New Research on Shining Low-Mass X-Ray Binary

Astronomers utilizing a range of space observatories have conducted comprehensive X-ray polarimetric and spectral studies on a prominent low-mass X-ray binary known as Serpens X-1. Their findings, released on August 29 via the preprint server arXiv, shed light on the intricate details of this celestial system.

X-ray binaries (XRBs) are systems where a normal star or a white dwarf transfers material to a compact neutron star or black hole. These binaries are divided into low-mass X-ray binaries (LMXB) and high-mass X-ray binaries (HMXB) based on the companion star's mass.

Among neutron star LMXBs, there are atoll and Z sources. Atoll-type systems, while sharing some traits with black hole LMXBs in terms of X-ray spectra and timing, are notably different in radio luminosity, being 27 times less luminous in radio waves.

Serpens X-1, located about 25,000 light-years from Earth, is a distinguished atoll-type neutron star LMXB. It is well-known for its consistent high luminosity and soft X-ray spectral state, with a luminosity around 67 undecillion erg/s. This makes it an excellent candidate for X-ray polarimetry research.

A research team led by Francesco Ursini from Roma Tre University in Rome, Italy, carried out these observations using the Imaging X-ray Polarimetry Explorer (IXPE), Neutron Star Interior Composition Explorer (NICER), and Nuclear Spectroscopic Telescope Array (NuSTAR).

The observations revealed that the X-ray polarization degree for Serpens X-1 is below 2.0% in the 2–8 keV energy range, likely due to its low inclination angle of about 25 degrees.

Furthermore, the study identified four type I X-ray bursts from Serpens X-1, which matched previously recorded properties. This marks the first detection of type I X-ray bursts by IXPE, with the maximum burst polarization reaching 80%.

Additionally, the research confirmed the presence of a relativistic reflection component in the X-ray spectrum, highlighted by a broad iron line. The iron abundance was found to be roughly five times greater than that of the Sun, consistent with earlier estimates.

In conclusion, the study's results indicate that the polarization and spectral characteristics of Serpens X-1 are similar to those of other atoll-type neutron star LMXBs observed by IXPE. This suggests that Serpens X-1 shares comparable geometrical properties with these other systems.