# Vela X-1 in depth: a review article on an outstanding object

Silvia Martínez-Núñez, Peter Kretschmar, I. El Mellah, F. Fuerst, V. Grinberg, A. Sander, J. van den Eijnden, N. Degenaar, J. Maíz Apellániz, F. Jiménez Esteban, et al.

► To cite this version:

Silvia Martínez-Núñez, Peter Kretschmar, I. El Mellah, F. Fuerst, V. Grinberg, et al. Vela X-1 in depth: a review article on an outstanding object. M. Manteiga,; L. Bellot,; P. Benavidez,; A. de Lorenzo-Cáceres,; M. A. Fuente,; M. J. Martínez,; M. Vázquez Acosta,; C. Dafonte. XV Scientific Meeting of the Spanish Astronomical Society, Sep 2022, La Laguna, Spain. Zenodo, Highlights of Spanish Astrophysics XI, 2023, 10.5281/zenodo.7038485. [https://doi.org/10.5281/zenodo.7038485](https://doi.org/10.5281/zenodo.7038485)

HAL Id: hal-04965438

https://hal.science/hal-04965438v1

Submitted on 25 Feb 2025

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Distributed under a Creative Commons Attribution 4.0 International License
# Vela X-1 in depth: a review article on an outstanding object

S. Martínez-Núñez (IFCA)*, P. Kretschmar (ESAC), I. El Mellah (Institut de Planétologie et d'Astrophysique de Grenoble), F. Fürst (ESAC), V. Grinberg (ESA/ESTEC), A. A. C. Sander (ZAH), J. van den Eijnden (Oxford U.), N. Degenaar (API), J. Maíz Apellániz (CAB), F. Jiménez Esteban (CAB), M. Ramos-Lerate (ESAC) & E. Utrilla (ESAC)

*silvia.martinez.nunez@gmail.com Twitter: @Silviamn75

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The paper - **Revisiting the archetypical wind accretor Vela X-1 in depth. Case study of a well-known X-ray binary and the limits of our knowledge** - provided a **thoughtful compilation of the knowledge of the Vela X-1 system** and **its modelling** published over decades; added **new information** derived from public data or catalogues, including an **updated distance and spectral classification** based on the Gaia EDR3 release; and identified **specific avenues of future research** that could help to clarify open questions.

This publication triggered a discussion in the editorial board of A&A journal, **leading to the creation of a new type of review article on uniquely interesting objects.**

## Vela X-1: a well-known classical wind-fed supergiant binary system?

A bright and persistent X-ray source and a challenge of scales - from the order of **millions of km** for the global system down to less than a **km** for the X-ray emission created in the accretion column -

<table><tr><td>Distance <sup>(1)</sup></td><td>1.99 (+0.13,-0.11) kpc</td></tr><tr><td>Donor star type, mass & radius <sup>(1)(2)(3)</sup></td><td>B0.2 Ia, 17-27M<sub>☉</sub>, 28-33R<sub>☉</sub></td></tr><tr><td>Mass-loss by stellar wind<sup>(2)(4)</sup></td><td>4x10<sup>-7</sup> - 2x10<sup>-6</sup> M<sub>☉</sub> per year</td></tr><tr><td>Accretor (NS) <sup>(2)</sup></td><td>1.9+0.7-0.5 M<sub>☉</sub></td></tr><tr><td>Orbital period <sup>(3)(5)(6)</sup></td><td>8.964357±0.000029 d</td></tr><tr><td>a sin i <sup>(2,5)</sup></td><td>113.89 lt-sec, i > 70 deg</td></tr><tr><td>Eccentricity <sup>(5)</sup></td><td>0.0898±0.0012</td></tr><tr><td>Pulse period</td><td>~283 s (erratic variations)</td></tr></table>

(1) Kretschmar+ (2021)

(2) Giménez-García+ (2016)

(3) Falanga+ (2015)

(4) Watanabe+ (2006)

(5) Bildsten+ (1997)

(6) Kreykenbohm+ (2008)

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Courtesy of Felix Furst

## X-ray properties: an extremely variable source

* Observed flux variations on many time scales: days; hours & minutes

* No two orbits are the same, but there are stable mean patterns

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* Random walk long-term pulse period evolution

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* Various satellites find **strong absorbing column density variations** along orbit, but same phases can look very differently at different times!

## Distance and OB association

* Gaia EDR3 parallax $\omega = 496.2 \pm 15.2$ µas + prior for runaway OB stars (Maíz Apellániz (2001); Maíz Apellániz et al. (2005); Maíz Apellániz et al. (2008))

→ updated distance to 1.99 (+0.13,-0.11) kpc

* Vela OB1 distance is compatible with a possible origin of Vela X-1. However, large uncertainties prevent us from reaching a firm conclusion about the origin of Vela X-1. **Future Gaia data releases need to confirm the association!**

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Temporal evolution of distance estimates derived with different approaches.

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Comparison of the proposed tracks for the Vela X-1 system relative to the Vela OB1 association.

## Key parameters to improve for future studies

* Estimated distance → still ~30% uncertainty in absolute luminosities

* Ephemeris with a recent zero-point

* Inclination (within ~20° wide interval) is a strong source of uncertainty for the determination of the masses & donor radius

* Stellar parameters: detailed SED modelling of high-resolution spectroscopic data; well-calibrated photometry and/or spectrophotometry to independently estimate the effective temperature

* A firmer constraint of $M_{NS}$ would be of high interest - frequently quoted as an example of a heavy NS

* The NS magnetic field (constrained ~10%) is a key parameter that drives the accretion physics & the physics of the emission region

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