Linking Burst-Only X-Ray Binary Sources to Faint X-Ray Transients

Burst-only sources are X-ray sources discovered thanks to their bursting activity with no associated emission (at least with the monitoring instrument that led to their discovery). This bursting activity consists in one single short (tens of seconds to minutes) burst of X-ray emission, with spectral and timing properties consistent with thermonuclear (type I) bursts usually occurring on the surface of a neutron star. This likely provides a tight link between burst-only sources and neutron star X-ray binary transients. We carried out a series of snapshot observations of the entire sample of burst-only sources with the Swift satellite. We found a few sources in outburst and detected faint candidates, likely representing their quiescent counterparts. To provide a more comprehensive view, we analyzed data for three quasi-persistent faint X-ray binary transients, another sub-class closely related to burst-only sources. We discuss burst-only sources and quasi-persistent sources in the framework of neutron star transients, providing clues on their nature.