Lucas Guillemot

Context. Optical pulsations from millisecond pulsars (MSPs) have recently been detected in a transitional MSP and an accreting MSP. Evidence for optical pulsations has also been found for a rotation-powered redback MSP. Observations suggest that optical pulsed emission is produced by different mechanisms depending on whether an accretion disk is present in the system. Aims. We explored a sample of rotation-powered MSPs in close binaries (P _orb < 1 day; redbacks and black widows), searching for optical periodic signals with the SiFAP2 photometer mounted on the Telescopio Nazionale Galileo (TNG). Methods. We performed pulsation searches using the epoch-folding method with the most up-to-date (radio or gamma-ray) orbital solution for each system. These solutions were based either on previously published works or recent observations from Nançay, the Giant Metrewave Radio Telescope, the Parkes Observatory, and Fermi -LAT. Results. We update or confirm radio and gamma-ray ephemerides for five of the presented systems. No optical pulsations were detected with a significance exceeding a 3 σ confidence level in any of our searches, despite achieving sensitivities close to the instrument’s limit. This places correspondingly stringent upper limits on the systems’ optical pulsed magnitude and conversion efficiency. Our upper limits are consistent with the pulsed optical luminosity of optical pulsations from the redback PSR J2339-0533. Compared to accreting and transitional MSPs, redback and black widow systems show much lower efficiency in converting their spin-down power into pulsed optical emission. This supports the recent suggestion that the accretion disk around the neutron star plays a role in accelerating the particles responsible for producing bright optical pulsed emission.

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.

Context. The diversity of pulsar light curves and radio polarisation properties originates in the structure of the magnetic field close to the stellar surface. For millisecond pulsars, this complexity is particularly puzzling. Fortunately, some means exist to uncover the magnetic field topology that indeed impacts the emission within the magnetosphere but also on the surface through its hot spot thermal radiation. Aims. We aim at deducing a plausible magnetic field geometry for the millisecond pulsar J0437−4715 by using combined information from the soft X-ray hot spot geometry deduced from NICER observations by pulse profile modelling and from radio and γ -ray pulse profile fitting. We also check the consistency between the obtained geometry and the radio polarisation data. Methods. Our γ -ray light-curve shapes rely on the striped wind model, whereas the radio polarisation fits rely on the rotating vector model. The magnetosphere structure is obtained from dipolar force-free magnetosphere simulations. Results. We demonstrate that a slightly off-centred dipole augmented by a small-scale dipole located on one polar cap explains simultaneously the shape of the hot spot and the radio and γ -ray data with a magnetic obliquity of α ≈ (42 ± 5)° and a line-of-sight inclination angle of ζ ≈ (136 ± 5)°. Conclusions. Our simple dipole model reproduces all the radio and γ -ray characteristics of PSR J0437−4715, including its radio polarisation data. It shows that the radio emission could be produced in regions where the magnetic field is mainly of dipolar nature.

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.

Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement ? de 20 MeV à plus de 300 GeV. Avant la mise en service du LAT, six pulsars jeunes et énergétiques étaient connus dans le domaine ? . Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l’étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu’alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux ? pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d’étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d’amas globulaires, sans succès à présent. L’analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement ? est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non-identi?ées sont des MSPs, pour l’instant inconnus.

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.

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.

Four neutron star radius measurements have already been obtained by modeling the X-ray pulses of rotation-powered millisecond pulsars observed by the Neutron Star Interior Composition ExploreR (NICER). We report here the radius measurement of PSR J0614−3329 employing the same method with NICER and XMM-Newton data using Bayesian inference. For all different models tested, including one with unrestricted inclination prior, we retrieve very similar nonantipodal hot region geometries and radii. For the preferred model, we infer an equatorial radius of R eq = 10.2 9 − 0.86 + 1.01 km for a mass of M = 1.4 4 − 0.07 + 0.06 M ⊙ (median values with equal-tailed 68% credible interval), the latter being essentially constrained from radio timing priors obtained by MeerKAT. A more complex model, fitting the data equally well, resulted in a consistent inferred radius. We find that, for all different models, the pulse emission originates from two hot regions, one at the pole and the other at the equator. The resulting radius constraint is consistent with previous X-ray and gravitational wave measurements of neutron stars in the same mass range. Equation of state inferences, including previous NICER and gravitational wave results, slightly soften the equation of state with PSR J0614−3329 included and shift the allowed mass–radius region toward lower radii by ∼300 m, which is compatible with previous analyses to within less than one standard deviation.

Low-frequency radio data improve the sensitivity of pulsar timing arrays (PTAs) to propagation effects such as dispersion measure (DM) variations, enabling better noise characterization essential for detecting the stochastic gravitational wave background (GWB). We combined LOFAR (100-200 MHz) and NenuFAR (30-90 MHz) observations with the recent European and Indian PTA release (DR2new+) into a new dataset, DR2low, spanning ~11 years for 12 pulsars. DR2low allows updated noise models, increasing PTA sensitivity to the GWB. Using Libstempo and Enterprise, we applied standard noise models including red noise (RN) and time-variable DM (DMv) as power laws, and performed Bayesian model selection over RN, DMv, and an additional chromatic noise term (CN4). Compared to DR2new+, DR2low improves DM constraints and separates DM and RN contributions. 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 CN4 in eight pulsars, while none showed such evidence in DR2new+. The analysis also reveals unmodelled solar wind effects, particularly near solar conjunction, with residual delays absorbed into the DM component, highlighting the importance of accurately modelling the solar wind in PTA datasets.