Unprecedented gamma-ray flare from PG1553+113 in spring 2012.

PG1553 is a well-known TeV blazar, which redshift is still uncertain (z>0.4 [1]). The source has been monitored in Very High Energy (VHE, E>100 GeV) gamma-rays by MAGIC telescopes since February 2005, and detected on a regular basis in a quiescent state with modest flux variations, lying in the range from 4% to 11% of the Crab Nebula flux above 150 GeV [2]. In March and April 2012, strong VHE gamma ray activity was detected with the MAGIC telescopes. The flux state reached unprecedented level of the Crab Nebula flux for PG 1553+113 [3]. A multiwavlength campaign during the flaring episodes has been carried out in radio by Metsa¨hovi and OVRO, in optical by KVA telescope, in optical-UV by Swift/UVOT, in X-rays by Swift/XRT, in infrared by REM telescope and in gamma-rays by Fermi/LAT. In addition, optical polarization measurements were carried out by Liverpool telescope. The multiwavelength behavior is particularly interesting, since the source was active in most of the bands. [ABSTRACT FROM AUTHOR]