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

Abstract

We present deep optical and X-ray follow-up observations of the bright unassociated Fermi-LAT gammaray 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\sim2 mag 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 > 1 mag and 0.5 mag respectively, over our total six nights of observations from 2012 March and May. Combined with Swif t UVOT data, the optical-UV spectrum is consistent with a blackbody temperature, kT \sim1 eV, and the emission volume radius Rbb\sim 1.5x10^4 km. In contrast, deep Suzaku observations conducted in 2009 and 2011 revealed strong X-ray flares with a lightcurve characterized with a power spectrum density of P(f) propto f^(-2) 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 milli-second pulsar candidate after 1FGL J2339.7-0531, although the origin of flaring X-ray and optical variability remains an open question.

8 pages, 10 figures, to appear in the Astrophysical Journal (ApJ)

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• Population synthesis of ultracompact X-ray binaries in the Galactic Bulge
• The Gamma-ray Blazar Quest: new optical spectra, state of art and future perspectives
• Radio Detection of the Fermi LAT Blind Search Millisecond Pulsar J1311-3430
• Refining the associations of the Fermi Large Area Telescope Source Catalogs
• 1FGL J0523.5-2529: A New Probable Gamma-ray Pulsar Binary
• Multiband Diagnostics of Unidentified 1FGL Sources with Suzaku and Swift X-ray Observations
• Orbital Phase Dependent Gamma-ray emissions from the Black Widow Pulsar
• A Likely Redback Millisecond Pulsar Counterpart of 3FGL J0838.8-2829
• High-Energy Variability of PSR J1311-3430
• Optical spectroscopic observations of gamma-ray blazar candidates. IX. Optical archival spectra and further observations from SOAR and OAGH
• Multi-wavelength observations of a new redback millisecond pulsar candidate: 3FGL J0954.8-3948
• The $\gamma$-ray sky seen at X-ray energies I. Searching for the connection between X-rays and $\gamma$-rays in Fermi BL Lac objects
• Discovery of Gamma-Ray Orbital Modulation in the Black Widow PSR J1311-3430
• Multi-wavelength observations of the black widow pulsar 2FGL J2339.6-0532 with OISTER and Suzaku
• A puzzling 2-hour X-ray periodicity in the 1.5-hour orbital period black widow PSR J1311-3430
• Crab pulsar spectrum: a non-extensive approach
• Late-time evolution of ultracompact X-ray binaries
• Formation of the planet orbiting the millisecond pulsar J1719-1438
• Radio counterparts of gamma-ray pulsars