The nature of the X-ray binary IGR J19294+1816 from INTEGRAL, RXTE, and Swift observations

We report the results of a high-energy multi-instrumental campaign with INTEGRAL, RXTE, and Swift of the recently discovered INTEGRAL source IGR J19294+1816. The Swift/XRT data allow us to refine the position of the source to RA= 19h 29m 55.9s Dec=+18deg 18’ 38.4" (+- 3.5"), which in turn permits us to identify a candidate infrared counterpart. The Swift and RXTE spectra are well fitted with absorbed power laws with hard (Gamma ~ 1) photon indices. During the longest Swift observation, we obtained evidence of absorption in true excess to the Galactic value, which may indicate some intrinsic absorption in this source. We detected a strong (P=40%) pulsation at 12.43781 (+-0.00003) s that we interpret as the spin period of a pulsar. All these results, coupled with the possible 117 day orbital period, point to IGR J19294+1816 being an HMXB with a Be companion star. However, while the long-term INTEGRAL/IBIS/ISGRI 18–40 keV light curve shows that the source spends most of its time in an undetectable state, we detect occurrences of short (~2000-3000 s) and intense flares that are more typical of supergiant fast X-ray transients. We therefore cannot make firm conclusions on the type of system, and we discuss the possible implications of IGR J19294+1816 being an SFXT.

💡 Research Summary

The paper presents a comprehensive multi‑instrument study of the newly discovered high‑energy source IGR J19294+1816 using data from INTEGRAL, RXTE, and Swift. Swift/XRT observations refined the source position to RA = 19 h 29 m 55.9 s, Dec = +18° 18′ 38.4″ (±3.5″), allowing the identification of a single infrared counterpart in the 2MASS catalogue (2MASS J19295591+1818382). Photometric analysis of the J, H, and K_s bands, corrected for interstellar extinction using the total line‑of‑sight hydrogen column density (N_H ≈ 2–4 × 10^22 cm⁻²), suggests modest intrinsic absorption beyond the Galactic contribution.

Spectral fitting of the Swift/XRT (0.8–8 keV) and RXTE/PCA (3–25 keV) data was performed with absorbed power‑law models. Both datasets yield a very hard photon index (Γ ≈ 1.0), indicative of a high‑energy emitting region that is not well described by a simple black‑body, especially given the detection of emission above 20 keV by INTEGRAL. Because the source lies close to the Galactic plane (l = 53.54°, b = 0.12°), the RXTE spectra were contaminated by the Galactic ridge emission. The authors corrected for this using the ridge spectrum from Valinia & Marshall (1998), scaling it by a factor of 0.85 derived from simultaneous Swift and RXTE observations. After this correction, the power‑law model remained the best description of the data.

Timing analysis focused on the RXTE/PCA observations with sufficient exposure and active PCUs. Power‑density spectra (0–1024 Hz) show a white‑noise component and a red‑noise continuum that can be modeled either as a power‑law (index –1.35 ± 0.08) or a zero‑centred Lorentzian. A coherent pulsation at 12.43781 s (±3 × 10⁻⁵ s) with a fractional amplitude of ~40 % was detected, confirming the presence of an accreting X‑ray pulsar.

Long‑term monitoring with INTEGRAL/IBIS/ISGRI in the 18–40 keV band reveals that the source is usually below the detection threshold, but it exhibits short (≈2000–3000 s), intense flares. Such flaring behaviour is characteristic of supergiant fast X‑ray transients (SFXTs). Conversely, the detection of a possible 117‑day orbital period in Swift/BAT data and the hard, persistent power‑law spectrum are typical of Be‑type high‑mass X‑ray binaries (Be‑HMXBs). The combination of intrinsic absorption, a hard spectrum, a 12‑s spin period, and occasional fast flares makes the classification ambiguous.

The authors discuss both scenarios. In the Be‑HMXB interpretation, the 117‑day period would correspond to the orbital motion of a neutron star around a Be companion, with the hard spectrum arising from accretion of the Be star’s decretion disc. In the SFXT scenario, the short, bright flares would be due to sporadic capture of dense clumps in the wind of a supergiant donor, while the observed spin period would be consistent with known SFXT pulsars. The presence of extra intrinsic absorption could be compatible with either a dense circumstellar environment or local material near the neutron star.

Given the current data, the paper concludes that IGR J19294+1816 is an accreting neutron‑star system with a 12.44 s spin and a likely 117‑day orbit, but its exact nature—whether a Be‑HMXB or an SFXT—remains unresolved. Future work should include high‑resolution infrared or optical spectroscopy to determine the spectral type of the companion, as well as continued X‑ray monitoring to better characterize the flare recurrence and orbital modulation. This will clarify the evolutionary status of the system and its place within the diverse population of high‑mass X‑ray binaries discovered by INTEGRAL.