Description
Since its launch in 2008, Fermi-LAT has been uncovering a new nearby population of compact
binary millisecond pulsars, also known as “spiders”. Expanding the currently known spider population
holds the key to finding the fastest-rotating and most massive neutron stars, which have
crucial implications for nuclear physics and gravitational wave astronomy. Although many of
these systems are found via radio follow-up of Fermi sources, searching for the variable optical
emission of their companions has also proven successful, and holds potential for revealing a hidden
population of radio-obscured spiders.
We present the discovery of nine variable optical counterparts to pulsar-like Fermi unidentified
sources, which we identify as new spider candidates. Their optical light curves show flux modulation
larger than 0.2 mag in the sub-day period range and effective temperatures of 4000−6000 K, all
properties consistent with the known redback subtype of binary millisecond pulsars. Among our
newly discovered candidates, 4FGL J0736.9-3231, if confirmed as millisecond pulsar, would be the
closest known spider to Earth (D=659 pc, from Gaia-DR3 parallax). Unlike the mildly irradiated
companion in the previous system, 4FGL J1639.3-5146—one of our most promising candidates—
shows strong irradiation, with temperature variations up to 1500 K. We also find an X-ray source
at the location of another redback candidate, 4FGL J1748.8-3915, which shows direct evidence of
the intrabinary shock between the pulsar and its companion’s winds. These results facilitate radio
observations of these candidates currently being acquired with the GBT, but also enable neutron
star mass estimates via optical light curve modelling, supported by upcoming GTC and VLT spectroscopic
follow-ups.