Astronomers Discovered Long-period, Relatively Faint X-ray Pulsar (Cosmology)

A team of international astronomers discovered a new long period, relatively faint X-ray pulsar SRGA J204318.2+443815 (or SRGe J204319.0+443820) in the Be binary system using ART-XC telescope. Their study recently appeared in Arxiv.

The key task of Spectrum Roentgen Gamma (SRG) mission is the deepest all-sky survey in X-rays, both at soft energies 0.3-10 keV with the eROSITA telescope and the hard ones 4-30 keV with the Mikhail Pavlinsky ART-XC telescope. ART-XC telescope provides a unique possibility to study the population of faint/low-luminosity transients that would be otherwise missed, being too weak for the wide field-of-view telescopes and all-sky monitors (like INTEGRAL/IBIS, Swift/BAT, MAXI). One of the most intriguing families of such a population are X-ray pulsars or accretion-powered pulsars. These pulsars are a class of astronomical objects that are X-ray sources displaying strict periodic variations in X-ray intensity. An X-ray pulsar consists of a magnetized neutron star in orbit with a normal stellar companion and is a type of binary star system. In these binary systems, the X-ray emission is powered by the release of gravitational potential energy as material is accreted from a massive companion.

Now, a team of astronomers with the help of ART-XC telescope discovered another long period, relatively faint X-ray pulsar, SRGA J204318.2+443815, which is about 26,000 light years away from the Earth.

Spectra of SRGA J204318.2+443815 measured with SRG instruments. Credit: Lutovinov et al.

Follow-up observations of this pulsar with space telescopes like XMM-Newton, NICER and NuSTAR revealed pulsations of SRGA J204318.2+443815 with a period of about 742 seconds. The source has a bolometric luminosity of 4 × 1035 erg/s¯1, hard X-ray spectrum with the exponential cutoff; a number of emission lines (Hα, He i, Pashen and Braket series) were detected in the optical and infrared spectra of its companion star.

Additionally, based on the optical and IR spectral measurements they suggested that the pulsar’s companion is a Be-star most likely of the B0-B2e spectral type at an estimated distance of 4-7.5 kpc. Finally, astronomers assume that SRGA J204318.2+443815 may be a new member of the subclass of persistent low-luminosity Be X-ray binary (BeXRB) systems presumably accreting from the “cold” accretion disk.

Summing up the results, the researchers demonstrated the importance of the SRG spacecraft in the discovery of faint pulsars like SRGA J204318.2+443815.

Reference: Lutovinov et al., “SRG/ART-XC discovery of SRGA J204318.2+443815: towards the complete population of faint X-ray pulsars”, arXiv:2107.05587 [astro-ph.HE]

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