Spectral classification of the mass donors in the high-mass X-ray binaries EXO 1722-363 and OAO 1657-415

Abstract

Aims. We report near-infrared (NIR) observations of the mass donors of the eclipsing high-mass X-ray binary (HMXB) systems EXO 1722-363 and OAO 1657-415 in order to derive their accurate spectral classifications. Methods. ESO/VLT observations of the targets with the NIR spectrometer ISAAC were compared with several published NIR spectral atlases of O and B supergiants, an identification of each object's spectral characteristics was made, enabling the refinement of spectral classification of the mass donors. Results. We determined that EXO 1722-363 was of spectral type B0-B1Ia, positioned at a distance 8.0 +2.5 −2.0 kpc with a progenitor mass in the range 30-40 M . Luminosity calculations imply that L X ∼ 10 35 -10 37 erg s −1 for this distance range. We conclude that EXO 1722-363 shares many of the properties associated with other X-ray binary B-type supergiant donors. We found that OAO 1657-415 correlates closely with the spectra of a class of transitional objects, the Ofpe/WNL stars, an intermediate evolutionary stage between massive O type stars leaving the main sequence and evolving into Wolf-Rayets. Due to the wide range in luminosity displayed by Ofpe/WNL stars, (log(L/L ) ∼ 5.3-6.2) distance determinations are problematic. For OAO 1657-415 we report a distance of 4.4 ≤ d ≤ 12 kpc, implying an X-ray luminosity of 1.5 × 10 36 ≤ L X ≤ 10 37 erg s −1 . We have used our new classification of OAO 1657-415 to explain the physical processes responsible for its unique position within the Corbet diagram.Ofpe/WNL stars demonstrate a high rate of mass-loss through a dense stellar wind combined with a low terminal velocity. This combination of wind properties leads to a high accretion rate and transfer of angular momentum to the neutron star in this system. We believe this in turn leads to a smaller instantaneous equilibrium spin period with respect to normal OB supergiants.

Preliminary determinations of the masses of the neutron star and mass donor in the high mass X-ray binary system EXO 1722–363

“…First, the resulting neutron star mass is consistent with the canonical mass of 1.4 M measured in most other eclipsing HMXBs, except for that in Vela X-1, (Quaintrell et al 2003). Second, the measured mass and radius of the supergiant, M ∼ 13−15 M and R ∼ 25−28 R , support the B0-1 Ia spectral classification that we have previously determined (Mason et al 2009). This is illustrated by the HertzsprungRussell diagram plotted in Fig.…”

“…Radial velocities were determined by cross-correlating the region around the HeI 2.112/3 μm absorption line in each of the 11 archive spectra against the high signal-to-noise K s -band spectrum of EXO 1722-363, which we had previously obtained for spectral classification purposes (Mason et al 2009). The resulting velocities were then corrected to the solar system barycentre and are reported in Table 1.…”

“…The resulting velocities were then corrected to the solar system barycentre and are reported in Table 1. The spectrum highlighted in bold is that used as the reference spectrum (our previously obtained high signal-to-noise K s -band spectrum of EXO 1722-363 Mason et al 2009). In obtaining these final velocities, we determined the absolute velocity of the reference spectrum by fitting the positions of its absorption lines.…”

High-mass X-ray binaries in the Milky Way

“…The source position was refined with INTEGRAL Walter et al, 2004) and further with XMM-Newton, which allowed an association with an infrared counterpart (Zurita . Its infrared spectrum was identified with that of a supergiant B0-B1Ia star, located at a distance of 7.1 − 7.9 kpc Mason et al, 2009Mason et al, , 2010. The orbital period of 9.742d, determined with RXTE (Markwardt and Swank, 2003;Corbet et al, 2005b) and refined with INTEGRAL thanks to the presence of X-ray eclipses, established the system as a sgHMXB.…”

The masses of the neutron and donor star in the high-mass X-ray binary IGR J18027-2016

“…Examining the K s band, the absence of lines due to the C iv triplet (2.069, 2.078 and 2.083 μm) combined with the He i 2.058 μm line in emission implies IGR J18027-2016 is not an O type supergiant (Mason et al 2009). Further constraining the spectral type, we can see that He i 2.058 μm is in emission and when combined with the observed He i 2.112 μm line in absorption, this is indicative of a B0-B2 I classification.…”