Ionized emission and absorption in a large sample of ultraluminous X-ray sources

Author:

Kosec P12,Pinto C34,Reynolds C S1ORCID,Guainazzi M4,Kara E2ORCID,Walton D J1ORCID,Fabian A C1ORCID,Parker M L15ORCID,Valtchanov I6

Affiliation:

1. Institute of Astronomy, Madingley Road, CB3 0HA Cambridge, UK

2. MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA 02139, USA

3. INAF - IASF Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy

4. ESTEC/ESA, Keplerlaan 1, NL-2201AZ Noordwijk, The Netherlands

5. European Space Astronomy Centre (ESAC), Science Operations Department, E-28692 Villanueva de la Cañada, Madrid, Spain

6. Operations Department, Telespazio Vega UK for ESA, European Space Astronomy Centre,E-28691 Villanueva de la Cañada, Spain

Abstract

ABSTRACT Most ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion on to stellar-mass compact objects. Accretors in this extreme regime are naturally expected to ionize copious amounts of plasma in their vicinity and launch powerful radiation-driven outflows from their discs. High spectral resolution X-ray observations [with reflection grating spectrometer (RGS) gratings onboard XMM–Newton] of a few ULXs with the best data sets indeed found complex line spectra and confirmed such extreme (0.1–0.3c) winds. However, a search for plasma signatures in a large ULX sample with a rigorous technique has never been performed, thereby preventing us from understanding their statistical properties such as the rate of occurrence, to constrain the outflow geometry, and its duty cycle. We developed a fast method for automated line detection in X-ray spectra and applied it to the full RGS ULX archive, rigorously quantifying the statistical significance of any candidate lines. Collecting the 135 most significant features detected in 89 observations of 19 objects, we created the first catalogue of spectral lines detected in soft X-ray ULX spectra. We found that the detected emission lines are concentrated around known rest-frame elemental transitions and thus originate from low-velocity material. The absorption lines instead avoid these transitions, suggesting they were imprinted by blueshifted outflows. Such winds therefore appear common among the ULX population. Additionally, we found that spectrally hard ULXs show fewer line detections than soft ULXs, indicating some difference in their accretion geometry and orientation, possibly causing overionization of plasma by the harder spectral energy distributions of harder ULXs.

Funder

European Space Agency

National Aeronautics and Space Administration

Science and Technology Facilities Council

European Research Council

ESA

NASA

California Institute of Technology

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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