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 Δm ∼ 2mag with a period of ≃1.5 hr in the Rc, r′, and g′ bands. Moreover, we found that the amplitude of the modulation and peak intensity changed by ≳1mag and ∼ 0.5mag, 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≃ 1eV and the emission volume radius R _bb ≃ 1.5 × 10⁴ d _kpc km (d _kpc is the distance to the source in units of 1kpc). 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) 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.