First Detection of $γ$-Ray Emission from the Compact Symmetric Object JVAS J1311+1658

TL;DR

First detection of GeV gamma-ray emission from the compact symmetric object JVAS J1311+1658 is reported. Analyzing 16.4 years of Fermi-LAT data in the 0.1–300 GeV band reveals a 100-day flare at MJD 60032.6–60132.6 with TS ≈ 43 (~$6.2σ$) and a photon index $Γ = 2.15 \\pm 0.19$; the 0.1–300 GeV flux is $(1.6 \\pm 0.6) imes 10^{-8}$ ph cm^-2 s^-1. The gamma-ray emission cannot be explained by IC scattering in the radio lobes, pointing instead to emission from newly launched sub-kpc-scale jets at the core, given the strong flux and ~100-day variability. The work places JVAS J1311+1658 among a small set of gamma-ray–emitting CSOs and underscores the role of early jet activity in the evolution of young radio galaxies, motivating continued multiwavelength monitoring.

Abstract

We report the first detection of $γ$-ray emission from the young radio galaxy JVAS~J1311+1658, classified as a compact symmetric object (CSO). This detection is characterized by a recent GeV $γ$-ray flare identified in Fermi-LAT data during MJD~60032.6--60132.6, with a $γ$-ray source detected at a significance level of $\sim6.2σ$. The average 0.1--300~GeV flux is measured to be $(1.6 \pm 0.6)\times10^{-8}\,\mathrm{ph\,cm^{-2}\,s^{-1}}$, with a photon spectral index of $Γ= 2.15 \pm 0.185$. We find that a radiative model of the radio lobes significantly underestimates the observed $γ$-ray emission. The strong flux and short-term variability over $\sim$100 days suggest that the emission likely originates from newly launched sub-kiloparsec-scale jets at the core. This detection provides a unique window into the extreme environments and early-stage jet activity of young radio galaxies, offering insights into their initial evolution and the formation of relativistic jets in the earliest phases of galaxy growth.

Figures (7)

• Figure 1: Upper panels: Smoothed 100 MeV--300 GeV $\gamma$-ray TS maps of a $1^{\circ} \times 1^{\circ}$ region with a pixel size of $0.01^{\circ}$, where the target source is excluded from the background model. Left panel: TS map derived from the first 14 years of Fermi-LAT data (MJD 54683--59796), during which no significant $\gamma$-ray emission is detected. Right panel: TS map corresponding to the 100-day $\gamma$-ray flaring interval (MJD 60032.6--60132.6). The magenta cross and circle indicate the best-fit position of the $\gamma$-ray source and its 95% confidence-level localization uncertainty, respectively. The green cross marks the radio position of JVAS J1311+1658. Bottom panel: Same as the upper right panel, but showing the TS map in the 1--300 GeV energy range for the flaring interval (MJD 60032.6--60132.6).
• Figure 2: Upper panel:$\gamma$-ray light curve of the target source obtained with 100-day time bins, covering the period from MJD 54733 to MJD 60683 (approximately 16.4 years of Fermi-LAT observations). Blue circles denote the measured $\gamma$-ray fluxes, red triangles indicate 95% C.L. flux upper limits (TS $<10$), and the semi-transparent red bars represent the corresponding TS values. Bottom panels:Left: zoomed-in view of the same light curve using 50-day time bins. Right: zoomed-in view using 10-day time bins. The time interval marked by the black dashed lines corresponds to the 100-day $\gamma$-ray flaring episode.
• Figure 3: A zoomed-in view of the 50-day binned $\gamma$-ray light curve, shown together with several known Fermi-LAT sources in the vicinity of the target.
• Figure 4: The $\gamma$-ray spectral energy distribution (SED) of the target source in the 100 MeV–300 GeV range. The black points represent the energy flux, the red triangles indicate the upper limits (TS $<$ 10), the black dashed line shows the best-fit model, and the gray shaded area denotes the 1$\sigma$ uncertainty range.
• Figure 5: Left panel: Redshift versus WISE W1-band luminosity diagram. The red points represent CSOs with detected $\gamma$-ray emission, while the blue points denote those without $\gamma$-ray detection. Except for DA 362, the redshifts of the CSOs are taken from the spectroscopic measurements reported by 2024ApJ...961..240K2024ApJ...961..242R. However, since the redshifts of JVAS J1311+1658 and JVAS J0855+5751 are incorrect, these two sources are excluded from the plot. For comparison, DA 362 is included, although the reliability of its redshift remains uncertain. Right panel: Redshift distribution of CSOs. The red histogram represents the redshift distribution of CSO 1s, while the black histogram shows the overall distribution of all CSOs (including CSO 1 and CSO 2). All data are taken from 2024ApJ...961..242R. Similarly, JVAS J1311+1658 and JVAS J0855+5751 are excluded from the plot due to their incorrect redshift values.