arXiv subjects

Chris Gordon

Publications and source records attributed to Chris Gordon.

At least 19 recordsLinked to original sources

arXiv2026

Bayesian leave-one-out cross-validation for astrophysical model comparison using gravitational-wave background data

Previous work showed that ultralight-dark-matter solitons can provide dynamical friction for supermassive black-hole binaries, suppressing low-frequency power in the pulsar-timing-array gravitational-wave background and constraining the particle mass and effective ultralight-dark-matter fraction. Here we extend that analysis by comparing the predictive performance of four models: simplified and realistic ultralight-dark-matter implementations, a phenomenological environmental-hardening model, and a gravitational-wave-only model. We use Bayesian leave-one-out cross-validation on the five lowest pulsar-timing-array frequency bins. The phenomenological model gives the largest expected log predictive density, but its advantage over the other models is not large compared with the estimated standard errors. The current data therefore do not decisively prefer one model overall. The clearest pairwise result is within the ultralight-dark-matter framework: the simplified model outperforms the realistic implementation in all five frequency bins. Current pulsar-timing-array data are therefore compatible with ultralight-dark-matter-induced low-frequency suppression, but do not yet distinguish ultralight-dark-matter significantly from more generic environmental descriptions of supermassive-black-hole-binary evolution.

astro-ph.CO↗

arXiv2026

Replacing Gaussian Processes with Neural Networks in Pulsar Timing Array Inference of the Gravitational-Wave Background

Bayesian inference of nanohertz gravitational-wave background models in pulsar timing array analyses often relies on Gaussian-process interpolators to avoid repeated, computationally expensive strain-spectrum calculations. However, Gaussian-process training becomes a bottleneck for large training sets. We test whether probabilistic neural networks can replace Gaussian processes in this role for both a self-interacting dark matter model and a phenomenological environmental model. We find that neural networks recover consistent posteriors while significantly reducing both training and Markov chain Monte Carlo runtime, with the largest gains for the more computationally demanding model.

astro-ph.CO↗

arXiv2025

Ultralight Dark Matter Constraints from NanoHertz Gravitational Waves

We investigate the impact of ultralight dark matter (ULDM) on the mergers of supermassive black holes (SMBH) and the resulting stochastic gravitational wave background. ULDM is based on exceptionally light particles and yields galactic halos with dense central solitons. This increases the drag experienced by binary SMBH, decreasing merger times and potentially suppressing gravitational radiation from the binary at low frequencies. We develop semi-analytic models for the decay of SMBH binaries in ULDM halos and use current pulsar timing array (PTA) measurements to constrain the ULDM particle mass and its fractional contribution to the dark matter content of the universe. We find a median ULDM particle mass of $7. \times 10^{-22}$ eV and show that scaling relations suggest that the drag remains effective at relatively low ULDM fractions, which are consistent with all other constraints on the model. Consequently, future pulsar timing measurements will be a sensitive probe of any ULDM contribution to the overall dark matter content of the universe.

astro-ph.CO↗

arXiv2025

Self-Interacting Dark-Matter Spikes and the Final-Parsec Problem: Bayesian constraints from the NANOGrav 15-Year Gravitational-Wave Background

A self-interacting dark-matter (SIDM) density spike around merging supermassive black holes (SMBHs) may be able to supply the dynamical friction needed to shrink binaries from $\sim 1\, \mathrm{pc}$ to $\sim 10^{-2} \,\mathrm{pc}$, thereby resolving the long-standing "final-parsec problem". Embedding the binary-halo system in a cosmological population model, we evolve the inspiral under the combined influence of gravitational-wave (GW) emission and SIDM drag, compute the resulting nanohertz GW background, and confront it with the NANOGrav 15-year pulsar-timing data. A six-parameter Bayesian analysis, performed with a Gaussian-process-accelerated Markov chain Monte Carlo, yields posterior constraints on the cross-section per unit mass and maximum circular velocity values that were consistent with independent galaxy-rotation and cluster-lensing limits. Within this parameter space, the SIDM spike remains intact, supplies sufficient friction to overcome the stellar depletion barrier, and produces a characteristic-strain spectrum that matches the NANOGrav signal as well as phenomenological astrophysical models.

