Contributed Talk - Splinter HotStars

Friday, 15 September 2023, 17:10   (H 2036)

Analysis of the UV spectrum of Holmberg II X-1

Sabela Reyero Serantes
University of Potsdam

UltraluminousX-ray sources (ULXs) are high mass X-ray binaries with an X-ray luminosity above 3x10^39 erg/s, which would exceed the Eddington limit for a 20 Msun black hole (BH). Alternatively to adopting such high BH masses, super-critical accretion has been proposed for explaning their X-ray brightness. On long terms, an ULX might become the progenitor of a BH merger like those observed by gravitational waves. The ULX Holmberg II X-1 (Ho II X-1) is a perfect test case to unravel its true nature, as it is relatively close and bright. Several studies have been devoted already to analysing its properties both in the X-ray and optical regime. Now we went one step further and obtained the first ever ultraviolet spectrum of an ULX with the HST COSspectrograph.These new UV spectra of Ho II X-1 and its surrounding nebula, complemented by optical data, are analysed in detail using stellar atmosphere (PoWR) and nebula (CLOUDY) models.