Alexander J. Dittmann

Constraints on neutron star radii provide insight into the properties of the cold, dense matter in their interiors. Previous studies using synthetic Neutron Star Interior Composition Explorer (NICER) pulse waveform data have demonstrated that radius inferences derived therefrom are robust against several classes of modeling systematics. Here we explore the consequences of assuming the wrong atmospheric composition, using synthetic data based on the ∼2.1 M _⊙ pulsar PSR J0740+6620. We find that the assumption of a hydrogen atmosphere when the synthetic data assumed a helium atmosphere, or vice versa, produces little bias in the inferred radius at the spot-to-background ratio of the actual PSR J0740+6620 data. However, when we increase the spot-to-background ratio by a factor of ∼20 while keeping the total number of counts fixed at the observed ∼5.5 × 10 ⁵ , we find that composition mismatch can produce significantly biased radius estimates while remaining hidden inside a fit with statistically acceptable residuals. Even in these cases, the Bayesian evidence consistently identifies the correct atmospheric model. Our findings reinforce the importance of using the Bayesian evidence for model comparison and goodness-of-fit tests.

Stars embedded in the accretion disks of active galactic nuclei (AGN) can accrete rapidly from their surroundings, dramatically altering their structure and evolution. However, feedback from the release of gravitational potential energy and radiative enthalpy by accreting gas can limit accretion rates, as recently demonstrated in radiation hydrodynamics simulations. To determine the importance of these effects neglected in earlier stellar evolution calculations, we incorporate these feedback processes into a semianalytical model of stellar structure and evolution and conduct a suite of calculations spanning a broad parameter space of AGN disk conditions drawn from α -disk models with central black hole masses M _• / M _⊙ ∈ [10 ⁶ , 10 ⁹ ]. We find that accretion feedback limits stellar accretion rates below ∼10 ⁻¹ M _⊙ yr ⁻¹ , reducing the sensitivity of stellar evolution to disk conditions. This suppression eliminates runaway accretion in models where it would otherwise occur, broadening the parameter space over which stars can reach long-lived “immortal” equilibria between accretion and mass loss. When gap opening is also accounted for, accretion feedback significantly alters stellar properties: it can reduce accretion and mass-loss rates by over an order of magnitude, reducing the strength of accretion shocks and thereby increasing equilibrium stellar masses and radii. These higher masses correspond to higher intrinsic luminosities, suggesting that neglecting accretion feedback may lead to an underestimate of disk chemical enrichment rates. Additionally, accretion feedback is important for predicting the properties of stellar populations within AGN disks, and associated transient phenomena.

Accurate and precise measurements of neutron star radii provide invaluable information about the cold, dense matter in neutron star cores. Analyses of synthetic X-ray pulse waveform data for nonaccreting neutron stars of the type collected by the Neutron Star Interior Composition Explorer (NICER) have indicated that the mass and radius estimates extracted therefrom are robust against potential sources of systematic error, such as errors in the assumed pattern of the thermal X-ray emission from the surface of these stars. A potentially important but unexplored source of systematic error is misparameterization of unmodulated background components, which might bias the inferred radius, particularly when data from different telescopes are analyzed. In this study, we investigate the effects of the background model on radius estimates by jointly analyzing synthetic NICER and XMM-Newton data, using the ∼2.1 M _⊙ pulsar PSR J0740+6620 as a prototypical example. Our analysis shows that even if the background assumed in the model underestimates the actual background by a factor of more than 5, the resulting shift of the radius posterior from the true value of the radius corresponds to only ∼1 σ . In all the cases we examined, the Bayesian evidence for the correct background model is greater than for the incorrect background model. These results add to the evidence that analyses of NICER-like data provide accurate measurements of radii when the statistical sampling is thorough and the model fits the data well.

