An international team of astronomers examined the recent outburst of an accreting millisecond X-ray pulsar known as SAX J1808.43658. They have used the Neutron Star Interior Composition Explorer (NICER) onboard the International Space Station (ISS). The study’s findings were published on the preprint server arXiv on December 19. It shed more light on the nature of this source.
X-ray pulsars have periodic variations in X-ray intensity that can last as little as a fraction of a second. Accreting millisecond X-ray pulsars (AMXPs) are a type of X-ray pulsar that has short spin periods due to long-lasting mass transfer from a low-mass companion star through an accretion disc onto a slow-rotating neutron star.
Astronomers see AMXPs as astrophysical laboratories that could help us learn more about thermonuclear burst processes. However, AMXPs are relatively rare. Only a few dozen of these objects have been identified using NICER and other space observatories to date.
SAX J1808.43658 (or SAX J1808 for short) was discovered in 1996 during an X-ray outburst by the X-ray satellite BeppoSAX. When X-ray pulsations were detected from this source, it was identified as the first accreting millisecond X-ray pulsar. The object is 11,400 light years away. It is made up of a neutron star in orbit with a brown dwarf companion. The system’s orbital period was calculated to be around 2.01 hours.
Since its discovery, SAX J1808 has had ten month-long outbursts with a 2-3 year recurrence period. On August 19, 2022, another outburst of this pulsar began. The outbrust was detected by the ISS’s Monitor of All-sky X-ray Image (MAXI) instrument. A group of astronomers led by Giulia Illiano of the Rome Observatory in Italy began monitoring SAX J1808 after the outburst began. They hoped to learn more about the pulsar’s properties.
During the most recent outburst, SAX J1808 entered a one-month-long reflaring stage after decaying from a peak luminosity of approximately one undecillion erg/s in about a week. Previous outbursts displayed similar behaviour.
The astronomers confirmed that the spin frequency of SAX J1808 decreased at an average rate of 0.001 pHz/s. This result is consistent with the energy losses expected from a rotating magnetic dipole of 100 YG cm3.
The study discovered that the orbital phase evolution of SAX J1808 has contracted since the last epoch for the first time in the last twenty years. Astronomers discovered that the orbit’s long-term behaviour can be described by an 11-second modulation with a period of 21 years.
The next SAX J1808 outburst is expected in 2025. The authors of the paper stressed the importance of observing this upcoming outburst in order to confirm their findings about the orbital evolution of this pulsar.
More information: Giulia Illiano et al, Timing analysis of the 2022 outburst of the accreting millisecond X-ray pulsar SAX J1808.4−3658: hints of an orbital shrinking, arXiv (2022). DOI: 10.48550/arxiv.2212.09778