arXiv subjects

Anna-Malin Lemke

Publications and source records attributed to Anna-Malin Lemke.

7 recordsLinked to original sources

arXiv2026

Probing Quadratically Coupled Ultralight Dark Matter with the Laser Interferometer Space Antenna

Ultralight dark matter can interact with Standard Model particles via gravitational and non-gravitational interactions. Through such interactions, it can leave distinctive signals in gravitational-wave experiments. In this work, we investigate signals induced by ultralight dark matter quadratically coupled to the Standard Model in the future space-borne gravitational-wave detector, the Laser Interferometer Space Antenna (LISA). Due to the quadratic nature of the coupling, dark matter signals appear at two distinct frequencies: the frequency corresponding to twice the dark matter mass, and frequencies below the typical dark matter kinetic energy. We analyze both contributions and show that LISA can surpass current constraints from terrestrial and astrophysical probes in certain mass ranges. We also find that dark matter signals in LISA are free from screening effects which significantly limit the sensitivity of terrestrial experiments.

hep-ph↗

arXiv2026

Detecting Gravitational-Wave Anisotropies with Simulation-Based Inference

Over the last five years, multiple Pulsar Timing Array (PTA) collaborations have reported mounting evidence for a gravitational-wave background (GWB) at nanohertz frequencies. Measuring anisotropies in the sky distribution of the GWB power is one of the most promising ways to identify and characterize its source. These anisotropies are expected to manifest as deviations from the Hellings-Downs (HD) correlations between the timing residuals of different pulsars. Current search strategies include Bayesian methods, which model anisotropies in the timing residuals likelihood, and faster frequentist approaches, which construct correlation estimators from timing residuals and use these to test the isotropic assumption. However, frequentist methods rely on the assumption that correlation estimators are Gaussian-distributed, an assumption that is not justified and that -- as we will show -- severely limits detection sensitivity. In this work, we present a Simulation-Based Inference (SBI) framework that replaces the analytic Gaussian likelihood used in frequentist searches with a neural network classifier trained on synthetic data. This approach captures the non-Gaussian structure of the data and significantly improves performance. Specifically, we find that the probability of $3σ$ detection increases by approximately 90% for single-hotspot scenarios and by 200% for double-hotspot scenarios compared to standard frequentist methods.

astro-ph.CO↗

arXiv2026

Prospects and Limitations of PTAs Anisotropy Searches -- The Frequentist Case

Recent findings from several Pulsar Timing Array (PTA) collaborations point to the existence of a Gravitational Wave Background (GWB) at nanohertz frequencies. A key next step towards characterizing this signal and identifying its origin is to map the sky distribution of its power. Several strategies have been proposed to reconstruct this distribution using PTA data. In this work, we compare these different strategies to determine which one is best suited to detect GWB anisotropies of different topologies. We find that, for both localized and large-scale anisotropies, reconstruction methods based on pixel and radiometer maps are the most promising. However, in both scenarios, even the optimistically large anisotropic signals discussed in this work remain challenging to detect with near-future PTA sensitivities. For example, we find that for a GWB hotspot contributing to $80\%$ of the GWB power in the second frequency bin, detection probabilities reach at most $\mathcal{O}(10\%)$ for a PTA with noise properties comparable with the ones of the upcoming IPTA third data release. Finally, we consider the fundamental limitations that cosmic variance poses to these kinds of searches by deriving the smallest deviations from isotropy that could be detected by an idealized PTA with no experimental or pulsar noise.

astro-ph.CO↗

arXiv2025

Mapping the Gravitational-wave Background Across the Spectrum with a Next-Generation Anisotropic Per-frequency Optimal Statistic

With pulsar timing arrays (PTAs) having observed a gravitational wave background (GWB) at nanohertz frequencies, the focus of the field is shifting towards determining and characterizing its origin. While the primary candidate is a population of GW-emitting supermassive black hole binaries (SMBHBs), many other cosmological processes could produce a GWB with similar spectral properties as have been measured. One key argument to help differentiate an SMBHB GWB from a cosmologically sourced one is its level of anisotropy; a GWB sourced by a finite population will likely exhibit greater anisotropy than a cosmological GWB through finite source effects (``shot noise'') and potentially large-scale structure. Current PTA GWB anisotropy detection methods often use the frequentist PTA optimal statistic for its fast estimation of pulsar pair correlations and relatively low computational overhead compared to spatially-correlated Bayesian analyses. However, there are critical limitations with the status quo approach. In this paper, we improve this technique by incorporating three recent advancements: accounting for covariance between pulsar pairwise estimates of correlated GWB power; the per-frequency optimal statistic to dissect the GWB across the spectrum; and constructing null-hypothesis statistical distributions that include cosmic variance. By combining these methods, our new pipeline can localize GWB anisotropies to specific frequencies, through which anisotropy detection prospects -- while impacted by cosmic variance -- are shown to improve in our simulations from a $p$-value of $\sim0.2$ in a broadband search to $\sim0.01$ in the per-frequency search. Our methods are already incorporated in community-available code and ready to deploy on forthcoming PTA datasets.

