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

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 days 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 ǫ ≈ 3.5 × 10 −4 (M Ch /M ) consistent with a massive white dwarf, and a disrupted minor body mass M mb ≈ 1.9 × 10 27 (M/M Ch ) g in the terrestrial-icy planet regime. These numbers yield an inner disc temperature of the order kT in ≈ 0.04 keV, consistent with the blackbody temperature of kT in ≈ 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 to 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.

Very hard states in neutron star low-mass X-ray binaries

“…Therefore, we have not determined a best-fit NH for SAX J1808.4−3658. Our values for the other sources are consistent with those reported in the literature Papitto et al 2013;Linares et al 2014;Del Santo et al 2014;Bozzo et al 2015;Ludlam et al 2016;Pintore et al 2016;Tetarenko et al 2016;Degenaar et al 2016).…”

A Comparison of the X-Ray Emission from Tidal Disruption Events with those of Active Galactic Nuclei

“…In addition, we find that our non-jetted TDE sample indicated by the filled circles in our plot, have a peak X-ray luminosity that is significantly softer than that expected from an AGN. For our jetted TDE sample represented by the star symbols, and the emission from IGR J17361-444 which was originally detected in the hard X-ray energy band using INTEGRAL (Del Santo et al 2014), we have the opposite case. Here the emission arising from these sources is harder than that seen from AGN at the same redshift.…”

The Drop During Less Than 300 Days of a Dusty White Dwarf's Infrared Luminosity

“…For comparison, there exist dusty white dwarfs with much larger inner holes, e.g., PG 1225-079 and G166-58, possibly also due to planetesimal impacts (Farihi et al 2010). Recently, the tidal disruption of an entire planet by a white dwarf has been invoked to explain the light curve of a source in the center of a globular cluster (Del Santo et al 2014).…”