On the Gamma-ray Efficiency of Superluminous Supernovae: Potential Detections and Population-Level Constraints
Milena Crnogorčević, Tim Linden, Ariel Goobar, Brian D. Metzger
Superluminous supernovae (SLSNe) are among the most energetic stellar explosions, yet their central power source remains uncertain. Models invoking magnetar spin-down or circumstellar interaction predict GeV gamma-ray emission once the ejecta becomes transparent to high-energy photons. We search for such emission from 223 hydrogen-poor SLSNe using 17 years of Fermi-LAT data, defining source-specific search windows based on the Bethe--Heitler transparency time. We find no significant (≥5σ) GeV emission. A joint-likelihood analysis constrains the GeV-to-optical efficiency to η < 1.3×10-3, two orders of magnitude below the predictions for weakly magnetized magnetar nebulae. A hierarchical population analysis shows that fewer than 0.7% of SLSNe-I can have η > 10-2. SN 2017egm, however, shows a suggestive excess (~4σ), with Lγ/Lopt ~ 0.68 exceeding hadronic expectations by over an order of magnitude, favoring a magnetar origin. The non-detection of the similarly nearby SN 2018bsz disfavors simple uniform-efficiency scenarios, potentially pointing to diversity in the underlying powering mechanisms.
Are X-Ray Detected Active Galactic Nuclei in Dwarf Galaxies Gamma-Ray Bright?
Milena Crnogorčević, Tim Linden, Annika H. G. Peter
Black holes don't just live in massive galaxies---many dwarf galaxies also host smaller "intermediate-mass" black holes, which could be the seeds that eventually grow into the giants we see at the centers of galaxies today. My collaborators and I used 15 years of observations from NASA's Fermi Gamma-ray Space Telescope to look for high-energy gamma-ray emission from active black holes in 74 dwarf galaxies identified through X-ray studies from eROSITA. While none of the galaxies showed strong gamma-ray signals on their own, our combined analysis revealed a small hint of very soft gamma-ray emission that may be linked to black hole mass. Although this result falls short of a firm detection, it provides the first constraints on gamma-ray activity in dwarf galaxies and opens the door to future discoveries with next-generation telescopes.
First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
Tim Linden, Jung-Tsung Li, Bei Zhou, Isabelle John, Milena Crnogorčević, Annika H. G. Peter, John F. Beacom
The Sun isn't just a source of optical light---it also shines in high-energy gamma rays when cosmic rays interact with its atmosphere and with sunlight. We analyze 15 years of data from NASA’s Fermi Gamma-ray Space Telescope to map the faint “solar halo,” created when cosmic-ray electrons scatter sunlight into gamma rays. We detected this halo all the way out to 45° from the Sun, and across energies from 30 MeV to 100 GeV---the first such measurement spanning an entire solar cycle. Our results reveal how the magnetic fields modulate cosmic rays, opening a new window into both space weather and fundamental physics near the Sun.