Deep-Survey Constraints on X-ray Outbursts from Galactic Nuclei

TL;DR

This study conducted a deep survey of 24,668 galaxies to constrain the rate of high-energy X-ray outbursts from galactic nuclei, finding no events but setting upper limits that inform models of black hole activity.

Contribution

It provides the first constraints on X-ray outbursts with harder spectra and higher redshifts, extending previous surveys and refining the understanding of nuclear black hole fueling.

Findings

No X-ray outbursts detected in the survey.

Upper limit on outburst rate is ~10^{-4} per galaxy per year for luminous events.

Results suggest the X-ray luminosity function of active galactic nuclei is not mainly due to stellar tidal disruptions.

Abstract

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 24668 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 upon the adopted outburst X-ray luminosity. For an outburst with X-ray luminosity $\ga 10^{43}$ ergs/s and a duration of 6 months, the upper limit on its event rate is ~10^{-4} /galaxy/yr, 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.