Very high energy $\gamma$-ray emission from RBS 0679

TL;DR

This paper reports the detection of very high energy gamma-ray emission from the BL Lac object RBS 0679 using Fermi LAT, revealing variability in gamma-ray flux and spectral index, and highlighting the source's potential for further ground-based observations.

Contribution

First detection of VHE gamma-ray emission from RBS 0679 with Fermi LAT, demonstrating its variability and spectral characteristics over 5.3 years.

Findings

RBS 0679 emits VHE gamma rays at 6.9 sigma significance.

Gamma-ray flux and spectral index are variable with a softer-when-brighter trend.

Detected VHE photons are consistent with the average spectral index, not associated with spectral hardening.

Abstract

In this paper we report the \textit{Fermi} Large Area Telescope (LAT) detection of Very High Energy (VHE; $E_{\gamma}>100$ GeV) $\gamma$-ray emission from the BL Lac object RBS 0679. $5.3$ years of LAT observations revealed the presence of three VHE photon events within 0.1 degrees of RBS 0679, with a subsequent unbinned likelihood analysis finding RBS 0679 to be a source of VHE photons at $6.9$ standard deviations ($\sigma$). An unbinned likelihood analysis of the $0.1-100$ GeV data, binned in 28-day periods, finds both the flux and spectral index to be variable, with a `softer-when-brighter' trend in the global $\gamma$-ray characteristics. On the other hand, the 28-day periods in which the VHE photons were detected have spectral indices that are consistent with the $5.3$ year average suggesting that the observed VHE emission is not associated with a spectral hardening event. The discovery of RBS 0679 in the $100-300$ GeV energy range, combined with the non-detection above 390 GeV with the H.E.S.S. telescope array, suggest RBS 0679 to be an intriguing source that requires further follow-up observations with ground-based $\gamma$-ray observatories.