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Abstract <p>We searched for very-high-energy (VHE, <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(E&gt;100\)</EquationSource> <!--BPhysMGU2570156Kovankin-m1--> </InlineEquation> GeV) signals from weak sources and bright transients in 16 years of Fermi–LAT data at <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(|b|&gt;10^{\circ}\)</EquationSource> <!--BPhysMGU2570156Kovankin-m2--> </InlineEquation> using the DBSCAN algorithm. We identified 107 clusters with <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\geq 5\)</EquationSource> <!--BPhysMGU2570156Kovankin-m3--> </InlineEquation> events and 114 transient doublets (duration <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\leq 3\)</EquationSource> <!--BPhysMGU2570156Kovankin-m4--> </InlineEquation> 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<InlineEquation ID="IEq5"> <EquationSource Format="TEX">\({}^{1}\)</EquationSource> <!--BPhysMGU2570156Kovankin-m5--> </InlineEquation> (five are associated with 3FHL catalog sources and one is ‘‘orphan’’). Some flares reachied isotropic VHE luminosities of <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(10^{47}\)</EquationSource> <!--BPhysMGU2570156Kovankin-m6--> </InlineEquation> erg s<InlineEquation ID="IEq7"> <EquationSource Format="TEX">\({}^{-1}\)</EquationSource> <!--BPhysMGU2570156Kovankin-m7--> </InlineEquation>. No systematic HE (<InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(E&gt;10\)</EquationSource> <!--BPhysMGU2570156Kovankin-m8--> </InlineEquation> 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.</p>

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

  • 期刊论文

  • 发表日期：

    2025年12月1日

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  • A. S. Kovankin,
  • M. S. Pshirkov

摘要

Abstract

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 \(|b|>10^{\circ}\) using the DBSCAN algorithm. We identified 107 clusters with \(\geq 5\) events and 114 transient doublets (duration \(\leq 3\) 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.