Timothy Dolch

We test the impact of an evolving supermassive black hole mass scaling relation (MBH–Mbulge) on the predictions for the gravitational-wave background (GWB). The observed GWB amplitude is 2–3 times higher than predicted by astrophysically informed models, which suggests the need to revise the assumptions in those models. We compare a semi-analytic model’s ability to reproduce the observed GWB spectrum with a static versus evolving-amplitude MBH–Mbulge relation. We additionally consider the influence of the choice of galaxy stellar mass function (GSMF) on the modeled GWB spectra. Our models are able to reproduce the GWB amplitude with either a large number density of massive galaxies or a positively evolving MBH–Mbulge amplitude (i.e., the MBH/Mbulge ratio was higher in the past). If we assume that the MBH–Mbulge amplitude does not evolve, our models require a GSMF that implies an undetected population of massive galaxies (M⋆ ≥ 1011M⊙ at z > 1). When the MBH–Mbulge amplitude is allowed to evolve, we can model the GWB spectrum with all fiducial values and an MBH–Mbulge amplitude that evolves as α(z) = α0(1 + z)1.04±0.5.

Free-floating objects (FFOs) in interstellar space—rogue planets, brown dwarfs, and large asteroids that are not gravitationally bound to any star—are expected to be ubiquitous throughout the Milky Way. Recent microlensing surveys have discovered several free-floating planets that are not bound to any known stellar systems. Additionally, three interstellar objects, namely 1I/’Oumuamua, 2I/Borisov, and 3I/ATLAS, have been detected passing through our solar system on hyperbolic trajectories. In this work, we search for FFOs on hyperbolic orbits that pass near millisecond pulsars (MSPs), where their gravitational influence can induce detectable perturbations in pulse arrival times. Using the NANOGrav 15 yr narrow band dataset, which contains high-precision timing data for 68 MSPs, we conduct a search for such hyperbolic scattering events between FFOs and pulsars. Although no statistically significant events were detected, this nondetection enables us to place upper limits (ULs) on the number density of FFOs as a function of their mass within our local region of the Galaxy. For example, the UL on the number density for Jupiter-mass FFOs (∼10 −2.5 –10 −3.5 M ⊙ ) obtained from different pulsars ranges from 5.25 × 10 6 pc −3 to 5.37 × 10 9 pc −3 , while the UL calculated by combining results from all the pulsars is 6.03 × 10 5 pc −3 . These results represent the first constraints on FFO population derived from pulsar timing data.

The NANOGrav 15-year (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nanohertz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black-hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this paper, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant-power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index gamma. Concurrently, it better approximates physically realistic GWB spectra -- especially those of cosmological origin -- than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.) that are ultimately combined in a Bayesian model average. The data products resulting from our analysis provide the basis for fast refits of spectral GWB models.

Pulsar timing arrays (PTAs) have recently entered the detection era, quickly moving beyond the goal of simply improving sensitivity at the lowest frequencies for the sake of observing the stochastic gravitational wave background (GWB), and focusing on its accurate spectral characterization. While all PTA collaborations around the world use Fourier-domain Gaussian processes to model the GWB and intrinsic long time-correlated (red) noise, techniques to model the time-correlated radio frequency-dependent (chromatic) processes have varied from collaboration to collaboration. Here we test a new class of models for PTA data, Gaussian processes based on time-domain kernels that model the statistics of the chromatic processes starting from the covariance matrix. As we will show, these models can be effectively equivalent to Fourier-domain models in mitigating chromatic noise. This work presents a method for Bayesian model selection across the various choices of kernel as well as deterministic chromatic models for non-stationary chromatic events and the solar wind. As PTAs turn towards high frequency (>1/yr) sensitivity, the size of the basis used to model these processes will need to increase, and these time-domain models present some computational efficiencies compared to Fourier-domain models.

Wideband timing of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) datasets, where a single time-of-arrival (TOA) and a single dispersion measure (DM) are measured using the entire bandwidth of each observation, was first done for the 12.5-year dataset, and proved to be invaluable for characterizing the time-varying dispersion measure, reducing the data volume, and for improving the overall timing precision. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope has been observing most NANOGrav millisecond pulsars (MSPs) at nearly daily cadence (compared to roughly monthly cadence for other NANOGrav observations) since 2019 with the objective of integration into future pulsar timing array (PTA) datasets. In this paper, we show the results of integration of high-cadence, low-observing-frequency CHIME data with data from the NANOGrav experiment for an isolated MSP PSR~J0645 $+$ 5158 and three binary MSPs PSR~J1012 $+$ 5307, PSR~J2145 $-$ 0750, and PSR~J2302 $+$ 4442. Using a wideband timing pipeline which we also describe, we present updated timing results for all four sources, including improvements in measurements of relativistic post-Keplerian parameters for the three binary pulsars in this analysis. For PSR~J2302 $+$ 4442, we report an updated strong detection of Shapiro delay from which we measured a companion mass of$0.35^{+0.05}_{-0.04}\ M_{\odot}$ , a pulsar mass of$1.8^{+0.3}_{-0.3}\ M_{\odot}$ , and an orbital inclination of${80^{\circ}}^{+1}_{-2}$ . We also report updated constraints on the reflex motion for PSR~J2145 $-$ 0750 using a combination of Very Long Baseline Array astrometry and our updated measurement of the time derivative of the projected semi-major axis of the pulsar orbit as a prior.

