How much can 56Ni be synthesized by the magnetar model for long gamma-ray bursts and hypernovae?

A rapidly rotating neutron star with strong magnetic fields, called a magnetar, is a possible candidate for the central engine of long gamma-ray bursts and hypernovae (HNe). We solve the evolution of a shock wave driven by the wind from a magnetar and we evaluate the temperature evolution, by which we estimate the amount of 56Ni that produces the bright emission of HNe. We obtain a constraint on the magnetar parameters (i.e. the poloidal magnetic field strength Bp and initial angular velocity Ωi) for synthesizing enough 56Ni mass to explain HNe (⁠|$M_{^{56}\mathrm{Ni}}\gtrsim 0.2\,\mathrm{M}_{{\odot }}$|⁠), that is, (Bp/1016 G)1/2 (Ωi/104 rad s−1) ≳ 0.7.

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