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

Abstract

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 which has been reported to be surrounded by abnormally high [O I]$\lambda$6300/H$\alpha >$ 0.1 ratios and for which high-resolution spectroscopy in X-rays has hinted the presence of powerful outflows. We report observations taken with the Multi-Unit Spectroscopic Explorer 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. We detect a bubble of 452 $\times$ 266\,pc in size, roughly centred around the ULX, which shows clear evidence for shock ionisation in the outer edges. We estimate shock velocities to be in the $\approx160-180$\,km/s range based on the line ratios. This suggests that an average and continuous outflow power of $\sim(2-4.5) \times 10^{40}$ erg/s over a timescale of $(4.5-7.8) \times10^5$\,yr is required to inflate the bubble. In the interior of the bubble and closer to the ULX we detect an extended ($\sim$140\,pc) X-ray ionised region. Additionally, we detect two supernova remnants coincidentally close to the bubble of which we also report age and explosion energy estimates. 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 or higher than the X-ray luminosity of the source, providing additional evidence in support of NGC 1313 X--1 harbouring a super-Eddington accretor.