Astronomers observe a giant outburst of X-ray binary RX J0440.9+4431
Tomasz Nowakowski
astronomy writer
Sadie Harley
scientific editor
Robert Egan
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Astronomers from Wuhan University in China have employed the Insight-HXMT satellite to observe a giant outburst that occurred in the X-ray binary RX J0440.9+4431 about three years ago. , presented July 24 on the pre-print server arXiv, provide a wealth of data regarding the properties and behavior of this system.
X-ray binaries (XRBs) are a class of binary stars luminous in X-rays. They are composed of a normal star or a white dwarf transferring mass onto a compact neutron star or a black hole. Based on the mass of the companion star, these systems can be divided into low-mass X-ray binaries (LMXB) and high-mass X-ray binaries (HMXB).
Be/X-ray binaries (BeXRBs) are one of the subclass of HMXBs in which the optical star is a dwarf, subgiant or giant OBe star. Detecting and investigating X-ray outbursts from BeXRBs could be essential in order to improve our understanding of the nature of X-ray binaries and their behavior.
RX J0440.9+4431 is a BeXRB discovered in 1997 along with its optical companion LS +44 17/BSD 24-491, during the ROSAT Galactic Plane Survey. The system, which harbors a pulsar, is located some 8,000 light years away and showcases X-ray pulsations with a period of 202.5 seconds.
In late December 2022, a giant outburst of RX J0440.9+4431 started, which lasted till March 2023. Wuhan University's Prahlad R. Epili and Wei Wang decided to observe the outburst with Insight-HXMT, hoping to shed more light on the X-ray spectral and timing variability of this binary.
"The Insight-HXMT has observed this source at several instances of the entire outburst in 2022–2023. We used these bright outburst observations of the pulsar to study its X-ray spectral and timing variability," the researchers wrote in the paper.
The observations found that the pulse profiles during the progress and declining phases of the 2022–2023 outburst at similar luminosities show similar variations in shape. This indicates a strong luminosity dependence of pulse profiles in RX J0440.9+4431.
The net X-ray luminosity of the pulsar in RX J0440.9+4431 during the outburst spanned within the range of 4.4–28 undecillion erg/s in 1–120 keV. The astronomers noted that the phase-averaged spectra of the pulsar in 1–120 keV can be explained with an absorbed cutoff power-law continuum model.
Furthermore, the high cadence hard X-ray observations of the pulsar reveal the presence of a variable cyclotron resonance scattering feature (CRSF) in the energies of (33.6–41.6) keV. During the declining phases of the giant outburst, the researchers identified a second cyclotron line varying in the range of about 64.6–75.3 keV.
The study also found that the pulsar in RX J0440.9+4431 has a relatively strong magnetic field. It varies throughout the outburst, between 4.8 and 15.4 trillion Gauss.
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More information: Prahlad R. Epili et al, Insight-HXMT Spectral and Timing Studies of a Giant Outburst in RX J0440.9+4431, arXiv (2025).
Journal information: arXiv
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