Scientists from the Institute of High Energy Physics recently announced that a strong gamma-ray flare was detected by the

Fig. 1 Left panel shows the significance map of Mrk 501 observed by the ARGO-YBJ experiment. The top-right panel shows the differential (upper) and accumulated (lower) light curves measured by SWIFT at 15-50 keV (x-rays). The bottom-right panel shows the similar light curves measured by the ARGO-YBJ experiment at energies above 300 GeV.

ARGO-YBJ experiment at energies above 300 GeV with a statistical significance of 6.1 standard deviations, from Active Galactic Nuclei (AGN) Mrk501

This flare started on Oct 17th 2011 and was still on-going, with an average flux approximately a factor of 3 stronger than the emission from the Crab Nebula (the standard candle in the northern sky). During the flare for a period of 10-days, the photon flux from Mrk501 was enhanced by a factor of 10 comparing with its long-term low states.

The ARGO-YBJ experiment, the largest scientific collaboration between CAS and INFN, is located at Yangbajing, Tibet, China, at 4300m a.s.l., which owns the most sensitive gamma-ray survey apparatus at very high energies (VHE) in the world due to its full-coverage detector array in air shower detections.

Since its operation started from July 2006, the ARGO-YBJ experiment has successfully detected 3 large VHE gamma ray flares from AGN Mrk421. For the first time in the world, such a long-term VHE observation of Mrk421 enabled a multi-wavelength analysis together with the gamma rays at GeV region, hard and soft x-rays.

This was the first time for the ARGO-YBJ experiment to detect VHE flares from Mrk501. No observation has been reported from Imagine Atmospheric Cherenkov Telescopes in this month-long flaring event because the source was in the light of the moon and the sun. This shed light on the irreplaceable role of the ARGO-YBJ experiment in modern VHE gamma ray astronomy due to its wide field of view, high duty cycle and low threshold.

As one of recommended major basic scientific research facilities to be built in next five years, the LHAASO detectors will be the next generation of the gamma ray astronomic survey facility, a factor of 30 more sensitive than ARGO-YBJ. The similar flaring phenomena could be well measured by LHAASO in minutes. This will enable a detailed investigation of transient phenomena such as AGNs and gamma ray bursts.

An AGN is a compact region at the centre of a galaxy which has a much higher luminosity than normal galaxies over the whole electromagnetic spectrum. The radiation from an AGN is believed to be a result of accretion of mass by the super-massive black hole at the centre of the host galaxy. High energy non-thermal radiation from X-ray to gamma-ray, relativistic jets and large outburst etc. are all observed associated with the AGNs. Hence AGNs are nature laboratories of high energy particles for their acceleration and radiative mechanism. VHE gamma-rays are important probes to realize the study with those labs.