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ATel #10919; Hiroyuki Maehara (Okayama Astrophysical Observatory, NAOJ, NINS) on 2 Nov 2017; 02:31 UTCredential Certification: Hiroyuki Maehara (firstname.lastname@example.org)
Subjects: Optical, Microlensing Event, Star, Transient, Variables
Referred to by ATel #: 10921, 10923, 10934
We report the spectroscopic observation of TCP J05074264+2447555 which was discovered by Tadashi Kojima at 10.8 mag on 2017-10-31.7340 (http://www.cbat.eps.harvard.edu/unconf/followups/J05074264+2447555.html). The object is identical to a 14 mag star UCAC4 574-013830 (V=14.086, B-V=0.908). The light curves of this object from All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014 and Kochanek et al. 2017) is similar to that of a microlensing event. The V-Ic color index of the object from the Kamogata-Kiso-Kyoto Wide-field Survey (KWS; ~1.0 on 2017-11-01) suggests that the object did not show significant color variations associated with the brightening.
We obtained a high-resolution spectrum of the object with the 188-cm telescope and the High-Dispersion Echelle Spectrograph (Kambe et al. 2013) at Okayama Astrophysical Observatory on 2017-11-01. No strong emission features are present in the spectrum. The overall appearance of the spectrum between 500 nm and 620 nm is similar to that of the rapidly-rotating F5V star HD 31845. The spectrum around the Mg I b triplet lines is posted at the following URL:
These results suggest that the object is a bright microlensing event in progress. According to the latest ASAS-SN and KWS data, the object has brightened to V=11.4 mag. Follow-up observations are strongly encouraged.