28 research outputs found
Strongly Lensed Supermassive Black Hole Binaries as Nanohertz Gravitational-Wave Sources
Supermassive black hole binary systems (SMBHBs) should be the most powerful
sources of gravitational waves (GWs) in the Universe. Once Pulsar Timing Arrays
(PTAs) detect the stochastic GW background from their cosmic merger history,
searching for individually resolvable binaries will take on new importance.
Since these individual SMBHBs are expected to be rare, here we explore how
strong gravitational lensing can act as a tool for increasing their detection
prospects by magnifying fainter sources and bringing them into view. Unlike for
electromagnetic waves, when the geometric optics limit is nearly always valid,
for GWs the wave-diffraction-interference effects can become important when the
wavelength of the GWs is larger than the Schwarzchild radius of the lens, i.e.
. For the GW frequency
range explored in this work, the geometric optics limit holds. We investigate
GW signals from SMBHBs that might be detectable with current and future PTAs
under the assumption that quasars serve as bright beacons that signal a recent
merger. Using the black hole mass function derived from quasars and a
physically motivated magnification distribution, we expect to detect a few
strongly lensed binary systems out to . Additionally, for a range
of fixed magnifications , strong lensing adds up to
30 more detectable binaries for PTAs. Finally, we investigate the
possibility of observing both time-delayed electromagnetic signals and GW
signals from these strongly lensed binary systems -- that will provide us with
unprecedented multimessenger insights into their orbital evolution.Comment: 13 pages, 7 figures (now 16 pages, 8 figures); updated method/results
& pre-publication revisions, references adde
Reliable Identification of Binary Supermassive Black Holes from Rubin Observatory Time-Domain Monitoring
Periodic signatures in time-domain observations of quasars have been used to
search for binary supermassive black holes. These searches, across existing
time-domain surveys, have produced several hundred candidates. The general
stochastic variability of quasars, however, can masquerade as a false-positive
periodic signal, especially when monitoring cadence and duration are limited.
In this work, we predict the detectability of binary supermassive black holes
in the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST). We
apply computationally inexpensive sinusoidal curve fits to millions of
simulated LSST Deep Drilling Field light curves of both single, isolated
quasars and binary quasars. Period and phase of simulated binary signals can
generally be disentangled from quasar variability. Binary amplitude is
overestimated and poorly recovered for two-thirds of potential binaries due to
quasar accretion variability. Quasars with strong intrinsic variability can
obscure a binary signal too much for recovery. We also find that the most
luminous quasars mimic current binary candidate light curves and their
properties: false positive rates are 60\% for these quasars. The reliable
recovery of binary period and phase for a wide range of input binary LSST light
curves is promising for multi-messenger characterization of binary supermassive
black holes. However, pure electromagnetic detections of binaries using
photometric periodicity with amplitude greater than 0.1 magnitude will result
in samples that are overwhelmed by false positives. This paper represents an
important and computationally inexpensive way forward for understanding the
true and false positive rates for binary candidates identified by Rubin.Comment: 21 pages, 14 figures, 3 table
Discovery and Characterization of Galactic-scale Dual Supermassive Black Holes Across Cosmic Time
The hierarchical structure formation paradigm predicts the formation of pairs
of supermassive black holes in merging galaxies. When both (or one) members of
the SMBH pair are unobscured AGNs, the system can be identified as a dual (or
offset) AGN. Quantifying the abundance of these AGN pairs as functions of
separation, redshift and host properties is crucial to understanding SMBH
formation and AGN fueling in the broad context of galaxy formation. The High
Latitude Wide Area Survey with Roman, with its unprecedented combination of
sensitivity, spatial resolution, area and NIR wavelength coverage, will
revolutionize the study of galactic-scale environments of SMBH pairs. This
white paper summarizes the science opportunities and technical requirements on
the discovery and characterization of SMBH pairs down to galactic scales (i.e.,
less than tens of kpc) over broad ranges of redshift (1<z<7) and luminosity
(Lbol>1E42 erg/s).Comment: Roman Core Community Survey White Paper, focusing on the High
Latitude Wide Area Surve
Massive Black Hole Binaries as LISA Precursors in the Roman High Latitude Time Domain Survey
With its capacity to observe faint active galactic nuclei
(AGN) out to redshift , Roman is poised to reveal a population of
black holes during an epoch of vigorous galaxy
assembly. By measuring the light curves of a subset of these AGN and looking
for periodicity, Roman can identify several hundred massive black hole binaries
(MBHBs) with 5-12 day orbital periods, which emit copious gravitational
radiation and will inevitably merge on timescales of years. During
the last few months of their merger, such binaries are observable with the
Laser Interferometer Space Antenna (LISA), a joint ESA/NASA gravitational wave
mission set to launch in the mid-2030s. Roman can thus find LISA precursors,
provide uniquely robust constraints on the LISA source population, help
identify the host galaxies of LISA mergers, and unlock the potential of
multi-messenger astrophysics with massive black hole binaries.Comment: White Paper for the Nancy Grace Roman Space Telescope's Core
Community Surveys (https://roman.gsfc.nasa.gov/science/ccs_white_papers.html
The NANOGrav 15-year Data Set: Search for Anisotropy in the Gravitational-Wave Background
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav)
has reported evidence for the presence of an isotropic nanohertz gravitational
