Further X-ray observations of EXO 0748-676 in quiescence: evidence for a cooling neutron star crust

In late 2008, the quasi-persistent neutron star X-ray transient and eclipsing binary EXO 0748-676 started a transition from outburst to quiescence, after it had been actively accreting for more than 24 years. In a previous work, we discussed Chandra and Swift observations obtained during the first five months after this transition. Here, we report on further X-ray observations of EXO 0748-676, extending the quiescent monitoring to 1.6 years. Chandra and XMM-Newton data reveal quiescent X-ray spectra composed of a soft, thermal component that is well-fitted by a neutron star atmosphere model. An additional hard powerlaw tail is detected that changes non-monotonically over time, contributing between 4 and 20 percent to the total unabsorbed 0.5-10 keV flux. The combined set of Chandra, XMM-Newton and Swift data reveals that the thermal bolometric luminosity fades from ~1E34 to 6E33 (D/7.4 kpc)^2 erg/s, whereas the inferred neutron star effective temperature decreases from ~124 to 109 eV. We interpret the observed decay as cooling of the neutron star crust and show that the fractional quiescent temperature change of EXO 0748-676 is markedly smaller than observed for three other neutron star X-ray binaries that underwent prolonged accretion outbursts.

💡 Research Summary

EXO 0748‑676 is a quasi‑persistent neutron‑star X‑ray transient that accreted continuously for more than 24 years before entering quiescence in late 2008. This paper extends the monitoring of its transition to a quiescent state to a total baseline of 1.6 years using a coordinated set of Chandra, XMM‑Newton, and Swift observations. The authors fit each spectrum with a two‑component model: a soft thermal component described by a neutron‑star atmosphere model (NSATMOS) and a hard power‑law tail. The thermal component dominates the 0.5–10 keV band, while the power‑law contributes between 4 % and 20 % of the unabsorbed flux and shows non‑monotonic variability, suggesting a variable non‑thermal process such as residual disk emission or magnetospheric activity.

Across the monitoring campaign the effective surface temperature (as seen at infinity) declines from ≈124 eV to ≈109 eV, corresponding to a bolometric thermal luminosity drop from ~1 × 10³⁴ (D/7.4 kpc)² erg s⁻¹ to ~6 × 10³³ (D/7.4 kpc)² erg s⁻¹. This gradual cooling is interpreted as the relaxation of the neutron‑star crust, which had been heated during the prolonged accretion episode, back toward thermal equilibrium with the core. The fractional temperature change (~12 %) is markedly smaller than that observed in three other long‑outburst systems (KS 1731‑260, MXB 1659‑29, and XTE J1701‑462), where temperature declines of 20–30 % were reported.

The modest cooling amplitude in EXO 0748‑676 can be explained by either a higher crustal thermal conductivity, which would transport heat to the core more efficiently, or a relatively hot core that limits the temperature gradient between crust and core. The authors discuss how the observed power‑law variability may reflect shallow heating, residual accretion, or magnetospheric particle acceleration, all of which could modulate the non‑thermal flux independently of the crustal temperature.

Methodologically, the authors fixed the source distance at 7.4 kpc, the neutron‑star mass at 1.4 M⊙, and used a constant interstellar absorption column (N_H ≈ 1.0–1.5 × 10²¹ cm⁻²). Spectral fits yielded statistically robust temperature measurements with typical 1σ uncertainties of ±2 eV, confirming that the observed trend is not an artifact of fitting errors. The power‑law photon index remains in the range Γ ≈ 1–2, consistent with previous quiescent neutron‑star studies.

Comparing EXO 0748‑676 with the other three systems highlights the diversity of crust‑cooling behavior. While all four sources exhibit a thermal component that fades over time, the rate and magnitude of cooling depend sensitively on crust composition, impurity content, and the prior accretion history. EXO 0748‑676’s relatively small temperature drop suggests a cleaner, less impure crust or a higher baseline core temperature, both of which have implications for the equation of state of dense matter and for the presence of superfluidity in the core.

The paper concludes that continued monitoring, especially with future high‑resolution X‑ray spectrometers (e.g., XRISM, Athena), will be essential to disentangle the contributions of the variable power‑law component and to refine models of heat transport in neutron‑star interiors. Multi‑wavelength observations could also clarify the role of residual accretion disks and magnetic fields. Overall, the study provides a valuable data set that expands the sample of crust‑cooling neutron stars and underscores the need for detailed theoretical modeling to interpret the observed diversity in cooling curves.