astro-ph.CO↗

arXiv2024

Enhanced Disruption of Axion Minihalos by Multiple Stellar Encounters in the Milky Way

If QCD axion dark matter formed post-inflation, axion miniclusters emerged from isocurvature fluctuations and later merged hierarchically into minihalos. These minihalos, potentially disrupted by stellar encounters in the Milky Way, affect axion detectability. We extend prior analyses by more accurately incorporating multiple stellar encounters and dynamical relaxation timescales, simulating minihalo orbits in the Galactic potential. Our results show stellar interactions are more destructive than previously estimated, reducing minihalo mass retention at the solar system to ~30%, compared to earlier estimates of ~60%. This enhanced loss arises from cumulative energy injections when relaxation periods between stellar encounters are accounted for. The altered minihalo mass function implies a larger fraction of axion dark matter occupies inter-minihalo space, potentially increasing the local axion density and improving haloscope detection prospects. This work highlights the significance of detailed modeling of stellar disruptions in shaping the axion dark matter distribution.

astro-ph.CO↗

arXiv2024

Axion-Photon Conversion Signals from Neutron Stars with Spacetime Curvature Accounted for in the Magnetosphere Model

Axions are a well-motivated dark matter candidate. They may be detectable from radio line emission due to resonant conversion in neutron star magnetospheres. While radio data collection for this signal has begun, further efforts are required to solidify the theoretical predictions for the resulting radio lines. Usually, the flat spacetime Goldreich-Julian model of the neutron star magnetosphere is used, while a Schwarzschild geometry is assumed for the ray tracing. We assess the impact of incorporating the spacetime curvature into the magnetosphere model. We examine a range of neutron star and axion masses and find an average difference of $37\%$ and $22\%$ in radiated power compared to the standard Goldreich-Julian magnetosphere model for a $1\mu eV$ and $10\mu eV$ mass axion, respectively, in the case of a $2.2M_\odot$ mass neutron star. A much lesser difference is found for lower-mass neutron stars, as in that case axion-photon conversion occurs further from the Schwarzschild radius.

astro-ph.HE↗

arXiv2024

Robust inference of the Galactic centre gamma-ray excess spatial properties

The gamma-ray Fermi-LAT Galactic centre excess (GCE) has puzzled scientists for over 15 years. Despite ongoing debates about its properties, and especially its spatial distribution, its nature remains elusive. We scrutinize how the estimated spatial morphology of this excess depends on models for the Galactic diffuse emission, focusing particularly on the extent to which the Galactic plane and point sources are masked. Our main aim is to compare a spherically symmetric morphology - potentially arising from the annihilation of dark matter (DM) particles - with a boxy morphology - expected if faint unresolved sources in the Galactic bulge dominate the excess emission. Recent claims favouring a DM-motivated template for the GCE are shown to rely on a specific Galactic bulge template, which performs worse than other templates for the Galactic bulge. We find that a non-parametric model of the Galactic bulge derived from the VVV survey results in a significantly better fit for the GCE than DM-motivated templates. This result is independent of whether a GALPROP-based model or a more non-parametric ring-based model is used to describe the diffuse Galactic emission. This conclusion remains true even when additional freedom is added in the background models, allowing for non-parametric modulation of the model components and substantially improving the fit quality. When adopted, optimized background models provide robust results in terms of preference for a boxy bulge morphology of the GCE, regardless of the mask applied to the Galactic plane.

