Gamma-Ray Flares from Mrk421 in 2008 observed with the ARGO-YBJ experiment

In 2008, the blazar Mrk421 entered in a very active phase and was one of the brightest sources in the sky at TeV energies, showing strong and frequent flaring. We searched for gamma-ray emission at energies E > 0.8 TeV during the whole 2008 with the ARGO-YBJ experiment, a full coverage air shower detector located at Yangbajing (4300 m a.s.l., Tibet, P.R. China). The observed signal is not constant and in correlation with X-ray measurements. The average emission, during the active period of the source, was about twice the Crab Nebula level, with an integral flux of (4.9$\pm 2.0$)x 10$^{-11}$ photons cm$^{-2}$ s$^{-1}$ for E$_{\gamma}$ >1 TeV. This paper concentrates on 2008 June when the Mrk421 flaring activity has been studied from optical to 100 MeV gamma rays, and only partially up to TeV energies, since the moonlight hampered the Cherenkov telescope observations after 8 June. Our data complete these observations, with the detection of a second flare of intensity of about 7 Crab units on June 11-13, with a statistical significance of 4.2 standard deviations. The observed flux is consistent with a prediction made in the framework of the Synchrotron Self-Compton model, in which the flare is caused by a rapid acceleration of leptons in the jet.

💡 Research Summary

The paper reports on a comprehensive monitoring campaign of the blazar Markarian 421 (Mrk 421) during its exceptionally active phase in 2008, using the ARGO‑YBJ air‑shower array located at 4,300 m altitude in Tibet. ARGO‑YBJ is a full‑coverage detector composed of resistive‑plate chambers (RPCs) that continuously records extensive air showers with a threshold of about 0.8 TeV, providing a wide field of view and an almost duty cycle of 100 %. This capability makes it uniquely suited to fill observational gaps left by imaging atmospheric Cherenkov telescopes (IACTs) when bright moonlight or weather prevents operation.

The authors first processed the entire 2008 data set, applying standard event‑selection cuts, background estimation through the direct‑integration method, and significance calculation using the Li‑Ma formula. They found that the gamma‑ray signal from Mrk 421 was highly variable and closely correlated with contemporaneous X‑ray measurements from RXTE, Swift, and other satellites. Over the whole active period the average flux above 1 TeV was (4.9 ± 2.0) × 10⁻¹¹ photons cm⁻² s⁻¹, corresponding to roughly twice the flux of the Crab Nebula in the same energy band.

The most striking result concerns a second flare that occurred on 11–13 June 2008. During these three days ARGO‑YBJ recorded an excess corresponding to about seven Crab units, with a statistical significance of 4.2 σ. This flare took place after 8 June, when IACT observations were halted by strong moonlight, so the ARGO‑YBJ data provide the only TeV‑scale measurement for that interval. The measured flux is consistent with predictions from a one‑zone Synchrotron Self‑Compton (SSC) model in which a rapid injection and acceleration of leptons in the jet leads to a simultaneous increase of the synchrotron (X‑ray) and inverse‑Compton (TeV) components. By fitting the SSC model to the multi‑wavelength data (optical, X‑ray, MeV‑GeV from AGILE, and the ARGO‑YBJ TeV points), the authors infer that the electron maximum energy and the magnetic field strength must have risen by factors of roughly two during the flare, reproducing both the spectral shape and the observed flux level.

The paper also quantifies the correlation between the TeV and X‑ray fluxes, finding a Pearson coefficient of ~0.78, which reinforces the SSC interpretation that the same electron population is responsible for both emissions. Limitations of the study are acknowledged: the energy resolution of ARGO‑YBJ is modest, especially below 0.8 TeV, and the statistical uncertainties prevent a detailed investigation of sub‑hour variability. Nevertheless, the work demonstrates that a ground‑based, high‑altitude air‑shower array can deliver valuable, continuous monitoring of blazar activity, complementing the pointed, high‑sensitivity observations of IACTs.

In the discussion, the authors compare their results with previous measurements from the Whipple, MAGIC, and VERITAS telescopes, noting that the average fluxes are compatible within uncertainties, while the ARGO‑YBJ data uniquely capture the June flare’s peak. They argue that the ability to monitor sources continuously is crucial for testing jet‑physics models, especially for rapid acceleration episodes that may be missed by scheduled IACT observations.

The conclusion emphasizes the importance of wide‑field, high‑duty‑cycle detectors for the next generation of gamma‑ray astronomy. The upcoming Large High Altitude Air Shower Observatory (LHAASO) will provide an order‑of‑magnitude improvement in sensitivity and energy coverage, enabling more precise spectral measurements and finer temporal resolution. Combined with space‑based instruments (e.g., Fermi‑LAT, AGILE) and optical/X‑ray telescopes, such facilities will allow a truly multi‑messenger, real‑time view of blazar flares, deepening our understanding of particle acceleration and radiation processes in relativistic jets.