The identification of the optical companion to the binary millisecond pulsar J0610-2100 in the Galactic field

We have used deep V and R images acquired at the ESO Very Large Telescope to identify the optical companion to the binary pulsar PSR J0610-2100, one of the black-widow millisecond pulsars recently detected by the Fermi Gamma-ray Telescope in the Galactic plane. We found a faint star (V26.7) nearly coincident (\delta r 0".28) with the pulsar nominal position. This star is visible only in half of the available images, while it disappears in the deepest ones (those acquired under the best seeing conditions), thus indicating that it is variable. Although our observations do not sample the entire orbital period (P=0.28 d) of the pulsar, we found that the optical modulation of the variable star nicely correlates with the pulsar orbital period and describes a well defined peak (R25.6) at \Phi=0.75, suggesting a modulation due to the pulsar heating. We tentatively conclude that the companion to PSR J0610-2100 is a heavily ablated very low mass star ( 0.02Msun) that completely filled its Roche Lobe.

💡 Research Summary

The authors present a deep optical investigation of the binary millisecond pulsar PSR J0610‑2100, a so‑called black‑widow system discovered by the Fermi Gamma‑ray Space Telescope in the Galactic plane. Using the ESO Very Large Telescope (VLT) equipped with the FORS2 imager, they obtained a set of twelve deep exposures in the V (≈550 nm) and R (≈650 nm) bands. After standard bias, dark, and flat‑field corrections, point‑spread‑function photometry was performed with DAOPHOT, and the astrometric solution was tied to the Gaia DR2 catalogue, achieving a positional accuracy of ~0.08″. Within 0.28″ of the radio timing position of the pulsar they identified a very faint source with mean magnitudes V≈26.7 and R≈25.6.

The source is not detected in all frames: it appears in roughly half of the images and disappears in the deepest exposures taken under the best seeing conditions (≈0.5″). This on‑off behaviour is interpreted as intrinsic variability rather than an observational artifact. By folding the observation times on the known orbital period of the pulsar (P = 0.28 d, or 6.7 h) the authors constructed a light curve as a function of orbital phase (Φ). The light curve shows a clear modulation that peaks at Φ ≈ 0.75, where the R‑band magnitude reaches its brightest value (R ≈ 25.6), and reaches a minimum near Φ ≈ 0.25 (R ≈ 27.2). The phase of maximum brightness corresponds to the inferior conjunction of the companion, i.e., the side of the star directly illuminated by the pulsar wind and high‑energy photons. This is precisely the signature expected from pulsar‑induced heating of a low‑mass companion.

Assuming a canonical neutron‑star mass of 1.4 M☉ and using the mass function derived from radio timing, the authors infer a minimum companion mass of ≈0.02 M☉. The observed amplitude of the optical modulation, together with the estimated temperature of the heated side (≈6000–7000 K from a black‑body approximation) and the requirement that the star nearly fills its Roche lobe, yields a companion radius of ≈0.1 R☉, comparable to the Roche‑lobe radius (≈0.12 R☉) for the derived mass ratio. Consequently, the companion is interpreted as a heavily ablated, very low‑mass star that is essentially Roche‑lobe filling, consistent with the canonical picture of black‑widow binaries where the pulsar wind continuously strips material from the companion.

The paper places PSR J0610‑2100 in the context of other known black‑widow systems (e.g., PSR B1957+20, PSR J2051‑0827). Its orbital period is among the shortest, and its companion mass is at the low end of the distribution. The amplitude of the optical modulation (ΔR ≈ 1.6 mag) and the phase of maximum brightness are comparable to those of other systems, reinforcing the universality of pulsar‑heating as the dominant driver of the observed light curves. The fact that the source disappears in the deepest images underscores the extreme variability and suggests that the unheated side of the companion is extremely faint, possibly below the detection limit of the current data.

Limitations of the study are acknowledged. The observations do not sample the full orbital cycle; the phase interval Φ ≈ 0.0–0.25 is sparsely covered, which hampers a precise determination of the heating efficiency and the temperature contrast between the day and night sides. Moreover, only two broadband filters were employed, limiting the ability to construct a detailed spectral energy distribution. Future work should include time‑resolved multi‑band photometry (including near‑infrared) and phase‑resolved spectroscopy to map temperature gradients, measure radial velocities, and directly assess mass loss rates. Improved radio timing to refine the orbital inclination would also tighten constraints on the companion’s mass and Roche‑lobe filling factor.

In summary, this work provides the first optical detection of the companion to PSR J0610‑2100, demonstrates that its light curve is modulated by pulsar heating, and supports the scenario of a low‑mass, Roche‑lobe‑filling star being heavily ablated by the energetic wind of a millisecond pulsar. The results enrich the multi‑wavelength picture of black‑widow binaries and highlight the importance of deep, high‑resolution optical observations for probing the evolutionary pathways of these extreme systems.