Investigating off-pulse gamma-ray emission from millisecond pulsars

Lei, Mingyu

Significant off-pulse emission has been detected in some millisecond pulsars (MSPs)， and exploring their physical origin could greatly benefit the study of radiation mechanisms. Motivated by this， we report our analysis of 15 years of Fermi-LAT data for 35 MSPs， focusing particularly on the emission in the off-pulse phase. Twelve MSPs exhibit substantial off-pulse emission (TS ≥ 25)， and eight of them show a significant cutoff (TScutoff ≥ 9) in their energy spectra， suggesting that the emission originates from the magnetosphere. The remaining four MSPs have power-law shaped energy spectra (TScutoff < 9) and point-like spatial distributions (TSext < 16). To investigate whether the emission originates from other radiation processes， such as ICS of electrons， we added an extra power-law component to the original model and refitted the data for these four MSPs. No significant ICS component was found for any of them， suggesting that the emission is also of magnetospheric origin. This approach provides a new way to discern the origin of pulsar radiation， and the detection of significant off-pulse emission from the magnetosphere can serve as a powerful discriminator for emission models. Moreover， we obtained the phase-resolved spectra of PSR J0614−3329， the brightest source in our samples， to provide another important diagnostic for model comparisons. Finally， we investigated high-energy emission above 25 GeV and found that only two MSPs， J0614−3329 and J1536−4948， exhibit high TS values.