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BVI photometric confirmation of a microlensing nature for ASASSN-16oe

ATel #9879; U. Munari (INAF Padova), F.-J. Hambsch and A. Frigo (ANS Collaboration)
on 19 Dec 2016; 16:58 UT
Credential Certification: U. Munari (ulisse.munari@oapd.inaf.it)

Subjects: Optical, Microlensing Event, Transient

ASASSN-16oe was discovered on UT 2016-11-30.35 at V~13.3 mag. The progenitor is present in many all sky surveys, including Gaia DR1. The quiescence brightness according to APASS is B=16.83 and V=15.93 mag. Strader et al. (Atel #9860) have recently suggested that ASASSN-16oe could be a microlensing event based on the preliminary ASASSN photometry and a spectrum they obtained for the object.

We are monitoring ASASSN-16oe in BVIc bands with a 40cm robotic telescope in Atacama (Chile). Our photometry, listed below, indeed supports a microlensing event: (i) the B-V=+0.89 and V-I=+1.12 colors remain constant throughout the event, (ii) they match the quiescence B-V=+0.90 from APASS survey, and (iii) the shape of the lightcurve perfectly fits that of a microlensing event for an Einstein time scale parameter of 4.72 days and t(0) = 2016 Dec 3.5 UT.

Strader et al. classify their spectrum as that of an F-type star, and provide access to it in graphical form. Albeit quite compressed, their picture allows us to refine the spectral classification by comparison with the Asiago MK Spectral Atlas, obtaining G0V as the spectral type. The strong intensity of NaI doublet suggests a significant reddening, which is confirmed by the observed B-V=+0.89 and V-Ic=1.12 colors. Comparing with the intrinsic colors for G0V stars from Fitzgerald (1970) and Bessell (1990), the resulting reddening is E(B-V)=0.27 and E(V-Ic)=0.40. The ratio of the two nicely match the value of 1.51 expected for a Sun-like energy distribution and the diffuse interstellar medium characterized by a R_v=3.1 reddening law.

 
December  B      V      Ic 
 (UT) 
03.368 13.970 13.092 12.039  
04.365 14.042 13.110 12.048  
05.365 13.946 13.172 12.079  
06.365 14.198 13.249 12.158  
07.364 14.265 13.375 12.298 
08.364 14.451 13.500 12.398 
09.364 14.531 13.592 12.442 
10.364 14.491 13.695 12.589 
11.364 14.665 13.787 12.684 
12.365 14.699 13.940 12.718 
13.365 14.874 14.037 12.887 
14.365 14.966 14.078 12.954 
16.364 15.172 14.248 13.135 
17.366 15.278 14.302 13.216 
18.366 15.215 14.380 13.253