The<i>Swift</i>-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

Parola, V. La; Cusumano, G.; Segreto, A.; D’Aí, A.

doi:10.1093/mnras/stw1915

Cited by 32 publications

(23 citation statements)

References 31 publications

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“…3.1.5) as the fundamental cyclotron line, we can use Eq. (7) to infer a magnetic field strength in the line-forming region of 2 × (1 + z)10 12 G. As has been shown by Fürst et al (2010) and La Parola et al (2016), the first harmonic (at about 55 keV) shows a correlation with luminosity, indicating that the line-forming region changes in altitude above the surface of the neutron star. Using this correlation and theoretical calculations by Becker et al (2012) and Fürst et al (2010) inferred a surface magnetic field of B = 2.6 × 10 12 G. However, this value should be taken only as an estimate because it depends on the exact physical conditions within the accretion column, in particular, on the dominating process of decelerating the in-falling material, as well as on the mass and radius of the neutron star.…”

Section: Strength Of the Magnetic Field Of The Neutron Starmentioning

confidence: 60%

“…Fürst et al (2014) found a positive correlation of the harmonic (higher-energy) line with flux, as expected for a source in the subcritical accretion regime as defined by Becker et al (2012). La Parola et al (2016) undertook a long-term study of Swift/BAT data of Vela X-1. While the fundamental line was not evident in their 15-150 keV spectra, the first harmonic of the CRSF was clearly detected between ∼53 and ∼58 keV, and with an apparent positive correlation of the line energy with luminosity, flattening above L 1−150 keV ≈ 5 × 10 36 erg s −1 .…”

Section: Cyclotron Resonance Scattering Featuresmentioning

confidence: 67%

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Revisiting the archetypical wind accretor Vela X-1 in depth

Kretschmar

Mellah

Martínez-Núñez

et al. 2021

A&A

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Context. The Vela X-1 system is one of the best-studied X-ray binaries because it was detected early, has persistent X-ray emission, and a rich phenomenology at many wavelengths. The system is frequently quoted as the archetype of wind-accreting high-mass X-ray binaries, and its parameters are referred to as typical examples. Specific values for these parameters have frequently been used in subsequent studies, however, without full consideration of alternatives in the literature, even more so when results from one field of astronomy (e.g., stellar wind parameters) are used in another (e.g., X-ray astronomy). The issues and considerations discussed here for this specific, very well-known example will apply to various other X-ray binaries and to the study of their physics. Aims. We provide a robust compilation and synthesis of the accumulated knowledge about Vela X-1 as a solid baseline for future studies, adding new information where available. Because this overview is targeted at a broader readership, we include more background information on the physics of the system and on methods than is usually done. We also attempt to identify specific avenues of future research that could help to clarify open questions or determine certain parameters better than is currently possible. Methods. We explore the vast literature for Vela X-1 and on modeling efforts based on this system or close analogs. We describe the evolution of our knowledge of the system over the decades and provide overview information on the essential parameters. We also add information derived from public data or catalogs to the data taken from the literature, especially data from the Gaia EDR3 release. Results. We derive an updated distance to Vela X-1 and update the spectral classification for HD 77518. At least around periastron, the supergiant star may be very close to filling its Roche lobe. Constraints on the clumpiness of the stellar wind from the supergiant star have improved, but discrepancies persist. The orbit is in general very well determined, but a slight difference exists between the latest ephemerides. The orbital inclination remains the least certain factor and contributes significantly to the uncertainty in the neutron star mass. Estimates for the stellar wind terminal velocity and acceleration law have evolved strongly toward lower velocities over the years. Recent results with wind velocities at the orbital distance in the range of or lower than the orbital velocity of the neutron star support the idea of transient wind-captured disks around the neutron star magnetosphere, for which observational and theoretical indications have emerged. Hydrodynamic models and observations are consistent with an accretion wake trailing the neutron star. Conclusions. With its extremely rich multiwavelength observational data and wealth of related theoretical studies, Vela X-1 is an excellent laboratory for exploring the physics of accreting X-ray binaries, especially in high-mass systems. Nevertheless, much room remains to improve the accumulated knowledge. On the observational side, well-coordinated multiwavelength observations and observing campaigns addressing the intrinsic variability are required. New opportunities will arise through new instrumentation, from optical and near-infrared interferometry to the upcoming X-ray calorimeters and X-ray polarimeters. Improved models of the stellar wind and flow of matter should account for the non-negligible effect of the orbital eccentricity and the nonspherical shape of HD 77581. There is a need for realistic multidimensional models of radiative transfer in the UV and X-rays in order to better understand the wind acceleration and effect of ionization, but these models remain very challenging. Improved magnetohydrodynamic models covering a wide range of scales are required to improve our understanding of the plasma-magnetosphere coupling, and they are thus a key factor for understanding the variability of the X-ray flux and the torques applied to the neutron star. A full characterization of the X-ray emission from the accretion column remains another so far unsolved challenge.

