arXiv subjects

T. Joseph W. Lazio

Publications and source records attributed to T. Joseph W. Lazio.

At least 19 recordsLinked to original sources

arXiv2026

Constraints on Supermassive Black Hole Binaries from the Lack of Resolvable Sources in the NANOGrav 15-yr Dataset

Pulsar timing arrays (PTAs) have found evidence of a gravitational wave background (GWB) consistent with a population of supermassive black hole binaries (SMBHBs), but have not yet resolved a continuous wave (CW) from an individual binary. We present the first SMBHB population constraints incorporating PTA upper limits on CWs, based on the NANOGrav 15-year (NG15) data. Using semi-analytic models to generate simulated populations, we calculate each sample's GWB likelihood by free spectral comparison and CW nondetection (nCW) likelihoods by (1) mapping each realization's highest CW signal-to-noise ratio (S/N) to a detection probability and (2) counting the fraction of realizations without a binary close enough that NG15 would have detected it. CW nondetection favors populations in which many lower-mass sources, rather than a few ultramassive ones, dominate the GWB. This information is complementary to that provided by the GWB amplitude, breaking the degeneracy between the masses and number of binaries required to reproduce the observed spectrum. Relative to GWB-only constraints, the joint posteriors favor higher galaxy merger rates, lower $M_\mathrm{BH}$-$M_\mathrm{bulge}$ scatter, shorter binary lifetimes, and steeper small-separation hardening. The most detectable sources in the S/N-weighted joint posterior have chirp masses $\sim 10^{9.6} M_\odot$, luminosity distances $\sim$1300 Mpc, and frequencies $\sim$6 nHz, while the nCW-only constraints favor lower frequencies, coinciding with the NG15 4 nHz candidate. These constraints will tighten if CW nondetection continues.

astro-ph.HE↗

arXiv2026

The MegaWave Radio Surveyor

Several Decadal-level questions in astrophysics, exoplanets, astrobiology, and cosmology can be addressed only at low radio frequencies inaccessible from Earth. The MegaWave Radio Surveyor would open this largely-unexplored region of the electromagnetic spectrum with a space-based interferometer to (1)~Track the space weather of other stars; (2)~Detect magnetically-generated emission from exoplanets to probe their interiors and assess magnetic shielding of their atmospheres; (3)~Probe the Universe's evolution during the Dark Ages via the highly-redshifted HI hyperfine line; and (4)~Assess the role of cosmic rays and magnetic fields in the cosmic web. An Astrophysics Strategic Technology & Research Accelerator (ASTRA) Initiative concept, the MegaWave Radio Surveyor's science objectives respond to the Pathways to Discovery Decadal Survey and three other National Academies studies, and it would serve as a Formative Era mission in the Enduring Quests, Daring Visions roadmap. Developments in U.S. space industries enable this observatory to be realized. The MegaWave Radio Surveyor would offer a versatile, scalable, and resilient architecture capable of sensitive and simultaneous observations below 45~MHz and unprecedented angular resolution at these frequencies. The concept builds upon NASA's Sun Radio Interferometer Space Experiment (SunRISE), Star-Planet Activity Research CubeSat (SPARCS), and Lunar Surface Electromagnetics Experiment (LuSEE-Night). The MegaWave Radio Surveyor could leverage multiple elements of the Artemis program, such as access to and beyond cislunar space and communications, and there are opportunities to infuse new autonomy/AI modes for mission operations. By opening one of the last windows in the electromagnetic spectrum and pioneering space interferometry at unprecedented scales, the MegaWave Radio Surveyor would establish a transformational capability.

astro-ph.IM↗

arXiv2026

Lunar Reflective Interferometry

We present the method of lunar reflection interferometry (LRI) in which a virtual interferometer can be formed by using a spacecraft-borne antenna in lunar orbit that receives both direct rays and those reflected from the lunar surface. The technique exploits the method of images and is akin to the classic "sea cliff" interferometer, with the Moon's surface, primarily the lunar Maria, replacing the ocean surface. We describe the method in detail, demonstrate that significant portions of the Moon's surface are sufficiently smooth at low radio frequencies, $ν\lesssim 10\,\mathrm{MHz}$, for the technique to work, and outline a spacecraft instrument implementation. We describe potential systematic errors and how they could be mitigated. We present several astrophysics applications of the method.

astro-ph.IM↗

arXiv2026

Discovering and Characterising Exoplanets and Ultracool Dwarfs with the Square Kilometre Array

