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Article Dans Une Revue Mon.Not.Roy.Astron.Soc. Année : 2024

A multiband look at ultraluminous X-ray sources in NGC 7424

Résumé

We studied the multiband properties of two ultraluminous X-ray sources (2CXO J225728.9-410211 = X-1 and 2CXO J225724.7-410343 = X-2) and their surroundings, in the spiral galaxy NGC 7424. Both sources have approached X-ray luminosities L_X~ 10^{40} erg/s at some epochs. Thanks to a more accurate astrometric solution (based on Australia Telescope Compact Array and Gaia data), we identified the point-like optical counterpart of X-1, which looks like an isolated B8 supergiant (M ~ 9 Msun, age ~ 30 Myr). Instead, X-2 is in a star-forming region (size of about 100 pc x 150 pc), near young clusters and ionized gas. Very Large Telescope long-slit spectra show a spatially extended region of HeII 4686 emission around the X-ray position, displaced by about 50 pc from the brightest star cluster, which corresponds to the peak of lower-ionization line emission. We interpret the HeII 4686 emission as a signature of X-ray photo-ionization from the ULX, while the other optical lines are consistent with UV ionization in an ordinary HeII region. The luminosity of this He^{++} nebula puts it in the same class as other classical photo-ionized ULX nebulae such as those around Holmberg II X-1 and NGC 5408 X-1. We locate a strong (5.5-GHz luminosity nu L_{nu}~ 10^{35} erg/s), steep-spectrum, unresolved radio source at the peak of the low-ionization lines, and discuss alternative physical scenarios for the radio emission. Finally, we use WISE data to obtain an independent estimate of the reddening of the star-forming clump around X-2.

Dates et versions

hal-04489765 , version 1 (05-03-2024)

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Citer

Roberto Soria, Siying Cheng, Manfred W Pakull, Christian Motch, Thomas D Russell. A multiband look at ultraluminous X-ray sources in NGC 7424. Mon.Not.Roy.Astron.Soc., 2024, 529 (2), pp.1169-1186. ⟨10.1093/mnras/stae551⟩. ⟨hal-04489765⟩
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