Searching for $\gamma$-ray Counterparts to Very Faint X-Ray Transient Neutron Star binaries

TL;DR

This study used Fermi LAT data to search for gamma-ray counterparts of very faint X-ray transients, aiming to identify potential transitional millisecond pulsars, but found no significant gamma-ray detections.

Contribution

The paper provides the first systematic gamma-ray analysis of 12 VFXTs, setting upper limits and constraining their possible nature as transitional MSPs.

Findings

No gamma-ray counterparts detected for the 12 VFXTs.

Luminosity upper limits are comparable to known transitional MSPs.

Results suggest VFXTs are unlikely to contain rotation-powered pulsars.

Abstract

Very faint X-ray transients (VFXTs) are a group of X-ray binaries with low luminosities, the peak X-ray luminosities during their outbursts being only 10$^{34}$--10$^{36}$ erg\,s$^{-1}$. Using the $\gamma$-ray data obtained with the Large Area Telescope (LAT) onboard {\it Fermi Gamma-Ray Space Telescope}, we investigate their possible nature of containing rotation-powered pulsars, or more specifically being transitional millisecond pulsars (MSPs). Among more than 40 known VFXTs, we select 12 neutron star systems. We analyze the LAT data for the fields of the 12 VFXTs in 0.2--300 GeV energy range, but do not find any counterparts likely detected by {\it Fermi}. We obtain the luminosity upper limits for the 12 sources. While the distances to the sources are largely uncertain, the upper limits are comparable to the luminosities of two transitional systems PSR J1023$-$0038 and XSS J12270$-$4859. From our study, we conclude that no evidence is found at $\gamma$-rays for the suggestion that some of VFXTs could contain rotation-powered MSPs (or be transitional MSP systems).