A search for the presence of magnetic fields in the two supergiant fast X-ray transients, IGRJ08408-4503 and IGRJ11215-5952

Abstract

Asignificant fraction of high-mass X-ray binaries are supergiant fast X-ray transients (SFXTs). The prime model for the physics governing their X-ray behaviour suggests that the winds of donor OB supergiants are magnetized. To investigate if magnetic fields are indeed present in the optical counterparts of such systems, we acquired low-resolution spectropolarimetric observations of the two optically brightest SFXTs, IGRJ08408-4503 and IGR J11215-5952, with the ESO FORS 2 instrument during two different observing runs. No field detection at a significance level of 3σ was achieved for IGRJ08408-4503. For IGRJ11215-5952, we obtain 3.2σ and 3.8σ detections (〈Bz〉hydr = -978 ± 308G and 〈Bz〉hydr = 416 ± 110 G) on two different nights in 2016. These results indicate that the model involving the interaction of a magnetized stellar wind with the neutron star magnetosphere can indeed be considered to characterize the behaviour of SFXTs. We detected long-term spectral variability in IGR J11215-5952, whereas for IGRJ08408-4503, we find an indication of the presence of short-term variability on a time-scale of minutes.