et al. (2015) Moore, C. J., Cole, R. H., & Berry, C. P. L. 2015, Classical and Quantum Gravity, 32, 015014
  * Moore et al. (2017) Moore, C. J., Mihaylov, D. P., Lasenby, A., & Gilmore, G. 2017, Phys. Rev. Lett., 119, 261102
  * Muñoz et al. (2019) Muñoz, D. J., Miranda, R., & Lai, D. 2019, ApJ, 871, 84
  * Müller-Sánchez et al. (2015) Müller-Sánchez, F., Comerford, J. M., Nevin, R., et al. 2015, ApJ, 813, 103
  * Muñoz & Lai (2016) Muñoz, D. J., & Lai, D. 2016, arXiv.org, arXiv:1604.00004
  * Nair et al. (2005) Nair, S., Jin, C., & Garrett, M. A. 2005, MNRAS, 362, 1157
  * Nandra et al. (2013) Nandra, K., Barret, D., Barcons, X., et al. 2013, arXiv e-prints, arXiv:1306.2307
  * Narayan & Yi (1995) Narayan, R., & Yi, I. 1995, ApJ, 452, 710
  * Nevin et al. (2016) Nevin, R., Comerford, J., Müller-Sánchez, F., Barrows, R., & Cooper, M. 2016, ApJ, 832, 67
  * Nguyen & Bogdanović (2016) Nguyen, K., & Bogdanović, T. 2016, ApJ, 828, 68
  * Nguyen et al. (2019) Nguyen, K., Bogdanović, T., Runnoe, J. C., et al. 2019, ApJ, 870, 16
  * Nguyen et al. (2020) —. 2020, ApJ, 894, 105
  * Noble et al. (2021) Noble, S. C., Krolik, J. H., Campanelli, M., et al. 2021, ApJ, 922, 175
  * Noble et al. (2009) Noble, S. C., Krolik, J. H., & Hawley, J. F. 2009, ApJ, 692, 411
  * Noble et al. (2012) Noble, S. C., et al. 2012, ApJ, 755, 51
  * Ogiya et al. (2020) Ogiya, G., Hahn, O., Mingarelli, C. M. F., & Volonteri, M. 2020, MNRAS, 493, 3676
  * O’Neill et al. (2022) O’Neill, S., Kiehlmann, S., Readhead, A. C. S., et al. 2022, ApJL, 926, L35
  * Osterbrock & Mathews (1986) Osterbrock, D. E., & Mathews, W. G. 1986, ARA&A, 24, 171
  * Owen et al. (1985) Owen, F. N., O’Dea, C. P., Inoue, M., & Eilek, J. A. 1985, ApJL, 294, L85
  * Palenzuela et al. (2010) Palenzuela, C., Lehner, L., & Liebling, S. L. 2010, Science, 329, 927
  * Pardo et al. (2023) Pardo, K., Chang, T.-C., Doré, O., & Wang, Y. 2023, arXiv e-prints, arXiv:2306.14968
  * Perets et al. (2007) Perets, H. B., Hopman, C., & Alexander, T. 2007, ApJ, 656, 709
  * Peterson (2014) Peterson, B. M. 2014, SSR, 183, 253
  * Pflueger et al. (2018) Pflueger, B. J., Nguyen, K., Bogdanović, T., et al. 2018, ApJ, 861, 59
  * Piro et al. (2023) Piro, L., Colpi, M., Aird, J., et al. 2023, MNRAS, 521, 2577
  * Popović et al. (2012) Popović, L. Č., Jovanović, P., Stalevski, M., et al. 2012, A&A, 538, A107
  * Predehl et al. (2021) Predehl, P., Andritschke, R., Arefiev, V., et al. 2021, A&A, 647, A1
  * Preto et al. (2011) Preto, M., Berentzen, I., Berczik, P., & Spurzem, R. 2011, ApJL, 732, L26
  * Qian et al. (2019) Qian, S. J., Britzen, S., Krichbaum, T. P., & Witzel, A. 2019, A&A, 621, A11
