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The optical counterpart to the new accreting pulsar Swift J0243.6+6124 is a Be star

ATel #10822; K. Kouroubatzakis (U. of Crete), P. Reig (FORTH), J. Andrews (FORTH), A. Zezas (FORTH))
on 5 Oct 2017; 17:47 UT
Credential Certification: Pablo Reig (pau@physics.uoc.gr)

Subjects: Neutron Star, Transient, Pulsar

Referred to by ATel #: 10866, 10907, 10968

We report optical spectroscopic observations of the optical counterpart to the 9.87-s accreting neutron star transient Swift J0243.6+6124 (ATel#10809, ATel#10812) from the 1.3-m telescope of the Skinakas Observatory (Greece). The observations were made on the night of 4 October 2017 and covered the spectral range 5460-6744 angstroms and 3880-5315 angstroms. The spectra show Halpha and Hbeta in emission with an equivalent width of -10.3+-0.4 and -1.0+-0.1 angstroms, respectively. He I lines at 5875 and 6678 angstroms also appear in emission. The low S/N of the blue-end spectrum prevents us from performing a detailed spectral classification analysis but we tentatively detect HeI and He II lines. Hgamma and Hdelta are also affected by fill-in emission. The 3880-5315 A spectrum is consistent with a late Oe-type or early Be-type star. This spectral type is typical of accreting pulsars with a Be star's companion. We conclude that Swift J0243.6+6124 is most likely a new Be/X-ray binary.