Abstract
The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. This scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. The maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the Landau–Rabi quantization of electron motion. Numerical results are also presented using experimental, as well as theoretical nuclear data. Whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. Most sources are found to be concentrated at densities 1010–1011 g cm−3 with heat power W∞∼1035–1036 erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. The change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. This suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. The present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the Brussels-Montreal unified equations of state.
Highlights
Magnetars form a subclass of known neutron stars exhibiting various astrophysical phenomena powered by their extreme magnetic field [1] and currently consisting of 12 confirmed soft gamma-ray repeaters and 12 anomalous X-ray pulsars [3] according to the McGill Online Magnetar Catalog1
With ages typically of the order of a few kyr according to the Magnetar Outburst Online Catalog5, we found that the magnetic fields have decayed by δB /B ∼ τ/τB ∼ 10−3, which is comparable to the change of magnetic field required by nuclear reactions
We investigated electron captures and pycnonuclear fusion reactions in the outer crust of a magnetar induced by the compression of matter accompanying the decay of the magnetic field and the spin-down of the star
Summary
Magnetars form a subclass of known neutron stars exhibiting various astrophysical phenomena powered by their extreme magnetic field [1] (see, e.g., [2] for a recent review) and currently consisting of 12 confirmed soft gamma-ray repeaters and 12 anomalous X-ray pulsars [3] according to the McGill Online Magnetar Catalog. We focused on the scenario proposed in [9], who pointed out that the decay of the magnetic field may trigger exothermic nuclear reactions (see, e.g., [6] for a critical review of other scenarios) This mechanism is similar to deep crustal heating in accreting neutron stars (see, e.g., [10] and the references therein), the compression of matter being induced here by the loss of magnetic pressure (and more generally by the local rearrangement of the magnetic field lines), rather than from the accumulation of accreted material onto the stellar surface (the existence of accreting magnetars—conceivably in the form of ultraluminous X-ray sources—remains a matter of debate; see, e.g., [11,12,13,14] and the references therein).
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