  * Qian et al. (2021) —. 2021, A&A, 653, A7
  * Qian et al. (2017) Qian, S. J., Britzen, S., Witzel, A., Krichbaum, T. P., & Gan, H. Q. 2017, A&A, 604, A90
  * Qian et al. (2018) Qian, S. J., Britzen, S., Witzel, A., Krichbaum, T. P., & Kun, E. 2018, A&A, 615, A123
  * Qian et al. (2009) Qian, S.-J., Witzel, A., Zensus, J. A., et al. 2009, Research in Astronomy and Astrophysics, 9, 137
  * Ragusa et al. (2020) Ragusa, E., Alexander, R., Calcino, J., Hirsh, K., & Price, D. J. 2020, MNRAS, 499, 3362
  * Ragusa et al. (2016) Ragusa, E., Lodato, G., & Price, D. J. 2016, MNRAS, 460, 1243
  * Rantala et al. (2018) Rantala, A., Johansson, P. H., Naab, T., Thomas, J., & Frigo, M. 2018, ApJ, 864, 113
  * Ravi et al. (2012) Ravi, V., Wyithe, J. S. B., Hobbs, G., et al. 2012, ApJ, 761, 84
  * Reardon et al. (2023) Reardon, D. J., Zic, A., Shannon, R. M., et al. 2023, ApJL, 951, L6
  * Rees (1978) Rees, M. J. 1978, Nature, 275, 516
  * Ren et al. (2021) Ren, G.-W., Ding, N., Zhang, X., et al. 2021, MNRAS, 506, 3791
  * Ricarte & Natarajan (2018) Ricarte, A., & Natarajan, P. 2018, MNRAS, 481, 3278
  * Ricarte et al. (2016) Ricarte, A., Natarajan, P., Dai, L., & Coppi, P. 2016, MNRAS, 458, 1712
  * Rieger (2004) Rieger, F. M. 2004, ApJL, 615, L5
  * Robinson et al. (1965) Robinson, I., Schild, A., & Schucking, E. L., eds. 1965, Quasi-stellar sources and gravitational collapse
  * Rodriguez et al. (2006) Rodriguez, C., Taylor, G. B., Zavala, R. T., et al. 2006, ApJ, 646, 49
  * Roedig et al. (2014) Roedig, C., Krolik, J. H., & Miller, M. C. 2014, ApJ, 785, 115
  * Roland et al. (2008) Roland, J., Britzen, S., Kudryavtseva, N. A., Witzel, A., & Karouzos, M. 2008, A&A, 483, 125
  * Roland et al. (2020) Roland, J., Gattano, C., Lambert, S. B., & Taris, F. 2020, A&A, 634, A101
  * Romero et al. (2000) Romero, G. E., Chajet, L., Abraham, Z., & Fan, J. H. 2000, A&A, 360, 57
  * Roming et al. (2005) Roming, P. W. A., Kennedy, T. E., Mason, K. O., et al. 2005, SSR, 120, 95
  * Roos (1988) Roos, N. 1988, ApJ, 334, 95
  * Roos et al. (1993) Roos, N., Kaastra, J. S., & Hummel, C. A. 1993, ApJ, 409, 130
  * Rosado et al. (2015) Rosado, P. A., Sesana, A., & Gair, J. 2015, MNRAS, 451, 2417
  * Rossi et al. (2010) Rossi, E. M., Lodato, G., Armitage, P. J., Pringle, J. E., & King, A. R. 2010, MNRAS, 401, 2021
  * Runnoe et al. (2015) Runnoe, J. C., Eracleous, M., Mathes, G., et al. 2015, ApJS, 221, 7
  * Runnoe et al. (2017) Runnoe, J. C., Eracleous, M., Pennell, A., et al. 2017, MNRAS, 468, 1683
  * Ryan & MacFadyen (2017) Ryan, G., & MacFadyen, A. 2017, ApJ, 835, 199
