COBIPULSE: A Systematic Search for Compact Binary Millisecond Pulsars

We report here the results obtained from a systematic optical photometric survey aimed at finding new compact binary millisecond pulsars (also known as "spiders"): the COmpact BInary PULsar SEarch (COBIPULSE). We acquired multi-band optical images over one year around \(33\) unidentified Fermi-LAT sources, selected as pulsar candidates based on their curved GeV spectra and steady \(\gamma\)-ray emission. We present the discovery of four optical variables coinciding with the Fermi sources 3FGL J0737.2\(-\)3233, 3FGL J2117.6\(+\)3725 (two systems in this field) and 3FGL J2221.6\(+\)6507, which we propose as new candidate spider systems. Indeed, they all show optical flux modulation consistent with orbital periods of \(0.3548(5) \ \mathrm{d}\), \(0.25328(6) \ \mathrm{d}\), \(0.441961(2) \ \mathrm{d}\), and \(0.165(4) \ \mathrm{d}\), respectively, with amplitudes \(\gtrsim 0.3 \ \mathrm{mag}\) and colors compatible with companion star temperatures of \(5000\)--\(6000 \ \mathrm{K}\). These properties are consistent with the "redback" sub-class of spider pulsars. If confirmed as a millisecond pulsar, 3FGL J0737.2\(-\)3233 will be the closest known spider to Earth (\(D=659_{-20}^{+16} \ \mathrm{pc}\), from Gaia-DR3 parallax). We searched and did not find any X-ray sources matching our four candidates, placing \(3\sigma\) upper limits of \(\sim10^{31}\)--\(10^{32} \ \mathrm{erg} \ \mathrm{s}^{-1}\) (\(0.3\)--\(10 \ \mathrm{keV}\)) on their soft X-ray luminosities. We also present and discuss other multi-wavelength information on our spider candidates, from infrared to X-rays.