NuSTAR results + conclusions Methods Motivations Mathematisch- Naturwissenschaftliche Fakultät Using Vela X-1 to understand accretion and wind structure in High-Mass X-ray binaries (HMXBs) 60 58 56 54 52 50 48 12 10 8 6 4 2 35 30 25 20 ECRSF,H [keV] Luminosity [1036erg s−1] ECRSF,F [keV] 1 0.8 0.6 0.4 0.2 0 100 80 60 40 20 0 CF NH,1 [1022cm−2] • Vela X-1: convenient physical parameters (close distance, high inclination, small orbital separation) which facilitate the observation and analysis of the system • Two observations taken on January (NuSTAR) and May 2019 (simultaneous NuSTAR+XMM-Newton) at orbital phases ~0.68–0.78 and ~0.36–0.52, respectively 0.6 0.4 0.2 0.0 0.8 0.6 1400 1200 1000 800 600 400 200 0 Rate [mCrab] Phase single-absorption model best-ﬁt model without 10 keV feature best-ﬁt model without CRSF best-ﬁt model 1 0.1 0.01 10-3 a) 0 b) 1 0.5 c) 1.2 1 d) 70 50 30 20 15 10 7 5 4 3 1.2 1 Counts s−1 keV−1 χ Ratio Ratio Energy (keV) Ratio References XMM-Newton PN analysis • Preliminary XMM-Newton results showing a strong variation of the absorption column density of the stellar wind along the orbital phase when the wakes are passing through the line of sight • drops after flaring episodes, which can be explained by accretion of clumps thus reducing the density of the material. NH,1 [1] C.M. Diez, V. Grinberg, et al. 2022, Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1, A&A 660, A19 [2] El Mellah, et al. 2018, Accretion from a clumpy massive-star wind in supergiant X- ray binaries, MNRAS 475, 3240–3252 [3] Mushtukov, et al. 2014, On the origin of cyclotron lines in the spectra of X-ray pulsars, EPJ Web of Conferences 64, 02005 [4] Fürst, F., Pottschmidt, K., Wilms, J., et al. 2014, NuSTAR discovery of a luminosity dependent cyclotron line energy in Vela X-1, ApJ, 780, 133 [5] C.M. Diez, V. Grinberg, I. El Mellah, et al., paper in prep. [3] [1] Fig. 6: Spectral parameter as function of the 3-79 keV luminosity based on our orbit-by-orbit spectroscopy results ( for observation I, for observation IIa, and for observation IIb) together with ks-integrated spectral fits from Fig. 7 in [4] ( ). Upper panel: photon index . The black dot-dashed line is a linear regression through all data points, meant to guide the eye. Middle panel: energy of the harmonic cyclotron line . The black solid line is the theoretical prediction for = 1, = 30 keV and = 1.9 . The above and below dot-dashed lines are for = 1.7 and = 2.1 respectively. Lower panel: energy of the fundamental line . Γ ECRSF,H Λ ENS MNS M⊙ MNS M⊙ MNS M⊙ ECRSF,F Fig. 5: Stellar wind absorption as function of the covering fraction based on our orbit-by-orbit and pulse-by-pulse spectroscopy results. are the results of the orbit-by-orbit analysis and of the pulse- by-pulse analysis both for observation I, are the results of the orbit-by- orbit spectroscopy and of the pulse-by-pulse spectroscopy both for observation IIa, are the results of the orbit-by-orbit spectroscopy and X of the pulse-by-pulse spectroscopy both for observation IIb. NH,1 CF Fig. 4: NuSTAR 3–79 keV flux for observation I (red) on January 2019 and observation II on May 2019 (blue) and Swift/BAT 15—50 keV flux averaged over all data since 2005 (black solid line) plotted over the neutron star’s orbital phase where 0 is defined with T90. Fig. 3: Time-averaged spectrum for observation I. FPMA data in red and FPMB data in blue, models for both focal plane modules are shown in the same color and are hardly distinguishable with the naked eye. The first panel shows the count spectrum. Top-down description of the residual panels: a) best-fit model (black), b) best-fit model with CRSFs (yellow) turned off but not fitted again, c) best-fit model with 10-keV feature (green) turned off but not fitted again, d) single-absorption model (magenta). C. M. Diez1, V. Grinberg1,2, F. Fürst3, E. Sokolova-Lapa4,5, A. Santangelo1, J. Wilms4, K. Pottschmidt6,7, S. Martínez-Núñez8, C. Malacar