The search for high energy gamma-ray emission from two white dwarf pulsar candidates Euve J0317-85.5 and J1912-4410 using fermi-LAT data

Abstract

The discovery of the first fast spinning, highly magnetic neutron star or "pulsar" (Hewish, Bell, Pilkington, Scott, and Collins (1968)) opened the doors to the possibility that fast spinning, highly magnetic white dwarfs could mimic the same behavior seen in these pulsars. Indeed, the first discovery of a binary "white dwarf pulsar" AR Scorpii by Marsh et al. (2016) confirmed that possibility. EUVE J0317-85.5 (J0317) is an isolated magnetic white dwarf which showed optical (Barstow et al. (1995)) and far-ultraviolet (FUV, Ferrario, Vennes, Wickramasinghe, Bailey, and Christian (1997)) pulsations at a spin period of 725 seconds (12 minutes) and its first harmonic ~362.9 seconds. This was also confirmed with new optical data from the BOOTES-6 and 7 robotic telescopes. Using archival X-ray data from ROSAT, possible soft X-ray pulsations were also found at the first harmonic. Observations from 𝘍𝘦𝘳𝘮𝘪-LAT revealed low-level γ-ray pulsations in the 0.5-10 GeV energy range at both the spin period 𝘗=724.65±0.54s at ~4.01σ (~3.72σ using the H-test Test Statistic) and its first harmonic 𝘗=362.65±0.29s at ~3.77σ (~3.61σ using H-test Test Statistic). The γ-ray light curve phase-folded on the rotation period was phase-aligned with the new BOOTES-6 and BOOTES-7 optical folded light curves using the same spin ephemeris suggesting a similar origin of the optical and γ-ray photons in J0317. 𝘍𝘦𝘳𝘮𝘪-LAT observations of the con firmed white dwarf pulsar J191213.72-441045.1 (J1912) revealed γ-ray pulsations in the 0.5-10 GeV energy range at a period 𝘗=319.99±0.35s at ~5.74σ (~4.73σ with H-test Test Statistic) or which the phase-folded γ-ray light curve is also in phase with the optical folded light curves using the same spin ephemeris provided by Pelisoli, Marsh, et al. (2023). In both of these systems, the γ-rays could be produced by the curvature radiation mechanism in which electrons slide along the magnetic field line emitting γ-rays modulated at the white dwarf rotation period.