arXiv subjects

Manuel Linares

Publications and source records attributed to Manuel Linares.

At least 19 recordsLinked to original sources

arXiv2026

Exploring the optical properties of redback pulsars: The case of J1717+4308A in the globular cluster M92

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 ($f_\mathrm{sd}$), which for M92A lies close to the boundary between ellipsoidal- and irradiation-dominated regimes ($f_\mathrm{sd} \approx 2.71$). 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 $\sim7200$ 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.

astro-ph.HE↗

arXiv2026

Modes in Transitional Millisecond Pulsars: Evidence of Pulsar Wind-Induced Disk Heating from GRMHD and Radiative Transfer

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 the 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 tMSP magnetosphere. For the first time, we post-process 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 propeller regime and lacks this heating process. 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.

astro-ph.HE↗

arXiv2026

Gamma-ray Modes in a Transitional Pulsar

Transitional millisecond pulsars (tMSPs) exhibit a unique sub-luminous disk state, at the cross-roads 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 anti-correlated 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 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 is dominant in GeV gamma-rays.

astro-ph.HE↗

arXiv2026

Massive hybrid stars within the extended three-flavor quark-meson diquark model

We discuss the properties of the extended three-flavor quark-meson diquark (EQMD) model as a renormalizable low-energy effective model for QCD. The effective degrees of freedom are quarks, scalar- and pseudoscalar mesons, diquarks, vector- and axial-vector mesons. We calculate the equation of state (EoS) in the mean-field approximation at $T=0$ imposing charge neutrality for electric and color charges. We match the EoS with a low-density nuclear equation of state. We discuss how the choice of parameters in the model affects the EoS and thereby the mass-radius for hybrid stars. We show that it is possible to construct hybrid stars whose masses and radii are in agreement with recent astrophysical observations and perturbative QCD (pQCD). The addition of vector and axial vector mesons to the quark-meson diquark is essential, since it makes the EoS sufficiently stiff for intermediate densities. Our results suggest that stars with a mass larger than $M\sim2M_{\odot}$ have a quark core with a central density $n_B\geq 3.9n_{\rm sat}$, where $n_{\rm sat}\approx0.165$fm$^{-3}$ is the saturation density. The speed of sound has a double-peak structure and relaxes to the conformal limit from above for large baryon chemical potentials $\mu_B$. This structure is caused by the decrease in the mass of the $s$ quark as $\mu_B$ increases.

hep-ph↗

arXiv2026

Gamma-ray Orbital Modulation in Spider Pulsars: Three Discoveries and a Universal Modulated Fraction

Compact binary millisecond pulsars (also known as spiders) allow us to probe pulsar winds in their innermost regions, between the light cylinder (radius $\sim10^{7}$ cm) and the companion star (at $\sim10^{11}$ 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 $22.0\pm2.6\%$. Including eclipsing systems seen edge-on, we find no clear dependence of the modulated fraction on the orbital inclination (within $\simeq$45-90$^\circ$). 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.

astro-ph.HE↗

arXiv2025

COBIPLANE: A Systematic Search for Compact Binary Millisecond Pulsars at Low Galactic Latitudes

We present the main results obtained from the COmpact BInary Pulsar search in the low-LAtitude NEighborhood (COBIPLANE), 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 (Fermi-LAT) catalog, selected for their pulsar-like $γ$-ray properties. Extending to Galactic latitudes as low as $\pm3^{\circ}$, this survey reaches closer to the Galactic plane than its predecessor survey, the COmpact BInary PULsar SEarch (COBIPULSE). 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 $0.41576(6) \ \mathrm{d}$, $0.305(2) \ \mathrm{d}$, $0.204(7) \ \mathrm{d}$, $0.3(2) \ \mathrm{d}$, and $0.4395(1) \ \mathrm{d}$, respectively, and photometric temperatures of $4000$--$6000 \ \mathrm{K}$, consistent with the companion stars of `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 $1.5 \times 10^{32} \ (D / 3.9 \ \mathrm{kpc})^2 \ \mathrm{erg} \ \mathrm{s}^{-1}$ ($0.3$--$10 \ \mathrm{keV}$) for 4FGL J1748.8-3915, and upper limits of $\sim10^{31}$--$10^{33} \ \mathrm{erg} \ \mathrm{s}^{-1}$ ($0.2$--$12 \ \mathrm{keV}$) for the others -- we classify these sources as new spider candidate systems.

astro-ph.HE↗

arXiv2025

SpiderCat: A Catalog of Compact Binary Millisecond Pulsars

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.

astro-ph.HE↗

arXiv2025

Very high-energy gamma-ray and neutrino emission from hadronic interaction in compact binary millisecond pulsars

