The Swift-BAT hard X-ray sky monitoring unveils the orbital period of the HMXB IGR J16493-4348

IGR J16493-4348 is a supergiant high mass X-ray binary discovered by INTEGRAL in 2004. The source is detected at a significance level of $\sim21$ standard deviations in the Swift-BAT survey data collected during the first 54 months of the Swift mission. The timing analysis reveals an orbital period of $\sim$6.78 days and the presence of a full eclipse of the compact ob\ ject. The dynamical range (variability up to a factor $\sim$20) observed during the BAT monitoring suggests that IGR J16493-4348 is a wind-fed system. The derived semi-major axis of the binary system is $\sim55 R_{\sun}$ with an orbit eccentr\ icity lower than 0.15.

💡 Research Summary

This paper presents a comprehensive timing analysis of the supergiant high‑mass X‑ray binary (sgHMXB) IGR J16493‑4348 using 54 months of Swift‑BAT survey data in the 15–50 keV band. The source, originally discovered by INTEGRAL in 2004 and later identified with a B0.5 Ib supergiant companion (2MASS J1642695‑4349090), is detected at a significance of ~21 σ in the BAT all‑sky map.

The authors extracted a light curve with the maximum available time resolution (~300 s) and corrected each time stamp to the Solar System barycentre. The average count rate is 1.03 × 10⁻⁴ counts s⁻¹, but short‑term excursions reach up to ~7 σ, corresponding to flux increases of a factor of ~20.

To search for periodicities, a folding technique was applied over the 0.5–100 day range using 16 phase bins per trial period. The period resolution is given by ΔP = P²/(NΔT) with N = 16 and a total time span ΔT ≈ 1.4 × 10⁸ s. The χ² periodogram displays a dominant peak at P₀ = 6.782 ± 0.005 days with χ² ≈ 240, far exceeding the noise level. Harmonic and sub‑harmonic features (P₀/2, 2P₀, 3P₀, etc.) are also present, the former arising from the deep eclipse seen in the folded profile.

Statistical significance was assessed by two independent methods. First, the χ² trend was modelled with a second‑order polynomial, subtracted, and the residual “z” distribution was examined. Fitting the high‑z tail (z > 25) with an exponential yields a chance probability of 4.6 × 10⁻¹⁸ for the observed z = 196.6, equivalent to ~8.5 σ. Second, 1 000 Monte‑Carlo simulations were performed by scrambling the observed rates while preserving the time stamps; the highest χ² obtained in any simulated periodogram was 97.9, implying a false‑alarm probability <2 × 10⁻⁸ (≈5.5 σ). Both approaches confirm the robustness of the detection.

The folded light curve at P₀ shows a flat out‑of‑eclipse level interrupted by a total eclipse centered at phase 0.319 ± 0.015, corresponding to MJD = 54175.92 ± 0.10. The eclipse duration is ~12 % of the orbital period, consistent with a near‑edge‑on geometry (inclination ≈ 90°) and a low orbital eccentricity.

Using Kepler’s third law and assuming a supergiant mass M★ ≈ 47 M⊙ (typical for a B0.5 Ib star) and radius R★ ≈ 32 R⊙, together with a canonical neutron‑star mass MX ≈ 1.4 M⊙, the semi‑major axis is derived as a ≈ 55 R⊙, i.e. roughly twice the stellar radius. This separation is typical for classical sgHMXBs and supports a wind‑fed accretion scenario. By evaluating the position of the inner Lagrangian point L₁ as a function of orbital phase for various eccentricities (following Paczyński 1971), the authors constrain the eccentricity to e < 0.15; higher eccentricities would place L₁ outside the stellar surface, contradicting the wind‑fed interpretation.

Swift‑XRT observations (0.2–10 keV) performed on 2006‑03‑11 covered orbital phases 0.45–0.51. The source displayed a persistent count rate of 0.376 ± 0.015 counts s⁻¹ with variability up to a factor of three, but no coherent pulsations were detected in the 5–1000 s range after correcting for the binary motion using the derived orbital parameters.

The paper concludes that the long‑term BAT monitoring has unambiguously revealed the orbital period and eclipse of IGR J16493‑4348, establishing it as a wind‑fed sgHMXB with a nearly circular, edge‑on orbit. The observed short‑term variability (up to a factor of ~20 on 300 s timescales) and the modest orbital modulation (factor ~4) are consistent with accretion from the supergiant’s stellar wind. The authors suggest that future high‑resolution spectroscopy and continued timing campaigns could further constrain the wind properties and potentially uncover the neutron‑star spin period.