Astronomers study nature of bright, low-mass X-ray binary system

Using various space observatories, astronomers have made polarimetric and X-ray spectral observations of a bright, low-mass binary star known as Serpens X-1. Results from the observing campaign, published on August 29 on the preprint server arXivprovide important information about the nature 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. Depending on the mass of the companion star, astronomers divide them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs).

Neutron star (NS) LMXBs are further divided into atoll sources and Z sources. So-called atoll-type systems share some characteristics with black hole LMXBs as they have similar X-ray spectra and temporal properties. However, they differ in their radio properties, as atoll sources are 27 times less luminous.

Located approximately 25,000 light-years away, Serpens X-1 (or Ser X-1 for short) is a bright and persistent NS LMXB atoll. It is a well-studied source, observed regularly in a soft, high-luminosity spectral state, with a luminosity of approximately 67 undecillion erg/s. Overall, Serpens X-1 is one of the brightest X-ray atolls known to date and is therefore an excellent target for X-ray polarimetry.

This is why a team of astronomers led by Francesco Ursini of the Roma Tre University in Rome, Italy, decided to conduct such studies of Serpens X-1 and its spectral observations, using the Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array (NuSTAR).

Observations revealed that the degree of polarization of X-rays from Serpens X-1 is less than 2.0% in the energy band from 2 to 8 keV. This may be due to the small tilt angle (about 25 degrees) of the source.

In addition, the observations detected four type I X-ray bursts from Serpens X-1, whose properties are consistent with those of previous studies. The astronomers noted that this is the first time that IXPE has identified type I X-ray bursts. The upper limit of the bursts’ polarization was found to be 80%.

The study also confirmed the presence of a relativistic reflection component in the X-ray spectrum: a broad iron line. The researchers found that this iron line requires an iron abundance about five times that of the Sun, which is consistent with previous estimates.

Summarizing the results, the authors of the paper conclude that the obtained polarization and spectral properties of Serpens X-1 are, in general, comparable to those of other low-mass atoll-type X-ray binaries studied with IXPE. This suggests a similar geometry of Serpens X-1 to these systems.

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