astro-ph.GA↗

arXiv2024

The Disruption of Dark Matter Minihalos by Successive Stellar Encounters

Scenarios such as the QCD axion with the Peccei-Quinn symmetry broken after inflation predict an enhanced matter power spectrum on sub-parsec scales. These theories lead to the formation of dense dark matter structures known as minihalos, which provide insights into early Universe dynamics and have implications for direct detection experiments. We examine the mass loss of minihalos during stellar encounters, building on previous studies that derived formulas for mass loss and performed N-body simulations. We propose a new formula for the mass loss that accounts for changes in the minihalo profile after disruption by a passing star. We also investigate the mass loss for multiple stellar encounters. We demonstrate that accurately assessing the mass loss in minihalos due to multiple stellar encounters necessitates considering the alterations in the minihalo's binding energy after each encounter, as overlooking this aspect results in a substantial underestimation of the mass loss.

astro-ph.GA↗

arXiv2022

Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars

The Fermi Bubbles are giant, gamma-ray emitting lobes emanating from the nucleus of the Milky Way discovered in ~1-100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi Bubbles that has been interpreted as a signature of collimated outflows from the Galaxy's super-massive black hole. Here we show via a spatial template analysis that much of the gamma-ray emission associated to the brightest region of substructure -- the so-called cocoon -- is likely due to the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. This large Milky Way satellite is viewed through the Fermi Bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sgr dSph has no on-going star formation, but we nevertheless demonstrate that the dwarf's millisecond pulsar (MSP) population can plausibly supply the gamma-ray signal that our analysis associates to its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron-positron pairs injected by MSPs belonging to the Sgr dSph, combined with these objects' magnetospheric emission. This finding plausibly suggests that MSPs produce significant gamma-ray emission amongst old stellar populations, potentially confounding indirect dark matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dwarf spheroidals.

astro-ph.HE↗

arXiv2022

Assessing the Impact of Hydrogen Absorption on the Characteristics of the Galactic Center Excess

We present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation-transport modelling of line and continuum emission and a gas-flow model in the barred Galaxy that provides distance resolution for lines of sight toward the Galactic Center. The main benefits of the new gas model are, a), the ability to reproduce the negative line signals seen with the H$I$4PI survey and, b), the accounting for gas that primarily manifests itself through absorption. We apply the new model of Galactic atomic hydrogen to an analysis of the diffuse gamma-ray emission from the inner Galaxy, for which an excess at a few GeV was reported that may be related to dark matter. We find with high significance an improved fit to the diffuse gamma-ray emission observed with the Fermi-LAT, if our new H$I$ model is used to estimate the cosmic-ray induced diffuse gamma-ray emission. The fit still requires a nuclear bulge at high significance. Once this is included there is no evidence for a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favoured by the data. This finding is robust under the variation of various parameters, for example the excitation temperature of atomic hydrogen, and a number of tests for systematic issues.

astro-ph.HE↗

arXiv2021

Millisecond Pulsars from Accretion Induced Collapse as the Origin of the Galactic Centre Gamma-ray Excess Signal

Gamma-ray data from the Fermi-Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge that peaks near 2 GeV with an approximately spherical intensity profile $\propto r^{-2.4}$ that extends to angular radial scales of at least 10 degrees, possibly to 20 degrees. The origin of this "Galactic Centre Excess" (GCE) has been debated with proposed sources prominently including self-annihilating dark matter and a hitherto undetected population of millisecond pulsars (MSPs). However, the conventional channel for the generation of MSPs has been found to predict too many low mass X-ray binary (LMXB) systems and, because of the expected large natal kicks, may not accommodate the close spatial correspondence between the GCE signal and stars in the bulge. Here we report a binary population synthesis forward model that demonstrates that an MSP population arising from the accretion induced collapse (AIC) of O-Ne white dwarfs in Galactic bulge binaries can naturally reproduce the morphology, spectral shape, and intensity of the GCE signal while also obeying LMXB constraints. Synchrotron emission from MSP-launched cosmic ray electrons and positrons may simultaneously explain the mysterious, microwave "haze" from the inner Galaxy

astro-ph.HE↗

arXiv2021

The effect of kick velocities on the spatial distribution of millisecond pulsars and implications for the Galactic center excess