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Section: Strength Of the Magnetic Field Of The Neutron Starmentioning

confidence: 60%

Section: Cyclotron Resonance Scattering Featuresmentioning

confidence: 67%

Revisiting the archetypical wind accretor Vela X-1 in depth

Kretschmar

Mellah

Martínez-Núñez

et al. 2021

A&A

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“…The red line is a fit to these two data sets defining σ = 0.18 E cyc . The individual sources are GX 304−1 (Klochkov et al 2015), Swift J1626.6−5156 (DeCesar et al 2013, Vela X-1 (La Parola et al 2016), and Her X-1 (same as Fig. 4).…”

Section: Changes In E Cyc With Fluxmentioning

confidence: 96%

Cyclotron line energy in Hercules X-1: stable after the decay

Staubert

Ducci

et al. 2020

A&A

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We summarize the results of a dedicated effort made between 2012 and 2019 to follow the evolution of the cyclotron line in Her X-1 through repeated NuSTAR observations. The previously observed nearly 20-year-long decay of the cyclotron line energy has ended in 2012: from then on, the pulse-phase-averaged flux-corrected cyclotron line energy has remained stable and constant at an average value of Ecyc = (37.44 ± 0.07) keV (normalized to a flux level of 6.8 RXTE/ASM-cts s−1). The flux dependence of Ecyc discovered in 2007 is now measured with high precision, giving a slope of (0.675 ± 0.075) keV/(ASM-cts s−1), corresponding to an increase of 6.5% of Ecyc for an increase in flux by a factor of two. We also find that all line parameters as well as the continuum parameters show a correlation with X-ray flux. While a correlation between Ecyc and X-ray flux (both positive and negative) is now known for several accreting binaries with various suggestions for the underlying physics, the phenomenon of a long-term decay has so far only been seen in Her X-1 and Vela X-1, with far less convincing explanations.

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“…In this time window the source is detected in the 60-110 keV band at 3.5 σ . Finally, we also associated to this spectrum a systematic error of the order of 2 per cent using deviations from the Crab spectrum (see La Parola et al 2016).…”

Section: Spectral Analysis Of the Single Datasetsmentioning

confidence: 99%

A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

D’Aí

Cusumano

Santo

et al. 2017

Monthly Notices of the Royal Astronomical Society

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The accretion-powered X-ray pulsar 4U 1626-67 is one of the few highly magnetized pulsars that accretes through Roche-lobe overflow from a low-mass companion. The characteristics of its broadband spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary systems, with a broad resonant cyclotron scattering feature (CRSF) at ∼ 37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broadband spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broadband model for the disk reflected spectrum. In the softer X-ray band, we need to add a soft black-body component with kT bb ∼ 0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at ∼ 61 keV.

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TheSwift-BAT monitoring reveals a long-term decay of the cyclotron line energy in Vela X-1

Cited by 32 publications

References 31 publications

Revisiting the archetypical wind accretor Vela X-1 in depth

Revisiting the archetypical wind accretor Vela X-1 in depth

Cyclotron line energy in Hercules X-1: stable after the decay

A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

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