Origin of the X-ray off-states in Vela X-1

Abstract

$Context$: Vela X-1 is the prototype of the classical sgHMXB systems. Recent continuous and long monitoring campaigns revealed a large hard X-rays variability amplitude with strong flares and off-states. This activity has been interpreted invoking clumpy stellar winds and/or magnetic gating mechanisms. $Aims$: We are probing if the observed behaviour could be explained by unstable hydrodynamic flows close to the neutron star rather than the more exotic phenomena. $Methods$: We have used the hydrodynamic code VH-1 to simulate the flow of the stellar wind with high temporal resolution and to compare the predicted accretion rate with the observed light-curves. $Results$: The simulation results are similar to the observed variability. Off-states are predicted with a duration of 5 to 120 minutes corresponding to transient low density bubbles forming around the neutron star. Oscillations of the accretion rate with a typical period of $\sim$ 6800 sec are generated in our simulations and observed. They correspond to the complex motion of a bow shock, moving either towards or away from the neutron star. Flares are also produced by the simulations up to a level of $10^{37}$ erg/s. $Conclusions$: We have qualitatively reproduced the hard X-ray variations observed in Vela X-1 with hydrodynamic instabilities predicted by a simple model. More sophisticated phenomena, such as clumpy winds or the magnetic gating mechanism, are not excluded but not required to explain the basic phenomenology.

arXiv commentAccepted for publication in A&A

References in corpus (20)

• ISGRI: the INTEGRAL Soft Gamma-Ray Imager
• Observations of Accreting Pulsars
• Calibration of the Rossi X-ray Timing Explorer Proportional Counter Array
• Non-linear X-ray variability in X-ray binaries and active galaxies
• A Review of Bondi--Hoyle--Lyttleton Accretion
• Density probability distribution in one-dimensional polytropic gas dynamics
• Theory of quasi-spherical accretion in X-ray pulsars
• The mass of the neutron star in Vela X-1 and tidally induced non-radial oscillations in GP Vel
• Probing clumpy stellar winds with a neutron star
• High variability in Vela X-1: giant flares and off states
• High resolution X-ray spectroscopy of bright O type stars
• Clumping in the inner winds of hot, massive stars from hydrodynamical line-driven instability simulations
• Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization
• Bondi-Hoyle Accretion in a Turbulent Medium
• X-ray variation statistics and wind clumping in Vela X-1
• Witnessing the magnetospheric boundary at work in Vela X-1
• On the nature of `off' states in slowly rotating low-luminosity X-ray pulsars
• Revisiting the Flip-Flop Instability of Hoyle-Lyttleton Accretion
• Hoyle-Lyttleton Accretion in Three Dimensions
• X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion

Cited by in corpus (28)

• High-Mass X-ray Binaries in the Milky Way: A closer look with INTEGRAL
• Towards a unified view of inhomogeneous stellar winds in isolated supergiant stars and supergiant high mass X-ray binaries
• Advances in Understanding High-Mass X-ray Binaries with INTEGRAL and Future Directions
• Wind Roche lobe overflow in high mass X-ray binaries : a possible mass transfer mechanism for Ultraluminous X-ray sources
• Accretion form a clumpy massive-star wind in Supergiant X-ray binaries
• Formation of wind-captured discs in Supergiant X-ray binaries : consequences for Vela X-1 and Cygnus X-1
• The clumpy absorber in the high-mass X-ray binary Vela X-1
• Bondi-Hoyle-Lyttleton accretion in Supergiant X-ray binaries: stability and disk formation
• Revisiting the archetypical wind accretor Vela X-1 in depth -- A case study of a well-known X-ray binary and the limits of our knowledge
• Clumpy wind accretion in supergiant neutron star high mass X-ray binaries
• Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system III. Dipping in the low/hard state
• Constraining Compact Object Formation with 2M0521
• Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1
• High-resolution X-ray spectroscopy of the stellar wind in Vela X-1 during a flare
• The stellar wind velocity field of HD 77581
• Observing the onset of the accretion wake in Vela X-1
• Looking through the photoionisation wake: Vela X-1 at ${\phi }_{\mathrm{orb}}\approx 0.75$ with Chandra/HETG
• Detection of the orbital modulation of Fe K$\alpha$ fluorescence emission in Centaurus X-3 using the high-resolution spectrometer Resolve onboard XRISM
• Detection of nearly periodic spin period reversals in Vela X-1 on long time-scales: inkling of solar-like cycle in the donor star?
• Spectral variation in the supergiant fast X-ray transient SAX J1818.6-1703 observed by XMM-Newton and INTEGRAL
• A semi-analytical treatment to wind accretion in neutron star supergiant high mass X-ray binaries: I. eccentric orbits
• Discovery of pulsation dropout and turn-on during the high state of the accreting X-ray pulsar LMC X-4
• X-ray irradiation of the stellar wind in HMXBs with B supergiants: Implications for ULXs
• Variable structures in the stellar wind of the HMXB Vela X-1
• Variability study of the High Mass X-ray Binary IGR J18027--2016 with {\it Swift}--XRT
• X-ray Pulsars
• Variability in high-mass X-ray binaries
• Wind accretion onto compact objects