Tuesday, April 2, 2013

Black Hole Rips Giant Planet

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.

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.

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.

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