Comparing the space densities of millisecond-spin magnetars and fast X-ray transients

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL, The Netherlands
² SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, Leiden 2333 CA, The Netherlands
³ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, 20742 Maryland, USA
⁴ Department of Physics, University of Warwick, Coventry CV4 7AL, UK

^⋆ Corresponding author: sumedha.biswas@ru.nl

Received: 19 February 2025
Accepted: 12 June 2025

Abstract

Context. Fast X-ray transients (FXTs) are bright X-ray flashes with durations of a few minutes to hours, peak isotropic luminosities of L_X, peak ∼ 10⁴² − 10⁴⁷ erg s⁻¹, and total isotropic energies of E ∼ 10⁴⁷ − 10⁵⁰ erg. They have been detected with space-based telescopes such as Chandra, XMM-Newton, Swift-XRT, and Einstein Probe in the soft X-ray band. Einstein Probe detected > 50 in its first year of operation. While several models have been proposed, the nature of many FXTs is currently unknown. One model predicts that FXTs are powered by the spin-down energy of newly formed millisecond magnetars. In this context, they are usually thought to form in a binary neutron star (BNS) merger. However, the rates seem to be in tension: the BNS volumetric rate is estimated to be ∼10² Gpc⁻³ yr⁻¹, which barely overlaps with the estimated FXT volumetric rate of 10³ − 10⁴ Gpc⁻³ yr⁻¹; thus, even in the small range of overlap, BNS mergers would need to produce FXTs with nearly 100% efficiency.

Aims. We explore the maximum volumetric formation rate of millisecond spin period magnetars, including several possibilities beyond the BNS channel, comparing it with the volumetric rate of FXTs to determine what fraction of FXTs could have a millisecond magnetar origin.

Methods. We compiled the estimated rate densities for several different suggested formation channels of rapidly spinning magnetars, including the accretion-induced collapse of white dwarfs, binary white dwarf mergers, neutron star–white dwarf mergers, and the collapse of massive stars. We converted the Milky Way event rates to volumetric rates, wherever necessary, by considering either the star formation rate or the stellar mass density distributions as a function of redshift.

Results. We find that the highest possible rates among these possibilities come from binary white dwarf mergers and the collapse of massive stars. However, both scenarios may be unfavourable for FXT production due to uncertainties in the resultant spin and magnetic field distributions of the newly formed neutron stars and several observational constraints. Moreover, in all the scenarios, we find that the fraction of neutron stars that meet both criteria of rapid rotation and a strong magnetic field is either very low or highly uncertain. We conclude that millisecond magnetars are not the most viable progenitors of FXTs and can account for at most 10% of the entire FXT population.