2014MNRAS.444...93D

On 2011 August 11, INTEGRAL discovered the hard X-ray source IGR J17361-4441 near the centre of the globular cluster NGC 6388. Follow-up observations with Chandra showed the position of the transient was inconsistent with the cluster dynamical centre, and thus not related to its possible intermediate mass black hole. The source showed a peculiar hard spectrum (Γ ~ 0.8) and no evidence of QPOs, pulsations, type-I bursts, or radio emission. Based on its peak luminosity, IGR J17361-4441 was classified as a very faint X-ray transient, and most likely a low-mass X-ray binary. We re-analysed 200 d of Swift/XRT observations, covering the whole outburst of IGR J17361-4441 and find a t-5/3 trend evident in the light curve, and a thermal emission component that does not evolve significantly with time. We investigate whether this source could be a tidal disruption event, and for certain assumptions find an accretion efficiency ε ~ 3.5x10^–4(MCh/M) consistent with a massive white dwarf, and a disrupted minor body mass Mmb ~ 1.9x10²⁷(M/MCh) g in the terrestrial-icy planet regime. These numbers yield an inner disc temperature of the order kTin ~ 0.04 keV, consistent with the blackbody temperature of kTin ~ 0.08 keV estimated by spectral fitting. Although the density of white dwarfs and the number of free-floating planets are uncertain, we estimate the rate of planetary tidal disruptions in NGC 6388 to be in the range 3x10^–6-3x10^–4/yr. Averaged over the Milky Way globular clusters, the upper limit value corresponds to 0.05/yr, consistent with the observation of a single event by INTEGRAL and Swift.