astro-ph.IM↗

arXiv2025

The impact of cosmic variance on PTAs anisotropy searches

Several Pulsar Timing Array (PTA) collaborations have recently found evidence for a Gravitational Wave Background (GWB) by measuring the perturbations that this background induces in the time-of-arrivals of pulsar signals. These perturbations are expected to be correlated across different pulsars and, for isotropic GWBs, the expected values of these correlations (obtained by averaging over different GWB realizations) are a simple function of the pulsars' angular separations, known as the Hellings-Downs (HD) correlation function. On the other hand, anisotropic GWBs would induce deviations from these HD correlations in a way that can be used to search for anisotropic distributions of the GWB power. However, even for isotropic GWBs, interference between GW sources radiating at overlapping frequencies induces deviations from the HD correlation pattern, an effect known in the literature as "cosmic variance". In this work, we study the impact of cosmic variance on PTA anisotropy searches. We find that the fluctuations in cross-correlations related to cosmic variance can lead to the miss-classification of isotropic GWBs as anisotropic, leading to a false detection rate of ~50% for frequentist anisotropy searches. We also observe that cosmic variance complicates the reconstruction of the GWB sky map, making it more challenging to resolve bright GW hotspots, like the ones expected to be produced from a Supermassive Black Hole Binaries population. These findings highlight the need to refine anisotropy search techniques to improve our ability to reconstruct the GWB sky map and accurately assess the significance of any isotropy deviations we might find in it.

astro-ph.CO↗

arXiv2024

Detecting Gravitational Wave Anisotropies from Supermassive Black Hole Binaries

Several Pulsar Timing Array (PTA) collaborations have recently found evidence for a gravitational wave background (GWB) permeating our galaxy. The origin of this background is still unknown. Indeed, while the gravitational wave emission from inspiraling supermassive black hole binaries (SMBHB) is the primary candidate for its origin, several cosmological sources have also been proposed. One distinctive feature of SMBHB-generated backgrounds is the presence of GWB anisotropies stemming from the binaries distribution in the local Universe. However, none of the anisotropy searches performed to date reported a detection. In this work, we show that the lack of anisotropy detection is not currently in tension with a SMBHB origin of the background. We accomplish this by calculating the anisotropy detection probability of present and future PTAs. We find that a PTA with the noise characteristics of the NANOGrav 15-year data set had only a $~2\%-11\%$ probability of detecting SMBHB-generated anisotropies, depending on the properties of the SMBHB population. However, we estimate that for the IPTA DR3 data set these probabilities will increase to $~4\%-28\%$, putting more pressure on the SMBHB interpretation in case of a null detection. We also identify SMBHB populations that are more likely to produce detectable levels of anisotropies. This information could be used together with the spectral properties of the GWB to characterize the SMBHB population.

astro-ph.HE↗

arXiv2024

Beyond the Background: Gravitational Wave Anisotropy and Continuous Waves from Supermassive Black Hole Binaries

Pulsar timing arrays have found evidence for a low-frequency gravitational wave background (GWB). Assuming the GWB is produced by supermassive black hole binaries (SMBHBs), the next gravitational wave (GW) signals astronomers anticipate are Continuous Waves (CWs) from single SMBHBs and their associated GWB anisotropy. The prospects for detecting CWs and anisotropy are highly dependent on the astrophysics of SMBHB populations. Thus, information from single sources can break degeneracies in astrophysical models and place much more stringent constraints than the GWB alone. We simulate and evolve SMBHB populations, model their GWs, and calculate their anisotropy and detectability. We investigate how varying components of our semi-analytic model, including the galaxy stellar mass function, the SMBH--host galaxy relation ($M_\mathrm{BH}$--$M_\mathrm{bulge}$), and the binary evolution prescription impact the expected detections. The CW occurrence rate is greatest for few total binaries, high SMBHB masses, large scatter in $M_\mathrm{BH}$--$M_\mathrm{bulge}$, and long hardening times. The occurrence rate depends most on the binary evolution parameters, implying that CWs offer a novel avenue to probe binary evolution. The most detectable CW sources are in the lowest frequency bin for a 16.03-year PTA, have masses from $\sim\!\!10^9-10^{10}\mathrm{M}_\odot$, and are $\sim\!\!1$ Gpc away. The level of anisotropy increases with frequency, with the angular power spectrum over multipole modes $\ell$ varying in low-frequency $C_{\ell>0}/C_0$ from $\sim\!\!5\times 10^{-3}$ to $\sim\!\!2\times10^{-1}$, depending on the model; typical values are near current upper limits. Observing this anisotropy would support SMBHB models for the GWB over cosmological models, which tend to be isotropic.

astro-ph.HE↗