Denis González-Caniulef

Context. The detection of likely thermal ultraviolet emission from a few old neutron stars suggests that at least one internal heating mechanism is present in these stars. One such proposed mechanism is rotochemical heating, which describes a scenario where the continuous contraction of the neutron star due to its spin-down produces chemical imbalances that induce Urca reactions, resulting in the latter depositing heat in the neutron star core. If the protons in the star are superconducting, their energy gap suppresses the reactions, except in microscopic magnetized regions (such as quantized flux tubes), where the protons act as if they were normal. Therefore, the strength of the internal magnetic field controls the rate at which reactions proceed and thus affects the thermal evolution of the neutron star. Aims. Here, we present the first comprehensive study of the effect of an internal magnetic field in the superconducting interior on rotochemical heating. Methods. We simulated the evolution of neutron stars for different internal magnetic field strengths and neutron energy gaps, and we compared the results to Hubble Space Telescope observations of old neutron stars. Results. Within the proposed model, the observationally inferred temperatures of the millisecond pulsar (MSP) PSR J0437−4715 and the classical pulsar (CP) PSR B0950+08 as well as the upper limits for the MSP PSR J2124−3358 and the CPs PSR J0108−1431 and PSR J2144−3933 can be accounted for if the proton energy gap is large (∼1.5 MeV) and the neutron energy gap is small (≲0.1 MeV) or vanishing, while the MSP PSR J0437−4715 needs to have a very weak internal magnetic field. Conclusions. Our results suggest that rotochemical heating alone can account for the observed thermal emission of old neutron stars if neutron-star cores are characterized by a large proton pairing gap and a small or vanishing neutron gap, and MSPs have very weak internal magnetic fields.

Pulse-profile modeling of rotation-powered millisecond pulsars targeted by NICER has enabled mass–radius constraints of several neutron star sources, with implications for the dense-matter equation of state. For the bright isolated pulsar PSR J0030+0451, the inferred mass and radius were previously found to depend strongly on the assumed hot-spot model. These hot-spot models yielded different mass–radius constraints, with the statistically preferred model exhibiting some mild tension with results inferred for PSR J0437−4715, PSR J0614−3329, and GW170817. We present an updated pulse-profile analysis of PSR J0030+0451 using new NICER observations obtained between 2017 July to 2023 January, increasing the number of X-ray counts by about 50% compared to previous analyses. We jointly analyze the NICER data with archival XMM-Newton observations to better constrain the source spectrum and background. The new analysis, using more conservative nested-sampling settings together with the new NICER observations, significantly reduces the discrepancy between the hot-spot models. The inferred mass and radius are M = 1.4 3 − 0.17 + 0.20 M ⊙ and R eq = 12.6 8 − 1.04 + 1.31 km (68% credible intervals), reducing the tension with the results from other sources. In addition, the inferred hot-spot configurations suggest the presence of intraspot temperature gradients.

Passively cooling neutron stars (NSs) are expected to reach undetectably low surface temperatures, T _s < 10 ⁴ K, within less than 10 ⁷ yr. However, Hubble Space Telescope (HST) observations have revealed likely thermal ultraviolet emission from the gigayear-old millisecond pulsars PSR J0437−4715 and PSR J2124−3358 and the ∼10 ^7 − 8 year-old classical pulsars PSR B0950+08 and PSR J0108−1431, implying temperatures of T _s ∼ 10 ⁵ K and thus suggesting the presence of some heating mechanism. In this work, the thermal evolution for each of these NSs was computed considering rotochemical heating in the NS core with normal or Cooper-paired matter, vortex creep in the inner crust, and crustal heating through nuclear processes. The results were contrasted with the observations while also including the stringent upper limit on the temperature of PSR J2144−3933. No single heating mechanism alone was found to account for all the observations. The high temperature of PSR J0437−4715 can be explained by rotochemical heating in the presence of a large Cooper pairing gap, Δ _i ∼ 1.5 MeV, for either neutrons or protons (or an equivalent combination of both), but this mechanism would require an unrealistically short initial rotation period, P ₀ ≲ 1.8 ms, to account for the high temperature of the classical pulsar PSR B0950+08. Conversely, the latter can be explained by rotochemical heating with modified Urca reactions in normal matter or by vortex creep with an excess angular momentum parameter, J ∼ 3 × 10 ⁴³ erg s, but these models are insufficient to account for the former. However, a model that includes rotochemical heating with a large Cooper pairing gap together with vortex creep is consistent with the temperature measurements of these two pulsars as well as the upper limits for the other three. Moreover, this scenario predicts that the temperatures of the other three pulsars should be close to these upper limits, suggesting that deeper observations and/or a wider wavelength coverage for these and other old nearby pulsars should yield a strong probe of this model.

