Deep-survey constraints on X-ray outbursts from galactic nuclei

Luminous X-ray outbursts with variability amplitudes as high as ∼ 1000 have been detected from a small number of galactic nuclei. These events are likely associated with transient fueling of nuclear supermassive black holes. In this paper we constrain X-ray outbursts with harder spectra, higher redshifts, and lower luminosities than have been studied previously. We performed a systematic survey of 24,668 optical galaxies in the Chandra Deep Fields to search for such X-ray outbursts; the median redshift of these galaxies is ∼0.8. The survey spans 798 days for the Chandra Deep Field-North, and 1828 days for the Chandra Deep Field-South. No outbursts were found, and thus we set upper limits on the rate of such events in the universe, which depend on the adopted outburst X-ray luminosity. For an outburst with X-ray luminosity ≳ 10⁴³ ergs s^_1 and a duration of 6 months, the upper limit on its event rate is ∼10^-4 galaxy^-1 yr^-1, roughly consistent with theoretical predictions. Compared to previous survey results, our harder band and deeper survey suggests that the outburst rate may increase by a maximum factor of 10 when considering both obscured X-ray outbursts and redshift evolution from z ∼ 0 to z ∼ 0.8. Our results also suggest that the X-ray luminosity function for moderate-luminosity active galactic nuclei is not primarily due to stellar tidal disruptions.