The majority of the Solar System planets are sources of bright radio emission, driven by energetic electrons trapped within each planet's magnetic field. Detection of this emission from exoplanets provides a unique opportunity to characterise their magnetic fields, which is key to determining the atmospheric evolution of exoplanets. However, a conclusive detection of radio emission from an exoplanet remains at large, primarily due to a lack of sensitivity at low radio frequencies. On the other hand, planet-like radio signatures have been detected on objects called ultracool dwarfs (UCDs) for over two decades. UCDs are of comparable sizes to Jupiter, but are more massive. They also possess similar interior structures to Jupiter, the region where magnetic fields are generated. Therefore, UCDs are ideal targets to study to advance our understanding of how magnetic fields manifest at planetary scales. In this Chapter, we outline the revolutionary role that the Square Kilometre Array will play in the study of exoplanets and UCDs. We anticipate that it will facilitate the first detection of radio emission from giant exoplanets with strong magnetic fields, and will deliver thousands of detections of UCDs within a few hundred parsecs. Combined with very long baseline interferometry, we also expect that astrometric monitoring will enable the detection of planets of a few Earth masses orbiting nearby radio-emitting UCDs. These findings will open a new window into how planets form and evolve in extrasolar systems.

astro-ph.EP↗

arXiv2026

Pulsar Timing Array Sensitivity to Anisotropy: Empirical Sensitivity Curves, Scaling Relations, and the Multi-Resolution Pixel Basis

We quantify pulsar timing array (PTA) sensitivity to anisotropy in the gravitational wave background using the cross-correlation based Fisher information matrix in the pixel and spherical harmonic bases. We use a set of simulations to empirically determine scaling relations of a PTA's sensitivity to anisotropy with the number of pulsars $N_\mathrm{psr}$ in the array, the error $δt$ on the times of arrival, the frequency $f_\mathrm{GW}$ of the gravitational waves, and the angular scale $ΔΩ$ of the anisotropy. The sensitivity scales approximately as $N_\mathrm{psr}^{0.8}$, $δt^{-0.08}$, and $ΔΩ^{1.6}-ΔΩ^{2.1}$ (depending on the ranges of $\ell$ and $m$ under consideration). In addition, we use realistic simulations to project the NANOGrav PTA sensitivity to a 30-year baseline and quantify the growth in sensitivity at several timeslices. Except at the lowest frequencies, we find negligible effect on sensitivity through increasing the observation duration only. Finally, we introduce a multi-resolution pixel basis motivated by the large dependence of the sensitivity on sky location, and demonstrate the operation of the basis through a set of injections and recoveries.

astro-ph.IM↗

arXiv2026

The NANOGrav 15 yr Data Set: Impacts of Customized Chromatic Noise Models on Gravitational Wave Analyses

We report updated nHz gravitational wave (GW) significance, characterization, and interpretations using the customized chromatic-noise models (CNMs) developed in Larsen, Baier et al. (2026). for the NANOGrav 15-year data set. We find increased evidence for the Hellings-Downs (HD) correlation signature of the stochastic gravitational wave background (GWB), with a Bayes factor of $1571\pm14$ for HD-correlations over a common uncorrelated red-noise process using a power-law model with $14$ Fourier modes. We find this $\sim8\times$ increase in Bayes factor from Agazie et al. (2023a) is a result of improved noise mitigation. Assuming an analytic null distribution for the frequentist interpulsar correlation statistic, this corresponds to a slightly more significant measurement from $3.16σ$ to $3.32σ$ against the no-correlation scenario. Spectral inference with CNMs brings the power-law GWB amplitude down to $A_{\rm GWB} = 2.1^{+0.6}_{-0.5}\times10^{-15}$ at fixed $γ_{\rm GWB} = 13/3$. In a varied-$γ$ analysis, the spectral index increases to $γ_{\rm GWB}=3.5^{+0.7}_{-0.6}$. We report updates on an all-sky continuous gravitational wave (CW) search as well as select targeted searches and calculate a $3.2\times$ larger detection volume for the NANOGrav detector. With CNMs, we find reduced evidence for a non-Einsteinian, scalar-transverse mode of gravity. Finally, we reinterpret the GWB first with the assumption of an astrophysical background sourced by SMBHBs and then assuming the more exotic origins of cosmic inflation, a first-order cosmological phase transition, and stable cosmic strings. Under both the SMBHB hypothesis and the cosmological hypotheses, we see only marginal shifts in model parameter posteriors which are consistent with the slightly quieter and steeper power-law GWB spectrum.

astro-ph.CO↗

arXiv2026

Gravitational Wave Measurement of the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ Intrinsic Scatter at High Redshift

