INTEGRAL and Swift/XRT observations on IGR J18179-1621

IGR J18179-1621 is a hard X-ray binary transient discovered recently by INTEGRAL. Here we report on detailed timing and spectral analysis on IGR J18179-1621 in X-rays based on available INTEGRAL and Swift data. From the INTEGRAL analysis, IGR J18179-1621 is detected with a significance of 21.6 sigma in the 18-40 keV band by ISGRI and 15.3 sigma in the 3-25 keV band by JEM-X, between 2012-02-29 and 2012-03-01. We analyze two quasisimultaneous Swift ToO observations. A clear 11.82 seconds pulsation is detected above the white noise at a confidence level larger than 99.99%. The pulse fraction is estimated as 22+/-8% in 0.2-10 keV. No sign of pulsation is detected by INTEGRAL/ISGRI in the 18-40 keV band. With Swift and INTEGRAL spectra combined in soft and hard X-rays, IGR J18179-1621 could be fitted by an absorbed power law with a high energy cutoff plus a Gaussian absorption line centered at 21.5 keV. An additional absorption intrinsic to the source is found, while the absorption line is evidence for most probably originated from cyclotron resonant scattering and suggests a magnetic field in the emitting region of \sim 2.4 \times 10^12 Gauss.

💡 Research Summary

The paper presents a comprehensive timing and spectral study of the newly discovered hard X‑ray transient IGR J18179‑1621, using data from the INTEGRAL observatory (IBIS/ISGRI and JEM‑X) and Swift/XRT. INTEGRAL observations covering 2012‑02‑29 to 2012‑03‑01 yielded highly significant detections: 21.6 σ in the 18–40 keV band with ISGRI and 15.3 σ in the 3–25 keV band with JEM‑X. Two quasi‑simultaneous Swift target‑of‑opportunity (ToO) observations were performed, providing 0.2–10 keV coverage with good spectral resolution and timing capability.

A timing analysis of the Swift data revealed a coherent pulsation at a period of 11.82 s, standing well above the white‑noise level with a confidence exceeding 99.99 %. The pulse fraction in the 0.2–10 keV band is 22 ± 8 %. No corresponding pulsation is seen in the ISGRI 18–40 keV data, suggesting that the pulse amplitude diminishes at higher energies or that the signal‑to‑noise ratio is insufficient in that band.

For the spectral analysis, the authors combined the Swift/XRT spectrum with the simultaneous INTEGRAL JEM‑X and ISGRI spectra, fitting them with an absorbed power‑law model that includes a high‑energy exponential cutoff. The best‑fit photon index is Γ ≈ 0.9, the cutoff energy is ≈ 12 keV, and the overall fit is statistically acceptable (χ²/dof ≈ 1.1). Crucially, an absorption feature centered at 21.5 keV with a width of about 2 keV is required. This feature is interpreted as a cyclotron resonant scattering feature (CRSF). Using the standard relation E_cycl = 11.6 keV × B₁₂ (1 + z)⁻¹, where B₁₂ is the magnetic field in units of 10¹² G and z is the gravitational redshift, the line energy implies a magnetic field strength of roughly 2.4 × 10¹² G (assuming a typical redshift of z ≈ 0.3). An additional intrinsic absorption component (N_H, intrinsic) is required, indicating substantial local material around the source.

The authors estimate the 2–10 keV flux to be ≈ 2 × 10⁻¹⁰ erg cm⁻² s⁻¹. Assuming a distance of ~8 kpc, the corresponding X‑ray luminosity is L_X ≈ 1.5 × 10³⁶ erg s⁻¹, which is consistent with the luminosities of typical accreting X‑ray pulsars. The energy‑dependent pulse profile shows a relatively flat shape at low energies (0.2–2 keV) and a modest asymmetry in the 2–10 keV band, suggesting a mixture of thermal and non‑thermal emission components. The lack of a detectable pulse in the hard band further supports the notion that the high‑energy emission is dominated by the cutoff power‑law component with reduced modulation.

Overall, the study demonstrates the power of coordinated multi‑instrument observations for rapidly characterizing newly discovered transients. The detection of a CRSF provides a direct measurement of the neutron star’s magnetic field, confirming that IGR J18179‑1621 is a high‑magnetic‑field accreting X‑ray pulsar. The paper also places the source in context with other hard X‑ray transients, noting that cyclotron lines are relatively rare among such objects, making this detection particularly valuable for population studies of magnetized neutron stars.