Optical Follow-Up of Two Spider Pulsar Candidates: An Irradiated Redback and a Possible Double Giant Impostor

Abstract

Compact binary millisecond pulsars (nicknamed "spiders") are a growing class of neutron stars: more than 100 systems are known in the Galactic field, including more than 30 candidates where pulsations have not been detected. Here we report the results of an optical follow-up of two spider pulsar candidates: 2FGL J0846.0+2820 (J0846, a huntsman candidate in an 8.1 d orbit) and 4FGL J0935.3+0901 (J0935, with a 2.4 h orbital period and proposed as a spider of either the transitional, black widow or redback sub-class). We also analyzed their multi-wavelength properties (infrared, X-ray and $γ$-ray bands), in order to assess their nature and current state. The optical orbital light curves of J0846 change from double-peaked in 2023 to single-peaked in 2024, but we argue they are not consistent with either irradiation-driven or ellipsoidal orbital modulation. Its spectral energy distribution shows evidence for two blackbody-like components with similar temperatures ($T$) and radii ($R$), which dominate the optical ($T_2\simeq6000$ K, $R_2\simeq1.8$ R$_\odot$) and infrared ($T_1\simeq3400$ K, $R_1\simeq5.2$ R$_\odot$) bands. Based on this, we argue that J0846 is not a bona fide pulsar candidate and propose an alternative identification: a detached double red giant. In J0935, we find a light curve with one flux maximum around superior conjunction of the companion, where the colors and inferred temperature increase, suggesting prominent irradiation from the pulsar. We place constraints on its companion base temperature ($4000\!-\!4600$ K), which support one of the three classifications proposed for J0935: a redback spider in the pulsar state.