Very Large Telescope MUSE Observations of the Bubble Nebula around NGC 1313 X-2 and Evidence for Additional Photoionization

Abstract

The bubble nebula surrounding NGC 1313 X-2 is believed to be powered by high velocity winds from the central ultraluminous X-ray source (ULX) as a result of supercritical accretion. With the Multi-Unit Spectroscopic Explorer (MUSE) observation of the nebula, we find enhanced [O iii] emission at locations spatially coincident with clusters of stars and the central X-ray source, suggesting that photoionization in addition to shock ionization plays an important role in powering the nebula. The X-ray luminosity of the ULX and the number of massive stars in the nebula region can account for the required ionizing luminosity derived with MAPPINGS V, which also confirms that pure shocks cannot explain the observed emission line ratios.

Identification of a Helium Donor Star in NGC 247 ULX-1

“…The velocity dispersion is typically ∼30 km s −1 for Bubble A − C, ∼100 km s −1 for Bubble B, and ∼70 km s −1 for Bubble C. The relatively high velocity dispersions in particular for Bubbles B and C suggest that they are more likely due to shock-ionization than photoionization. Even for Bubble A, which has the smallest velocity dispersion, its regular elliptical shape (similar to the bubble around NGC 1313 X-2; Pakull & Mirioni 2002;Zhou et al 2022) indicates that it is powered by winds from the ULX. In Figure 7, we plot the diagnostic diagram using emission line flux ratios (Baldwin et al 1981) for the three bubbles together with other known ULX nebulae.…”

“…Ultraluminous X-ray sources (ULXs) are non-nuclear accreting compact objects with an apparent luminosity above the Eddington limit for stellar-mass black holes (Fabbiano 1989;Makishima et al 2000). Identifications of highvelocity (∼0.1c-0.4c) outflows (Pinto et al 2016(Pinto et al , 2017(Pinto et al , 2021Walton et al 2016;Kosec et al 2018), energetic shock-ionized bubble nebulae (Pakull & Mirioni 2002Ramsey et al 2006;Abolmasov & Moiseev 2008;Russell et al 2011;Belfiore et al 2020;Soria et al 2021;Zhou et al 2022), and neutron star accretors (Bachetti et al 2014;Fürst et al 2016;Israel et al 2017aIsrael et al , 2017bCarpano et al 2018;Sathyaprakash et al 2019;Rodríguez Castillo et al 2020) suggest that the majority of them are powered by supercritical accretion.…”

“…If these winds with a high mechanical power are injected into the interstellar medium, a forward shock will be produced appearing as an optical nebula (Siwek et al 2017). Indeed, optical bubble nebulae with a size of some 100 pc and an expansion velocity about 100 km s −1 have been observed around many ULXs (Pakull & Mirioni 2002Ramsey et al 2006;Abolmasov & Moiseev 2008;Russell et al 2011;Soria et al 2021;Guo et al 2023;Gúrpide et al 2022;Zhou et al 2022). The emission line properties suggest that many of them are driven by winds or jets with an ultraluminous mechanical power (∼10 39 -10 40 erg s −1 ) over a duration typically of Myr.…”

Bubble in the Whale: Identifying the Optical Counterparts and Extended Nebula for the Ultraluminous X-Ray Sources in NGC 4631

“…The extended structures around ULXs may originate from photoionization or shock ionization, both of which could coexist while playing major roles in different parts of the structure (Moon et al 2011;Gúrpide et al 2022;Zhou et al 2022). Generally, in the photoionization process, the line flux ratio [O III]/Hβ tends to peak at or near the ionizing source and declines outward.…”

“…For the other few cases, the high-ionization features (e.g., He II λ4686) in the spectra of the optical nebulae imply that the photoionization could be the major origin of the extended structure (Pakull & Mirioni 2002). Both shock ionization and photoionization may certainly be working at the same time while dominating different parts of the same optical nebula (Gúrpide et al 2022;Zhou et al 2022).…”