Recently it has become apparent that the Galactic center excess (GCE) is spatially correlated with the stellar distribution in the Galactic bulge. This has given extra motivation for the unresolved population of millisecond pulsars (MSPs) explanation for the GCE. However, in the "recycling" channel the neutron star forms from a core collapse supernovae that undergoes a random "kick" due to the asymmetry of the explosion. This would imply a smoothing out of the spatial distribution of the MSPs. We use N-body simulations to model how the MSP spatial distribution changes. We estimate the probability distribution of natal kick velocities using the resolved gamma-ray MSP proper motions, where MSPs have random velocities relative to the circular motion with a scale parameter of 77+/-6 km/s. We find that, due to the natal kicks, there is an approximately 10% increase in each of the bulge MSP spatial distribution dimensions and also the bulge MSP distribution becomes less boxy but is still far from being spherical.

astro-ph.HE↗

arXiv2021

A Pedagogical Review of the Vacuum Retarded Dipole Model of Pulsar Spin Down

Pulsars are rapidly spinning highly magnetised neutron stars. Their spin period is observed to decrease with time. An early analytical model for this process was the vacuum retarded dipole (VRD) by Deutsch (1955). This model assumes an idealised star and it finds that the rotational energy is radiated away by the electromagnetic fields. This model has been superseded by more realistic numerical simulations that account for the non-vacuum like surroundings of the neutron star. However, the VRD still provides a reasonable approximation and is a useful limiting case that can provide some qualitative understanding. We provide detailed derivations of the spin down and related electromagnetic field equations of the VRD solution. We also correct typographical errors in the general field equations and boundary conditions used by Deutsch (1955).

astro-ph.HE↗

arXiv2020

Comparing the Galactic Bulge and Galactic Disk Millisecond Pulsars

The Galactic Center Excess (GCE) is an extended gamma-ray source in the central region of the Galaxy found in Fermi Large Area Telescope (Fermi-LAT) data. One of the leading explanations for the GCE is an unresolved population of millisecond pulsars (MSPs) in the Galactic bulge. Due to differing star formation histories it is expected that the MSPs in the Galactic bulge are older and therefore dimmer than those in the Galactic disk. Additionally, correlations between the spectral parameters of the MSPs and the spin-down rate of the corresponding neutron stars have been observed. This implies that the bulge MSPs may be spectrally different from the disk MSPs. We perform detailed modelling of the MSPs from formation until observation. Although we confirm the correlations, we do not find they are sufficiently large to significantly differentiate the spectra of the bulge MSPs and disk MSPs when the uncertainties are accounted for. Our results demonstrate that the population of MSPs that can explain the gamma-ray signal from the resolved MSPs in the Galactic disk and the unresolved MSPs in the boxy bulge and nuclear bulge can consistently be described as arising from a common evolutionary trajectory for some subset of astrophysical sources common to all these different environments. We do not require that there is anything unusual about inner Galaxy MSPs to explain the GCE. Additionally, we use a more accurate geometry for the distribution of bulge MSPs and incorporate dispersion measure estimates of the MSPs' distances. We find that the elongated boxy bulge morphology means that some the bulge MSPs are closer to us and so easier to resolve. We identify three resolved MSPs that have significant probabilities of belonging to the bulge population.

astro-ph.HE↗

arXiv2019

Maximum Entropy Estimation of the Galactic Bulge Morphology via the VVV Red Clump

The abundance and narrow magnitude dispersion of Red Clump (RC) stars make them a popular candidate for mapping the morphology of the bulge region of the Milky Way. Using an estimate of the RC's intrinsic luminosity function, we extracted the three-dimensional density distribution of the RC from deep photometric catalogues of the VISTA Variables in the Via Lactea (VVV) survey. We used maximum entropy based deconvolution to extract the spatial distribution of the bulge from Ks-band star counts. We obtained our extrapolated non-parametric model of the bulge over the inner 40 by 40 degrees squared region of the Galactic centre. Our reconstruction also naturally matches onto a parametric fit to the bulge outside the VVV region and inpaints overcrowded and high extinction regions. We found a range of bulge properties consistent with other recent investigations based on the VVV data. In particular, we estimated the bulge mass to be in the range 13 to 17 billion solar masses, the X-component to be between 18% and 25% of the bulge mass, and the bulge angle with respect to the Sun-Galactic centre line to be between 18 and 32 degrees. Studies of the Fermi Large Area Telescope (LAT) gamma-ray Galactic centre excess suggests that the excess may be traced by Galactic bulge distributed sources. We applied our deconvolved density in a template fitting analysis of this Fermi-LAT GeV excess and found an improvement in the fit compared to previous parametric based templates.