The observed GWB spectrum is higher in amplitude than model predictions by a factor of 2-3. Using a semi-analytic model, we evaluate the effect of a high-scatter supermassive black hole (SMBH) scaling relation ($M_\mathrm{BH}$-$M_\mathrm{bulge}$) on models of the nanohertz gravitational wave background (GWB). By implementing an intrinsic scatter of the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation, which is larger at higher redshift, but matches local observations, we find that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. This amplitude increase is not uniform across frequencies, a strongly evolving scatter preferentially increases the number density of the most massive SMBHs which, in the GWB spectrum, minimizes the strength of the low-frequency turnover. Our models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at $4 < z < 6$ without changing the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. We conclude that the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter that evolves as $\varepsilon(z) = \varepsilon_0 + (0.56 \pm 0.4) \log_{10}(1 + z)$ and normalization that evolves as $α(z) = α_0 (1 + z)^{0.84 \pm 0.35}$. The results of this work imply that the $M_\mathrm{BH}$-$M_\mathrm{bulge}$ relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH-galaxy evolution.

astro-ph.HE↗

arXiv2026

Solar System Technosignatures

NASA has five robotic space probes on escape trajectories from the Solar System, and the Interstellar Probe concept was considered in the recent U.S. Solar & Space Physics Decadal Survey. While none of these robotic probes will be operational when they reach another star, it is natural to ask whether another civilization also might have sent out interstellar probes. Serious consideration of interstellar probes dates at least to R. Bracewell in the early 1960s, and the discovery of three interstellar objects has rekindled some of that interest. I consider current limits on signatures of extraterrestrial technology in the Solar System, both objects on various orbits ("probes") and surface artifacts, using data from planetary exploration and astronomical sky surveys. Perhaps not surprisingly, the completeness to which the Solar System has been searched varies as a function of distance from the Sun. I conclude that only extremely crude upper limits can be placed on the existence of technosignatures in the Solar System and that, in some cases, relatively large probes or surface artifacts would have escaped detection. I also highlight areas that might be profitable for improving these limits considerably.

astro-ph.EP↗

arXiv2026

All-Sky Ultra-Narrowband Spectral Imaging with the OVRO-LWA: Technosignature Constraints and Axion-Like Particle Prospects

We present an imaging-domain search for technosignatures at decametric wavelengths with the OVRO-LWA, targeting ultra-narrowband continuous-wave signals between 50 and 86 MHz. We implement an offline GPU pipeline that processes raw voltage data with upchannelization to approximately 10 Hz frequency resolution, producing all-sky images for each fine channel and totaling more than 3 x 10^6 images for a single 30 s epoch. Candidate selection is performed using multi-kernel matched filtering across frequency, empirical noise standardization, and false-discovery-rate control. After applying quality cuts that remove extended sources, corrupted images, and obvious RFI, three narrowband candidates with signal-to-noise ratios above 10 sigma were selected for detailed analysis. By re-imaging these candidates with finer temporal and spectral resolution, we resolved their structure and found them to be inconsistent with compact celestial narrowband emitters. Consequently, we report no detection of extraterrestrial technosignatures. The representative sensitivity of the search is ~100 Jy per channel across the entire visible hemisphere. For an unresolved emitter, this corresponds to 10 sigma equivalent isotropic radiated power (EIRP) limits of about 10^14 W at a distance of 10 pc and 10^18 W at 1 kpc. The wide field of view and ultra-fine spectral resolution of this approach enable simultaneous probing of technosignature signals from millions of stellar systems. This method further establishes a scalable framework for deeper integrations and stacked searches toward neutron-star targets relevant to axion-like particle (ALP) line conversion.

astro-ph.IM↗

arXiv2026

Searching for Exotrojans in Pulsar Systems

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 parts of the Solar System. Despite up to planetary mass Trojans being predicted in extrasolar systems (i.e. exotrojans), they remain unconfirmed, although strong candidate evidence has emerged recently. For the first time, we extend the search for exotrojans to radio pulsars with low-mass ($\sim0.01\,\rm{M}_\odot$) companions 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 $\sim 1\,\rm{M}_\oplus$ upper mass constraints on potential exotrojans around eight pulsars observed in the NANOGrav 15-year data set. We find weak evidence consistent with $\sim2$--4$\,\rm{M}_{\rm J}$ exotrojans in the PSR~J0023+0923 and PSR~J1705$-$1903 binary 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~J1641+8049 system by looking for an inconsistency between the times of superior conjunction as measured by optical light curves and those predicted by radio timing. These results offer initial observational constraints on the existence of exotrojans around pulsars, while their possible formation mechanisms remain unexplored.

astro-ph.HE↗

arXiv2026

A Commensal Radio-Only Cosmic Ray Detector at the Owens Valley Radio Observatory Long Wavelength Array

