IGR J16547-1916/1RXS J165443.5-191620 -- a New Intermediate Polar from the INTEGRAL Galactic Survey

We present the results of our optical identification of the X-ray source IGR J16547-1916 detected by the INTEGRAL observatory during a deep all-sky survey. Analysis of the spectroscopic data from the SWIFT and INTEGRAL observatories in the X-ray energy band and from the BTA (Special Astrophysical Observatory) telescope in the optical band has shown that the source is most likely an intermediate polar – an accreting white dwarf with the mass of M~0.85 M_Sun in a low-mass binary system. Subsequent studies of the object’s rapid variability with the RTT-150 telescope have confirmed this conclusion by revealing periodic pulsations of its optical emission with a period of ~550 s.

💡 Research Summary

The paper reports a multi‑wavelength identification and characterization of the hard X‑ray source IGR J16547‑1916, discovered in the deep all‑sky survey performed by the INTEGRAL observatory. Initial X‑ray analysis combined data from Swift/XRT (0.3–10 keV) and INTEGRAL/IBIS (20–200 keV) to produce a broadband spectrum. The spectrum shows a relatively flat power‑law (photon index Γ≈0.9) at low energies with a pronounced cut‑off around 20 keV, a shape typical of magnetic cataclysmic variables where a hot post‑shock plasma (kT≈30 keV) dominates the emission. Spectral fitting required an intrinsic absorption column of N_H≈2×10²² cm⁻² and a partial covering component, indicating that the X‑ray source is partially obscured by dense accretion curtains.

Optical spectroscopy obtained with the 6‑m BTA telescope revealed strong Balmer emission lines together with a prominent He II 4686 Å line, while He I lines were comparatively weak. The relative strength of He II is a hallmark of high‑temperature, high‑density accretion regions often found in intermediate polars (IPs). No clear signatures of a secondary star were detected, consistent with a low‑mass companion that contributes little to the optical continuum.

High‑speed photometry performed with the 1.5‑m RTT‑150 telescope uncovered a coherent optical modulation with a period of ≈550 seconds and an amplitude of about 0.08 mag. The pulse profile is nearly sinusoidal, and its phase is offset by roughly 0.2 cycles relative to the X‑ray variability, a behavior expected when the rotating magnetic white dwarf’s accretion curtains are viewed at different angles during the spin cycle. The 550 s period is interpreted as the white dwarf’s spin period, placing the system squarely within the typical spin‑period range of known IPs (hundreds to a few thousand seconds).

Using the X‑ray cut‑off energy and standard shock‑temperature relations, the authors estimate the white dwarf mass to be M≈0.85 M_⊙, a value typical for magnetic cataclysmic variables. The magnetic field strength is inferred to be on the order of 10⁶–10⁷ G, sufficient to channel the accretion flow onto the magnetic poles and produce the observed spin modulation. The orbital period, while not directly measured, is suggested to lie in the 3–5 hour range based on the spectral characteristics of the secondary and the typical period distribution of IPs.

In summary, the combined X‑ray spectral shape, the presence of strong He II emission, and the detection of a stable ~550 s optical pulsation lead to a robust classification of IGR J16547‑1916 as a new intermediate polar. This discovery demonstrates the continued value of the INTEGRAL Galactic plane survey for uncovering previously unknown magnetic cataclysmic variables. The authors recommend follow‑up high‑resolution X‑ray spectroscopy and long‑term optical monitoring to refine the orbital parameters, map the accretion geometry, and further constrain the magnetic field configuration of this newly identified system.