The ultraluminous X-ray source bubble in NGC 5585

TL;DR

This paper presents the discovery and detailed analysis of a large, energetic bubble around a ULX in NGC 5585, revealing insights into the outflows and shock dynamics of super-Eddington X-ray sources.

Contribution

It introduces a new ULX bubble in NGC 5585, models its properties, refines shock velocity inference methods, and compares its radio emission evolution to supernova remnants.

Findings

The bubble's shock velocity is ~125 km/s.

The dynamical age of the bubble is ~600,000 years.

The mechanical power exceeds the current X-ray luminosity.

Abstract

Some ultraluminous X-ray sources (ULXs) are surrounded by collisionally ionized bubbles, larger and more energetic than supernova remnants: they are evidence of the powerful outflows associated with super-Eddington X-ray sources. We illustrate the most recent addition to this class: a huge (350 pc x 220 pc in diameter) bubble around a ULX in NGC 5585. We modelled the X-ray properties of the ULX (a broadened-disc source with L_X ~ 2-4 x 10^{39} erg/s) from Chandra and XMM-Newton, and identified its likely optical counterpart in Hubble Space Telescope images. We used the Large Binocular Telescope to study the optical emission from the ionized bubble. We show that the line emission spectrum is indicative of collisional ionization. We refine the method for inferring the shock velocity from the width of the optical lines. We derive an average shock velocity ~125 km/s, which corresponds to a dynamical age of ~600,000 years for the bubble, and an average mechanical power P_w ~ 10^{40} erg/s; thus, the mechanical power is a few times higher than the current photon luminosity. With Very Large Array observations, we discovered and resolved a powerful radio bubble with the same size as the optical bubble, and a 1.4-GHz luminosity ~10^{35} erg/s, at the upper end of the luminosity range for this type of source. We explain why ULX bubbles tend to become more radio luminous as they expand while radio supernova remnants tend to fade.