Discovery and analysis of a ULX nebula in NGC 3521

Abstract

We present Very Large Telescope/X-shooter and Chandra X-ray observatory/ACIS observations of the ULX [SST2011] J110545.62+000016.2 in the galaxy NGC 3521. The source identified as a candidate near-infrared counterpart to the ULX in our previous study shows an emission line spectrum of numerous recombination and forbidden lines in the visible and near-infrared spectral regime. The emission from the candidate counterpart is spatially extended (∼34 pc) and appears to be connected with an adjacent Hii region, located ∼138 pc to the NE. The measured velocities of the emission lines confirm that both the candidate counterpart and Hii region reside in NGC 3521. The intensity ratios of the emission lines from the ULX counterpart show that the line emission originates from the combined effect of shock and photoionisation of low metallicity (12 + log (O/H) = 8.19 ± 0.11) gas. Unfortunately, there is no identifiable spectral signature directly related to the photosphere of the mass-donor star in our spectrum. From the archival Chandra data, we derive the X-ray luminosity of the source in the 0.3-7 keV range to be (1.9 ± 0.8) × 10 40 erg cm −2 s −1 , almost a factor of four higher than what is previously reported.

NIR counterparts to ULXs (III): completing the photometric survey and selected spectroscopic results★

“…Pakull & Grisé 2008) and these ULX nebulae often show strong emission lines in the NIR (e.g. Heida et al 2016;López et al 2019; this work; see Section 5.4). In conclusion, neither source A nor source B have definite characteristics that prove whether one of them is the donor star to the ULX.…”

“…Additionally, we discovered that four counterparts, initially classified as RSG based on photometry, are actually nebulae partially powered by the X-ray emission of the ULX: ULX J022727+333443 (in NGC 925), ULX J120922+295551 (in NGC 4136), ULX Ho II X-1 (in Holmberg II; Heida et al 2016), and [SST2011] J110545. 62+000016.2 (in NGC 3521;López et al 2019). This means that, at the time of writing this manuscript, the photometric and spectroscopic observations provided 3 confirmed RSGs, 12 candidate RSGs, 4 confirmed nebulae, and 14 candidate stellar clusters/active galactic nuclei (AGNs).…”

“…Three sources turned out to be nebulae (in NGC 925, in Holmberg II and NGC 4136). López et al (2019) followed up a candidate RSG in NGC 3521 identified by López et al (2017). Its spectral signatures single this source out as a nebula.…”

“…At the time of writing this manuscript, only seven counterparts had been spectroscopically followed up by Heida et al (2015aHeida et al ( , 2016 and López et al (2019). All seven sources have an absolute magnitude consistent with being RSGs.…”

“…The preliminary classification of the NIR candidate counterparts is based on their absolute magnitudes, WISE colours, spatial extent, and/or visual inspection of the NIR image. The spectroscopic classification of 12 sources is based on the spectra that we obtained for them, reported in Heida et al (2015aHeida et al ( , 2016 and López et al (2019). the ULX is H, K s < −11 mag, due to crowding or high background from the host galaxies.…”