X-ray binaries (XRBs) consist of a normal star or white dwarf transferring mass to a compact neutron star or black hole.
Using the AstroSat and NuSTAR space telescopes, astronomers have observed an X-ray binary known as XTE J1739-285 during its recent period of explosive activity. The results of the observation campaign, published on March 23 on the arXiv preprint server, provide crucial information about the behavior of this system.
X-ray binaries (XRBs) consist of a normal star or white dwarf transferring mass to a compact neutron star or black hole. Based on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).
Some LMXBs exhibit transient outbursts, during which an increase in X-ray luminosity is observed. When these outbursts are characterized as type I X-ray outbursts (thermonuclear explosions that take place in the surface layers of neutron stars) , obviously confirm the presence of neutron stars in such binaries.
Discovered in 1999 by the Rossi X-ray Timing Explorer (RXTE), XTE J1739−285 is a transient LMXB with a neutron star as a companion. Since its discovery, the source has experienced dozens of X-ray bursts. More recently, in 2019, it entered a rebrightness phase, during which new X-ray bursts were identified. A team of astronomers led by Aru Beri from the Indian Institute of Scientific Education and Research (IISER) Mohali in India, began observing XTE J1739-285 in October 2019, when the system was in its outburst period. They used India’s AstroSat and NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft to perform a detailed spectral and temporal survey of XTE J1739-285.
“In this paper, we report our results from AstroSat and NuSTAR observations of XTE J1739-285 during its 2019 and 2020 outbursts. We have performed a detailed time and spectrum study of this source,” the researchers wrote. X-ray light curves during 2019 observations of XTE J1739-285 by Beri’s team indicate the presence of flares. In addition, the observations identified accretion-driven X-ray pulsations at 386 Hz during very short intervals (0.5 to 1 s) of these X-ray flashes, making XTE J1739-285 an intermittent X-ray pulsar. .
AstroSat observations of XTE J1739-285 in 2020 revealed the presence of a thermonuclear X-ray burst, leading to the detection of coherent burst oscillations at 383 Hz during the decay phase of the burst. Therefore, XTE J1739-285 turns out to be one of the few neutron star LMXBs that experience both nuclear and accretionary pulsations.
The observations also detected a quasi-periodic oscillation (QPO) at 0.83 Hz with a root mean square (rms) variability of around 7% during the hard state of XTE J1739−285 in 2020. The astronomers noted that no found a similar feature during the soft state of the source, in the previous year. Furthermore, X-ray spectroscopy revealed significant changes in the X-ray spectra of XTE J1739-28 during the 2019 and 2020 burst.