First measurements of linear polarization of MAXI J1820+070

ATel #11445; A. Berdyugin, (Tuorla Obs., Dept. of Physics and Astronomy, University of Turku), A. Veledina(Tuorla Obs., Dept. of Physics and Astronomy, University of Turku; Nordita, Stockholm, Sweden), I. Kosenkov(Tuorla Obs., Dept. of Physics and Astronomy, University of Turku), J. Poutanen(Tuorla Obs., Dept. of Physics and Astronomy, University of Turku), S. Tsygankov(Tuorla Obs., Dept. of Physics and Astronomy, University of Turku), J. J.E. Kajava(FINCA, University of Turku), M. Kagitani(Graduate School of Science, Tohoku University, Aoba-ku, Sendai), & T. Sakanoi(Graduate School of Science, Tohoku University, Aoba-ku, Sendai)
on 20 Mar 2018; 06:10 UT
Credential Certification: Andrei Berdyugin (andber@utu.fi)

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Following the detection of the newly discovered X-ray transient MAXI J1820+070 (ATel #11399, #11400, #11403, #11404, #11406, #11418), we have started a polarimetric monitoring campaign of the source. Here we report the first detection of the linear polarization obtained on March 18, 2018, around 14:52 UTC (JD 2458196.120). The measurements were done with the Dipol-2 polarimeter, which is currently installed on the Tohoku 60cm (T60) telescope at Haleakala Observatory, Hawaii. Observations have been made simultaneously in the Johnson-Cousins B, V and R-bands over the time interval of approximately two hours. The total number of single measurements of the normalized Stokes parameters is 48. The average values of linear polarization and polarization angle for the B, V and R-bands are given below: B-band: P = 0.74 +/- 0.02(%), Theta = 53.0 +/- 0.7(deg); V-band: P = 0.71 +/- 0.03(%), Theta = 53.6 +/- 1.1(deg); R-band: P = 0.78 +/- 0.02(%), Theta = 54.5 +/- 0.7(deg). Thus the measured magnitude of linear polarization is below 1%. The value of interstellar polarization in the direction of the MAXI J1820+070 can be estimated from the reddening of the source, given that normally polarization degree PD(ISM) < 9 x E(B-V). An estimate of the reddening was given in ATel#11418 E(B-V) = 0.167, that corresponds to a Hydrogen column density of NH = 1.12 x 10^21 cm-2. However, in the Swift/XRT spectrum taken on March 13, 2018 (OBSID: 00010627002) the best fitting hydrogen column was an order of magnitude lower, NH = 1.2 (\pm 0.4) x 10^20 cm-2, which gives a much lower reddening estimate, AV= 0.055 (Guver & Ozel, 2009) or AV= 0.067, if one uses the relation of Predehl and Schmitt NH=AV x 1.8E21. For these lower values we get E(B-V) = AV/RV = 0.015 or 0.019, respectively, assuming RV=3.6. The discrepancy between the extinction values suggests that MAXI J1820+070 is closer to the Earth than the majority of the ISM in that direction. If the correct reddening corresponds to these low values measured from the X-ray spectrum, the measured polarization degrees are significantly above the expected interstellar polarization level. For the higher reddening values, the observed polarization is consistent with that coming from the ISM. Further observations of the source are planned in order to detect variations and to verify if the measured polarization is intrinsic.