COBIPULSE: A Systematic Search for Compact Binary Millisecond Pulsars

Abstract

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 $\sim 10^{31}$--$10^{32}\mathrm{erg}{\mathrm{s}}^{-1}$ ($0.3$--$10\mathrm{keV}$) on their soft X-ray luminosities. We also present and discuss other multi-wavelength information on our spider candidates, from infrared to X-rays.

38 pages, 93 figures, 4 tables. This paper has been accepted for publication in ApJ

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