Hunting for Weak VHE Sources and Bright Transients in 16-Years Fermi-LAT Data

We searched for very-high-energy (VHE, E>100 GeV) signals from weak sources and bright transients in 16 years of Fermi–LAT data at |b|>10^ using the DBSCAN algorithm. We identified 107 clusters with 5 events and 114 transient doublets (duration 3 days). Two clusters, associated with 4FGL J0039. 1–2219 and 4FGL J0212. 2–0219, represent new weak VHE BL Lacs, making them promising targets for next-generation IACTs. Six VHE flares lack counterparts in the TeVCat catalog ^1 (five are associated with 3FHL catalog sources and one is ‘‘orphan’’). Some flares reachied isotropic VHE luminosities of 10^47 erg s ^-1. No systematic HE (E>10 GeV) –VHE timing pattern is found: HE activity can lead, coincide with, or lag behind VHE emission, and some VHE flares lack any HE counterpart. Remarkably, the detection flares from Seyfert I galaxy NGC 1218 (3FHL J0308. 4+0408) and LINER NGC 5549 (the ‘‘orphan‘‘ flare) augments the small population of nonblazar AGN at VHE energies.