Astronomers have received data on the outbreak, which repeats every three years

Using the Neutron Star InternalComposition Explorer (NICER) aboard the International Space Station (ISS), astronomers studied the recent burst of an accreting millisecond X-ray pulsar known as SAX J1808.4−3658. 

X-ray pulsars are characterized by strictperiodic changes in the intensity of the radiation of the same name, which amount to fractions of a second. Accreting millisecond X-ray pulsars (AMXP) are a unique type of pulsar in which short rotation periods are caused by long-term mass transfer from a low-mass companion star through an accretion disk to a slowly rotating neutron star.

Astronomers perceive AMXP as astrophysicallaboratories that are crucial for studying processes during thermonuclear explosions. However, AMXPs are relatively rare, and to date only a few dozen objects of this type have been identified using NICER and other space observatories.

Temporal evolution of the SAX J1808 flare in 2022, monitored by NICER. 
Credit: Iliano et al., 2022

SAX J1808.4−3658 (or SAX J1808 for short) was discovered in 1996 using the BeppoSAX X-ray satellite. He detected a flash of X-ray radiation. Two years later, it was classified as the first accreting millisecond X-ray pulsar. The object, located about 11,400 light-years from Earth, consists of a neutron star in orbit with a brown dwarf companion (with a mass of about 0.05 solar masses). The orbital period of the system is just over two hours.

Since its discovery, SAX J1808 has recordedten outbreaks lasting about a month with a recurrence period of 2–3 years. Recently, on August 19, 2022, another flare of this pulsar began, which was noticed by the MAXI instrument on board the ISS. A team of astronomers led by Giulia Illiano of the Observatory of Rome in Italy began observing SAX J1808 shortly after the outburst began, hoping to gain more information about the pulsar's properties.

Astronomers have confirmed that the rotation frequency of SAXJ1808 decreased at an average rate of −0.001 pHz/s. The result is consistent with the energy loss expected from a rotating magnetic dipole of this type.

For the first time in twenty years of observation, the studyshowed that the orbit of SAX J1808 has shrunk since the last epoch. Astronomers noted that the long-term behavior of the orbit can be described by an 11-second modulation with a period of 21 years.

New SAX J1808 flare expected to occurin 2025 . The authors of the paper emphasized that observations of this event are crucial to confirm their conclusions about the evolution of the orbit of this pulsar.

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Cover: Artist's impression of the Kevin Gill pulsar/flickr.com