Ph.D. candidate GUO Yunlang and Prof. WANG Bo from the Yunnan Observatories of the Chinese Academy of Sciences investigated the formation of black widow pulsars through ultracompact X-ray binaries (UCXBs) with He star companions.
This work was published in Monthly Notices of the Royal Astronomical Society.
Millisecond pulsars (MSPs) are old radio pulsars with spin periods less than 30 ms. The eclipsing MSPs have orbital periods ranging from about 0.1 to 1.0 days, and their companion stars are stripped of material by the pulsar’s pulsing radiation, as if they are spiders cannibalizing their companions. Thus, scientists name these eclipsing MSPs black widows (BWs) and redbacks (RBs).
BWs have companions with masses less than 0.05 M☉ (the mass of the Sun), while RBs have companions with masses in the range of 0.1–1.0 M☉. It has been suggested that BWs may be the evolutionary link between the accreting X-ray pulsars and the isolated MSPs, in which the companion is ablated by the γ-ray and the energetic particles emitted by MSPs.
Recent observations indicate that BWs can be divided into two sub-types: one is the BWs with companion mass (Mtwo) in the range of 0.01–0.05 M☉while the other sub-type have Mtwo ≤ 0.01 M☉. The first subtype can be explained by the pulsar radiation evaporating normal stars. However, the origin of the second sub-type is not well understood.
In this study, the researchers explored the origin of BWs with Mtwo ≤ 0.01 M☉ by considering the evaporation process based on the UCXB channel.
They calculated the long-term evolution of NS+He star systems by considering different He star masses and evaporation efficiencies. The evaporation efficiencies will affect the evolutionary track of the binary system when the evaporation process begins. The He star mass will decrease faster and the binary orbits widen more quickly as the evaporation efficiencies increase, owing to the more efficient evaporation.
They found that the BWs with Mtwo ≤ 0.01 M☉ can be explained by the UCXB channel with He star companions. To further reproduce the properties of BWs, they considered different input parameters for initial models. The BW properties can be well reproduced by using appropriate evaporation efficiencies and initial NS spin periods.
In addition, they found that the mass of He star companions can decrease to ☉ or even -5 M☉ within the Hubble time. Previous studies suggested that the He degenerate companion will undergo the tidal disruption before the companion mass decrease to 0.004 M☉. Therefore, the BWs through the UCXB channel with He star companions are the potential progenitors of isolated MSPs. Moreover, they found that the number of isolated MSPs through this channel is in the range of 10–80.
This work explains the formation of BWs with Mtwo ≤ 0.01 M☉and proposed an alternative formation channel for producing isolated MSPs.
Researchers explain formation of millisecond pulsars with long orbital periods
Yunlang Guo et al, Formation of black widows through ultra-compact X-ray binaries with He star companions, Monthly Notices of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac1917
Provided by Chinese Academy of Sciences
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