MUSE spectroscopy of the ULX NGC 1313 X-1: A shock-ionised bubble, an X-ray photoionised nebula, and two supernova remnants

Abstract

Context. The presence of large ionised gaseous nebulae found around some ultraluminous X-ray sources (ULXs) provides the means to assess the mechanical and radiative feedback of the central source, and hence constrain the efficiency and impact on the surroundings of the super-Eddington regime powering most of these sources. NGC 1313 X-1 is an archetypal ULX, reported to be surrounded by abnormally high [O I]λ6300/Hα > 0.1 ratios, and for which high-resolution spectroscopy in X-rays has hinted at the presence of powerful outflows. Aims. We report observations taken with the integral field unit Multi-Unit Spectroscopic Explorer (MUSE) mounted at the Very Large Telescope of NGC 1313 X-1 in order to confirm the presence of a nebula inflated by the winds, investigate its main sources of ionisation and estimate the mechanical output of the source. Methods. We investigated the morphology, kinematics, and sources of ionisation of the bubble through the study of the main nebular lines. We compared the main line ratios with spatially resolved Baldwin'Phillips'Terlevich diagrams and with the prediction from radiative shock libraries, which allows us to differentiate regions excited by shocks from those excited by extreme ultraviolet and X-ray radiation. Results. We detect a bubble of 452 × 266 pc in size, roughly centred around the ULX, which shows clear evidence of shock ionisation in the outer edges. We estimate shock velocities to be in the-160-180 km s-1 range based on the line ratios. This suggests that an average and continuous outflow power of ∼(2-4.5)×-1040 erg s-1 over a timescale of (4.5-7.8)×-105 yr is required to inflate the bubble. In the interior of the bubble and closer to the ULX we detect an extended (∼140 pc) X-ray ionised region. Additionally, we detect two supernova remnants coincidentally close to the ULX bubble of which we also report age and explosion energy estimates. Conclusions. The elongated morphology and the kinematics of the bubble strongly suggest that the bubble is being inflated by winds and/or jets emanating from the central source, supporting the presence of winds found through X-ray spectroscopy. The estimated mechanical power is comparable to or higher than the X-ray luminosity of the source, which provides additional evidence in support of NGC 1313 X-1 harbouring a super-Eddington accretor.