High-resolution X-ray spectroscopy of the stellar wind in Vela X-1 during a flare

¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
e-mail: Maria.Lomaeva.19@ucl.ac.uk
² ESA European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
³ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴ Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
⁵ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁶ Dr. Remeis Sternwarte & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁷ European Space Astronomy Centre (ESAC), Science Operations Department, 28692 Villanueva de la Cañada, Madrid, Spain
⁸ Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías, Universiad de Alicante, Alicante 0380, Spain
⁹ Instituto de Física de Cantabria (CSIC-Universidad de Cantabria), 39005 Santander, Spain

Received: 24 February 2020
Accepted: 6 July 2020

Abstract

Context. We present a ~130 ks observation of the prototypical wind-accreting, high-mass X-ray binary Vela X-1 collected with XMM-Newton at orbital phases between 0.12 and 0.28. A strong flare took place during the observation that allows us to investigate the reaction of the clumpy stellar wind to the increased X-ray irradiation.

Aims. To examine the wind’s reaction to the flare, we performed both time-averaged and time-resolved analyses of the RGS spectrum and examined potential spectral changes.

Methods. We focused on the high-resolution XMM-Newton RGS spectra and divided the observation into pre-flare, flare, and post-flare phases. We modeled the time-averaged and time-resolved spectra with phenomenological components and with the self-consistent photoionization models calculated via CLOUDY and XSTAR in the pre-flare phase, where strong emission lines due to resonant transitions of highly ionized ions are seen.

Results. In the spectra, we find emission lines corresponding to K-shell transitions in highly charged ions of oxygen, neon, magnesium, and silicon as well as radiative recombination continua (RRC) of oxygen. Additionally, we observe potential absorption lines of magnesium at a lower ionization stage and features identified as iron L lines. The CLOUDY and XSTAR photoionization models provide contradictory results, either pointing towards uncertainties in theory or possibly a more complex multi-phase plasma, or both.

Conclusions. We are able to demonstrate the existence of a plethora of variable narrow features, including the firm detection of oxygen lines and RRC that RGS enables to observe in this source for the first time. We show that Vela X-1 is an ideal source for future high-resolution missions, such as XRISM and Athena.