The giant X-ray outbursts in NGC 5905 and IC 3599: Follow-up observations and outburst scenarios

TL;DR

This paper investigates the cause of giant X-ray outbursts in NGC 5905 and IC 3599, analyzing multi-wavelength data and comparing various models, ultimately supporting tidal disruption events as the likely explanation for NGC 5905's outburst.

Contribution

It provides a comprehensive multi-wavelength analysis and compares different outburst scenarios, strongly supporting tidal disruption as the cause in NGC 5905.

Findings

NGC 5905 shows no signs of persistent Seyfert activity.

IC 3599 exhibits signs of ongoing activity.

NGC 5905 is a prime candidate for a tidal disruption event.

Abstract

(abridged) Huge amplitude X-ray outbursts in a few galaxies were reported in the last few years. As one of the exciting possibilities to explain these observations, tidal disruption of a star by a supermassive black hole has been proposed. In the present paper, we perform a detailed discussion of this and other possible scenarios for the X-ray outburst in NGC 5905, and a comparison of NGC 5905 and IC 3599 in outburst as well as in quiescence. To this end we present (i) a thorough analysis of all ROSAT observations of NGC 5905, (ii) optical photometry of NGC 5905 quasi-simultaneous to the X-ray outburst, (iii) the first post-outburst optical spectra of NGC 5905 and high-resolution post-outburst spectra of IC 3599, and (iv) photoionization models for the high-excitation emission lines that were discovered in the optical outburst spectrum of IC 3599. The investigated outburst models include, besides the tidal disruption event, a supernova in dense medium, an accretion-disk instability, an event of extreme gravitational lensing, and the X-ray afterglow of a GRB. The optical spectra of both galaxies in quiescence are carefully examined for signs of permanent low-level Seyfert-activity. Whereas IC 3599 shows several signs of activity, none is revealed for NGC 5905. At present, and among the X-ray bursts, this makes NGC 5905 the only safe candidate for a tidal disruption event in an otherwise non-active galaxy. We briefly comment on a search for further highly variable objects on the basis of ROSAT observations. Several with factors 10-20 are found.