Optical polarization monitoring of the black-hole candidate Swift J1727.8-1613
ATel #16245; V. Kravtsov, A. P. Nitindala, A. Veledina, A. Berdyugin, J. Poutanen, V. Piirola (University of Turku, Finland)
on 18 Sep 2023; 18:35 UT
Credential Certification: Vadim Kravtsov (vakrau@utu.fi)
Subjects: Optical, Binary, Black Hole, Transient
We perform optical polarimetric monitoring of the the X-ray transient Swift J1727.8-1613 (originally GRB 230824A: GCN #34536, #34537, ATel #16205, #16206, #16207, #16210, #16215, #16217 #16225, #16208, #16209 #16211 #16228, #16230) with DIPol-2 BVR polarimeter starting from 2 September 2023. DIPol-2 is installed on the remotely-controlled 60 cm Tohoku telescope (T60, Haleakala Observatory, Hawaii). The source was observed for about 1 hour each night, with 24 to 48 independent measurements of normalized Stokes parameters q and u simultaneously in B, V and R bands.
We find the 12 night average polarization degree PD = 0.92 ± 0.03 % and polarization angle PA = 70 ± 1 deg, which are consistent with the average polarization of two field stars (PD ~1% and PA ~70 deg, blue circles on Figure 1, see https://github.com/vadim-kravtsov/Swift1727_pol), located at distances in the range 1 - 3 kpc. This indicates the absence of a high (P_int > 1%) intrinsic polarization of the source. Nevertheless, we see the significant changes in the observed polarization between the observations, suggesting the presence of a small (less than 0.5 %), variable intrinsic polarization. Similar intrinsic PD values have previously been observed in MAXI J1820+070 (see Kosenkov et al. 2020, MNRAS Lett., 496, 96).
Taking the field stars polarization as a proxy for the interstellar medium component, we obtain the following intrinsic values: PD_B= 0.11 ± 0.03% with PA_B = 0 ± 8 deg, PD_R = 0.32 ± 0.04 % with PA_R = -12 ± 5 deg, and no clear average intrinsic polarization in V, despite the presence of significant variability (see Figure 2). We note that our PA_B is consistent with the PA in submillimeter (-4 ± 2 deg, ATel #16230) and in X-rays (PA = 3 ± 2 deg, ATel #16242). We continue monitoring the source and the field stars to obtain a more reliable ISM polarization estimate.
Link to the figures.
