Toward Identifying the Unassociated Gamma-Ray Source 1FGL J1311.7-3429 with X-Ray and Optical Observations

We present deep optical and X-ray follow-up observations of the bright unassociated Fermi-LAT gamma-ray source 1FGL J1311.7-3429. The source was already known as an unidentified EGRET source (3EG J1314-3431, EGR J1314-3417), hence its nature has remained uncertain for the past two decades. For the putative counterpart, we detected a quasi-sinusoidal optical modulation of Delta m similar to 2 mag with a period of similar or equal to 1.5 hr in the Rc, r', and g' bands. Moreover, we found that the amplitude of the modulation and peak intensity changed by greater than or similar to 1 mag and similar to 0.5 mag, respectively, over our total six nights of observations from 2012 March to May. Combined with Swift UVOT data, the optical-UV spectrum is consistent with a blackbody temperature, kT similar or equal to 1 eV and the emission volume radius R-bb similar or equal to 1.5 x 10(4) d(kpc) km (d(kpc) is the distance to the source in units of 1 kpc). In contrast, deep Suzaku observations conducted in 2009 and 2011 revealed strong X-ray flares with a light curve characterized with a power spectrum density of P(f) proportional to f(-2.0 +/- 0.4), but the folded X-ray light curves suggest an orbital modulation also in X-rays. Together with the non-detection of a radio counterpart, and significant curved spectrum and non-detection of variability in gamma-rays, the source may be the second "radio-quiet" gamma-ray emitting millisecond pulsar candidate after 1FGL J2339.7-0531, although the origin of flaring X-ray and optical variability remains an open question.