A search for the presence of magnetic fields in the two Supergiant Fast X-ray Transients IGR J08408-4503 and IGR J11215-5952

TL;DR

This study investigates magnetic fields in two supergiant fast X-ray transients using spectropolarimetry, finding evidence of magnetic fields in one system and variability in both, supporting models involving magnetized stellar winds.

Contribution

It provides the first spectropolarimetric measurements of these SFXTs, confirming the presence of magnetic fields in IGR J11215-5952 and supporting magnetized wind models.

Findings

Magnetic field detected in IGR J11215-5952 at 3.2-3.8 sigma significance.

No magnetic field detection in IGR J08408-4503 at 3 sigma.

Spectral variability observed in IGR J11215-5952 and short-term variability in IGR J08408-4503.

Abstract

A significant 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, IGR J08408-4503 and IGR J11215-5952 with the ESO FORS2 instrument during two different observing runs. No field detection at a significance level of 3sigma was achieved for IGR J08408-4503. For IGR J11215-5952, we obtain 3.2sigma and 3.8sigma detections (<B_z>_hydr = -978+-308G and <B_z>_hydr = 416+-110G) 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, while for IGR J08408-4503 we find an indication of the presence of short-term variability on a time scale of minutes.