arXiv subjects

Serena Valtolina

Publications and source records attributed to Serena Valtolina.

7 recordsLinked to original sources

arXiv2026

An updated constraint for the Gravitational Wave Background from the Gamma-ray Pulsar Timing Array

Fermi LAT observations of gamma-ray pulsars can be used to build a pulsar timing array (PTA) experiment to search for gravitational wave (GW) signals at nanohertz frequencies. At those frequencies, the dominant signal is expected to be a stochastic gravitational wave background (GWB) produced by the incoherent superposition of the quasi-monochromatic GW emissions from a population of supermassive black hole binaries. While the radio PTAs have recently announced compelling evidence for a GWB signal with a power law spectrum of strain amplitude $\approx2-3\times10^{-15}$ (at the frequency of $1 {\rm yr}^{-1}$), in 2022 an analysis of $12.5$ years of Fermi data for 35 pulsars led to an upper limit of $1\times10^{-14}$ for the GWB amplitude. The analysis was carried out on times-of-arrival (TOAs) obtained by folding from six months up to one year of photon observations. A photon-by-photon approach was also tested to infer constraints on the GWB amplitude from individual pulsars, but without accounting for the cross-pulsar correlations that a GWB would induce. Here, we reanalyse the same dataset using a regularized likelihood method that correctly models cross-pulsar correlations directly from the photons, while additionally marginalising over the uncertain pulse profile shape. While the two methods are not expected to have significant differences in sensitivity, we prove through simulations of gamma-ray PTA datasets that the photon-by-photon method for GWB recoveries is, statistically, more robust. The resulting upper limit obtained for the GWB strain amplitude is $1.2\times10^{-14}$, indicating that the improved method yields a consistent result with the previous analyses.

astro-ph.HE↗

arXiv2026

Timing Gamma-ray Pulsars using Gibbs Sampling

Timing analyses of gamma-ray pulsars in the Fermi Large Area Telescope data set can provide sensitive probes of many astrophysical processes, including timing noise in young pulsars, orbital period variations in redback binaries, and the stochastic gravitational wave background (GWB). These goals can require careful accounting of stochastic noise processes, but existing methods developed to achieve this in radio pulsar timing analyses cannot be immediately applied to the discrete gamma-ray arrival time data. To address this, we have developed a new method for timing gamma-ray pulsars, in which the timing model fit is transformed into a weighted least squares problem by randomly assigning each photon to an individual Gaussian component of a template pulse profile. These random assignments are then numerically marginalised over through a Gibbs sampling scheme. This method allows for efficient estimation of timing and noise model parameters, while taking into account uncertainties in the pulse profile shape. We simulated Fermi-LAT data sets for gamma-ray pulsars with power-law timing noise processes, showing that this method provides robust estimates of timing noise parameters. We also describe a Gaussian-process model for orbital period variations in black-widow and redback binary systems that can be fit using this new timing method. We demonstrate this method on the black-widow binary millisecond pulsar B1957+20, where the orbital period varies significantly over the LAT data, but which provides one of the most stringent gamma-ray upper limits on the GWB.

astro-ph.HE↗

arXiv2025

Regularizing the Pulsar Timing Array likelihood: A path towards Fourier Space

The recent announcement of evidence for a stochastic background of gravitational waves (GWB) in pulsar timing array (PTA) data has piqued interest across the scientific community. A combined analysis of all currently available data holds the promise of confirming the announced evidence as a solid detection of a GWB. However, the complexity of individual pulsar noise models and the variety of modeling tools used for different types of pulsars present significant challenges for a truly unified analysis. In this work we propose a novel approach to the analysis of PTA data: first a posterior distribution over Fourier modes is produced for each pulsar individually. Then, in a global analysis of all pulsars these posterior distributions can be re-used for a GWB search, which retains all information regarding the signals of interest without the added complexity of the underlying noise models or implementation differences. This approach facilitates combining radio and gamma-ray pulsar data, while reducing the complexity of the model and of its implementations when carrying out a GWB search with PTA data.

astro-ph.IM↗

arXiv2025

Dissecting the nanoHz gravitational wave sky: frequency-correlated anisotropy induced by eccentric supermassive black hole binaries

