Ultrahigh-Energy Cosmic Rays From the “En Caul” Birth of Magnetars*

Abstract

Rapidly spinning magnetars can potentially form through the accretion induced collapse of a white dwarf or by neutron star (NS) mergers if the equation of state of the nuclear density matter is such that two low-mass NSs can form a massive NS rather than a black hole. In either case, the newborn magnetar is an attractive site for the production of ultrahigh-energy cosmic rays (particles with individual energies exceeding 10 eV; 18 UHECRs). The short-period spin and strong magnetic field are able to accelerate particles up to appropriate energies, and the composition of material on and around the magnetar may naturally explain recent inferences of heavy elements in UHECRs. We explore whether the small amount of natal debris surrounding these magnetars allows UHECRs to escape easily. We also investigate the impact on the UHECRs of the unique environment around the magnetar, which consists of a bubble of relativistic particles and magnetic field within the debris. The rates and energetics of UHECRs are consistent with such an origin, even though the rates of events that produce rapidly spinning magnetars remain very uncertain. The low ejecta mass also helps the high-energy neutrino background associated with this scenario to be below current IceCube constraints over most of the magnetar parameter space. A unique prediction is that UHECRs may be generated in old stellar environments without strong star formation, in contrast to what would be expected for other UHECR scenarios, such as active galactic nuclei or long gamma-ray bursts.

High-energy Neutrinos from Millisecond Magnetars Formed from the Merger of Binary Neutron Stars

“…They also estimate the neutrino emission from the hadronic interaction between UHECRs and ejecta baryons. However, Piro & Kollmeier (2016b) ignored interaction with the radiation background, under the assumption that the cosmic rays are composed primarily of heavy nuclei, in which case the neutrino signal is much weaker signal due to energy losses being dominated by photo-disintegration instead of pion creation.…”

“…It is also important to keep in mind that mergers giving rise to magnetars instead of black holes may not produce detectable prompt gamma-ray emission, in which case the GRB sample could be biased (though mergerproduced magnetars should be accompanied by luminous optical/X-ray counterparts; Yu et al 2013;Metzger & Piro 2014;Siegel & Ciolfi 2016a,b;Ciolfi et al 2017b). Piro & Kollmeier (2016b) focused on the escape of UHECRs from low ejecta-mass explosions, suggesting stable magnetars from NS mergers as sources which can explain the rates and heavy composition of the UHECR measurement by the Auger Observatory (Aab et al 2015b). They also estimate the neutrino emission from the hadronic interaction between UHECRs and ejecta baryons.…”

“…Different from Gao et al (2013) which considers the secondary emission by particles accelerated in the shocked ejecta, we focus on a general scenario that cosmic rays accelerated in the magnetosphere confront a spherical hot nebula. We account for additional interaction and cooling channels of cosmic ray particles that were not considered in Piro & Kollmeier (2016a), which can crucially impact the high-energy emission of the merger remnant, as demonstrated by Murase et al (2009) in the case of magnetars formed in supernovae.…”

“…A millisecond magnetar also provides a promising site for accelerating particles to ultra-high energies Piro & Kollmeier 2016a). A magnetospheric voltage drop of magnitude Φ mag ∼ µ (2 π/P ) 2 /c 2 = 1.3×10 21 B 14 P −2 −3 V is produced across the open field lines that extend beyond the light cylinder located at R lc = c/Ω, where µ ∼ B R 3 * is the magnetic dipole moment, B = 10 14 B 14 G is the strength of the surface magnetic field, R * ∼ 10 km is the NS radius, and P = 10 −3 P −3 s is the spin period of the magnetar.…”

The Fate of Neutron Star Binary Mergers

“…This includes the outflow of neutron-rich material producing a "kilonova" (Metzger 2017; though see Berger et al 2013 andTanvir et al 2013 for a potential case), the spin down of a remnant magnetar producing high energy and later radio emission (Nakar & Piran 2011;Piro & Kulkarni 2013;Zhang 2013;Metzger & Piro 2014), and fast radio bursts (FRBs) from collapsing supramassive NSs (Falcke & Rezzolla 2014). In other studies, it has been postulated that post-merger magnetars may be an important source of ultrahigh-energy cosmic rays (UHECRs; Piro & Kollmeier 2016).…”