The First GeV Gamma-Ray Flares from the CSO-like Source 4C 76.03

Abstract

We report the first detection of GeV gamma-ray flaring activity from the compact symmetric object (CSO)-like source 4C 76.03, based on 17 years of Fermi-LAT observations. Its long-term, time-averaged gamma-ray properties are consistent with the 4FGL-DR4 catalog. However, a time-resolved analysis with 100-day binning reveals two prominent flares occurring on timescales of approximately 30 days and 20 days, separated by about 2.5 years, with nearly identical fluxes, test statistic (TS) values, and photon indices. The short-timescale variability indicates localized and transient energy dissipation in the nuclear region, likely associated with newly injected jet components. Although the gamma-ray emission does not directly trace the long-term jet power responsible for building the observed radio structure, it demonstrates that the central engine remains active. In the context of CSO evolution, 4C 76.03 may represent a rare transitional case, where repeated energy injections allow the source to exceed the canonical 500 pc scale of most CSOs, providing key insight into the early stages of radio jet evolution.

Figures (6)

• Figure 1: Upper panel: 100-day binned $\gamma$-ray light curve of the target source, covering MJD 54732.6 to MJD 60932.6 (approximately 17 years of Fermi-LAT observations). Blue circles represent the $\gamma$-ray flux measurements, red triangles indicate flux upper limits (TS $<$ 10), and the semi-transparent red bars show the corresponding TS values. The green horizontal line indicates the 17-year long-term average flux. The black dashed lines highlight a possible minor flaring episode during MJD 58632.6--58732.6. The lower portion shows the WISE W1 and W2 band light curves of the source. Bottom panels:Left: Zoomed-in 10-day binned view of the 100-day binned $\gamma$-ray light curve for the first flare (MJD 55532.6--55632.6). Right: Zoomed-in 10-day binned view of the 100-day binned $\gamma$-ray light curve for the second flare (MJD 56532.6--56632.6).
• Figure 2: Upper panels: Smoothed $\gamma$-ray TS maps in the 100 MeV--300 GeV energy range, with the target source excluded from the background model. Left panel: TS map derived from 30 days of Fermi-LAT data corresponding to the first flare (MJD 55602.6--55632.6). The magenta cross and circle indicate the best-fit position of the target $\gamma$-ray source and its 95% C.L. localization error circle, respectively. The black X marks the radio position of 4C 76.03, while the green cross and circle indicate the best-fit position of the target $\gamma$-ray source and its 95% C.L. localization error circle obtained from the $\sim$17-yr data set. Right panel: TS map derived from 20 days of Fermi-LAT data corresponding to the second flare (MJD 56532.6--56552.6). The symbols have the same meanings as in the left panel. Bottom panel: Residual TS map based on $\sim$17-yr of Fermi-LAT data, with the target source included in the model. The symbols have the same meanings as in the upper panels.
• Figure 3: The $\gamma$-ray SED of the target source in the 100 MeV--300 GeV energy range. Circular data points represent the energy flux, while triangles indicate upper limits on the energy flux (TS $< 10$). The dashed line shows the best-fit model, and the shaded region denotes the $1\sigma$ uncertainty range. The purple and green colors correspond to two different flaring periods, while the gray color represents the results from approximately 17 years of data.
• Figure 4: Comparison of the projected linear sizes and 1.4 GHz luminosities. Green points represent 17 CSO-2s, while the red point denotes the CSO2-like source 4C 76.03. The 1.4 GHz data are adopted from 1992ApJS...79..331W, and the sizes are taken from 2024ApJ...961..241K.
• Figure 5: Left panel: The relationship between the $\gamma$-ray luminosity and the $\gamma$-ray photon index for jetted AGNs with known redshifts included in the 4LAC-DR3 catalog 2022ApJS..263...24A. Different classes of jetted AGNs are represented by different symbols: FSRQs (orange points), BL Lacs (gray points), radio galaxies (blue points), and CSS sources (cyan points). The long-term averaged state (green star) and the short-term flaring state (red star) of 4C 76.03 are also shown. All $\gamma$-ray luminosities in the figure are K-corrected. Right panel: The $\gamma$-ray luminosity versus radio luminosity diagram. The 1.4 GHz radio data are taken from the NRAO VLA Sky Survey (NVSS; 1998AJ....115.1693C) and the Faint Images of the Radio Sky at Twenty Centimeters (FIRST; 2015ApJ...801...26H). We did not include DA 362 in our figures, as the origin of its redshift remains uncertain, which could introduce ambiguity in the interpretation 2025ApJ...979...97S. We also do not show JVAS J1311+1658, as its redshift is currently unknown 2026ApJ...999..182J. Similarly, 4C +39.23B is not shown because its $\gamma$-ray emission is likely contaminated by a flaring source located $\sim$0.1$^\circ$ away 2022ApJ...927..221G.