Revealing the nature of the nanoHz gravitational wave (GW) signal recently reported by Pulsar Timing Arrays (PTAs) collaborations around the world is the next goal of low-frequency GW astronomy. The signal likely originates from the incoherent superposition of GWs emitted by a cosmological population of supermassive black hole binaries (SMBHBs). Those binaries can be highly eccentric and/or strongly coupled to their nuclear environment, resulting in an attenuation of the overall GW signal at low frequencies. In this paper, we propose to use the correlation properties of the distributed GW power in the sky across the frequency spectrum as a smoking gun for eccentric SMBHBs thus allowing to break the spectral degeneracy between eccentricity and environmental effect. The simple underlying idea is that, contrary to circular binaries, eccentric ones emit a broadband spectrum thus resulting in similar sky maps at different frequencies. We first demonstrate the applicability of this simple concept on sky maps constructed directly from the theoretical sky distribution of the GWB power induced by realistic populations of SMBHBs. We then demonstrate the viability of this analysis on simulated SKA-like PTA data. By statistically comparing sky maps reconstructed from hundreds injected circular and highly eccentric SMBHB populations, we find that eccentricity can be detected at $3σ$ in more than $50\%$ of cases.

astro-ph.GA↗

arXiv2024

Source anisotropies and pulsar timing arrays

Pulsar timing arrays (PTA) hunt for gravitational waves (GW) by searching for the correlations that GWs induce in the time-of-arrival residuals from different pulsars. If the GW sources are of astrophysical origin, then they are located at discrete points on the sky. However, PTA data are often modeled, and subsequently analyzed, via a "standard Gaussian ensemble". That ensemble is obtained in the limit of an infinite density of vanishingly weak, Poisson-distributed sources. In this paper, we move away from that ensemble, to study the effects of two types of "source anisotropy". The first (a), which is often called "shot noise", arises because there are $N$ discrete GW sources at specific sky locations. The second (b) arises because the GW source positions are not a Poisson process, for example, because galaxy locations are clustered. Here, we quantify the impact of (a) and (b) on the mean and variance of the pulsar-averaged Hellings and Downs correlation. For conventional PTA sources, we show that the effects of shot noise (a) are much larger than the effects of clustering (b).

gr-qc↗

arXiv2024

Pulsar timing array source ensembles

The stochastic gravitational wave background for pulsar timing arrays is often modeled by a Gaussian ensemble which is isotropic and unpolarized. However, the Universe has a discrete set of polarized gravitational wave sources at specific sky locations. Can we trust that the Gaussian ensemble is an accurate description? To investigate this, we explicitly construct an ensemble containing $N$ individual binary sources with circular orbits. The orbital inclination angles are randomly distributed, hence the individual sources are elliptically polarized. We then compute the first two moments of the Hellings and Downs correlation, as well as the pulsar-averaged correlation mean and (cosmic) variance. The first moments are the same as for a previously studied ensemble of circularly polarized sources. However, the second moments, and hence the variances, are different for the two ensembles. While neither discrete source model is exactly described by a Gaussian ensemble, we show that in the limit of large $N$, the differences are small.

gr-qc↗

arXiv2024

Testing strengths, limitations and biases of current Pulsar Timing Arrays detection analyses on realistic data

State-of-the-art searches for gravitational waves (GWs) in pulsar timing array (PTA) datasets model the signal as an isotropic, Gaussian and stationary process described by a power-law. In practice, none of these properties are expected to hold for an incoherent superposition of GWs generated by a cosmic ensemble of supermassive black hole binaries (SMBHBs), which is expected to be the primary signal in the PTA band. We perform a systematic investigation of the performance of current search algorithms, using a simple power law model to characterize GW signals in realistic datasets. We use, as the baseline dataset, synthetic realisations of timing residuals mimicking the European PTA (EPTA) second data release (DR2). Thus, we include in the dataset uneven time stamps, achromatic and chromatic red noise and multi-frequency observations. We then inject timing residuals from an ideal isotropic, Gaussian, single power-law stochastic process and from a realistic population of SMBHBs, performing a methodical investigation of the recovered signal. We find that current search models are efficient at recovering the GW signal, but several biases can be identified due to the signal-template mismatch, which we identify via probability-probability (P-P) plots and quantify using Kolmogorov-Smirnov (KS) statistics. We discuss our findings in light of the signal observed in the EPTA DR2 and corroborate its consistency with an SMBHB origin.

astro-ph.HE↗