Monday, August 11, 2014

White Dwarf Shreds Planet in a Burst of X-Rays

The globular cluster NGC 6388 is believed to harbour an intermediate mass black hole (IMBH) with ~1000 times the Sun’s mass at its centre. On 11 August 2011, the INTEGRAL satellite detected a hard X-ray transient identified as IGR J17361-4441 near the centre of NGC 6388. The hard X-ray transient was thought to have originated from the IMBH at the centre of the globular cluster. However, follow-up observations reveal that the position of the hard X-ray transient was off-centre and could not possibly be related to the IMBH.

Figure 1: Artist’s impression of a planet in the dense stellar environment of a globular cluster.

A study by M. Del Santo et al. (2014) suggests that IGR J17361-4441 is consistent with a tidal disruption event (TDE). Its nature as a TDE is based on two pieces of empirical evidences. Firstly, the decline of the X-ray light curve is typical for a TDE (Figure 2). Secondly, observations of TDEs from the disruption of stars by supermassive black holes (SMBHs) show a thermal emission component that does not evolve significantly with time. Such a thermal emission was observed for IGR J17361-4441 (Figure 3).

Observational analysis and theoretical calculations indicate that IGR J17361-4441 is most likely a TDE involving a white dwarf and a disrupted planetary object. The mass of the disrupted planetary object depends on the accretion efficiency (i.e. how much of the planetary material that is accreting onto the white dwarf is converted into energy to power the hard X-ray transient). It turns out that the disrupted planetary object is estimated to have a mass that is on the order of one-third the Earth’s mass.

The TDE occurred when this free-floating planetary object came too close to the white dwarf. Nevertheless, the rate of such a TDE in a globular cluster is unknown since the densities of white dwarfs and free-floating planets in globular clusters are uncertain. It is reasonable to assume that free-floating planets are more common in globular clusters as the dense stellar environment makes it more likely for planetary systems to be perturbed by passing stars. Assuming such a TDE occurs in a given globular cluster at a rate of one every ~3,000 years and the total number of globular clusters in the Milky Way is roughly 150, then the total event rate is once per ~20 years.

Figure 2: Observed X-ray light-curve of IGR J17361-4441 fitted with a model expected from a TDE (red line). M. Del Santo et al. (2014).

Figure 3: Observed thermal emission component of IGR J17361-4441. M. Del Santo et al. (2014).

M. Del Santo et al. (2014), “The puzzling source IGR J17361-4441 in NGC 6388: a possible planetary tidal disruption event”, arXiv:1407.5081 [astro-ph.HE]