Manuel Linares

Transitional millisecond pulsars (tMSPs) exhibit a unique subluminous disk state, at the crossroads between accretion and rotation power, where they switch between two distinct X-ray modes. We present the discovery of gamma-ray modes in PSR J1023+0038, the first confirmed tMSP, from stacking Fermi-LAT data during the modes (which we identify using simultaneous X-ray observations). Surprisingly, we find that gamma rays and X-rays are anticorrelated during this mode switching: the gamma-ray flux is higher in the X-ray low mode, and vice versa. This contradicts the state-of-the-art model, which predicts bright gamma rays only in the X-ray high mode, from the interaction between the pulsar wind and surrounding disk via synchrotron and inverse Compton processes. Because the pulsar wind is likely absent in the gamma-ray high (X-ray low) mode, which is also brighter in the radio band, we suggest that jet emission may be dominant in GeV gamma rays. We also discuss an alternative interpretation where the gamma-ray high mode is powered by self-synchrotron Compton emission in the propeller state.

Transitional millisecond pulsars (tMSPs) alternate between radio and X-ray pulsar states, and can represent the missing link between rotation- and accretion-powered neutron stars. Their disk state switches stochastically between low and high X-ray modes, both of unknown physical origin and less luminous than low-mass X-ray binaries. To reveal the source of the X-ray emission, we carry out 2D axisymmetric general-relativistic magnetohydrodynamical simulations of the interaction between an accretion disk and a tMSP magnetosphere. For the first time, we postprocess tMSP simulations with a radiative transfer code that incorporates thermal synchrotron, absorption, and Compton scattering processes. By varying the disk density, hence the inflow rate, we explore two disk regimes: one truncated outside and another inside the light cylinder. In the former, most of the X-ray flux comes from the synchrotron emission powered by the wind heating the disk: this “wind” regime could correspond to the high X-ray mode . The latter is the classical propeller regime and lacks this heating. However, the propeller episodically expels the disk, activating the wind heating: a 70%–30% mixture of such propeller and wind regimes reproduces the X-ray spectrum of the low X-ray mode . The excess electromagnetic torque in the propeller regime increases the spin-down rate, averaged over both modes, by a few percent above the disk-free radio pulsar state, in agreement with observations. Overall, the system is more luminous in X-rays when the flow is truncated outside the light cylinder and supports a contribution from wind-induced disk heating in both low and high X-ray modes.

Binary millisecond pulsars (MSPs) in globular clusters (GCs) are key for binary and stellar evolution studies under extreme conditions. The identification of their optical companion stars is instrumental in order to characterise these systems and to constrain the possible recycling mechanisms. For this work, we searched for the optical counterpart to PSR J1717+4308A (hereafter M92A) in the GC M92. To this end, we exploited a multi-epoch, multi-wavelength dataset obtained with the Hubble Space Telescope. We constructed colour–magnitude diagrams, investigated proper motions to assess cluster membership, and modelled the observed light curves. We identified an object located at only 0.02 arcsec from the nominal radio position as the likely optical companion to M92A. The star is significantly bluer than the main sequence at the same luminosity level and exhibits clear photometric variability with a periodicity in agreement with the orbital motion of the binary. The light curve displays two maxima and two minima, indicative of strong tidal distortion and only mild irradiation. Such mild irradiation is consistent with the ratio of the pulsar spin-down to the companion flux ( f _sd ), which for M92A lies close to the boundary between ellipsoidal- and irradiation-dominated regimes ( f _sd ≈ 2.71). From the light curve modelling we inferred the main physical properties of the companion star. The best-fit model indicates a high-inclination system with a relatively low-mass companion and a massive neutron star. With a base temperature of ~7200 K, the companion ranks among the hottest redbacks known to date. This object therefore adds additional pieces to the puzzle of MSP companion properties and contributes to outlining the characteristics of redbacks across the different classes.

Compact binary millisecond pulsars (also known as spiders) allow us to probe pulsar winds in their innermost regions, between the light cylinder (radius ∼10 ⁷ cm) and the companion star (at ∼10 ¹¹ cm). Their flux is known to vary along the orbit, from radio to X-rays. During the past decade, gamma-ray orbital modulation (GOM) has been discovered in a handful of spiders, but its origin remains largely unknown. We present the results of a systematic search for GOM among 43 systems, selecting pulsed 0.1–1 GeV photons and using spin and orbital ephemeris from Fermi’s Third Pulsar Catalog. We discover GOM from three spiders—PSR J1124-3653, PSR J1946-5403, and PSR J2215+5135—and confirm four previous detections. In all seven cases so far, the GOM peaks near the pulsar’s superior conjunction. The X-ray orbital light curves are usually in antiphase, peaking when the pulsar is at inferior conjunction, but we find one case where both gamma rays and X-rays peak around superior conjunction: PSR J1946-5403. We measure the modulated fractions of the GOM and find consistent values for all seven spiders, with an average 22.0% ± 2.6%. Including eclipsing systems seen edge-on, we find no clear dependence of the modulated fraction on the orbital inclination (within ≃45°–90°). Our results challenge previous models proposed to explain GOM in spiders, based on inverse Compton and synchrotron emission close to the companion, since these predict a clear dependence on orbital inclination (stronger modulation at high inclinations). We nearly double the number of GOM detections in spiders, showing that it is more common than previously thought.

