Probing a magnetar origin for the population of extragalactic fast X-ray transients detected by Chandra

¹ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile
e-mail: jaquirola@uc.cl
² Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
³ Observatorio Astronómico de Quito, Escuela Politécnica Nacional, 170136 Quito, Ecuador
⁴ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA
⁵ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁶ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁷ Department of Astronomy & Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
⁸ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA
⁹ Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹¹ School of Astronomy and Space Science, Nanjing University, Nanjing, PR China
¹² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
¹³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, LC, Italy
¹⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
¹⁵ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
¹⁶ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
¹⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁸ SRON Netherlands Institute for Space Research, Postbus 800, 9700 AV Groningen, The Netherlands
¹⁹ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands

Received: 2 August 2023
Accepted: 28 December 2023

Abstract

Context. Twenty-two extragalactic fast X-ray transients (FXTs) have now been discovered from two decades of Chandra data (analyzing ∼259 Ms of data), with 17 associated with distant galaxies (≳100 Mpc). Different mechanisms and progenitors have been proposed to explain their properties; nevertheless, after analyzing their timing, spectral parameters, host-galaxy properties, luminosity function, and volumetric rates, their nature remains uncertain.

Aims. We interpret a sub-sample of nine FXTs that show a plateau or a fast-rise light curve within the framework of a binary neutron star (BNS) merger magnetar model.

Methods. We fit their light curves and derive magnetar (magnetic field and initial rotational period) and ejecta (ejecta mass and opacity) parameters. This model predicts two zones: an orientation-dependent free zone (where the magnetar spin-down X-ray photons escape freely to the observer) and a trapped zone (where the X-ray photons are initially obscured and only escape freely once the ejecta material becomes optically thin). We argue that six FXTs show properties consistent with the free zone and three FXTs with the trapped zone.

Results. This sub-sample of FXTs has a similar distribution of magnetic fields and initial rotation periods to those inferred for short gamma-ray bursts, suggesting a possible association. We compare the predicted ejecta emission fed by the magnetar emission (called merger-nova) to the optical and near-infrared upper limits of two FXTs, XRT 141001 and XRT 210423 where contemporaneous optical observations are available. The non-detections place lower limits on the redshifts of XRT 141001 and XRT 210423 of z ≳ 1.5 and ≳0.1, respectively.

Conclusions. If the magnetar remnants lose energy via gravitational waves (GWs), it should be possible to detect similar objects with the current advanced LIGO detectors out to a redshift z ≲ 0.03, while future GW detectors will be able to detect them out to z ≈ 0.5.