Trojan asteroids are found in the equilateral triangle Lagrange points of the Sun-Jupiter system in great number, though they also exist less prolifically in other Sun-planet systems. Despite up to planetary mass Trojans being predicted in extrasolar systems (i.e. exotrojans), they remain largely unconfirmed, though with recent strong candidate evidence emerging. We turn the current search for exotrojans to radio pulsars with low-mass companions ( $\sim0.01\,\rm{M}_\odot$ ) using accurately measured pulse times of arrival. With techniques developed for detecting the reflex motion of a star due to a librating Trojan, we place reasonable mass constraints ( $\sim 1\,\rm{M}_\oplus$ ) on potential exotrojans around binary pulsars observed in the NANOGrav 15-year data set. We find weak evidence consistent with$\sim1\,\rm{M}_{\rm J}$exotrojans in the PSR~J0023+0923 and PSR~J1705 $-$ 1903 systems, though the signals likely have a different, unknown source. We also place a libration-independent upper mass constraint of$\sim8$ \,M $_{\rm J}$on exotrojans in the PSR~1641+8049 binary system by looking for an inconsistency between the times of superior conjunction as measured by optical light curves and those predicted by radio timing.

Accurate pulsar astrometric estimates play an essential role in almost all high-precision pulsar timing experiments. Traditional pulsar timing techniques refine these estimates by including them as free parameters when fitting a model to observed pulse time-of-arrival measurements. However, reliable sub-milliarcsecond astrometric estimations require years of observations and, even then, power from red noise can be inadvertently absorbed into astrometric parameter fits, biasing the resulting estimations and reducing our sensitivity to red noise processes, including gravitational waves (GWs). In this work, we seek to mitigate these shortcomings by using pulsar astrometric estimates derived from Very Long Baseline Interferometry (VLBI) as priors for the timing fit. First, we calibrated a frame tie to account for the offsets between the reference frames used in VLBI and timing. Then, we used the VLBI-informed priors and timing-based likelihoods of several astrometric solutions consistent with both techniques to obtain a maximum-posterior astrometric solution. We found offsets between our results and the timing-based astrometric solutions, which, if real, would lead to absorption of spectral power at frequencies of interest for single-source GW searches. However, we do not find significant power absorption due to astrometric fitting at the low-frequency domain of the GW background.

We test the impact of an evolving supermassive black hole (SMBH) mass scaling relation (Mbh-Mbulge) on the predictions for the gravitational wave background (GWB). The observed GWB amplitude is 2-3 times higher than predicted by astrophysically informed models which suggests the need to revise the assumptions in those models. We compare a semi-analytic model's ability to reproduce the observed GWB spectrum with a static versus evolving-amplitude Mbh-Mbulge relation. We additionally consider the influence of the choice of galaxy stellar mass function on the modeled GWB spectra. Our models are able to reproduce the GWB amplitude with either a large number density of massive galaxies or a positively evolving Mbh-Mbulge amplitude (i.e., the Mbh / Mbulge ratio was higher in the past). If we assume that the Mbh-Mbulge amplitude does not evolve, our models require a galaxy stellar mass function that implies an undetected population of massive galaxies (Mstellar > 10^11 Msun at z > 1). When the Mbh-Mbulge amplitude is allowed to evolve, we can model the GWB spectrum with all fiducial values and an Mbh-Mbulge amplitude that evolves as alpha(z) = alpha_0 (1 + z)^(1.04 +/- 0.5).

We present the first catalog of targeted searches for continuous gravitational waves (CWs) from 114 active galactic nuclei (AGN) that may host supermassive black hole binaries (SMBHBs), using the NANOGrav 15 yr data set. By incorporating electromagnetic priors on sky location, distance, redshift, and CW frequency, our strain and chirp mass upper limits are on average 2.6$\times$ more constraining than sky-averaged limits. Bayesian model comparisons against a common uncorrelated red noise for the gravitational wave background (GWB) disfavor a CW signal for almost all targets, yielding a mean Bayes factor of $0.87 \pm 0.31$. There are two notable exceptions: SDSS J153636.22+044127.0, ``Rohan'' with $\mathrm{BF} = 3.37(5)$, and SDSS J072908.71+400836.6, ``Gondor'' with $\mathrm{BF} = 2.44(3)$. These Bayes factors correspond to p-values of $0.01$--$0.03$ ($1.9σ$--$2.3σ$) and $0.05$--$0.08$ ($1.4σ$--$1.6σ$), respectively, depending on the empirical null distribution. We outline the beginnings of a detection protocol by identifying and carrying out a battery of tests on Rohan and Gondor to verify their binary nature. Notably, when replacing the common uncorrelated red noise model with a Hellings--Downs correlated GWB, Rohan's Bayes factor drops to $1.25(7)$, while Gondor's increases to $3.2(1)$. Both have rich electromagnetic datasets, including optical and infrared variability and spectroscopic features that support their classification as SMBHB candidates, though this was discovered after the targeted searches were complete. Our results suggest more simulations are needed to confirm or refute the nature of these and future SMBHB candidates, while creating a roadmap for targeted CW detection.