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Determination of the distance to SWIFT J0243.6+6124

ATel #10968; I. Bikmaev, V. Shimansky, E. Irtuganov, M. Glushkov, N. Sakhibullin (KFU/AST, Kazan, Russia), I. Khamitov (TUG, Antalya, Turkey, KFU, Kazan, Russia), R. Burenin, A. Lutovinov, I. Zaznobin, M. Pavlinsky (IKI RAN, Moscow, Russia), R. Sunyaev (IKI RAN, Moscow, Russia, MPA, Garching, Germany), S. Dodonov, V. Afanasiev, S. Kotov (SAO RAS, NiznijArkhyz, Russia), V. Doroshenko (IAAT, Germany), S. Tsygankov (Tuorla Observatory, Turku, Finland)
on 14 Nov 2017; 09:21 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Alexander Lutovinov (aal@hea.iki.rssi.ru)

Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar

We have performed an optical photometry and spectroscopy of the Be counterpart of a newly discovered transient X-Ray pulsar SWIFT J0243.6+6124 (Kennea et al, 2017, ATel #10809) using facilities of the 1.5-meter Russian-Turkish telescope (RTT-150, TUBITAK National Observatory, Antalya, Turkey) and the 6-meter Russian telescope (BTA, Special Astrophysical Observatory of the Russian Academy of Sciences).

TFOSC instrument equipped with 2K x 2K ANDOR DW436 CCD was used at RTT-150 and SCORPIO-2 instrument equipped with 4K x 2K CCD was used at BTA. BVRI images and 2.5 A resolution echelle-spectra (4000-9000 AA) were obtained at RTT-150 in the period Oct 9 - 17, 2017, and 5 A resolution spectra (3600-10000 AA) were obtained at BTA in the night Oct 23/24, 2017.

Most of the hydrogen and helium lines show emission components from the Be-star disc in agreement with the results of Kouroubatzakis et al. (ATel #10822). At the same time presence of the HeII absorption lines in the spectra indicates an early B spectral class. In particular, HeI (5047) and HeII (4541, 4686) absorption lines and Bowen blend at 4630-4650 AA were detected in the spectra of the star. We calculated their profiles using the Kurucz grid of atmospheric models and our own original spectral synthesis code, which also includes non-LTE corrections.

As the result main parameters of the atmosphere of the B-star were determined: Teff = 29500 +/- 1000 K, logg = 3.7 - 4.0. Using the evolutionary tracks of Martins and Palacios (A&A, 2013, 560, A16) we have estimated parameters of the Be star itself: the mass M = 16+/-2 Msun, radius R=7+/-2 Rsun, luminosity log (L/Lsun) = 4.5 +/- 0.2 dex.

Using these parameters we can estimate the intrinsic color (B-V)0 = -0.30, bolometric luminosity Mbol = -6.45+/-0.5 mag, and the bolometric correction BC= -3.1+/-0.2 mag.

We have determined the reddening value to be in range of E(B-V) = 1.07 - 1.57 based on measured equivalent widths (EWs) of several DIBs absorption lines (at 5487.7, 5797.1, 5780.5, 6283.8, 6613.6 AA) and calibrations from Friedman et al. (ApJ, 2013, 723, 33). Independently, we obtained E(B-V) = 1.15 +/- 0.1 by fitting the observed spectral energy distribution (corrected for E(B-V)) with calculated flux based on model atmosphere. Note the N(H) ~1022 column density value, derived from HI Tables (Dickey and Lockman, 1990) and X-ray spectra of the source (Kennea et al., 2017, ATel #10809) corresponds to much higher value of E(B-V) ~ 2.0.

During our RTT-150 photometric observations the Be-star did not show any photometric variations within 0.02 mag. We have calibrated our photometric measurements against Landolt standards. The observed magnitudes are B = 13.67, V = 12.68, R = 12.07, I = 11.42. Using the observed (B-V)=+0.99+/-0.03 mag and intrinsic (B-V)0 =-0.30 mag values we determine E(B-V) = 1.29. Supposing the standard interstellar absorption with Rv = 3.1 we can estimate Av = 4.00 mag.

Based on the values of the absolute magnitude Mv = -3.35 mag, the observed magnitude V=12.68 mag, and Av=4.0 - we can estimate the distance to the SWIFT J0243.6+6124 as D = 2.5+/- 0.5 kpc. This distance and Galactic coordinates (LII = 136 deg, BII = 1.4 deg) indicate that source is likely located in the Perseus spiral arm of the Milky Way.

Obtained distance value allows us to estimate the X-ray luminosity of the source as ~5 x 1038 erg/s in the peak of the outburst. Such luminosity is typical for giant outbursts from Be/XRPs and supports the conclusion that SWIFT J0243.6+6124 entered into the super-Eddington regime (Doroshenko et al., 2017, arXiv:1710.10912; J. van den Eijnden et al., ATel #10946).