Gamma-ray outburst detected from the radio source 3C 216

Using NASA’s Fermi space telescope, Italian astronomers have observed a radio source known as 3C 216. As a result, they detected increased gamma-ray activity from this source, including a strong outburst. The finding is reported in a research paper published on the arXiv preprint server.

3C 216 is an extragalactic radio source at a redshift of approximately 0.67, with a projected linear size of about 182,500 light years. It has an overall steep radio spectrum and a relatively compact morphology. Therefore, it is classified as a compact steep spectrum (CSS) object.

Previous observations of 3C 216 have found that it is a radio galaxy consisting of a central component surrounded by a more extended structure, and has an inner relativistic jet. It turns out that this galaxy is associated with gamma-ray source 4FGL J0910.0+4257.

In late 2022, gamma-ray activity of 3C 216 started to increase, which was detected by Fermi’s Large Area Telescope (LAT). Now, a team of astronomers led by Federica Giacchino of the National Institute for Nuclear Physics in Rome, Italy, presents the new results of the monitoring campaign of this source with LAT.

“The LAT observed a pronounced enhancement in the gamma-ray activity from the direction of the source on 2022-12-08 and on 2023-05-01,” the paper reads.

LAT monitoring of 3C 216 recorded a strong gamma-ray outburst of 3C 216 on May 1, 2023, when its daily-averaged gamma-ray flux increased by a factor of 177 relative to the average flux, while its photon index decreased from the average value of 2.57 to 2.11. These findings represent both the highest daily flux and the hardest photon index recorded for this source by Fermi to date.

Furthermore, the astronomers analyzed multiwavelength data of 3C 216, finding simultaneous flaring activity in optical, ultraviolet, X-ray and gamma-ray bands between May 1–9, 2023. This confirms the association of the gamma-ray source with 3C 216.

In general, the study found that the spectral energy distribution of the flaring activity of 3C 216 evolves in a coherent way. This finding points to a common origin for the multi-frequency emissions.

Summing up the results, the authors of the paper conclude that the flaring activity of 3C 216 can be interpreted as an outburst of synchrotron self-Compton (SSC) radiation. Such an outburst is likely associated with the acceleration of a distribution of charged particles within the jet, which subsequently cool down through radiative losses.

“Our results also confirm that the SSC process can well explain the production of gamma-ray outbursts from this type of radio source,” the researchers add.

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