Observations of a GX 301–2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

Abarr, Quincy; Baring, Matthew G.; Beheshtipour, Banafsheh; Beilicke, M.; Geronimo, G. De; Dowkontt, P. F.; Errando, M.; Guarino, V. J.; Iyer, Nirmal; Kislat, Fabian; Kiss, Mózsi; Kitaguchi, Takao; Krawczynski, H.; Lanzi, James; Li, S.; Lisalda, Lindsey; Okajima, Takashi; Pearce, Mark; Press, L.; Rauch, B. F.; Stuchlik, David; Takahashi, H.; Tang, Jason; Uchida, Naoshige; West, Andrew; Jenke, P.; Krimm, H. A.; Lien, A. Y.; Malacaria, C.; Miller, J. M.; Wilson-Hodge, C.

doi:10.3847/1538-4357/ab672c

Cited by 37 publications

(14 citation statements)

References 85 publications

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“…A notable recent exception was the balloon-borne hard X-ray calorimeter, X-Calibur, which observed Vela X-1. Because the flight duration was much shorter than anticipated, however, no constraining data could be obtained (Abarr et al 2020).…”

Section: X-ray Polarizationmentioning

confidence: 96%

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: X-ray Polarizationmentioning

confidence: 96%

Revisiting the archetypical wind accretor Vela X-1 in depth

Kretschmar

Mellah

Martínez-Núñez

et al. 2021

A&A

View full text Add to dashboard Cite

show abstract

“…So far, for lack of sensitive X-ray polarimeters in space, there have been no observations of X-ray polarization parameters in Vela X-1 or similar systems from X-ray satellites. A notable recent exception was the balloon borne hard X-ray calorimeter, X-Calibur, which was observing Vela X-1; however, due to a much shorter flight duration than anticipated no constraining data could be obtained (Abarr et al 2020).…”

Section: X-ray Polarizationmentioning

confidence: 99%

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

Kretschmar,

Mellah,

Martínez-Núñez

et al. 2021

Preprint

View full text Add to dashboard Cite

Context. The Vela X-1 system is one of the best studied X-ray binaries, due to its early detection, persistent X-ray emission and a rich phenomenology at many wavelengths. The system is frequently quoted as an archetype of wind-accreting high-mass X-ray binaries and its parameters referred to as typical examples. Frequently though, specific values for these parameters have been used in subsequent studies, 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 the study of their physics. Aims. We aim to 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. Since this overview is targeted at a broader readership, we include more background information on the physics of the system and on methods than 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 currently possible. Methods. We explore the vast literature for Vela X-1 and on modelling efforts based on this system or close analogues. We describe the evolution of the system knowledge 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 spectra 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 tensions subsist. The orbit is in general very well determined, but a slight tension 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 towards lower velocities over the years. Recent results with wind velocities at the orbital distance in the range or 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 hints have emerged. Hydrodynamic models and observations are consistent with an accretion wake trailing the neutron star. Conclusions. With its extremely rich multi-wavelength observational data and wealth of related theoretical studies, Vela X-1 is an excellent laboratory to explore the physics of accreting X-ray binaries, especially in high-mass systems. Nevertheless, a lot of room remains to improve on the accumulated knowledge. On the observational...

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“…For the context of accreting X-ray pulsars, the reader can survey the papers of Ventura, Nagel, & Mészáros (1979); Mészáros & Bonazzola (1981); Mészáros et al (1988) and various articles cited in the book by Mészáros (1992) to discern the varied energy-dependent polarization signatures that define powerful observational probes of accretion geometry. This is generally the domain of hard X-ray polarimetry, and an introductory demonstration of diagnostic capability in this band was recently provided by the X-Calibur polarization measurement above 15 keV for the X-ray binary GX 301-2 (Abarr et al 2020).…”

Section: Introductionmentioning

confidence: 99%

Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions

Hu,

Baring,

Barchas

et al. 2022

Preprint

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The surfaces of neutron stars are sources of strongly polarized soft X rays due to the presence of strong magnetic fields. Radiative transfer mediated by electron scattering and free-free absorption is central to defining local surface anisotropy and polarization signatures. Scattering transport is strongly influenced by the complicated interplay between linear and circular polarizations. This complexity has been captured in a sophisticated magnetic Thomson scattering simulation we recently developed to model the outer layers of fully-ionized atmospheres in such compact objects, heretofore focusing on case studies of localized surface regions. Yet, the interpretation of observed intensity pulse profiles and their efficacy in constraining key neutron star geometry parameters is critically dependent upon adding up emission from extended surface regions. In this paper, intensity, anisotropy and polarization characteristics from such extended atmospheres, spanning considerable ranges of magnetic colatitudes, are determined using our transport simulation. These constitute a convolution of varied properties of Stokes parameter information at disparate surface locales with different magnetic field strengths and directions relative to the local zenith. Our analysis includes full general relativistic propagation of light from the surface to an observer at infinity. The array of pulse profiles for intensity and polarization presented highlights how powerful probes of stellar geometry are possible. Significant phase-resolved polarization degrees in the range of 10-60% are realized when summing over a variety of surface field directions. These results provide an important background for observations to be acquired by NASA's new IXPE X-ray polarimetry mission.

show abstract

Observations of a GX 301–2 Apastron Flare with the X-Calibur Hard X-Ray Polarimeter Supported by NICER, the Swift XRT and BAT, and Fermi GBM

Cited by 37 publications

References 85 publications

Revisiting the archetypical wind accretor Vela X-1 in depth

Revisiting the archetypical wind accretor Vela X-1 in depth

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

Intensity and Polarization Characteristics of Extended Neutron Star Surface Regions

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