We present the main results obtained from the Compact Binary Pulsar Search in the Low-Latitude Neighborhood, an optical photometric survey designed to find new “spider” binary millisecond pulsars. We conducted observations targeting 30 unidentified sources from the 4FGL-DR3 Fermi Large Area Telescope catalog, selected for their pulsar-like γ -ray properties. Extending to Galactic latitudes as low as ±3°, this survey reaches closer to the Galactic plane than its predecessor survey, the Compact Binary Pulsar Search. We report the discovery of five optical variables coincident with the localizations of 4FGL J0821.5−1436, 4FGL J1517.9−5233, 4FGL J1639.3−5146, 4FGL J1748.8−3915, and 4FGL J2056.4+3142. These systems show optical flux modulation at the presumed orbital periods of 0.41576(6) days, 0.305(2) days, 0.204(7) days, 0.3(2) days, and 0.4395(1) days, respectively, and photometric temperatures of 4000–6000 K, consistent with the companion stars of the “redback” subtype of spider pulsar binaries. Based on their optical light curve shapes and X-ray properties characteristic for spider systems—namely, a luminosity of 1.5 × 10 ³² ( D /3.9 kpc) ² erg s ⁻¹ (0.3–10 keV) for 4FGL J1748.8−3915, and upper limits of ∼10 ³¹ –10 ³³ erg s ⁻¹ (0.2–12 keV) for the others—we classify these sources as new spider candidate systems.

We present SpiderCat, a multiwavelength catalog of all publicly known compact binary millisecond pulsars (MSPs) in the Galactic field. These systems, colloquially known as “spiders,” consist of neutron stars in tight orbits with low-mass companions, which are gradually ablated by the pulsar wind. SpiderCat includes both primary subclasses—redbacks and black widows—distinguished by companion mass, as well as candidates and peculiar systems such as transitional, huntsman, and tidarren MSPs. As of this initial release, SpiderCat contains 111 entries: 30 redbacks, 50 black widows, two huntsmans, 23 redback candidates, five black widow candidates, and one huntsman candidate. In this paper, we compile and summarize key parameters for each system, including spin and orbital properties, and multiwavelength data from radio, optical, X-ray, and γ -ray observations. An interactive, publicly accessible web interface, at https://astro.phys.ntnu.no/SpiderCAT , enables exploration and visualization of the data. The rapid growth of the number of known spiders, accelerated by the Fermi Large Area Telescope survey and its ability to identify MSPs in γ rays, has opened the door to population-level studies. Utilizing SpiderCat, we analyze trends in spin period, orbital period, companion mass, emission properties, and spatial distribution. SpiderCat serves as a dynamic, multiwavelength repository for this unique class of binary pulsars, facilitating new discoveries and constraints on pulsar evolution, particle acceleration, and the neutron star equation of state.

Black widow and redback systems are millisecond pulsars in compact orbits with ultra-light and low-mass companions, respectively, collectively known as “spider pulsars.” In such systems, an intrabinary shock can form between the pulsar and the companion winds, serving as a site for particle acceleration and associated nonthermal emission. Assuming that protons can be extracted from the neutron star surface and accelerated at the intrabinary shock and/or within the pulsar wind, we model the very high-energy gamma-ray and neutrino emissions (0.1–10 ³ TeV) produced through interactions with the companion wind and the companion star. We first calculate the high-energy emissions using an optimistic combination of parameters to maximize the gamma-ray and neutrino fluxes. We find that, for energetic spider pulsars with a spin-down power ≳10 ³⁵ erg s ⁻¹ and a magnetic field of ∼10 ³ G in the companion region, the gamma-ray emission could be detectable as point sources by the Cherenkov Telescope Array and the Large High Altitude Air Shower Observatory, while the neutrino emission could be detectable by the future TRIDENT detector. Finally, we build a synthetic population of these systems, compute the cumulative neutrino flux expected from spider pulsars, and compare it with the Galactic neutrino diffuse emission measured by IceCube. We find that, under realistic assumptions on the fraction of the spin-down power converted into protons, the contribution of spiders to the diffuse Galactic neutrino flux is negligible.