Ismael Cognard

We use the Bayesian methods in the ENTERPRISE package to examine the fully general parameterization of pulsar timing models in tandem with noise. We investigate four pulsars, PSRs J1600–3053, J2043+1711, J0740+6620, and J1640+2224, through the lens of Bayesian timing. We selected these pulsars as they are well studied but exhibit interesting characteristics through the lens of Bayesian timing. We generally find good agreement between best-fit and maximum a posteriori values, and in many cases more constrained parameter estimations, when comparing our method to various other Bayesian and frequentist generalized timing methods. We find tighter constraints on the estimated pulsar mass for three pulsars when using NANOGrav 12.5 yr data releases as compared to their published results. These constraints (medians and 68% confidence intervals) for our fully general nonlinear Bayesian timing models are m _p = 1.6(1) M _⊙ for PSR J2043+1711 and m p = 2 . 3 − 0.7 + 0.9 M ⊙ for PSR J1600–3053 both using the NANOGrav 12.5 yr data release, and m _p = 2.06(6) M _⊙ for PSR J0740+6620 using the data from E. Fonseca et al. We investigate the effects of placing physical priors on timing model parameters, including restricting the upper limit on the pulsar mass for PSR J1640+2224, which has a mass often estimated to be greater than 3 M _⊙ . We find that this results in a pulsar mass of m _p = 2.2(5) M _⊙ for PSR J1640+2224 using the NANOGrav 12.5 yr data release. For the first time, we find evidence for intrinsic red noise in PSR J2043+1711. We show how fully general Bayesian timing can better model the interplay of the intrinsic noise and the timing parameters.

Rotating nonaxisymmetric neutron stars (NSs) are promising sources for continuous gravitational waves (CWs). CWs may, if detected, inform us about the internal structure and equation of state of NSs. Here, we present a narrowband search for CWs from known pulsars, for which a matched-filter search can be applied. Narrowband searches are robust to mismatches between electromagnetic (EM) and gravitational emissions, in contrast to fully targeted searches where they are assumed to be phase-locked. In this work, we search for the CW counterparts emitted by 34 pulsars using data from the first and second parts of the fourth LIGO–Virgo–KAGRA observing run. We use the 5n-vector narrowband pipeline, which applies frequency-domain matched filtering. In previous searches, it covered a narrow range in the frequency—frequency time derivative ( f — f ̇ ) space. Here, we also explore a range in the second time derivative of the frequency f ̈ around the EM observations. Additionally, for the first time, we target sources in a binary system with this kind of search. We find no evidence for CWs and therefore set upper limits on the strain amplitude emitted by each pulsar. For 20 analyses, we report an upper limit below the theoretical spin-down limit. The tightest constraint is for pulsar PSR J0534+2200 (the Crab pulsar), for which our strain upper limit on the CW amplitude corresponds to ≤0.04% of the spin-down power being radiated in the CW channel.

PSR J1455−3330 is a ∼7.98 ms pulsar in a ∼76.17 day nearly circular orbit with a white dwarf companion. In this work, we combine the available Lovell, Nançay decimetric Radio Telescope, Green Bank, and MeerKAT pulsar timing data spanning approximately 30 years to measure the kinematic and relativistic effects of PSR J1455−3330 to constrain its 3D orbital geometry and component masses. We detect a relativistic Shapiro delay signal. We measure a significant orthometric amplitude of h ₃ = 0.307 ^+0.022 _−0.026 μs and an orthometric ratio of ς = 0.551 ^+0.057 _−0.054 . We measured the change in projected semi-major axis, x ˙ = −202.1 ^+2.5 _−2.7 × 10 ⁻¹⁶ s s ⁻¹ , with high significance, parallax, ϖ = 1.11(6) mas, parallax derived distance 0.90(5) kpc, and a precise total proper motion magnitude of 12.432(2) mas yr ⁻¹ . A self-consistent analysis of all kinematic and relativistic effects, assuming general relativity, yielded two solutions: (1) a pulsar mass of M _p = 1.39 ^+0.38 _−0.18 M _⊙ , a companion mass of M _c = 0.293 ^+0.056 _−0.026 M _⊙ , an orbital inclination of i = 63(2)°, and a longitude of the ascending node of Ω = 212(12)° or (2) a pulsar mass of M _p = 1.53 ^+1.10 _−0.22 M _⊙ , a companion mass of M _c = 0.309 ^+0.163 _−0.026 M _⊙ , an orbital inclination of i = 123(4)°, and a longitude of the ascending node of Ω = 334(12)°. All uncertainties represent the 68.27% credibility region. These results strongly favour a helium-dominated white dwarf companion.

