Fast flaring observed from XMMU J053108.3−690923 by eROSITA: a supergiant fast X-ray transient in the Large Magellanic Cloud

Abstract

Context. Supergiant fast X-ray transients (SFXTs) are a peculiar class of supergiant high-mass X-ray binary (HMXB) systems characterised by extreme variability in the X-ray domain. In current models, this is mainly attributed to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. Aims. We studied the X-ray properties of the supergiant HMXB XMMU J053108.3−690923 in the Large Magellanic Cloud to understand its nature. Methods. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3−690923. Results. We confirm the putative pulsations previously reported for the source with high confidence, certifying its nature as a neutron star in orbit with a supergiant companion. We identify the extremely variable nature of the source in the form of flares seen in the eROSITA light curves. The source flux exhibits a total dynamic range of more than three orders of magnitude, which confirms its nature as an SFXT, and is the first such direct evidence from a HMXB outside our Galaxy exhibiting a very high dynamic range in luminosity as well as a fast flaring behaviour. We detect changes in the hardness ratio during the flaring intervals where the hardness ratio reaches its minimum during the peak of the flare and increases steeply shortly afterwards. This is also supported by the results of the spectral analysis carried out at the peak and off-flare intervals. This scenario is consistent with the presence of dense structures in the supergiant wind of XMMU J053108.3−690923 where the clumpy medium becomes photoionised at the peak of the flare leading to a drop in the photo-electric absorption. Further, we provide an estimate of the clumpiness of the medium and the magnetic field of the neutron star assuming a spin equilibrium condition.

RX J0529.8−6556: a BeXRB pulsar with an evolving optical period and out of phase X-ray outbursts

“…Among the LMC HMXBs, there is a high fraction of supergiant systems (Maitra et al 2019a;van Jaarsveld et al 2018;Antoniou & Zezas 2016). Two of these have been suggested to be probable Supergiant Fast X-ray Transients (SFXTs) based on their fast flaring properties (see Vasilopoulos et al 2018;Maitra et al 2020).…”

eROSITA calibration and performance verification phase: High-mass X-ray binaries in the Magellanic Clouds

“…As can be already seen from the image, apart from the persistently bright black hole system LMC X-1, only one source was sufficiently bright for a detailed spectroscopic and temporal analysis. This is the supergiant fast X-ray transient (SFXT) XMMU J053108.3−690923 (marked with its pulse period of 2013 s in the image), for which the eROSITA results are already presented in Maitra et al (2021b). One new BeXRB was discovered in this work (SRGEt J052829.5−690345, source 73 in Table 2 and Fig.…”

“…All other sources (we exclude LMC X-1 from our analysis) were detected in at least one of the observations with fluxes (luminosities) in the range of 4.3×10 −14 erg cm −2 s −1 to 1.7×10 −12 erg cm −2 s −1 (1.5×10 34 erg s −1 to 5.9×10 35 erg s −1 ). The brightest sources were the SFXT XMMU J053108.3−690923 (Maitra et al 2021b) and the supergiant HMXB candidate RX J0541.4−6936, the latter being undetected in one observation with an upper limit a factor of ∼30 below the two detections.…”

“…The observation also covers the supergiant fast X-ray transient (SFXT) XMMU J053108.3−690923 (Maitra et al 2021b) at a similar off-axis angle. The SFXT is securely identified with a supergiant in the LMC.…”

“…On the other hand, ∼60 HMXB candidates were identified in the LMC, with detected pulsations in about 20 of them. The fraction of SgXRBs is somewhat higher in the LMC as compared to the SMC (Maitra et al 2021b(Maitra et al , 2019avan Jaarsveld et al 2018;Vasilopoulos et al 2018) which is probably related to the different star formation history of the two Clouds (Antoniou & Zezas 2016;Antoniou et al 2010).…”