A Huge Drop in the X-Ray Luminosity of the Nonactive Galaxy RX J1242.6-1119A, and the First Postflare Spectrum: Testing the Tidal Disruption Scenario

Abstract

In recent years, indirect evidence has emerged suggesting that many nearby nonactive galaxies harbor quiescent supermassive black holes. Knowledge of the frequency of occurrence of black holes, of their masses and spins, is of broad relevance for studying black hole growth and galaxy and active galactic nuclei formation and evolution. It has been suggested that an unavoidable consequence of the existence of supermassive black holes, and the best diagnostic of their presence in nonactive galaxies, would be occasional tidal disruption of stars captured by the black holes. These events manifest themselves in the form of luminous flares powered by accretion of debris from the disrupted star into the black hole. Candidate events among optically nonactive galaxies emerged in the past few years. For the first time, we have looked with high spatial and spectral resolution at one of these most extreme variability events ever recorded among galaxies. Here we report measuring a factor of ~200 drop in luminosity of the X-ray source RX J1242-1119 with the X-ray observatories Chandra and XMM-Newton, and perform tests of the favored outburst scenario, tidal disruption of a star by a supermassive black hole. We show that the detected low-state emission has properties such that it must still be related to the flare. The power-law shaped postflare X-ray spectrum indicates a hardening compared to outburst. The inferred black hole mass, the amount of liberated energy, and the duration of the event favor an accretion event of the form expected from the (partial or complete) tidal disruption of a star.