Astronomers from the Open University in Milton Keynes, UK, have made a groundbreaking discovery of a new Be/X-ray binary system, shedding light on the mysterious cosmic phenomena. Designated as 4XMM J182531.5–144036, this system presents unique characteristics that distinguish it within the broader class of high-mass X-ray binaries (HMXBs). The findings, detailed in a paper published on the pre-print server arXiv on January 4, reveal the persistent X-ray emission and other intriguing features of this newly identified celestial object.
X-ray binaries are captivating celestial entities consisting of a normal star or a white dwarf that transfers mass onto a compact neutron star or a black hole. Astronomers categorize them into low-mass X-ray binaries (LMXBs) and high-mass X-ray binaries (HMXBs) based on the mass of the companion star. Be/X-ray binaries (Be/XRBs) form the largest subgroup of HMXBs, typically comprising Be stars and neutron stars, including pulsars.
The journey of discovery for 4XMM J182531.5–144036 began in April 2008 when it was initially detected as a hard X-ray source using the European Space Agency's XMM-Newton satellite. The nature of this enigmatic system remained undisclosed, prompting a team of astronomers led by Andrew Mason Jr. from the Open University to meticulously analyze data from various sources, including XMM-Newton, NASA's Chandra spacecraft, Very Large Telescope (VLT), and UKIDSS Galactic Plane Survey.
The study revealed that the newly discovered system aligns with the characteristics of Be stars. An infrared object associated with 4XMM J182531.5–144036 displayed near-infrared excess compared to the spectra of early B-type dwarf or giant stars, along with a notable hydrogen emission line. These features are hallmarks of Be stars, providing crucial insights into the nature of the binary system.
Astronomers detected coherent X-ray pulsation in 4XMM J182531.5–144036, with a period of 781 seconds. This pulsation is consistent with BeXRB pulsars. The asymmetric X-ray pulse profile observed in the system offers valuable information about the magnetic field structure of the associated neutron star. The researchers noted that the X-ray pulsation persists over time, as evidenced by consistent profiles observed in widely separated XMM-Newton and Chandra observations.
The orbital period of 4XMM J182531.5–144036 was calculated to fall within the range of 250–500 days, with a low eccentricity in the system's orbit. Despite the significant strides made in understanding this binary system, determining its precise distance remains challenging. Astronomers estimate a distance range between 3,300 and 23,000 light years, with the caveat that the system's faintness prevents detection by ESA's Gaia satellite, precluding an independent distance estimate.