The metallicity of the nebula surrounding the ultra‐luminous X‐ray source NGC 1313 X‐2

Abstract

Recent models of the formation of ultra-luminous X-ray sources (ULXs) predict that they preferentially form in lowmetallicity environments. We look at the metallicity of the nebula surrounding NGC 1313 X-2, one of the best-studied ULXs. Simple estimates, based on the extrapolation of the metallicity gradient within NGC 1313, or on empirical calibrations (relating metallicity to strong oxygen lines) suggest a quite low metal content (Z ∼ 0.1Z ). But such estimates do not account for the remarkably strong X-ray flux irradiating the nebula. Then, we build photoionization models of the nebula using CLOUDY; using such models, the constraints on the metallicity weaken substantially, as we find 0.15Z ≤ Z ≤ 0.5Z .

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

“…Each O star has an effective temperature of 40000 K. We vary the X-ray luminosity and O star quantity to fit the observation, and find that an X-ray luminosity of 1.6 × 10 40 erg s −1 plus 60 O stars with a total luminosity of 6.4 × 10 40 erg s −1 can reasonably match the observations (Table 3). The simulated [N II] λ6583/Hα is higher than that observed; this is consistent with previous studies by Ripamonti et al (2011) who propose that the nitrogen abundance in the nebula could be further lower. This ULX also displays a soft blackbody component, which could be the ionizing source.…”

“…We assume an ISM density of 1 cm −3 and a magnetic field of 3 μG (Han 2017); these two parameters are not sensitive to the results. We also assume Z = 0.5 Z e as suggested by previous studies (Ripamonti et al 2011;Pintore & Zampieri 2012) and will discuss its influence. The models include dust calculations and allow grain destruction; these two options also have a small impact on our conclusions.…”

“…NGC 1313 X-2 is a prototype ULX among the first discovered (Colbert et al 1995). A bubble nebula is found to be coincident with the X-ray source position and has been extensively studied (Pakull & Mirioni 2002Zampieri et al 2004;Mucciarelli et al 2005;Pakull et al 2006;Ramsey et al 2006;Ripamonti et al 2011). The nebula is elongated with a geometry of 590 pc × 410 pc (26 × 18 ).…”

THE LOCAL ENVIRONMENT OF ULTRALUMINOUS X-RAY SOURCES VIEWED BYXMM-NEWTON's OPTICAL MONITOR

“…Swartz et al (2009) used photometric data from the Sloan Digital Sky Survey to look for possible associations of 47 ULXs with SFRs or young superclusters. They found that statistically ULXs are indeed associated with recent star formation (within 1 The following is a list of references for some of the most famous objects with optical counterparts: NGC 5204 X-1 ), NGC 1313 X-1 (Yang et al 2011), ESO 243-49 HLX-1 ) NGC 1313 X-2 (Zampieri et al 2004;Liu et al 2007;Ripamonti et al 2011;Zampieri et al 2012), M81 X-6 (Liu et al 2002;Swartz et al 2003;Moon et al 2011), Holmberg IX X-1 (Grisé, Pakull, & Motch 2006;Moon et al 2011;Grisé et al 2011), Holmberg II X-1 Tao et al 2012b), NGC 5408 X-1 (Lang et al 2007;Grisé et al 2012), M101 X-1 (Kuntz et al 2005;Liu 2009), NGC 4559 , two ULXs in M51 , NGC 2403 X-1 , IC 342 X-1 (Feng & Kaaret 2008), the ULX in NGC 247 ), NGC 6946 X-1 and ULX P13 in NGC 7793 (Pakull et al 2010;Motch et al 2011). 100 pc distance), but no superclusters were detected given the poor spatial resolution of the instrument. Poutanen et al (2012) performed spectral and photometric analyses of clusters associated with the Antennae ULXs and found that almost all are very young (2.4 to 3.2 Myr), and that only one resides inside a cluster (see also Rangelov et al 2012).…”

The black hole in NGC 1313 X‐2

“…Therefore, although the metallicity in the environment of NGC 1313 X-2 appears to be definitely sub-solar, the actual value is still uncertain (see Ripamonti et al 2011; see also Pintore & Zampieri (2011) for an independent metallicity estimate from X-ray spectra).…”