The mass of the neutron star in Vela X-1

Abstract

Abstract.We measured the radial-velocity curve of HD 77581, the B-supergiant companion of the X-ray pulsar Vela X-1, using 183 high-resolution optical spectra obtained in a nine-month campaign. We derive radial-velocity amplitudes for different lines and wavelength regions, and find all are consistent with each other, as well as with values found in previous analyses. We show that one apparent exception, an anomalously low value derived from ultra-violet spectra obtained with the International Ultraviolet Explorer, was due to an error in the analysis procedures. We re-analyse all IUE spectra, and combine the resulting velocities with the ones derived from the new optical spectra presented here, as well as those derived from optical spectra published earlier. As in previous analyses, the radial velocities show strong deviations from those expected for a pure Keplerian orbit, with rootmean-square amplitudes of ∼7 km s −1 for strong lines of Si iv and N iii near 4100Å, and up to ∼20 km s −1 for weaker lines of N ii and Al iii near 5700Å. The deviations likely are related to the pronounced line-profile variations seen in our spectra. Our hope was that the deviations would average out when a sufficient number of spectra were added together. It turns out, however, that systematic deviations as a function of orbital phase are present as well, at the 3 km s −1 level, with the largest deviations occurring near inferior conjunction of the neutron star and near the phase of maximum approaching velocity. While the former might be due to a photo-ionisation wake, for which we observe direct evidence in the profiles of Hδ and Hα, the latter has no straightforward explanation. As a result, our best estimate of the radial-velocity amplitude, Kopt = 21.7 ± 1.6 km s −1 , has an uncertainty not much reduced to that found in previous analyses, in which the influence of the systematic, phase-locked deviations had not been taken into account. Combining our velocity amplitude with the accurate orbital elements of the X-ray pulsar, we infer Mns sin 3 i = 1.78 ± 0.15 M .

Merger Sites of Double Neutron Stars and Their Host Galaxies

“…We find, in contrast to Bethe & Brown (1998), that in most cases NSs do not accrete enough material to become BHs, and thus the channels with phases of CE are still open for the formation of coalescing NS-NS binaries. This discrepancy is due to two facts: (1) their assumption of M p max, NS , which we find very low in view of the high NS mass 1.5 M , estimates in Cyg X-2 ( ; Orosz & Kuulkers 1.78 ‫ע‬ 0.23 M , 1999) and in Vela X-1 ( M , ; Barziv et al 2001), and 1.9 ‫ע‬ 0.3 (2) their approximate 4 treatment of accretion onto NSs, which results in higher final compact object masses than in our exact numerical solution. Both our detailed treatment of the HCA and the variation of coalescence rates of compact object binaries for all our models are given in Belczynski et al (2002b).…”

Tidal effects on the radial velocity curve of HD 77581 (Vela X-1)

“…Barziv et al (2001) report the presence of a "blue excursion" in the Hδ data that produces a local minimum in the RV curve at φ ∼ 0.37. They interpret the "blue excursion" in terms of a photoionization wake.…”

“…In most observational investigations φ obs = 0 is defined at the orbital latitude 90 • from the line of nodes. This definition is such that φ obs = 0 at the midpoint of the X-ray eclipse (Bildsten et al 1997;van Kerkwijk et al 1995;Barziv et al 2001), and periastron passage occurs at φ obs = 0.17. Hence, φ obs = φ + 0.17.…”

“…Specifically, the lines develop asymmetries over the orbital cycle which lead to systematic deviations of the measured RV data points from a Keplerian RV curve. It is believed that the line-profile variability is associated with tidal forcing, non-radial pulsations and other interaction effects in the binary system (van Paradijs et al 1977b;van Kerkwijk et al 1995;Barziv et al 2001;Quaintrell et al 2003). This raises the question of whether these systematic deviations may not lead to an artificially large RV curve amplitude and, hence, an overestimate of the neutron star mass.…”

Irradiation Effects in Compact Binaries

“…In Nova Sco (Phillips, Shahbaz, & Podsiadlowski 1999) the residuals of a fit to the observed absorptionline curve when compared to a standard sinusoidal fit are as large as 20kms _ 1 (10%). In addition, in Vela X-l there are persistent residuals in the radialvelocity curve (5kms _ 1 , 25%, Barziv et al 2001) whose cause is unknown. These could be a direct observation of circulation currents on the secondary surface.…”