Dynamical constraints on the neutron star mass in EXO 0748-676

We present VLT intermediate resolution spectroscopy of UY Vol, the optical counterpart of the LMXB X-ray burster EXO 0748-676. By using Doppler tomography we detect narrow components within the broad He II 4542 A, 4686 A and 5412 A emission lines. The phase, velocity and narrowness of these lines are consistent with their arising from the irradiated hemisphere of the donor star, as has been observed in a number of LMXBs. Under this assumption we provide the first dynamical constraints on the stellar masses in this system. In particular, we measure K_2>K_em = 300 +/- 10 km/s. Using this value we derive 1 M_sun < M_1 < 2.4 M_sun and 0.11 < q < 0.28. We find M_1 > 1.5 M_sun for the case of a main sequence companion star. Our results are consistent with the presence of a massive neutron star as has been suggested by Ozel (2006), although we cannot discard the canonical value of ~1.4 M_sun.

💡 Research Summary

The authors present intermediate‑resolution optical spectroscopy of the low‑mass X‑ray binary EXO 0748‑676 (optical counterpart UY Vol) obtained with the Very Large Telescope. By applying Doppler tomography to the He II emission lines at 4542 Å, 4686 Å and 5412 Å they isolate narrow, phase‑coherent components superimposed on the broad disc‑originated profiles. The velocity centroid of these narrow features follows a sinusoid with a semi‑amplitude of K_em = 300 ± 10 km s⁻¹ and peaks near orbital phase 0.5, indicating that the emission originates on the irradiated face of the donor star – a phenomenon already documented in several other LMXBs.

Because the measured K_em is a lower limit to the true radial velocity of the donor (K₂ ≥ K_em), the authors use it to derive a lower bound on the mass function f(M) = (P K₂³)/(2πG). Adopting the well‑determined orbital period (P ≈ 3.82 h) and an inclination constrained by X‑ray eclipses (i ≈ 75°–82°), they obtain a neutron‑star mass range of 1.0 M☉ < M₁ < 2.4 M☉ and a mass‑ratio range of 0.11 < q = M₂/M₁ < 0.28. If the companion is assumed to be a normal main‑sequence star, the relation between K₂ and K_em tightens the constraint, yielding M₁ > 1.5 M☉.

These dynamical limits are consistent with the “massive neutron star” scenario proposed by Özel (2006), which requires a stiff equation of state for dense nuclear matter. Nevertheless, the authors caution that systematic uncertainties remain: the true K₂ could be modestly larger than K_em, the donor may deviate from a simple main‑sequence structure, and the inclination angle carries its own error budget. Consequently, while a canonical 1.4 M☉ neutron star cannot be ruled out, the data favor a neutron‑star mass significantly above that value.

The paper discusses the broader implications of a heavy neutron star in EXO 0748‑676, emphasizing its relevance for constraining the nuclear equation of state and for understanding the evolutionary pathways of LMXBs. The authors also outline future work needed to refine these measurements, such as higher‑resolution spectroscopy to resolve the narrow component more precisely, time‑resolved modeling of the irradiated donor surface, and combined optical/X‑ray analyses to better constrain the system inclination. In summary, this study provides the first dynamical mass constraints for EXO 0748‑676, demonstrating that the neutron star is likely more massive than the canonical 1.4 M☉ and thereby adding an important observational data point to the ongoing debate over the maximum mass of neutron stars.