First detection of VHE gamma-ray signal from the FSRQ TON 0599

TL;DR

The paper reports the first VHE gamma-ray detection of the FSRQ TON 0599 at $z=0.7247$ by MAGIC in 2017, revealing a flare with a cut-off around $E_c\approx50$ GeV that implies the emission region lies beyond the BLR. A joint MAGIC–Fermi-LAT analysis shows a smooth connection between HE and VHE spectra, with EBL-absorbed SEDs well fit by a power-law with exponential cut-off (EPWL). A comprehensive MWL campaign with Swift, WEBT, and optical spectroscopy demonstrates correlated variability across bands and provides constraints on the low-energy peak and BLR properties, while radio remains largely uncorrelated. The broadband modelling using a simple one-zone leptonic framework places the high-energy emission outside the BLR, dominated by EC scattering off torus photons, and highlights KN effects as a key factor shaping the observed cut-off; this yields insights into the location and physical conditions of the gamma-ray emitting region in this distant FSRQ.

Abstract

TON 0599 (z=0.7247) belongs to the few flat spectrum radio quasars (FSRQs) detected in the very high energy (VHE, $E > 100$ GeV) gamma-ray band. Its redshift makes it currently one of the farthest VHE gamma-ray sources. It was detected for the first time with the MAGIC telescopes on 2017 December 15, and observed until December 29. The flux reached a maximum of about 50 per cent of the Crab Nebula flux above 80 GeV on the second night of observation, after which we witnessed a gradual decrease of the flux. The VHE gamma-ray spectrum connects smoothly to the one in the high energy ($E > 100$ MeV) band obtained from simultaneous observations with Fermi-LAT. It features a cut-off at energies around 50 GeV, indicating the location of the gamma-ray emission zone beyond the broad line region. In addition, we were able to follow the spectral evolution during the fading phase of the flare. Multiwavelength analysis based on observations in optical, near-infrared, and radio bands acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from November to March, as well as observations in X-ray and optical$-$UV bands with instruments on board the Swift satellite, shows strong correlation between different bands. We model the broadband emission with a simple one-zone leptonic model, where the high-energy peak is predominantly produced by external Compton (EC) scattering of photons from the dusty torus.

Figures (8)

• Figure 1: Integrated flux measured by MAGIC above 80 GeV. Blue triangles represent upper limits for points whose relative uncertainties are greater than 0.5. Letters "A", "B", "C", and "D" mark the four periods, as described in the main text.
• Figure 2: Spectral energy distributions in the gamma-ray band for periods A through D. Full black circles are MAGIC measurements. Only points with excess significance greater than 2 are displayed. Black empty circles represent intrinsic (EBL de-absorbed) SED values. Blue butterflies represent Fermi-LAT SEDs, with parameters given in Table \ref{['tab:LAT_butterflies']}. Source intrinsic SEDs are represented with dashed red lines. The results of the fit are given in Table \ref{['tab:MAGIC-LAT_FitResults']}. Solid red lines represent the intrinsic spectra attenuated by the EBL absorption.
• Figure 3: From top to bottom: $B$, $V$, $R$, $I$ light curves (observed magnitudes) obtained with data from the WEBT collaboration (red circles) and Steward Observatory (black squares). Vertical dashed lines mark the beginning and the end of the MAGIC observations, vertical full lines mark the Swift pointings of 2017 December 15 (Period A) and 16 (Period B).
• Figure 4: Optical spectrum of TON 0599 acquired at TNG on 2018 February 22 with the Dolores instrument.
• Figure 5: MWL light curve around the MAGIC observations fitted with exponential function.