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Multiple X-ray quasi-periodic oscillations detected in X-ray binary IGR J19294+1816

Using the Hard X-ray Modulation Telescope (HXMT), Chinese astronomers have observed an X-ray binary system designated IGR J19294+1816. As a result, they discovered multiple X-ray quasi-periodic oscillations in this source. The finding was reported in a paper June 6 on the arXiv pre-print server.

In general, X-ray binaries are systems composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Taking into account the mass of the companion star in XRBs, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

The largest subgroup of HMXBs is known as Be/X-ray binaries (BeXRBs). They are composed of Be stars and, usually, neutron stars, including pulsars. Most of these binaries exhibit weak persistent X-ray emission interrupted by outbursts lasting several weeks.

IGR J19294+1816 is a BeXRB at a distance of some 36,000 light years, discovered during an outburst in 2009. It showcases pulsations with a period of 12.4 seconds and long-term flux variability with an orbital period of approximately 117.2 days.

In order to get more insights into the properties of IGR J19294+1816, Wen Yang and Wei Wang of the Wuhan University in China decided to investigate it with HXMT. The observations resulted in the detection of quasi-periodic oscillations (QPOs) from this system.

In general, QPOs are believed to arise from the interaction of matter in the accretion disk with the magnetosphere of a compact object such as a neutron star or a black hole.

The astronomers report that a significant QPO feature was identified at around 30.2 mHz in the 10–50 keV band, with the root-mean-square (rms) amplitude increasing with energy. Furthermore, they detected two QPOs at the frequency of about 51.1 mHz and 113.7 mHz in 25–50 keV, exhibiting an rms amplitude of around 12%.

"The ∼30 mHz QPO feature with ∼10% rms and quality factor of ∼8 is detected in the power density spectra and wavelet power spectra, and two QPOs at ∼50mHz and 110 mHz have a quality factor of ∼5 and ∼3 respectively," the researchers wrote in the paper.

According to the study, the QPO appears in the first 100 seconds and reappears after 200 seconds in the low-energy bands. The mHz QPO in the medium-energy bands, with a constant frequency near 30 mHz, persists throughout the entire duration of the good time interval (GTI).

The astronomers add that for the high-energy bands, there is also a QPO near 30 mHz lasting about 600 seconds in the early stage and another 200-second oscillation in the later phase.

Trying to explain the origin of QPOs in IGR J19294+1816, the authors of the paper take into account several hypotheses. One of them that they find the most plausible to explain such low-frequency mHz QPOs is the magnetic disk precession model.

"In this scenario, the inner region of the accretion disk experiences magnetic torques that can induce warping and precession of the disk. Under typical conditions in X-ray pulsars, these torques can overcome viscous damping, allowing the precessional instability to develop and potentially give rise to mHz QPOs," the scientists conclude.

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