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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., Comment: 15 pages, 9 figures, 3 tables. This paper has been accepted for publication in ApJ, and currently in press

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 $\gamma$-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 $3\sigma$ 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., Comment: 38 pages, 93 figures, 4 tables. This paper has been accepted for publication in ApJ

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., Comment: 18 pages, 11 figures

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 $\gamma$-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 $\gamma$-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., Comment: 12 pages, 11 figures, 3 tables. This article has been accepted for publication in MNRAS published by Oxford University Press on behalf of the Royal Astronomical Society. Typos corrected, acknowledgements added

We present the first multi-band optical light curves of PSR J1622-0315, among the most compact known redback binary millisecond pulsars, with an orbital period Porb=3.9 h. We find a flux modulation with two maxima per orbital cycle and a peak-to-peak amplitude of about 0.3 mag, which we attribute to the ellipsoidal shape of the tidally distorted companion star. The optical colours imply a late-F to early-G spectral type companion and do not show any detectable temperature changes along the orbit. This suggests that the irradiation of the star's inner face by the pulsar wind is unexpectedly missing despite its short orbital period. To interpret these results, we introduce a new parameter fsd, defined as the ratio between the pulsar wind flux intercepted by the companion star and the companion intrinsic flux. This flux ratio fsd, which depends on the spin-down luminosity of the pulsar, the base temperature of the companion and the orbital period, can be used to quantify the effect of the pulsar wind on the companion star and turns out to be the most important factor in determining whether the companion is irradiated or not. We find that the transition between these two regimes occurs at fsd=2-4 and that the value for PSR J1622-0315 is fsd=0.7, placing it firmly in the non-irradiated regime.