A puzzling 2-hour X-ray periodicity in the 1.5-hour orbital period black widow PSR J1311−3430

Abstract

Time-domain analysis of an archival XMM-Newton observation unveiled a very unusual variability pattern in the soft X-ray emission of PSR J1311−3430, a black widow millisecond pulsar in a tight binary (PB = 93.8 min) with a very low-mass (M ∼ 0.01 M⊙) He companion star, known to show flaring emission in the optical and in the X-rays. A series of six pulses with a regular recurrence time of ∼124 min is apparent in the 0.2−10 keV light curve of the system, also featuring an initial, bright flare and a quiescent phase lasting several hours. The X-ray spectrum does not change when the pulses are seen and it is consistent with a power law with photon index Γ ∼ 1.5, also describing the quiescent emission. The peak luminosity of the pulses is of several 1032 erg s−1. Simultaneous observations in the U band with the Optical Monitor onboard XMM and in the g′ band from the Las Cumbres Observatory do not show any apparent counterpart of the pulses and only display the well-known orbital modulation of the system. We consider different hypotheses to explain the recurrent pulses: we investigate their possible analogy with other phenomena already observed in this pulsar and in similar systems and we also study possible explanations related to the interaction of the energetic pulsar wind with intra-binary material, but we found none of these pictures to be convincing. We identify simultaneous X-ray observations and optical spectroscopy as a possible way to constrain the nature of the phenomenon.