Sub-Threshold Fermi-LAT Sources in the Vicinity of KM3-230312A

TL;DR

The KM3-230213A neutrino event motivates a search for accompanying GeV–TeV gamma-ray emission in Fermi-LAT data to reveal potential cascades or echoes. The authors analyze two LAT windows, a ~2-year post-event period and the full ~17-year archive, using TS maps and localization to identify uncatalogued sub-threshold sources within $3.5^{\circ}$. They find three such sources: a transient J0616.1-0428 and two persistent sources J0614.6-0731 and J0621.1-0610, with plausible counterparts suggesting microquasar or BL Lac associations, but none providing a compelling, unique link to the neutrino. The results indicate that delayed GeV–TeV signatures from cascades or echoes remain a possibility and emphasize the need for coordinated multiwavelength follow-up to constrain the neutrino origin and the ambient magnetic-field environment.

Abstract

The KM3NeT collaboration has recently reported the detection of an extraordinary ultra-high-energy neutrino event with an energy of 220 PeV. Ultrahigh energy neutrinos and gamma-rays are co-produced in ultrahigh energy cosmic-ray interactions. The ultrahigh energy gamma-rays produced alongside the KM3NeT neutrino may quickly cascade down to lower energies due to interactions with intergalactic photons and magnetic fields. Because of this, the KM3NeT neutrino could be accompanied by an observable GeV - TeV gamma-ray signal. We investigate the data collected by the Large Area Telescope (LAT) on-board the \textit{Fermi} Gamma-ray Space Telescope for transient and sub-threshold gamma-ray sources in the vicinity of the KM3NeT neutrino. We find three sub-threshold sources with TS $\gtrsim 16$ within $3.5^{\circ}$ of the neutrino event not included in any existing \textit{Fermi}-LAT catalogs. One of the three, J0616.1-0428, is a transient sub-threshold gamma-ray source that appears only after the neutrino observation, but may be the unrelated flaring of a nearby microquasar. Another sub-threshold source, J0621.1-0610, also exhibits fluctuations in gamma-rays immediately following the neutrino observation, and may be coincident with a radio blazar. We note that the number of sub-threshold sources observed around the KM3NeT neutrino could be expected at another sky region of the same Galactic latitude, and that the fluctuations they exhibit appear to be consistent with background.

Figures (3)

• Figure 1: Left panel: Fermi-LAT TS map (1 GeV -- 1 TeV) of the $10^{\circ} \times 10^{\circ}$ region centered on KM3-230213 using 17 years of Fermi LAT data prior to the neutrino detection showing the sub-threshold sources J0614.6-0731 and J0621.1-0610. Right panel: TS map generated using LAT data from the time of the neutrino detection to slightly more than 2 years after the event showing the transient sub-threshold gamma-ray source J0616.1--0428, which appears only after the observation of KM3-230213A. The large concentric circles indicate the 68%, 90%, and 99% error circles for the localization of KM3-230213A. In both the TS Maps, the circled sources are excluded from the model to highlight their emission. Both TS maps are fit assuming a point source model with a spectral index $\Gamma = 2$.
• Figure 2: Light curves for each of the sub-threshold sources discussed in this work. The light curves are produced in the 1 GeV - 1 TeV energy range. Within each time bin, an errorbar is placed if TS $\geq 4$, while a $2\sigma$ upper limit is estimated when TS $> 4$. Each time bin is $\sim 300$ days wide. The vertical dashed line indicates the observation time of the KM3NeT neutrino KM3-230213A. Both J0616.1-0428 and J0621.1-0610 appear to fluctuate immediately after the observation of KM3-230213A, while J0614.6-0731 only has upward fluctuations years before the neutrino observation. Note that for a sub-threshold source, fluctuations above $2\sigma$ are not inconsistent with statistical variation and should not be considered as an accurate proxy of the source's activity.
• Figure 3: Spectral energy distributions (SEDs) for the three sub-threshold sources uncovered in this analysis.