Using the NuSTAR spacecraft and NICER instrument, an international team of astronomers have discovered a new accelerating millisecond X-ray pulsar. The newly discovered object, designated IGR J17591-2342, is the latest addition to the still short list of known accreting millisecond X-ray pulsars. This is reported in a document posted on August 30th on the arXiv pre-print server.
X-ray pulsars exhibit severe periodic changes in X-ray intensity that can be shorter than a fraction of a second. Accretion of millisecond X-ray pulsars (AMXPs) is a type of X-ray pulsar in which short periods of rotation are caused by prolonged mass transfer from a star with low-mass companions through an accretion disk to a slowly rotating neutron star. Astronomers see AMXP as a kind of astrophysical laboratory that could be critical to improving our knowledge of fusion burst processes.
To date, only 21 AMXPs have been detected, with rotation periods ranging from 1.7 to 9.5 milliseconds. To expand the list of these specific objects, the scientific community is still actively looking for such sources using space observatories such as the NASA Astrophysical Telescope (NuSTAR).
Recently, a team of researchers led by Andrea Sanna of the University of Cagliari, Italy, used NuSTAR to identify a new AMXP. The source, named IGR J17591-2342, was originally classified as an X-ray transient and submitted to the European Space Agency's International Laboratory for Astrophysical Gamma-Ray Research (INTEGRAL) to scan the galactic center on August 10, 2018. The team observed this source using NuSTAR, which revealed data on the nature of this object. Additional observations of the pulsar were carried out using the Neutron Star Inner Composition Explorer (NICER) aboard the International Space Station (ISS).
“In this article, we describe a coherent analysis of the observation times of NuSTAR and NICER, showing the rotation period of the pulsar and binary ephemeris,” the researchers said in their article.
In data from NuSTAR and NICER, the team found coherent X-ray pulses of about 527.4 Hz (1.9 milliseconds) taken nearly 25 days after the onset of the 15 percent momentum fraction burst.
According to the document, IGR J17591-2342 has an orbital period of about 8.8 hours. The mass of a neutron star has been calculated to be about 1.4 solar masses, and the minimum mass of a companion is most likely 0.42 solar masses.
In addition, astronomers have noted that IGR J17591-2342 is located near the center of our Milky Way galaxy, about 28,000 light-years from Earth. The estimated accretion rate was found to be about 0.52 billion solar masses per year. IGR J17591-2342 is still the 22nd known AMXP. The authors of the article emphasized that their discovery enriches the census of these objects, which are necessary for understanding the later stages of stellar evolution.