  * Ryu et al. (2018) Ryu, T., Perna, R., Haiman, Z., Ostriker, J. P., & Stone, N. C. 2018, MNRAS, 473, 3410
  * Saade et al. (2020) Saade, M. L., Stern, D., Brightman, M., et al. 2020, ApJ, 900, 148
  * Saade et al. (2023) Saade, M. L., Brightman, M., Stern, D., et al. 2023, arXiv e-prints, arXiv:2304.06144
  * Salcido et al. (2016) Salcido, J., Bower, R. G., Theuns, T., et al. 2016, MNRAS, 463, 870
  * Sandrinelli et al. (2018) Sandrinelli, A., Covino, S., Treves, A., et al. 2018, A&A, 615, A118
  * Sato-Polito & Kamionkowski (2023) Sato-Polito, G., & Kamionkowski, M. 2023, arXiv e-prints, arXiv:2305.05690
  * Schmidt (1963) Schmidt, M. 1963, Nature, 197, 1040
  * Serafinelli et al. (2020) Serafinelli, R., Severgnini, P., Braito, V., et al. 2020, ApJ, 902, 10
  * Sesana et al. (2005) Sesana, A., Haardt, F., Madau, P., & Volonteri, M. 2005, ApJ, 623, 23
  * Sesana et al. (2018) Sesana, A., Haiman, Z., Kocsis, B., & Kelley, L. Z. 2018, ApJ, 856, 42
  * Sesana et al. (2012) Sesana, A., Roedig, C., Reynolds, M. T., & Dotti, M. 2012, MNRAS, 420, 860
  * Shakura & Sunyaev (1973) Shakura, N. I., & Sunyaev, R. A. 1973, A&A, 24, 337
  * Shankar et al. (2013) Shankar, F., Weinberg, D. H., & Miralda-Escudé, J. 2013, MNRAS, 428, 421
  * Shen et al. (2008) Shen, Y., Greene, J. E., Strauss, M. A., Richards, G. T., & Schneider, D. P. 2008, ApJ, 680, 169
  * Shen et al. (2019) Shen, Y., Hwang, H.-C., Zakamska, N., & Liu, X. 2019, ApJL, 885, L4
  * Shen et al. (2013) Shen, Y., Liu, X., Loeb, A., & Tremaine, S. 2013, ApJ, 775, 49
  * Shen & Loeb (2010) Shen, Y., & Loeb, A. 2010, ApJ, 725, 249
  * Shen et al. (2015) Shen, Y., Greene, J. E., Ho, L. C., et al. 2015, ApJ, 805, 96
  * Shi & Krolik (2015) Shi, J.-M., & Krolik, J. H. 2015, ApJ, 807, 131
  * Shi et al. (2012) Shi, J.-M., Krolik, J. H., Lubow, S. H., & Hawley, J. F. 2012, ApJ, 749, 118
  * Shields et al. (2009) Shields, G. A., Rosario, D. J., Smith, K. L., et al. 2009, ApJ, 707, 936
  * Shu et al. (2020) Shu, X., Zhang, W., Li, S., et al. 2020, Nature Communications, 11, 5876
  * Sirko & Goodman (2003) Sirko, E., & Goodman, J. 2003, MNRAS, 341, 501
  * Siwek et al. (2023) Siwek, M., Weinberger, R., Muñoz, D. J., & Hernquist, L. 2023, MNRAS, 518, 5059
  * Smallwood et al. (2022) Smallwood, J. L., Lubow, S. H., & Martin, R. G. 2022, MNRAS, 514, 1249
  * Song et al. (2021) Song, Z., Ge, J., Lu, Y., Yan, C., & Ji, X. 2021, A&A, 645, A15
  * Soyuer et al. (2021) Soyuer, D., Zwick, L., D’Orazio, D. J., & Saha, P. 2021, MNRAS, 503, L73