The Fermi Large Area Telescope (LAT) provides advantages for radio pulsar searches by enabling efficient target selection. We can confidently point radio telescopes to the positions of Fermi unidentified gamma-ray sources that have a high probability of hosting a pulsar. As part of Transients and Pulsars with MeerKAT (TRAPUM), we conducted a survey of Fermi-LAT sources using the Ultra High Frequency (UHF; 544–1088 MHz) receiver of the MeerKAT radio telescope. We observed 79 sources that were identified as pulsar-like candidates using a random forest technique from the Fermi-LAT Fourth Source Catalogue. We observed each target for 10 minutes at two separate epochs. As a result, we discovered nine new millisecond pulsars (MSPs) and six slow pulsars. Based on the radio discoveries, we also searched for gamma-ray pulsations, confirming that seven of the newly discovered MSPs are associated with Fermi-LAT sources, and performed joint radio and gamma-ray pulsar timing. Companion mass estimates and evidence of radio eclipses indicate that among the nine MSPs there are three black widows and three redbacks. Lastly, we compared the discovered pulsars in the MeerKAT UHF survey against the previous Fermi sources TRAPUM survey at L band, concluding the superiority of UHF observations in sensitivity to fainter pulsars and in detection rate than L band for finding new gamma-ray MSPs.

We present 15 yr of Nançay and Green Bank radio telescope timing observations for PSR J1231−1411. This millisecond pulsar is a primary science target for the Neutron Star Interior Composition Explorer telescope (NICER, which discovered its X-ray pulsations), has accumulated near-continuous γ -ray data since the Fermi-Large Area Telescope’s launch, and has been studied extensively with the Green Bank and Nançay radio telescopes. We have undertaken a campaign with the Green Bank Telescope targeting specific orbital phases designed to improve our constraint on the pulsar’s mass through the detection of a relativistic Shapiro delay. Both frequentist and Bayesian techniques—the latter incorporating priors from white dwarf binary evolution models—are applied to 15 yr of radio observations, yielding relatively weak constraints on the companion and pulsar masses of 0.2 3 − 0.06 + 0.09 M _⊙ and 1.8 7 − 0.67 + 1.11 M _⊙ , respectively (68.3% CI from Bayesian fits); however, the orbital inclination is measured to better relative precision ( 79.8 0 − 4.70 + 3.47 °). Restricting the maximum allowed pulsar mass to 3 M _⊙ improves the constraint and lowers the measured mass to 1.7 1 − 0.56 + 0.70 M _⊙ . A fully generalized Bayesian fit that simultaneously samples the noise and timing models yields a pulsar mass in close agreement with this value. While our radio-derived inclination result has informed recent NICER X-ray studies of J1231−1411, the lessons learned from this troublesome pulsar will also bolster future high-precision mass measurement campaigns and resulting constraints on the neutron star interior equation of state.

We perform searches for gravitational-wave memory in the data of two major Pulsar Timing Array (PTA) experiments located in Europe and Australia. Supermassive black hole binaries (SMBHBs) are the primary sources of gravitational waves in PTA experiments. We develop and carry out the first search for late inspirals and mergers of these sources based on full numerical relativity waveforms with null (nonlinear) gravitational-wave memory. Additionally, we search for generic bursts of null gravitational-wave memory, exploring the possibilities of reducing the computational costs of these searches through kernel density estimation and normalizing flow approximations of the posteriors. We rule out mergers of SMBHBs with a chirp mass of 10 ¹⁰ M _⊙ up to 700 Mpc over 18 yr of observation at 95% credibility. We rule out the observation of generic displacement memory bursts with strain amplitudes >10 ⁻¹⁴ in brief periods of observation time but across the sky, or over the whole observation time but for certain preferred sky positions, at 95% credibility.