Blackwidow 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 non-thermal 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 $\gtrsim 10^{35}\rm erg\, s^{-1}$ and a magnetic field of $\sim 10^{3}\, \rm G$ in the companion region, the gamma-ray emission could be detectable as point sources by CTA and LHAASO, 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.

astro-ph.HE↗

arXiv2025

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

astro-ph.HE↗

arXiv2025

The slowest spinning Galactic-field spider PSR J1932+2121: A history of inefficient mass transfer

The Five-hundred-meter Aperture Spherical Telescope is discovering hundreds of new pulsars, including a slowly spinning compact binary millisecond pulsar (spin period $P_{\rm spin}=14.2$\,ms) which showed radio eclipses and evidence of ablation of its companion: PSR J1932+2121. Its orbital period is $P_{\rm orb}=0.08$\,d and the minimum companion mass is estimated as 0.12\,\Msun. Hence, this pulsar is classified as part of the Galactic-field spider (redback) population. However, it spins almost an order of magnitude slower than other Galactic-field spiders. Using detailed evolutionary calculations with {\tt MESA}, we model the formation, mass-transfer and radio-pulsar phases, in order to explain the observed properties of PSR\,J1932+2121. We find that PSR\,J1932+2121 is a redback that has experienced an inefficient mass-transfer phase resulting in a lower accretion efficiency (in the range of 0.3 to 0.5) and subsequently slower spin compared to other spiders. We narrow down the initial range of $P_{\rm orb}$ that best reproduces its properties, to 2.0--2.6\,d. Current models of accretion-induced magnetic field decay are not able to explain its unusually high surface magnetic field of $2\times 10^{9}$\,G. Hence, PSR\,J1932+2121 provides a unique opportunity to study inefficient accretion-induced spin up and surface magnetic field decay of pulsars.

astro-ph.HE↗

arXiv2025

An energy approach to pulsar-disc interaction: disc stability and implications for transitional millisecond pulsars

The stability of an accretion disc surrounding a millisecond pulsar is analysed from an energetic point of view, using magnetohydrodynamic simulations that consider realistic disc structures and a variety of magnetic field inclination angles. The time-averaged components of the magnetic field interact with the disc through ohmic dissipation, which causes heating and partial evaporation of its innermost region. The stability of the disc right after the magnetic field is turned on is analysed as a function of the location of the inner radius of the disc and the magnetic inclination angle. Our results show that the disc is severely altered in those cases where its inner radius lies well beyond the light cylinder and the magnetic axis is not totally aligned with the neutron star spin axis. Overall, the results of the simulations agree with those obtained in previous works where analytical or semi-analytical energy models were also used to discuss the stability of the disc. The implications for the understanding of the transitional millisecond pulsars are discussed. We briefly mention implications of our results for low-mass X-ray binaries and supernova fallback discs.

astro-ph.HE↗

arXiv2025

Connecting the m-dots: accretion rates and thermonuclear burst recurrence times on neutron stars and white dwarfs

We present a compilation of observed recurrence times ($t_{\rm rec}$) and infer the corresponding local mass-accretion rates ($\dot m$) for type I X-ray bursts, milliHertz quasi-periodic oscillating sources and recurrent novae eruptions. We construct models of the $t_{\rm rec}-\dot m$ relation for accreting white dwarfs and neutron stars and find that both are roughly consistent with a global inverse linear relation, connecting for the first time thermonuclear runaways on neutron stars and white dwarfs. We find that theoretical models of pure He bursts are in agreement with the best $t_{\rm rec}$ measurements in ultra-compact X-ray binaries at low $\dot m$ (4U~$0614+09$ and 2S~0918-549). We suggest that the transient Z source XTE~J1701-462 is a slow rotator, based on its mHz QPO properties. Finally, we discuss the implications for thermonuclear ignition and point out that the difference in eruption/burst energy ($E_{b_{WD}}/E_{b_{NS}}=2\times 10^4$) is consistent with the difference in area between neutron stars and white dwarfs $\left((R_{WD}/R_{NS})^2=4\times 10^4\right)$. We conclude that ignitions of thermonuclear shell flashes on neutron stars and white dwarfs depend primarily on the specific mass accretion rate and do not depend on the nature of the underlying compact object.

astro-ph.HE↗

arXiv2025

Discovery of the variable optical counterpart of the redback pulsar PSR J2055+1545