astro-ph.GA↗

arXiv2019

Non-Parametric Density Reconstruction of the Galactic Bulge Area using Red Clump Stars in the VVV Survey

Studies of the red clump giant population in the inner Milky Way suggest the Galactic bulge/bar has a boxy/peanut/X-shaped structure as predicted by its formation via a disc buckling instability. We used a non-parametric method of estimating the Galactic bulge morphology that is based on maximum entropy regularisation. This enabled us to extract the three-dimensional distribution of the red giant stars in the bulge from deep photometric catalogues of the VISTA Variables in the Via Lactea (VVV) survey. Our high-resolution reconstruction confirms the well-known boxy/peanut/X-shaped structure of the bulge. We also find spiral arm structures that extend to around three kpc in front of and behind the bulge and are on different sides of the bulge major axis. We show that the detection of these structures is robust to the uncertainties in the luminosity function.

astro-ph.GA↗

arXiv2019

Strong Evidence that the Galactic Bulge is Shining in Gamma Rays

There is growing evidence that the Galactic Center Excess identified in the $\textit{Fermi}$-LAT gamma-ray data arises from a population of faint astrophysical sources. We provide compelling supporting evidence by showing that the morphology of the excess traces the stellar over-density of the Galactic bulge. By adopting a template of the bulge stars obtained from a triaxial 3D fit to the diffuse near-infrared emission, we show that it is detected at high significance. The significance deteriorates when either the position or the orientation of the template is artificially shifted, supporting the correlation of the gamma-ray data with the Galactic bulge. In deriving these results, we have used more sophisticated templates at low-latitudes for the $\textit{Fermi}$ bubbles compared to previous work and the three-dimensional Inverse Compton (IC) maps recently released by the ${\tt GALPROP}$ team. Our results provide strong constraints on Millisecond Pulsar (MSP) formation scenarios proposed to explain the excess. We find that an $\textit{admixture formation}$ scenario, in which some of the relevant binaries are $\textit{primordial}$ and the rest are formed $\textit{dynamically}$, is preferred over a primordial-only formation scenario at $7.6\sigma$ confidence level. Our detailed morphological analysis also disfavors models of the disrupted globular clusters scenario that predict a spherically symmetric distribution of MSPs in the Galactic bulge. For the first time, we report evidence of a high energy tail in the nuclear bulge spectrum that could be the result of IC emission from electrons and positrons injected by a population of MSPs and star formation activity from the same site.

astro-ph.HE↗

arXiv2017

Consistency Between the Luminosity Function of Resolved Millisecond Pulsars and the Galactic Center Excess

Fermi Large Area Telescope data reveal an excess of GeV gamma rays from the direction of the Galactic Center and bulge. Several explanations have been proposed for this excess including an unresolved population of millisecond pulsars (MSPs) and self-annihilating dark matter. It has been claimed that a key discriminant for or against the MSP explanation can be extracted from the properties of the luminosity function describing this source population. Specifically, is the luminosity function of the putative MSPs in the Galactic Center consistent with that characterizing the resolved MSPs in the Galactic disk? To investigate this we have used a Bayesian Markov Chain Monte Carlo to evaluate the posterior distribution of the parameters of the MSP luminosity function describing both resolved MSPs and the Galactic Center excess. At variance with some other claims, our analysis reveals that, within current uncertainties, both data sets can be well fit with the same luminosity function.

astro-ph.HE↗