wave background (GWB) in its 15 yr dataset. However, if the GWB is produced by
a population of inspiraling supermassive black hole binary (SMBHB) systems,
then the background is predicted to be anisotropic, depending on the
distribution of these systems in the local Universe and the statistical
properties of the SMBHB population. In this work, we search for anisotropy in
the GWB using multiple methods and bases to describe the distribution of the
GWB power on the sky. We do not find significant evidence of anisotropy, and
place a Bayesian upper limit on the level of broadband anisotropy such
that . We also derive conservative estimates on the
anisotropy expected from a random distribution of SMBHB systems using
astrophysical simulations conditioned on the isotropic GWB inferred in the
15-yr dataset, and show that this dataset has sufficient sensitivity to probe a
large fraction of the predicted level of anisotropy. We end by highlighting the
opportunities and challenges in searching for anisotropy in pulsar timing array
data.Comment: 19 pages, 11 figures; submitted to Astrophysical Journal Letters as
part of Focus on NANOGrav's 15-year Data Set and the Gravitational Wave
Background. For questions or comments, please email [email protected]
The NANOGrav 12.5 yr Data Set: Search for Gravitational Wave Memory
We present the results of a Bayesian search for gravitational wave (GW) memory in the NANOGrav 12.5 yr data set. We find no convincing evidence for any gravitational wave memory signals in this data set. We find a Bayes factor of 2.8 in favor of a model that includes a memory signal and common spatially uncorrelated red noise (CURN) compared to a model including only a CURN. However, further investigation shows that a disproportionate amount of support for the memory signal comes from three dubious pulsars. Using a more flexible red-noise model in these pulsars reduces the Bayes factor to 1.3. Having found no compelling evidence, we go on to place upper limits on the strain amplitude of GW memory events as a function of sky location and event epoch. These upper limits are computed using a signal model that assumes the existence of a common, spatially uncorrelated red noise in addition to a GW memory signal. The median strain upper limit as a function of sky position is approximately 3.3 × 10−14. We also find that there are some differences in the upper limits as a function of sky position centered around PSR J0613−0200. This suggests that this pulsar has some excess noise that can be confounded with GW memory. Finally, the upper limits as a function of burst epoch continue to improve at later epochs. This improvement is attributable to the continued growth of the pulsar timing array
The NANOGrav 15-year data set: Search for Transverse Polarization Modes in the Gravitational-Wave Background
Recently we found compelling evidence for a gravitational wave background
with Hellings and Downs (HD) correlations in our 15-year data set. These
correlations describe gravitational waves as predicted by general relativity,
which has two transverse polarization modes. However, more general metric
theories of gravity can have additional polarization modes which produce
different interpulsar correlations. In this work we search the NANOGrav 15-year
data set for evidence of a gravitational wave background with quadrupolar
Hellings and Downs (HD) and Scalar Transverse (ST) correlations. We find that
HD correlations are the best fit to the data, and no significant evidence in
favor of ST correlations. While Bayes factors show strong evidence for a
correlated signal, the data does not strongly prefer either correlation
signature, with Bayes factors when comparing HD to ST correlations,
and for HD plus ST correlations to HD correlations alone. However,
when modeled alongside HD correlations, the amplitude and spectral index
posteriors for ST correlations are uninformative, with the HD process
accounting for the vast majority of the total signal. Using the optimal
statistic, a frequentist technique that focuses on the pulsar-pair
cross-correlations, we find median signal-to-noise-ratios of 5.0 for HD and 4.6
for ST correlations when fit for separately, and median signal-to-noise-ratios
of 3.5 for HD and 3.0 for ST correlations when fit for simultaneously. While
the signal-to-noise-ratios for each of the correlations are comparable, the
estimated amplitude and spectral index for HD are a significantly better fit to
the total signal, in agreement with our Bayesian analysis.Comment: 11 pages, 5 figure
How to Detect an Astrophysical Nanohertz Gravitational-Wave Background
Analysis of pulsar timing data have provided evidence for a stochastic
gravitational wave background in the nHz frequency band. The most plausible
source of such a background is the superposition of signals from millions of
supermassive black hole binaries. The standard statistical techniques used to
search for such a background and assess its significance make several
simplifying assumptions, namely: i) Gaussianity; ii) isotropy; and most often
iii) a power-law spectrum. However, a stochastic background from a finite
collection of binaries does not exactly satisfy any of these assumptions. To
understand the effect of these assumptions, we test standard analysis
techniques on a large collection of realistic simulated datasets. The dataset
length, observing schedule, and noise levels were chosen to emulate the
NANOGrav 15-year dataset. Simulated signals from millions of binaries drawn
from models based on the Illustris cosmological hydrodynamical simulation were
added to the data. We find that the standard statistical methods perform
remarkably well on these simulated datasets, despite their fundamental
assumptions not being strictly met. They are able to achieve a confident
detection of the background. However, even for a fixed set of astrophysical
parameters, different realizations of the universe result in a large variance
in the significance and recovered parameters of the background. We also find
that the presence of loud individual binaries can bias the spectral recovery of
the background if we do not account for them.Comment: 14 pages, 8 figure