Where are the magnetar binary companions? Candidates from a comparison with binary population synthesis predictions

Abstract

It is well established that magnetars are neutron stars with extreme magnetic fields and young ages, but the evolutionary pathways to their creation are still uncertain. Since most massive stars are in binaries, if magnetars are a frequent result of core-collapse supernovae, some fraction are expected to have a bound companion at the time of observation. In this paper, we utilize literature constraints, including deep Hubble Space Telescope imaging, to search for bound stellar companions to magnetars. The magnitude and colour measurements are interpreted in the context of binary population synthesis predictions. We find two candidates for stellar companions associated with CXOU J171405.7–381031 and SGR 0755–2933, based on their J-H colours and H-band absolute magnitudes. Overall, the proportion of the Galactic magnetar population with a plausibly stellar near-infrared counterpart candidate, based on their magnitudes and colours, is between 5 and 10 per cent. This is consistent with a population synthesis prediction of 5 per cent, for the fraction of core-collapse neutron stars arising from primaries which remain bound to their companion after the supernova. These results are therefore consistent with magnetars being drawn in an unbiased way from the natal core-collapse neutron star population, but some contribution from alternative progenitor channels cannot be ruled out.

Strange magnetars admixed with fermionic dark matter

“…On the other hand, magnetars are usually found isolated [57,58]. However, since most massive stars are in binary systems, if core-collapse supernovae frequently give birth to magnetars, some fraction of them are expected to have a companion at the time of observation [86]. So, the merger of a system with at least one magnetar could provide us with useful information about their structure.…”

Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions

“…The scintillation of radio photons by electrons in the Milky Way is accounted for in our fits by adding the expected amplitude of scintillation in quadrature with the observational errors. The amplitude of scintillation is calculated using the method of Gompertz & Fruchter (2017), which is based on the model of Goodman (1997) and uses the free electron distribution model of Cordes & Lazio (2002). Our model parameterises the afterglow in terms of its isotropic equivalent kinetic energy ( k,iso ), the underlying distribution of electron energies, which is assumed to be a power-law with an index , the fraction of energy that is contained in the radiating electrons ( ), the fraction of energy that is contained in the magnetic fields ( ), the density of the circumburst medium ( for an ISM-like ∝ 0 or * for a wind-like ∝ −2 environment) and the half-opening angle of the jet .…”

“…Since most binary systems are unbound upon primary supernova (e.g. Eldridge et al 2011;Renzo et al 2019;Chrimes et al 2022), the assumption of similar ISM properties for primaries and secondaries should generally hold.…”

Towards an understanding of long gamma-ray burst environments through circumstellar medium population synthesis predictions

“…The scintillation of radio photons by electrons in the Milky Way is accounted for in our fits by adding the expected amplitude of scintillation in quadrature with the observational errors. The amplitude of scintillation is calculated using the method of Gompertz & Fruchter (2017), which is based on the model of Goodman (1997) and uses the free electron distribution model of Cordes & Lazio (2002). Our model parameterises the afterglow in terms of its isotropic equivalent kinetic energy (𝐸 k,iso ), the underlying distribution of electron energies, which is assumed to be a power-law with an index 𝑝, the fraction of energy that is contained in the radiating electrons (𝜖 𝑒 ), the fraction of energy that is contained in the magnetic fields (𝜖 𝐵 ), the density of the circumburst medium (𝑛 for an ISM-like 𝜌 ∝ 𝑟 0 or 𝐴 * for a wind-like 𝜌 ∝ 𝑟 −2 environment) and the half-opening angle of the jet 𝜃 𝑜 .…”