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 ≈ 3.5 × 10−4(MCh/M) consistent with a massive white dwarf, and a disrupted minor body mass Mmb ≈ 1.9 × 1027(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 3 × 10−6–3 × 10−4 yr−1. Averaged over the Milky Way globular clusters, the upper limit value corresponds to 0.05 yr−1, consistent with the observation of a single event by INTEGRAL and Swift.

The puzzling source IGR J17361-4441 in NGC 6388 : a possible planetary tidal disruption event / M. Del Santo, A.A. Nucita, G. Lodato, L. Manni, F. De Paolis, J. Farihi, G. De Cesare, A. Segreto. - In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY. - ISSN 0035-8711. - 444:1(2014), pp. 93-101.

The puzzling source IGR J17361-4441 in NGC 6388 : a possible planetary tidal disruption event

G. Lodato
Penultimo
;
2014

Abstract

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 ≈ 3.5 × 10−4(MCh/M) consistent with a massive white dwarf, and a disrupted minor body mass Mmb ≈ 1.9 × 1027(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 3 × 10−6–3 × 10−4 yr−1. Averaged over the Milky Way globular clusters, the upper limit value corresponds to 0.05 yr−1, consistent with the observation of a single event by INTEGRAL and Swift.
Accretion; Accretion disc; Globular clusters: individual: NGC 6388; White dwarfs; X-rays: individual: IGR J17361-4441
Settore FIS/05 - Astronomia e Astrofisica
2014
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/238990
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