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 PSRJ0610-2100, one of the black-widow millisecond pulsars recently detected by the Fermi Gamma-ray Space Telescope in the Galactic plane. We found a faint star (V ~26.7) nearly coincident (δr ~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 days) 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 (R ~25.6) at phi = 0.75, suggesting a modulation due to the pulsar heating. We tentatively conclude that the companion to PSRJ0610-2100 is a heavily ablated very low mass star (~0.02 Msol) that completely filled its Roche lobe.