We perform searches for gravitational wave memory in the data of two major Pulsar Timing Array (PTA) experiments located in Europe and Australia. Supermassive black hole binaries (SMBHBs) are the primary sources of gravitational waves in PTA experiments. We develop and carry out the first search for late inspirals and mergers of these sources based on full numerical relativity waveforms with null (nonlinear) gravitational wave memory. Additionally, we search for generic bursts of null gravitational wave memory, exploring possibilities of reducing the computational cost of these searches through kernel density and normalizing flow approximation of the posteriors. We rule out the mergers of SMBHBs with a chirp mass of 10^10 Solar Mass up to 700 Mpc over 18 years of observation at 95% credibility. We rule out the observation of generic displacement memory bursts with strain amplitudes > 10^-14 in brief periods of the observation time but across the sky, or over the whole observation time but for certain preferred sky positions, at 95%$credibility.

Context. Low radio frequency data are highly valuable for enhancing the sensitivity of pulsar timing arrays (PTAs) to propagation effects, such as dispersion measure (DM) variations. These low-frequency observations are particularly sensitive to DM fluctuations and can therefore significantly improve noise characterization in PTA datasets, which is essential for detecting the stochastic gravitational wave background (GWB). Aims. For this work we incorporated for the first time low-frequency observations from LOFAR (100 − 200 MHz) and NenuFAR (30 − 90 MHz) into a PTA context by combining them with the most recent data release from the European and Indian PTAs (in particular, with the subsample labeled DR2new+ , which includes only data from the new backends). This new combined dataset, labeled DR2low , consists of 12 pulsars observed over a time span of ∼11 years, with radio frequencies spanning the range 30 − 2500 MHz. The expanded frequency coverage of DR2low enables us to update and refine the noise models of DR2new+ , and this is crucial in order to increase the PTA sensitivity when searching for the stochastic gravitational wave background, which is the primary goal of PTA observations. This work is a milestone in the integration of low-frequency data into the upcoming third data release of the International PTA, which is posed to achieve the 5 σ detection of the GWB. Methods. We used the pulsar timing software packages L IBSTEMPO and E NTERPRISE to perform a noise analysis of DR2low . At first, we applied a standard noise model including red noise (RN) and time-variable dispersion measure (DMv) as power laws, with Fourier components up to 30 and 100 frequencies, respectively. Next, we performed a fully Bayesian model selection to identify the favored noise model for each pulsar and compute the Bayes factors across all combinations of RN, DMv, and a noise term with a chromatic index of 4 (CN ₄ ). Finally, we carried out a detailed analysis on the choice of the chromatic index for CN ₄ and the contribution of the solar wind. Results. The comparison between DR2low and DR2new+ using the standard noise model highlights the benefits of including low-frequency data. In particular, the additional frequency coverage improves the constraints on the DM variations and helps disentangle the DM and RN noise components in most pulsars. Through a Bayesian model selection, we found that the RN is required in the final model for 10 out of 12 pulsars, compared to only 5 in the DR2new+ dataset. The improved sensitivity to plasma effects provided by DR2low also favors the identification of significant CN ₄ in eight pulsars, while none showed such evidence in DR2new+ . The chromatic index of this process is consistent with four of the five pulsars, while two (PSRs J0030+0451 and J1022+1001) show significant deviations from such a value. We attribute this discrepancy to unmodeled contributions from the solar wind, especially because of the high DM sensitivity of LOFAR and NenuFAR and the high observing cadence provided by these datasets near solar conjunction. A dedicated analysis confirms that the current solar wind model fails to fully capture the observed delay, and residual power is absorbed into the DM component of the model.