In January 2011, the European Space Agency’s INTEGRAL space observatory detected an X-ray flare from a nearby galaxy called NGC 4845. The X-ray flare is designated IGR J12580+0134 and follow-up observations were conducted using XMM-Newton (a space based X-ray observatory), Swift (a space observatory designed to study gamma-ray bursts) and MAXI (an X-ray monitoring device aboard the International Space Station). The light curve of J12580+0134 shows a rise to a maximum in a few weeks, followed by a gradual decrease over a year or so. A supernova explosion is unlikely to produce an X-ray flare like J12580+0134. This is because the peak X-ray luminosity of J12580+0134 is ~100 times larger than from a typical supernova and its subsequent decline in luminosity is too rapid to be consistent with a supernova.
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Figure 1: Artist’s impression of a star being tidally
disrupted by a black hole.
Tidal disruptions of objects by black holes have been
extensively modeled and the decline in emission following peak luminosity tends
to follow a power law with a characteristic slope of -5/3. It turns out that J12580+0134
indeed shows such a characteristic and is consistent with a tidal disruption
event. As tidally disrupted matter plunges violently into the titanic
gravitational well of the black hole, it causes rapid variability of the X-ray
emission. Measurements of the fast X-ray variability observed near the peak of
the X-ray flare provided an estimate of the black hole’s mass to be no more
than 1,000,000 times the mass of the Sun. That would easily place this black
hole into the category of supermassive black holes.
Based on the light curve characteristics of the X-ray flare,
the tidally disrupted object involved in this event is likely to be a 14 to 30
Jupiter-mass sub-stellar object with about 10 percent of its mass being tidally
ripped off and accreted by the black hole. In that mass regime, the object is
either a free-floating giant planet or a brown dwarf. A more massive object
such as a star is unlikely to be the object responsible for this tidal
disruption event because the tidal disruption of a star would show a more rapid
decrease in emission following peak luminosity.
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Figure 2: The light curve of IGR J12580+0134 observed in the
17.3-80 keV energy band. Red squared points refer to INTEGRAL data; blue
crosses show Swift and XMM-Newton observations. The solid line shows a model
with a characteristic slope of -5/3, as expected for fallback of material after
a tidal disruption event. The long-short dash and dotted lines indicate the
predictions of simulations for the disruption of a sub-stellar object (γ = 5/3;
β = 0.7) and, respectively of a star (γ = 4/3; β = 0.65).
(Credit: Guillochon, J. & Ramirez-Ruiz, E. 2013, ApJ, 767, 25)
Supermassive black holes can tidally disrupt stars and also
less massive sub-stellar objects that happen to venture too close. In a galaxy,
the population of sub-stellar objects is expected to be at least as large as
the population of stars. For example, the population of free-floating
Jupiter-mass objects is estimated by Sumi et al. (2011) to be about twice as
common as stars. As a consequence, the tidal disruption of sub-stellar objects
could be at least as frequent as the tidal disruption of stars. The detection
of tidal disruption events in the vicinity of supermassive black holes may
serve as a means to estimate the population of sub-stellar objects.
References:
Marek Nikolajuk and Roland Walter (2013), “Tidal disruption
of a super-Jupiter by a massive black hole”, arXiv:1304.0397 [astro-ph.HE]
Sumi, T., Kamiya, K., Bennett, D. P., et al. (2011), “Unbound
or distant planetary mass population detected by gravitational microlensing”, Nature
473, 349-352