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Long-term temperature evolution of neutron stars undergoing episodic accretion outbursts

机译：经历积聚爆发的中子星的长期温度演化

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Context . Transient neutron star low-mass X-ray binaries undergo episodes of accretion, alternated with quiescent periods. During an accretion outburst, the neutron star heats up due to exothermic accretion-induced processes taking place in the crust. Besides the long-known deep crustal heating of nuclear origin, a likely non-nuclear source of heat, dubbed “shallow heating”, is present at lower densities. Most of the accretion-induced heat slowly diffuses into the core on a timescale of years. Over many outburst cycles, a state of equilibrium is reached when the core temperature is high enough that the heating and cooling (photon and neutrino emission) processes are in balance. Aims . We investigate how stellar characteristics and outburst properties affect the long-term temperature evolution of a transiently accreting neutron star. For the first time the effects of crustal properties are considered, particularly that of shallow heating. Methods . Using our code NSCool , we tracked the thermal evolution of a neutron star undergoing outbursts over a period of 10~(5)yr. The outburst sequence is based on the regular outbursts observed from the neutron star transient Aql X-1. For each model we calculated the timescale over which equilibrium was reached and we present these timescales along with the temperature and luminosity parameters of the equilibrium state. Results . We performed several simulations with scaled outburst accretion rates, to vary the amount of heating over the outburst cycles. The results of these models show that the equilibrium core temperature follows a logarithmic decay function with the equilibrium timescale. Secondly, we find that shallow heating significantly contributes to the equilibrium state. Increasing its strength raises the equilibrium core temperature. We find that if deep crustal heating is replaced by shallow heating alone, the core would still heat up, reaching only a 2% lower equilibrium core temperature. Deep crustal heating may therefore not be vital to the heating of the core. Additionally, shallow heating can increase the quiescent luminosity to values higher than previously expected. The thermal conductivity in the envelope and crust, including the potentially low-conductivity pasta layer at the bottom of the crust, is unable to significantly alter the long-term internal temperature evolution. Stellar compactness and nucleon pairing in the core change the specific heat and the total neutrino emission rate as a function of temperature, with the consequences for the properties of the equilibrium state depending on the exact details of the assumed pairing models. The presence of direct Urca emission leads to the lowest equilibrium core temperature and the shortest equilibrium timescale.
机译：语境。瞬态中子星低质量X射线双星经历增生，并处于静止期。在增生爆发期间，由于地壳中发生放热的增生诱导过程，中子星变热。除了众所周知的核深层地壳加热外，还存在密度较低的可能的非核热源，称为“浅层加热”。大多数吸积引起的热量会在数年的时间内缓慢扩散到岩心中。在许多爆发周期中，当核心温度足够高以至于加热和冷却（光子和中微子发射）过程达到平衡时，就会达到平衡状态。目的。我们研究了恒星特征和爆发特性如何影响瞬态吸积中子星的长期温度演化。首次考虑了地壳特性的影响，特别是浅层加热的影响。方法 。使用我们的代码NSCool，我们跟踪了在10〜（5）年内爆发的中子星的热演化。爆发序列是基于从中子星瞬变Aql X-1观察到的规则爆发。对于每个模型，我们都计算了达到平衡的时间尺度，并给出了这些时间尺度以及平衡状态的温度和发光度参数。结果。我们使用按比例确定的爆发增长速率进行了几次模拟，以改变爆发周期中的热量。这些模型的结果表明，平衡核心温度遵循对数衰减函数，并具有平衡时标。其次，我们发现浅层加热显着地促进了平衡状态。增加其强度会升高平衡堆芯温度。我们发现，如果仅用浅层加热代替深层地壳加热，堆芯仍会加热，仅使平衡堆芯温度降低2％。因此，深地壳加热对于岩心加热可能不是至关重要的。此外，浅层加热可将静态发光度增加到高于先前预期的值。外壳和外壳的导热性（包括外壳底部可能具有低导热性的面食层）无法显着改变长期内部温度的变化。核心中的恒星紧致度和核子配对会根据温度改变比热和总中微子发射速率，其对平衡态性质的影响取决于假定配对模型的确切细节。 Urca直接排放的存在导致最低的平衡核心温度和最短的平衡时间尺度。

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