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.

The Crab pulsar experienced two relatively small glitches separated by only 20 days in 2025 September and October. Imaging X-ray Polarimetry Explorer (IXPE) observed the source twice, with delay times since the glitch epoch ranging between 35 and 75 days, depending on the observation. We carried out a multimethod analysis to investigate whether there is evidence for significant changes in the polarization properties of the pulsar, underlying possible variations in the pulsar magnetosphere itself following the glitches. Specifically, we performed (1) phase-averaged polarimetry of the Crab pulsar before and after the glitches, following an approach similar to that adopted in 2019 by PolarLight, a nonimaging CubeSat-class photoelectric polarimeter which observed a change in the X-ray polarization within 100 days after a stronger glitch in 2019 July, and (2) a comparison, before and after the glitch, of phase-resolved X-ray polarimetry with IXPE, not possible with PolarLight. Furthermore, we investigated, by means of phase-resolved optical (OPTIMA) polarimetry, whether a significant change in the X-to-optical lag was present in the data before and after the glitch. We find no evidence of a change in the polarization for the pulsar emission before and after the glitch. We use the upper limits obtained to estimate the maximum change in magnetic obliquity allowed by the data, using the standard rotating vector model and assuming that the glitch is due to a neutron-star quake. We constrain this maximum change to be no greater than ±4° at the 95% confidence level.

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.

Magnetars are neutron stars that host huge, complex magnetic fields which require supporting currents to flow along the closed field lines. This makes magnetar atmospheres different from those of passively cooling neutron stars because of the heat deposited by backflowing charges impinging on the star surface layers. This particle bombardment is expected to imprint the spectral and, even more, the polarisation properties of the emitted thermal radiation. We present solutions for the radiative transfer problem for bombarded plane-parallel atmospheres in the high magnetic field regime. The temperature profile is assumed a priori, and selected in such a way to reflect the varying rate of energy deposition in the slab (from the impinging currents and/or from the cooling crust). We find that thermal X–ray emission powered entirely by the energy released in the atmosphere by the magnetospheric back–bombardment is linearly polarised and X-mode dominated, but its polarisation degree is significantly reduced (down to 10%–50%) when compared with that expected from a standard atmosphere heated only from the cooling crust below. By increasing the fraction of heat flowing in from the crust the polarisation degree of the emergent radiation increases, first at higher energies (∼10 keV) and then in the entire soft X-ray band. We use our models inside a ray-tracing code to derive the expected emission properties as measured by a distant observer and compare our results with recent IXPE observations of magnetar sources.

We develop two new highly efficient estimators to measure the polarization (Stokes parameters) in experiments that constrain the position angle of individual photons such as scattering and gas-pixel-detector polarimeters, and analyse in detail a previously proposed estimator. All three of these estimators are at least fifty percent more efficient on typical datasets than the standard estimator used in the field. We present analytic estimates of the variance of these estimators and numerical experiments to verify these estimates. Two of the three estimators can be calculated quickly and directly through summations over the measurements of individual photons.