The brief (10 nanoseconds) transient radio emission from cosmic ray air showers carries key information about the energy and mass composition of high energy cosmic rays, but anthropogenic radio frequency interference has historically prevented radio-based cosmic ray studies from being carried out independently from other types of detectors. We describe a cosmic ray detection system for the Owens Valley Radio Observatory Long Wavelength Array that searches for radio emission from cosmic ray air showers without relying on an external trigger, and runs alongside the other observing modes of the array. The OVRO-LWA, located in Eastern California, recently completed an expansion to 352 dual-polarization antennas and new signal processing infrastructure. In order to detect cosmic rays in the presence of radio frequency interference (RFI), initial event classification and RFI rejection is performed on Field Programmable Gate Array boards, which each process a sampled voltage timeseries from both polarizations of a subarray of 32 antennas. Each board uses dedicated RFI veto antennas outside the air shower radio footprint to reject RFI events. We present the trigger design, RFI flagging strategy, and candidate cosmic rays.

astro-ph.IM↗

arXiv2026

Upper Limits on Pulsed Radio Emission from Unseen Compact Objects in Six Galactic Stellar Binaries

We have conducted a search for radio pulsars in six Galactic stellar binary systems having unseen primary stars. All six systems have estimated primary masses in the range that could be consistent with neutron stars. We used the Green Bank Telescope at a center frequency of 350 MHz to search for dispersed periodicities and single pulses across a range of possible dispersion measures (DMs) and binary accelerations. No astrophysical signals were detected in our search. The estimated 400-MHz luminosity upper limits from the search are comparable to or smaller than the lowest values observed for almost all the known Galactic binary pulsars having cataloged 400 MHz radio luminosities. This implies that the systems we observed either do not harbor radio-emitting pulsars, contain pulsars that do not beam in our direction, or contain pulsars with luminosities that are significantly lower than this subset of the known Galactic binary pulsar population.

astro-ph.HE↗

arXiv2026

Chandra X-ray Observations of Quasars with Velocity-Offset Broad Lines: Assessing the Binary Supermassive Black Hole Hypothesis

During the final stages of a galaxy merger, dynamical friction acting on the supermassive black holes (SMBHs) in the post-merger remnant can lead to the formation of a gravitationally bound binary SMBH. In the event that at least one of these SMBHs is actively accreting, the system can appear phenomenologically as an active galactic nucleus (AGN) with a broad line region (BLR) kinematically offset from the host galaxy rest frame. Such velocity offsets have been interpreted as signatures of binary SMBHs, recoiling SMBHs, or BLR gas dynamics within a single-SMBH system. We present deep Chandra X-ray observations of five nearby (0.1 < z < 0.2) Sloan Digital Sky Survey quasars whose broad emission lines are Doppler-shifted relative to their host galaxies' systemic velocities, along with archival Chandra observations of 11 additional sources from the same sample. Using our Chandra data, we constrain SMBH masses with multiple independent techniques. We find systematic, method-dependent differences among black hole mass estimates, with masses inferred from the fundamental plane of black hole activity generally lower and single-epoch virial masses typically higher than those obtained using other methods. We also compare the X-ray photon indices and optical-to-X-ray spectral indices of our quasars to the broader quasar population. While we find no strong differences in optical-to-X-ray spectral indices, we do find systematically harder X-ray photon indices than typically observed in comparable quasars. These results constrain competing physical models but do not provide conclusive evidence for or against a binary SMBH origin of the velocity-offset BLRs.

astro-ph.GA↗

arXiv2026

NIAC project report: Solar system-scale VLBI to dramatically improve cosmological distance measurements

We investigate the feasibility and scientific potential of the Cosmic Positioning System (CPS), a space mission concept enabling purely geometric distance measurements to sources at hundreds of megaparsecs by directly detecting electromagnetic wavefront curvature. CPS consists of a constellation of radio antennas distributed across the outer Solar System, operating on baselines of tens of astronomical units. By precisely timing the arrival of repeating fast radio bursts (FRBs), CPS infers source distances via trilateration -- analogous to global navigation satellite systems such as GPS but on cosmological scales. We show that CPS distance measurements could result in sub-percent constraints on the Hubble constant with even a handful of detections, whereas we predict that 10-100 FRB sources are likely visible. We evaluate dominant sources of uncertainty -- wavefront timing precision, interstellar refractive delays, spacecraft positional knowledge, and onboard clock stability -- finding these controllable at required levels using near-term technologies. Our nominal design employs five spacecraft with 8 m deployable antennas, 3-6 GHz receivers with sub-30 K system temperatures, and space-qualified atomic clocks similar to those on GPS satellites, supported by a ground network for ranging calibration and FRB alerts. Beyond cosmic expansion, CPS may enable frontier measurements in astrophysics and fundamental physics, including constraints on small-scale dark matter structure, microhertz gravitational waves (bridging pulsar timing arrays and LISA), and the outer Solar System mass distribution. The most significant viability issue concerns FRB properties at several-GHz frequencies; we recommend observational campaigns to characterize repeating FRBs in this band.

