The observers investigate a short-period X-ray binary system

The observations investigate a short-period X-ray binary system

Details of the central region of NGC 4214, adapted from an HST/WFC3 image processed and published by the Hubble Heritage Team. The X-1’s optical counterpart is marked with an arrow at the bottom right of the image. It is on the outskirts of the present starburst region. Credits: NASA, ESA and Hubble Heritage (STScI/AURA)-ESA/ Hubble Collaboration.

Using the Hubble Space Telescope (HST) and the Chandra X-ray Observatory, astronomers took a closer look at a short-period, high-mass X-ray binary known as CXOU J121538.2+361921. Results of the observational campaign, presented July 13 on the prepress server arXivthrow more light on the properties of this system.

X-ray binaries consist of a normal star or white dwarf mass transferring 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 (LMXB) and high-mass X-ray binaries (HMXB).

Located about 9.8 million light-years away in the galaxy NGC 4214, CXOU J121538.2+361921 (or NGC 4214 X-1) is a bright HMXB, showing X-ray eclipse with a period of 3.62 hours. The period of the eclipse is, most likely, also the orbital period, making NGC 4214 X-1 the shortest period HMXB system known to date. However, while many studies have been done on this system, its properties are not well understood.

That’s why a team of astronomers led by Zikun Lin of the University of the Chinese Academy of Sciences in Beijing, China decided to investigate NGC 4214 X-1 with the Hubble and Chandra telescopes.

“We combined new and archival Chandra and HST data for a study of the short-period eclipsing X-ray binary NGC 4214 X-1,” the researchers wrote in the paper.

Observations confirmed that NGC 4214 X-1 is still active and still showing eclipse, with the brightness outside the eclipse at a level of about one duodecillion erg/s. The period of the eclipse and the mean duration time of the eclipse have been confirmed to be approximately 3.6 and 0.57 hours, respectively.

The fraction of the eclipse was calculated to be about 0.16, which allowed the researchers to estimate the system’s minimum mass ratio of about 2.0. This discovery further confirms the HMXB nature of NGC 4214 X-1.

The stellar density of the donor star has been calculated to be about 5.9 g/cm3. This result, together with the mass ratio and short binary period, suggests that the donor is a Wolf-Rayet (WR) star or an intermediate-mass stripped helium star.

Also, based on HST observations, Lin’s team found an optical counterpart of NGC 4214 X-1, with an apparent luminosity of 24 mag. The optical source consists of two clearly distinct components: a blue emitter (with a temperature of about 60,00080,000 K and a characteristic radius of 2.0 solar radii) and a red emitter (with a temperature of about 2,5003,000 K and a characteristic radius of about 400 solar radii).

The authors of the paper concluded that the blue component further supports the WR scenario for the donor star in NGC 4214 X-1. They added that the red component could be an irradiated circumbinary disk.

More information:
Zikun Lin et al, On the Short-Period Eclipsing High-Mas-ray-ray Binary in NGC 4214, arXiv (2023). DOI: 10.48550/arxiv.2307.06993

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