In August 2011 a new hard X-ray source, labelled IGR J17361-4441,
was discovered in the Globular Cluster NGC 6388 by INTEGRAL (see the
IBIS light curve in the top left figure, upper panel). This cluster
lies at about 13.2 kpc from the Earth (about 40000 light years). General
excitement arose, because the intermediate mass black hole in the centre
of NGC 6388 (see the Hubble image at the bottom right) could have been
the origin of this emission. The Swift/XRT observing campaign spanning
about 200 days showed an X-ray light curve following a t(-5/3)
trend (see the XRT light curve at the bottom left). Such an evolution in
intensity is usually observed in tidal disruption events (TDEs) of a star
by a supermassive black hole. Previous observations with Chandra, however,
revealed that the position of IGR J17361-4441 is not consistent with the
gravitational centre of the cluster, and the source of (hard) X-rays could
not have been the intermediate mass black hole.
So, which kind of object was captured and tidally disrupted, and by what?
Based on the observations (e.g., inferred black-body temperature from X-ray
spectral fits) and theoretical considerations it turns out that the disrupting
object is most likely a massive (~1.5 M☉) white dwarf, while the disrupted
object is most likely a terrestrial planet with a mass of about one third of
that of the Earth (see the artist's impression of the system in the top right;
credits: A. Nucita). This implies that the tidal force exerted by the white
dwarf becomes stronger than the self-gravity of the planet, causing the planet's
disruption. In the case of the event associated with IGR J17361-4441, half of
the planet mass is thrown out of the system at high velocity, while the remaining
debris are accreted onto the white dwarf. The resulting flare exhibits typical
black-body emission in the X-ray regime, plus a Compton up-scattered emission in
hard X-rays. Numerical simulations show that a significant population (from 1 to
100 per star) of free-floating planets, resulting from the dissolution of planetary
systems, is retained in globular clusters and the white dwarf density is estimated
in the range 104 - 105 pc-3. Combining these
values derived by numerical simulations with observational parameters of NGC 6388,
such as the core radius of ~0.5 pc, the rate of these kind of events in our Galaxy
can be estimated. Considering that there are more than 150 globular clusters in
our Galaxy, a planetary tidal disruption event by a white dwarf can happen as
frequently as once every 20 years at most.
Reference:
"The puzzling source IGR J17361-4441 in NGC 6388: a possible planetary tidal disruption event"
M. Del Santo et al., 2014, Monthly Notices of the Royal Astronomical Society, Vol. 444, Pag. 93
http://adsabs.harvard.edu/abs/2014MNRAS.444...93D
