Chinese researchers have observed the most recent outburst of the low-mass X-ray binary system GX 339-4 using the Hard X-ray Modulation Telescope (HXMT). In this source, they discovered fresh quasi-periodic oscillations. Their findings have been published on the pre-print server arXiv.
A regular star or white dwarf that has acquired mass transfers it to a compact neutron star or a black hole to form X-ray binaries (XRBs). Low-mass X-ray binaries (LMXB) and high-mass X-ray binaries (HMXB) are two categories based on the companion star’s mass.
Located some 39,000 light years away from the Earth, GX 339-4 is a recurrent black hole LMXB first detected in 1973. At least 5.8 times as enormous as the sun, according to estimates, is its black hole. Over the past thirty years, GX 3394 has frequently experienced outbursts, showed all the black hole accretion states, and endured quasi-periodic oscillations (QPOs).
GX 339-4 is one of the most researched black hole LMXBs due to the extensive study it has received at all wavelengths.
By using the data of Insight-HXMT from February to March 2021, we make the X-ray timing analysis of this new outburst. Based on the results of count rates, hardness-intensity diagram (HID), and power density spectrum (PDS), we confirm that the source exhibits spectral transitions from the low-hard state (LHS) to the hard-intermediate state (HIMS). During the transition from the LHS to the HIMS, low-frequency quasi-periodic oscillations (LFQPOs) are detected in the PDS.
Chinese researchers
In order to learn more about the behavior of this system, a group of astronomers led by Wei Wang of Wuhan University in China chose to conduct HXMT observations of the most recent outburst of GX 339-4, which started in early 2021. This resulted in the detection of new quasi-periodic oscillations (QPOs).
As gas swirls onto a compact object like a neutron star or a black hole, QPOs are thought to happen when X-rays are released close to the inner border of the accretion disk. QPOs in black hole X-ray binary systems are classified into Type-A, -B, or -C, based on properties like the quality factor and the shape of the noise associated with the oscillation.
“By using the data of Insight-HXMT from February to March 2021, we make the X-ray timing analysis of this new outburst. Based on the results of count rates, hardness-intensity diagram (HID), and power density spectrum (PDS), we confirm that the source exhibits spectral transitions from the low-hard state (LHS) to the hard-intermediate state (HIMS). During the transition from the LHS to the HIMS, low-frequency quasi-periodic oscillations (LFQPOs) are detected in the PDS,” the researchers explained.
The observations detected type-C QPOs with centroid frequencies evolving from 0.1 to 0.6 Hz in the low-hard state and in the 1−3 Hz frequency range in the hard-intermediate state. The authors of the paper noted that Type-C QPOs are the most common type of QPOs in black hole XRBs, and occur mostly in the low-hard and hard-intermediate states.
Furthermore, Wang’s team managed to identify a phase lag behavior (in the 50-100 keV band) in GX 339-4 for the first time. It turned out that when the QPO centroid frequency is below 0.2 Hz, the lag is negative, but becomes positive after that. However, more observations are required in order to understand the physical mechanisms such phase lags.
The study also found that in the root-mean-square (rms) energy spectrum, the QPO rms first remains flat at lower energy, and then decreases from about 10% below 10 keV to 2% above 50 keV.
The researchers also noted that during the 2006–2007 outburst, when the QPO frequency was less than 1 Hz, the same correlation between the rms of QPOs and the photon energy above 10 keV took place.
