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Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1
Journal article   Open access   Peer reviewed

Towards a dynamical mass of the ultraluminous X-ray source NGC 5408 X-1

D Cseh, F Grisé, P Kaaret, S Corbel, S Scaringi, P Groot, H Falcke and E Körding
Monthly notices of the Royal Astronomical Society, Vol.435(4), pp.2896-2902
11/11/2013
DOI: 10.1093/mnras/stt1484
url
https://doi.org/10.1093/mnras/stt1484View
Published (Version of record) Open Access

Abstract

We obtained multi-epoch Very Large Telescope optical spectroscopic data in 2011 and 2012 on the ultraluminous X-ray source NGC 5408 X-1. We confirm that the He ii λ4686 line has a broad component with an average full width at half-maximum of v = 780 ± 64 km s−1 with a variation of ∼13 per cent during observations spanning over four years, and is consistent with the origin in the accretion disc. The deepest optical spectrum does not reveal any absorption line from a donor star. Our aim was to measure the radial velocity curve and estimate the parameters of the binary system. We find an upper limit on the semi-amplitude of the radial velocity of K = 132 ± 42 km s−1. A search for a periodic signal in the data resulted in no statistically significant period. The mass function and constraints on the binary system imply a black hole mass of less than ∼510 M. Whilst, a disc irradiation model may imply a black hole mass smaller than ∼431-1985 M, depending on inclination. Our data can also be consistent with an unexplored orbital period range from a couple of hours to a few days, thus with a stellar-mass black hole and a subgiant companion.
 black hole physics X-rays: binaries accretion, accretion discs

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