First look at Vela X-1 with XRISM

A simultaneous campaign with XMM-Newton and NuSTAR

¹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² IRAP, Université de Toulouse, CNRS, CNES, UT3-PS, Toulouse, France
³ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁴ Quasar Science Resources SL for ESA, European Space Astronomy Centre (ESAC), Science Operations Department, 28692 Villanueva de la Cañada, Madrid, Spain
⁵ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, (RM), Italy

^⋆ Corresponding author: camille.m.diez@gmail.com

Received: 3 July 2025
Accepted: 3 September 2025

Abstract

Context. High-mass X-ray binaries (HMXBs) can serve as useful laboratories for exploring the behaviour of accreted matter onto compact objects and for probing the complex wind environments of massive stars. These investigations are essential for understanding stellar life cycles and the dynamics of the Milky Way, and they are prominent topics in the science cases for XRISM and NewAthena.

Aims. We report, for the first time, on a XRISM observation of the HMXB Vela X-1, conducted during the first cycle of the XRISM general observer programme and complemented by simultaneous XMM-Newton and NuSTAR coverage. This campaign targeted a critical orbital phase – when the neutron star is in inferior conjunction – during which significant changes in absorption are expected.

Methods. We performed absorption-resolved spectral analyses during two time intervals of interest: the soft and hard hardness ratio (HR) intervals, as it is strongly correlated with absorption variability.

Results.We observed a sudden transition in the HR from a soft to a hard state coinciding with an increase in the absorption column density. This is likely attributable to the onset of the accretion structure crossing our line of sight. With XRISM/Resolve, we also investigated the Fe K region, and we report for the first time the presence of a Fe Kα doublet in the spectrum of Vela X-1 together with the presence of already known Fe Kβ and Ni Kα lines that are produced in cold clumps embedded in the hot ionised wind. The measured line velocities of the order of 10² km s⁻¹ are consistent with production sites in the vicinity of the neutron star.

Conclusions.This precursor study with Vela X-1 shows the potential of XRISM to study in unprecedented detail the spectral evolution of wind-accreting X-ray binaries.