We present the discovery of the variable optical counterpart to PSR J2055+1545, a redback millisecond pulsar, and the first radial velocity curve of its companion star. The multi-band optical light curves of this system show a $0.4$$-$$0.6 \ \mathrm{mag}$ amplitude modulation with a single peak per orbit and variable colours, suggesting that the companion is mildly irradiated by the pulsar wind. We find that the flux maximum is asymmetric and occurs at orbital phase $\simeq0.4$, anticipating the superior conjunction of the companion (where the optical emission of irradiated redback companions is typically brightest). We ascribe this asymmetry, well fit with a hot spot in our light curve modelling, to irradiation from the intrabinary shock between pulsar and companion winds. The optical spectra obtained with the \textit{Gran Telescopio Canarias} reveal a G-dwarf companion star with temperatures of $5749 \pm 34 \ \mathrm{K}$ and $6106 \pm 35 \ \mathrm{K}$ at its inferior and superior orbital conjunctions, respectively, and a radial velocity semi-amplitude of $385 \pm 3 \ \mathrm{km}\ \mathrm{s}^{-1}$. Our best-fit model yields a neutron star mass of $1.7^{+0.4}_{-0.1} \ \mathrm{M_{sun}}$ and a companion mass of $0.29^{+0.07}_{-0.01} \ \mathrm{M_{sun}}$. Based on the close similarity between the optical light curve of PSR~J2055$+$1545 and those observed from PSR J1023+0038 and PSR J1227-4853 during their rotation-powered states, we suggest this system may develop an accretion disc in the future and manifest as a transitional millisecond pulsar.

astro-ph.HE↗

arXiv2025

Investigating cannibalistic millisecond pulsar binaries using MESA: New constraints from pulsar spin and mass evolution

Compact binary millisecond pulsars (MSPs) with orbital periods $\lesssim1$d are key to understanding binary evolution involving massive neutron stars (NSs). Due to the ablation of the companion by the rapidly spinning pulsar, these systems are also known as spiders and categorized into two main branches: redbacks (RBs; companion mass in the range of 0.1 to 0.5\,\Msun) and black widows (BWs; companion mass $\lesssim$\,0.1\,\Msun). We present models of low- and intermediate-mass X-ray binaries and compare them with observations of Galactic spiders (including the presence or absence of hydrogen lines in their optical spectra), and we constrain and quantify the interaction between the pulsar and the companion. Using MESA, we created the allowed initial parameter space. For the first time in MESA, we also included the detailed evolution of the pulsar spin and modeled the irradiation of the companion by the pulsar wind. Efficient mass accretion onto the NS (at least $70\%$ of the mass transferred is accreted) with an X-ray irradiated disk followed by strong irradiation of the companion can explain most of the properties of the observed spiders. Our RB evolutionary tracks continue to the BW regime, connecting the two branches of spiders. Our models explain the lack of hydrogen in some observed BWs with ultra-light companions. During accretion induced spin up, the mass required to spin up an NS to sub-milliseconds is high enough to collapse it into a black hole. Finally, after analyzing the formation of RB-like spiders with giant companions and orbital periods of several days (huntsmen), we conclude that they are unlikely to produce super-massive NSs (maximum accreted mass $\lesssim$0.5M$_{\odot}$). Cannibalistic MSP binary formation depends heavily on the interplay between accretion onto the pulsar and pulsar wind irradiation.

astro-ph.HE↗

arXiv2025

A GTC spectroscopic study of three spider pulsar companions: line-based temperatures, a new face-on redback, and improved mass constraints

We present GTC-OSIRIS phase-resolved optical spectroscopy of three compact binary MSPs, or 'spiders': PSR J1048+2339, PSR J1810+1744, and (for the first time) PSR J1908+2105. For the companion in each system, the temperature is traced throughout its orbit, and radial velocities are measured. The radial velocities are found to vary with the absorption features used when measuring them, resulting in different radial velocity curve semi-amplitudes: for J1048 ($K_\mathrm{metals, red} = 344 \pm 4$ km s$^{-1}$, $K_\mathrm{metals, blue} = 372 \pm 3$ km s$^{-1}$) and, tentatively, for J1810 ($K_\mathrm{Balmer} = 448 \pm 19$ km s$^{-1}$, $K_\mathrm{metals} = 491 \pm 32$ km s$^{-1}$). With existing inclination constraints, this gives the neutron star (NS) and companion masses $M_\mathrm{NS} = 1.50 - 2.04$ $M_\odot$ and $M_2 = 0.32 - 0.40$ $M_\odot$ for J1048, and $M_\mathrm{NS} > 1.7$ $M_\odot$ and $M_2 = 0.05 - 0.08$ $M_\odot$ for J1810. For J1908, we find an upper limit of $K_2 < 32$ km s$^{-1}$, which constrains its mass ratio $q = M_2 / M_\mathrm{NS} > 0.55$ and inclination $i < 6.0^\circ$, revealing the previously misunderstood system to be the highest mass ratio, lowest inclination redback yet. This raises questions for the origins of its substantial radio eclipses. Additionally, we find evidence of asymmetric heating in J1048 and J1810, and signs of metal enrichment in J1908. We also explore the impact of inclination on spectroscopic temperatures, and demonstrate that the temperature measured at quadrature ($ϕ= 0.25, 0.75$) is essentially independent of inclination, and thus can provide additional constraints on photometric modelling.