Circumbinary disks crucially affect the orbital and electromagnetic properties of binary systems across the Universe, from stars in our Galactic neighborhood to supermassive black hole binaries formed as the result of tumultuous galactic mergers. Previous simulations have focused nearly exclusively on thick accretion disks, appropriate for studying stellar binaries, and have found encouraging agreement with observations thereof. We present herein the first systematic study of eccentric binary systems accreting from thin disks, focusing on binary orbital evolution. Our main results are that (1) thinner disks not only drive binaries to rapidly inspiral but also excite binary eccentricities at much higher rates; (2) while thick disks may drive binaries to a stable fixed point of e ≈ 0.425, thinner disks pump binary eccentricities to e ≳ 0.6; and (3) the range of near-zero eccentricities that are damped toward zero depends on both disk thickness and viscosity, thinner disks and those with α viscosities driving binaries toward circularity over a much narrower range of eccentricities. These differences follow largely from the effects of pressure support on accretion streams and shocks within the inner regions of the accretion flow. Our results suggest that accreting binary black holes should have high eccentricities well into the frequency range probed by pulsar timing arrays and space-based gravitational wave interferometers, affecting the spectrum and isotropy of the gravitational wave background. Our results also suggest that circumbinary disks may play an important role in shaping the orbits of close binary stars, but much less so those of wider binaries.

Neutron Star Interior Composition Explorer (NICER) data have been used to estimate the masses and radii of the rotation-powered millisecond pulsars PSR J0030+0451, PSR J0740+6620, PSR J0437−4715, PSR J1231−1411, and PSR J0614−3329, sometimes in joint analyses with XMM-Newton data. These measurements provide invaluable information about the properties of cold, catalyzed matter beyond nuclear saturation density. Here, we present the results of our modeling of NICER data on PSR J0437−4715 using several different models of hot thermal X-ray-emitting spots on the stellar surface. For this pulsar, previous Nuclear Spectroscopic Telescope Array observations established that there is also a modulated nonthermal component to the emission, but the previously published analysis of NICER data did not model this component. We find that the Bayesian evidence is significantly higher when the modulated nonthermal component is included and that omission of this component leads to poorer fits to the bolometric NICER data and thus risks bias in the resulting radius estimates. Our models, which we pursue to inferential convergence, therefore have modulated nonthermal emission, and our headline model has in addition three uniform-temperature thermally emitting circular spots. Using this model, the symmetric 68% credible range in the radius is 11.9–15.5 km, which at the independently measured mass of M = 1.418 ± 0.044 M _⊙ is consistent with previous reports of the radius of the ∼1.4 M _⊙ pulsar PSR J0030+0451. We discuss the implications of this measurement for the equation of state of dense matter.

Upcoming space-based gravitational wave detectors such as LISA, the Laser Interferometer Space Antenna, will be sensitive to extreme- and intermediate-mass-ratio inspirals (EMRIs and IMRIs). These binaries are comprised of a supermassive black hole and a stellar-mass object or intermediate-mass black hole. Their detection will probe the structure of galactic nuclei and enable tests of general relativity. As these events will be observed over thousands of orbital cycles, they will be extremely sensitive to both the underlying spacetime and astrophysical environment, demanding exquisite theoretical models on both fronts to avoid biased or even erroneous results. In particular, many (E/)IMRIs are expected to occur within accretion discs around supermassive black holes, and the nonlinearities present when modeling these systems require numerical simulations. In preparation for future modeling of LISA sources, we have conducted a comparison between eight different hydrodynamical codes and applied them to the problem of a q = 10^{-4} mass ratio binary interacting with an accretion disc. Thicker discs appear more lenient, and all codes at sufficiently high resolutions are in good agreement with each other and analytical predictions. For thinner discs, beyond the reach of analytical models, we find substantial disagreement between 2D and 3D simulations and between different codes, including both the magnitude and sign of the torque. With time and energy efficiency in mind, codes that leverage moving meshes or grid-based Lagrangian remapping seem preferable, as do codes that can leverage graphical processing units and other energy-efficient hardware.