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Using various space observatories, astronomers have performed X-ray polarimetric and spectral observations of a bright low-mass X-ray binary known as Serpens X-1. Results of the observational campaign, published August 29 on the preprint server arXiv<\/i>, provide important insights into the nature of this system.<\/p>\n
X-ray binaries (XRBs) are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a 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).<\/p>\n
Neutron star (NS) LMXBs are further divided into atoll and Z sources. The so-called atoll-type systems share some characteristics with black hole LMXBs as they have similar X-ray spectra and timing properties. However, they differ in their radio properties, in that atoll sources are 27 times less radio luminous.<\/p>\n
Located some 25,000 light years away, Serpens X-1 (or Ser X-1 for short) is a bright, persistent atoll NS LMXB. It is a well-studied source, consistently observed in the high luminosity, soft spectral state, with a luminosity of about 67 undecillion erg\/s. All in all, Serpens X-1 is one of the X-ray-brightest atolls known to date and therefore it is an excellent target for X-ray polarimetry.<\/p>\n
That 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 also its spectral observations, using the Imaging X-ray Polarimetry Explorer (IXPE), the Neutron Star Interior Composition Explorer (NICER) and Nuclear Spectroscopic Telescope Array (NuSTAR).<\/p>\n
The observations found that the X-ray polarization degree for Serpens X-1 is less than 2.0% in the 2\u20138 keV energy band. This can be due to the low inclination angle (approximately 25 degrees) of the source.<\/p>\n
Furthermore, the observations detected four type I X-ray bursts from Serpens X-1, with properties that are consistent with previous studies. The astronomers noted that this is the first time that IXPE has identified type I X-ray bursts. The upper limit to the burst polarization was found to be 80%.<\/p>\n
The study also confirmed the presence of a relativistic reflection component in the X-ray spectrum\u2014a broad iron line. The researchers found that this iron line requires an iron abundance of about five times the solar one, which is consistent with previous estimates.<\/p>\n
Summing up the results, the authors of the paper concluded that the obtained polarization and spectral properties of Serpens X-1 are, in general, comparable to other atoll-type neutron star low-mass X-ray binaries investigated with IXPE. This suggests similar geometry of Serpens X-1 to these systems.<\/p>\n
More information:<\/strong> \n\t\t\t\t\t\t\t\t\t\t\t\t\tJournal information:<\/strong> \t\t\t\t\t\t\t\t\t\t\t\t\t\t\t
\n\t\t\t\t\t\t\t\t\t\t\t\tF. Ursini et al, X-ray spectropolarimetry of the bright atoll Serpens X-1, arXiv<\/i> (2024). DOI: 10.48550\/arxiv.2408.16713<\/p>\n
\n\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\t\tarXiv<\/cite><\/p>\n