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SUMMARY:Dynamics of nuclear reactions in the outer crust of transiently ac
 creting neutron stars
DTSTART;VALUE=DATE-TIME:20261029T063000Z
DTEND;VALUE=DATE-TIME:20261029T064500Z
DTSTAMP;VALUE=DATE-TIME:20261102T230149Z
UID:indico-contribution-6186@cern.ch
DESCRIPTION:Speakers: Nikita Zemlyakov (Ioffe Institute)\nSome neutron sta
 rs are located in low-mass X-ray binaries (LMXBs)\, i.e. have low-mass com
 panions. In these systems\, matter from the companion is transferred via R
 oche-lobe overflow and accretes onto the neutron star. The accretion proce
 ss can be either transient or persistent. During accretion\, the previousl
 y accreted material is compressed under the weight of newly accreted matte
 r. In the outer crust\, the pressure is primarily determined by degenerate
  electrons\; consequently\, this compression leads to an increase in the e
 lectron chemical potential. Such an increase can drive electron capture re
 actions out of equilibrium. Previous studies investigating the nuclear evo
 lution of accreted matter in the neutron star crust can be divided into tw
 o groups: (1) those that consider variable accretion rates but assume elec
 tron capture reactions occur instantaneously\; and (2) those that account 
 for finite reaction rates but assume a steady (stationary) accretion rate.
 \n\nA recent study [1] examined non‑stationary accretion while accountin
 g for finite electron capture rates. However\, the capture rate was comput
 ed using the zero‑temperature approximation. Under this approximation\, 
 the reaction rate is exactly zero below the threshold density and increase
 s sharply above it. Using this approach\, the authors of [1] demonstrated 
 that the crust structure is non‑stationary. During an accretion episode\
 , the composition of the outer crust evolves away from beta equilibrium du
 e to the accumulation of nuclei above the threshold. During a quiescent st
 ate\, these accumulated nuclei are burnt out\, which results in a nearly b
 eta‑equilibrium composition restored before the next accretion episode b
 egins. This leads to a cyclical pattern in the crust’s composition. \n\n
 In this work\, we investigate the dynamics of nuclear reactions under cond
 itions of non‑stationary accretion\, incorporating finite‑temperature 
 reaction rates. We show that this approach substantially alters the physic
 al picture. Due to thermal excitations\, electron capture reactions become
  effective even below the nominal threshold density. At the same time\, th
 e reverse reaction is suppressed: it is inhibited by a subsequent electron
  capture on the daughter nuclei. This second capture is superthreshold bec
 ause of nuclear pairing effects – the daughter nucleus is odd‑odd\, so
  the threshold density is low. Furthermore\, the dependence of the reactio
 n rates on density becomes less pronounced. Consequently\, the composition
  of the outer crust evolves more slowly compared to the model presented in
  [1]. As a result\, the composition at the onset of an accretion episode d
 epends on the long‑term accretion history of the system. In particular\,
  this effect can lead to a substantial release of energy during a strong a
 ccretion episode as a result of burning of nuclei that accumulated in the 
 crust during earlier accretion. \n\nThe work is supported by Russian Scien
 ce Foundation (grant No. 22-12-00048-P).\n\n[1] Suleiman\, L.\, Zdunik\, J
 . L.\, & Haensel\, P. (2024). Layers of electron captures in the crust of 
 accreting neutron stars. A&A\, 690\, A301.\n\nhttps://indico.particle.meph
 i.ru/event/626/contributions/6186/
LOCATION: Conference Hall 2
URL:https://indico.particle.mephi.ru/event/626/contributions/6186/
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