  * Stappers et al. (2018) Stappers, B. W., Keane, E. F., Kramer, M., Possenti, A., & Stairs, I. H. 2018, Philosophical Transactions of the Royal Society of London Series A, 376, 20170293
  * Stemo et al. (2020) Stemo, A., Comerford, J. M., Barrows, R. S., et al. 2020, ApJ, 888, 78
  * Stirling et al. (2003) Stirling, A. M., Cawthorne, T. V., Stevens, J. A., et al. 2003, MNRAS, 341, 405
  * Stockton & Farnham (1991) Stockton, A., & Farnham, T. 1991, ApJ, 371, 525
  * Sudarshan et al. (2022) Sudarshan, P., Penzlin, A. B. T., Ziampras, A., Kley, W., & Nelson, R. P. 2022, A&A, 664, A157
  * Tanaka et al. (2012) Tanaka, T., Menou, K., & Haiman, Z. 2012, MNRAS, 420, 705
  * Tang et al. (2018) Tang, Y., Haiman, Z., & MacFadyen, A. 2018, MNRAS, 476, 2249
  * Taylor et al. (2019) Taylor, S., Burke-Spolaor, S., Baker, P. T., et al. 2019, BAAS, 51, 336
  * Taylor (2021) Taylor, S. R. 2021, arXiv e-prints, arXiv:2105.13270
  * Taylor et al. (2020) Taylor, S. R., van Haasteren, R., & Sesana, A. 2020, PRD, 102, 084039
  * The International Pulsar Timing Array Collaboration et al. (2023) The International Pulsar Timing Array Collaboration, Agazie, G., Antoniadis, J., et al. 2023, arXiv e-prints, arXiv:2309.00693
  * The LIGO Scientific Collaboration et al. (2021) The LIGO Scientific Collaboration, the Virgo Collaboration, the KAGRA Collaboration, et al. 2021, arXiv e-prints, arXiv:2111.03606
  * Thomas et al. (2016) Thomas, J., Ma, C.-P., McConnell, N. J., et al. 2016, Nature, 532, 340
  * Thompson et al. (2005) Thompson, T. A., Quataert, E., & Murray, N. 2005, ApJ, 630, 167
  * Thorne & Braginskii (1976) Thorne, K. S., & Braginskii, V. B. 1976, ApJL, 204, L1
  * Tiede et al. (2020) Tiede, C., Zrake, J., MacFadyen, A., & Haiman, Z. 2020, ApJ, 900, 43
  * Tiede et al. (2022) —. 2022, ApJ, 932, 24
  * Tofflemire et al. (2017) Tofflemire, B. M., Mathieu, R. D., Herczeg, G. J., Akeson, R. L., & Ciardi, D. R. 2017, ApJL, 842, L12
  * Tremmel et al. (2018) Tremmel, M., Governato, F., Volonteri, M., Quinn, T. R., & Pontzen, A. 2018, MNRAS, 475, 4967
  * Tsai et al. (2013) Tsai, C.-W., Jarrett, T. H., Stern, D., et al. 2013, ApJ, 779, 41
  * Tsalmantza et al. (2011) Tsalmantza, P., Decarli, R., Dotti, M., & Hogg, D. W. 2011, ApJ, 738, 20
  * Valtonen & Pihajoki (2013) Valtonen, M., & Pihajoki, P. 2013, A&A, 557, A28
  * Valtonen et al. (2008) Valtonen, M. J., Lehto, H. J., Nilsson, K., et al. 2008, Nature, 452, 851
  * Valtonen et al. (2023) Valtonen, M. J., Zola, S., Gopakumar, A., et al. 2023, MNRAS, 521, 6143
  * Vaughan et al. (2016) Vaughan, S., Uttley, P., Markowitz, A. G., et al. 2016, MNRAS, 461, 3145
  * Villata & Raiteri (1999) Villata, M., & Raiteri, C. M. 1999, A&A, 347, 30