astro-ph.HE↗

arXiv2024

COBIPULSE: A Systematic Search for Compact Binary Millisecond Pulsars

We report here the results obtained from a systematic optical photometric survey aimed at finding new compact binary millisecond pulsars (also known as "spiders"): the COmpact BInary PULsar SEarch (COBIPULSE). We acquired multi-band optical images over one year around $33$ unidentified Fermi-LAT sources, selected as pulsar candidates based on their curved GeV spectra and steady $γ$-ray emission. We present the discovery of four optical variables coinciding with the Fermi sources 3FGL J0737.2$-$3233, 3FGL J2117.6$+$3725 (two systems in this field) and 3FGL J2221.6$+$6507, which we propose as new candidate spider systems. Indeed, they all show optical flux modulation consistent with orbital periods of $0.3548(5) \ \mathrm{d}$, $0.25328(6) \ \mathrm{d}$, $0.441961(2) \ \mathrm{d}$, and $0.165(4) \ \mathrm{d}$, respectively, with amplitudes $\gtrsim 0.3 \ \mathrm{mag}$ and colors compatible with companion star temperatures of $5000$--$6000 \ \mathrm{K}$. These properties are consistent with the "redback" sub-class of spider pulsars. If confirmed as a millisecond pulsar, 3FGL J0737.2$-$3233 will be the closest known spider to Earth ($D=659_{-20}^{+16} \ \mathrm{pc}$, from Gaia-DR3 parallax). We searched and did not find any X-ray sources matching our four candidates, placing $3σ$ upper limits of $\sim10^{31}$--$10^{32} \ \mathrm{erg} \ \mathrm{s}^{-1}$ ($0.3$--$10 \ \mathrm{keV}$) on their soft X-ray luminosities. We also present and discuss other multi-wavelength information on our spider candidates, from infrared to X-rays.

astro-ph.HE↗

arXiv2024

The Orbit and Companion of PSR J1622-0315: Variable Asymmetry and a Massive Neutron Star

The companion to PSR J1622-0315, one of the most compact known redback millisecond pulsars, shows extremely low irradiation despite its short orbital period. We model this system to determine the binary parameters, combining optical observations from NTT in 2017 and NOT in 2022 with the binary modeling code ICARUS. We find a best-fit neutron star mass of $2.3 \pm 0.4\,\text{M}_\odot $, and a companion mass of $0.15 \pm 0.02\,\text{M}_\odot$. We detect for the first time low-level irradiation from asymmetry in the minima as well as a change in the asymmetry of the maxima of its light curves over five years. Using star spot models, we find better fits than those from symmetric direct heating models, with consistent orbital parameters. We discuss an alternative scenario where the changing asymmetry is produced by a variable intrabinary shock. In summary, we find that PSR J1622-0315 combines low irradiation with variable light curve asymmetry, and a relatively high neutron star mass.

astro-ph.HE↗

arXiv2024

The invisible black widow PSR J1720-0534: implications for the electron density towards the North Polar Spur

Radio emission from pulsars can be used to map out their distances through dispersion measure (DM), which quantifies the amount of radio pulse dispersion. However, this method relies on accurately modelling the free electron density in the line of sight. Here, we present a detailed study of the multiwavelength emission from PSR J1720$-$0534, a black widow compact binary millisecond pulsar discovered in 2021, which the latest electron density model of the Galaxy (Yao et al. 2017) places at only 191 pc. We obtained and analysed deep multiwavelength observations in the $γ$-ray (Fermi-Large Area Telescope, 2008-2022), optical (Las Cumbres Observatory, 2.7 h), near-infrared (Nordic Optical Telescope, 3.5 h), and X-ray (Swift-X-Ray Telescope, 10 ks) bands. We found no significant detection of $γ$-ray, optical, near-infrared, or X-ray counterparts around the radio-timing position of PSR J1720$-$0534, which we thus nickname 'the invisible black widow'. Employing the most constraining near-infrared limit ($J>23.4$ mag), we established a lower limit on the source distance, $d>1.1$ kpc, assuming conservative properties for the black widow companion star. This distance lower limit differs drastically (by a factor of more than 5) from the Yao et al. DM distance estimate. We attribute this difference to the inclusion in the Yao et al. model of a large and dense component towards the North Polar Spur. Considering our results and recent parallax distances to other pulsars in this direction, we argue that such a local and large component in the electron density model of the Galaxy is unnecessary.

astro-ph.HE↗