astro-ph.IM↗

arXiv2026

Inference on inner galaxy structure via gravitational waves from supermassive binaries

The detection of a stochastic gravitational wave background by pulsar-timing arrays indicates the presence of a population of supermassive black hole binaries. Although the observed spectrum generally matches predictions for orbital evolution driven by gravitational-wave emission in circular orbits, there is a preference for a spectral turnover at the lowest observed frequencies, which may point to substantial hardening during a transition from early environmental influences to later stages dominated by emission. In the vicinity of these binaries, the ejection of stars or dark matter particles through gravitational three-body slingshots efficiently extracts orbital energy, leading to a low-frequency turnover in the spectrum. Here we model how the gravitational-wave spectrum depends on the initial inner galactic profile before scouring by binary ejections while accounting for a range of initial binary eccentricities. By analysing the NANOGrav 15-year data, we find that a parsec-scale galactic-centre density of around $10^6 M_{\odot} \mathrm{pc}^{-3}$ is favoured across most of the parameter space, thus shedding light on the environmental effects that shape black hole evolution and the combined matter density near galaxy centres.

astro-ph.HE↗

arXiv2026

The Tantalizing Case of the Quasar J0950+5128 -- I. Presentation of the Data and Detailed Exploration of the Binary Supermassive Black Hole Scenario

Spectroscopic observations of the quasar J0950$+$5128 spanning 22 years reveal monotonic radial velocity variations in its broad H$β$ emission line. Moreover, the line profile becomes broader over time, necessitating careful measurements. We present robust H$β$ velocity shift measurements obtained via cross correlation, applied to both the full spectra and to isolated broad H$β$ components derived from spectral decomposition. We also examine the light curves for variability consistent with the spectroscopic trends. Using Lomb-Scargle periodogram analysis we find no significant periodic signal. We consider several interpretations for the observed changes, including a binary supermassive black hole, dust-cloud obscuration, outflows, a recoiling black hole, and a single perturbed, disk-like broad-line region. We deem the binary and perturbed broad-line region scenarios to be physically plausible. The binary interpretation is the only one for which we can immediately compare a physical model to the available data. Thus, we incorporate radial velocity ''jitter'' to emulate typical quasar variability and fit the radial velocity curve with a Keplerian model to examine whether it can reproduce the observations. In this context, the available observations trace only a segment of the putative orbit. The fit yields a period of 33 years (observed frame) and an eccentricity of 0.65, with lower limits on the semi-major axis and black hole mass of $10^{-2}\;$pc and $10^7\;{\rm M}_\odot$, respectively. Thus, J0950$+$5128 is a binary candidate deserving further study. The single, perturbed broad-line region interpretation remains viable but requires additional observations and modeling for further evaluation. Continued monitoring is, therefore, essential.

astro-ph.GA↗

arXiv2026

The NANOGrav 15 yr Data Set: Piecewise Power-Law Reconstruction of the Gravitational-Wave Background

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.

astro-ph.HE↗

arXiv2026

The NANOGrav 15 yr Data Set: Targeted Searches for Supermassive Black Hole Binaries

We present the first targeted searches for continuous gravitational waves (CWs) from 114 active galactic nuclei (AGN) that may host supermassive black hole binaries, 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 typically improved by a factor of $\sim 2$ (median 2.2) relative to all-sky limits at the same frequency. Bayesian comparisons against a model including only a Hellings-Downs correlated background disfavors a CW signal for all targets, with a mean Bayes factor of $0.73 \pm 0.32$. Two targets have Bayes factors slightly above unity, but coherence tests, random targeting experiments, and a conservative accounting of the 114-target trials factor all indicate that they are consistent with noise. We use these two candidates as worked examples to illustrate an end-to-end targeted CW search analysis and a suite of follow up tests that future promising candidates would need to pass. We find that the electromagnetic interpretations of both candidates are ambiguous, and we update the constraints on a putative binary in 3C 66B, ruling out part of its previously allowed parameter space. Ultimately, our results demonstrate the current sensitivity of targeted pulsar timing array searches for CWs and define a roadmap for future multimessenger CW detections.

astro-ph.HE↗