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

S. Hubrig$^1$, L. Sidoli$^2$, K. Postnov$^3$, M. Sch\"oller$^4$, A.F. Kholtygin$^5$, S.P. J\"arvinen$^1$, P. Steinbrunner$^6$

$^1$ AIP, $^2$ INAF Milano, $^3$ Sternberg Institute, $^4$ ESO, $^5$ Saint-Petersburg State University, $^6$ Freie Universit\"at Berlin

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$-$4503 and IGR\,J11215$-$5952 with the ESO FORS\,2 instrument
during two different observing runs. No field detection at a significance level of 3$\sigma$ was achieved
for IGR\,J08408$-$4503. For IGR\,J11215$-$5952, we obtain 3.2$\sigma$ and 3.8$\sigma$ detections
($\left< B_{\rm z}\right>_{\rm hydr}=-978\pm308$\,G and $\left< B_{\rm z}\right>_{\rm hydr}=416\pm110$\,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$-$5952, while for IGR\,J08408$-$4503 we find
an indication of the presence of short-term variability on a time scale of minutes.

Reference: MNRAS
Status: Manuscript has been accepted