Probing the emission mechanism and nature of the pulsating compact object in the X-ray binary SAX J1324.4−6200

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, University of Tuebingen, Sand 1, 72076 Tübingen, Germany
e-mail: ducci@astro.uni-tuebingen.de
² ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
⁴ INAF – OAR, Via Frascati, 33, 00078 Monte Porzio Catone, Rome, Italy
⁵ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁶ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 20 December 2023
Accepted: 1 March 2024

Abstract

Recently, there has been renewed interest in the Be X-ray binary (Be/XRB) SAX J1324.4−6200 because of its spatial coincidence with a variable γ-ray source detected by Fermi/LAT. To explore more thoroughly its properties, new X-ray observations were carried out in 2023 by NuSTAR, XMM-Newton, and Swift satellites, jointly covering the energy range from 0.2 − 79 keV. SAX J1324.4−6200 was caught at an X-ray flux of ∼10⁻¹¹ erg cm⁻² s⁻¹. The X-ray spectrum fits well with an absorbed power law with a high energy cutoff. Other acceptable fits require an additional blackbody component (kT_bb ≈ 1.1 keV) or a Gaussian in absorption (E_gabs ≈ 6.9 keV). We measured a NuSTAR spin period of 175.8127 ± 0.0036 s and an XMM-Newton spin period of 175.862 ± 0.025 s. We show that all the available spin period measurements of SAX J1324.4−6200, spanning 29 yr, are highly correlated with time, resulting in a remarkably stable spin-down of Ṗ = 6.09 ± 0.06 × 10⁻⁹ s s⁻¹. We find that if SAX J1324.4−6200 hosts an accretion-powered pulsar, accretion torque models indicate a surface magnetic field of ∼10^{12 − 13} G. The X-ray properties emerging from our analysis strenghten the hypothesis that SAX J1324.4−6200 belongs to the small group of persistent Be/XRBs. We also performed radio observations with the Parkes Murriyang telescope, to search for radio pulsations. However, no radio pulsations compatible with the rotational ephemeris of SAX J1324.4−6200 were detected. We rule out the hypothesis that SAX J1324.4−6200 is a γ-ray binary where the emission is produced by interactions between the pulsar and the companion winds. Other models commonly used to account for the production of γ-rays in accreting pulsars cannot reproduce the bright emission from SAX J1324.4−6200. We examined other possible mechanisms behind the γ-ray emission and note that there is a ∼0.5% chance probability that an unknown extragalactic active galactic nucleus (AGN) observed through the Galactic plane may coincidentally fall within the Fermi/LAT error circle of the source and be responsible for the γ-ray emission.