  * Volonteri et al. (2020) Volonteri, M., Pfister, H., Beckmann, R. S., et al. 2020, MNRAS, 498, 2219
  * Wang et al. (2023a) Wang, H.-Y., Bai, X.-N., & Lai, D. 2023a, ApJ, 943, 175
  * Wang et al. (2023b) Wang, H.-Y., Bai, X.-N., Lai, D., & Lin, D. N. C. 2023b, MNRAS, 526, 3570
  * Wang et al. (2009) Wang, J.-M., Chen, Y.-M., Hu, C., et al. 2009, ApJL, 705, L76
  * Wang & Li (2020) Wang, J.-M., & Li, Y.-R. 2020, Research in Astronomy and Astrophysics, 20, 160
  * Wang et al. (2023c) Wang, J.-M., Songsheng, Y.-Y., Li, Y.-R., & Du, P. 2023c, MNRAS, 518, 3397
  * Wang et al. (2017) Wang, L., Greene, J. E., Ju, W., et al. 2017, ApJ, 834, 129
  * Wang et al. (2022a) Wang, L.-F., Shao, Y., Zhang, J.-F., & Zhang, X. 2022a, arXiv e-prints, arXiv:2201.00607
  * Wang et al. (2022b) Wang, Y., Zhai, Z., Alavi, A., et al. 2022b, ApJ, 928, 1
  * Westernacher-Schneider et al. (2022) Westernacher-Schneider, J. R., Zrake, J., MacFadyen, A., & Haiman, Z. 2022, PRD, 106, 103010
  * Will et al. (2023) Will, C. M., Naoz, S., Hees, A., et al. 2023, arXiv e-prints, arXiv:2307.16646
  * Witt et al. (2022) Witt, C. A., Charisi, M., Taylor, S. R., & Burke-Spolaor, S. 2022, ApJ, 936, 89
  * Wrobel & Lazio (2021) Wrobel, J. M., & Lazio, T. J. W. 2021, arXiv e-prints, arXiv:2108.07341
  * Wrobel & Lazio (2022) —. 2022, ApJ, 931, 12
  * Xin et al. (2020a) Xin, C., Charisi, M., Haiman, Z., & Schiminovich, D. 2020a, MNRAS, 495, 1403
  * Xin et al. (2020b) Xin, C., Charisi, M., Haiman, Z., et al. 2020b, MNRAS, 496, 1683
  * Xin & Haiman (2021) Xin, C., & Haiman, Z. 2021, MNRAS, 506, 2408
  * Xu et al. (2023) Xu, H., Chen, S., Guo, Y., et al. 2023, Research in Astronomy and Astrophysics, 23, 075024
  * Yan et al. (2015) Yan, C.-S., Lu, Y., Dai, X., & Yu, Q. 2015, ApJ, 809, 117
  * Yang et al. (2019) Yang, X., Joshi, R., Gopal-Krishna, et al. 2019, ApJS, 245, 17
  * Yu (2002) Yu, Q. 2002, MNRAS, 331, 935
  * Zevin et al. (2021) Zevin, M., Bavera, S. S., Berry, C. P. L., et al. 2021, ApJ, 910, 152
  * Zhang et al. (2022) Zhang, W., Basu-Zych, A., Bautz, M., et al. 2022, in AAS/High Energy Astrophysics Division, Vol. 54, AAS/High Energy Astrophysics Division, 108.45
  * Zhao et al. (2011) Zhao, W., Hong, X. Y., An, T., et al. 2011, A&A, 529, A113
  * Zheng et al. (2016) Zheng, Z.-Y., Butler, N. R., Shen, Y., et al. 2016, ApJ, 827, 56
  * Zhou et al. (2018) Zhou, J., Wang, Z., Chen, L., et al. 2018, Nature Communications, 9, 4599
  * Zhu & Thrane (2020) Zhu, X.-J., & Thrane, E. 2020, ApJ, 900, 117
  * Zrake et al. (2021) Zrake, J., Tiede, C., MacFadyen, A., & Haiman, Z. 2021, ApJL, 909, L13