TCP J05074264+2447555 as a bright microlensing event due to a binary system with very low mass ratio component: hint for a new planetary system?

ATel #10934; A. A. Nucita (Univ. of Salento, Dept. of Matemetics & Physics, Italy), D. Licchelli (R. P. Feynman Observatory, Gagliano del Capo, Italy), F. De Paolis (Univ. of Salento, Italy), G. Ingrosso (Univ. of Salento, Italy), F. Strafella (Univ. of Salento, Italy)
on 8 Nov 2017; 09:20 UT
Credential Certification: Achille Nucita (nucita@le.infn.it)

Tweet

The transient event labeled as TCP J05074264+2447555 (RA 05h07m42.64s, DEC +24°47'55.5" (J2000.0)) was first discovered by Kojima on UT 2017-10-25.688 in the Taurus region and suddenly recognized as something peculiar. Indeed, the event resembles a typical microlensing curve and was recognized as such by T. Jayasinghe et al. (ATel #10923) and by Hiroyuki Maehara’s (ATel #10919) follow-up spectroscopy. In the meantime K. Sokolovsky by means of Swift observations (ATel #10921) found no X-ray source at the target position and reported a brightening in the UV similar to the brightening observed in the optical band for other microlensing events. In ATel #10923, The ASAS-SN light curve for TCP J05074264+2447555, despite rare data points on the event peak, was clearly fitted by a single-lens microlensing model. In particular, the event is characterized by a time of closest approach t_0=58058.80 days (MJD), impact parameter u_0=0.091 and Einstein crossing time t_E=26.7 days. No evidence of blending was reported. Here we show that, taking into account the available AAVSO photometry in the V band and the B,V, r', i' data obtained at the R.P. Feynman Observatory (Gagliano del Capo, Italy) with a 300 mm aperture telescope (F/5.3) at the event peak and in the subsequent days, it is possible to distinguish features which are typical of a binary microlensing event. By modeling the event with a binary lens amplifying a background source it is possible to adequately describe the overall microlensing light curve and reproduce the observed features. The best fit procedure consists in adapting a seven free fit parameter model (i.e., the mass ratio q, the binary separation b in units of the Einster radius, the time t_0 of the projected closest separation between the source and the binary center of mass, the Einstein time t_E, the impact parameter u_0 and the source trajectory to binary axis angular separation theta, the baseline magnitude V_0) to the available data. Each model comes from the solution of the lens equation for a generic binary lens system. Restricting the fit search from the time 57980 MJD, we obtained the following best fit parameters: q=5e-5, b=0.92, t_0=58058.3 MJD, t_E= 29 days, u_0=0.089, theta = 1.5 radians, V_0 = 14.18. The best fit (solid blue line) together with available data (ASAS-SN, AAVSO and proprietary V data, red data points) are shown in Figure 1. The low component mass ratio makes the microlensing event as possibly due to a binary lens hosting an Earth-sized object. Subsequent analysis on the characteristics of the lens system parameters and the background source is in progress with the aim to refine the best fit parameters to further constrain the lens system and to constrain source charcateristics as its distance which, at the moment, we can estimate to be to in the range 1-3 kpc. More details on the TCP J05074264+2447555 microlensing event will be presented elsewhere. This work makes use of ASAS-SN data (Shappee et al. 2014 and Kochanek et al. 2017). We acknowledge with thanks the variable star observations from the AAVSO International Database contributed by observers worldwide and used in this research.

Light curve fitting model