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Discovery of Very High Energy Gamma-Ray Emission from the intermediate BL Lac S2 0109+22 with the MAGIC telescopes

The MAGIC collaboration reports the discovery of very high energy (VHE ; E>100 GeV) gamma-ray emission from the BL Lac S2 0109+22 (RA: 01h12m05.8s Dec:+22d44m39s, J2000). The object was observed with the MAGIC telescopes for 5.3 hours from 2015/07/22 to 2015/07/25, following the high-flux state spotted in the high-energy (E>10 GeV) domain of the public Fermi/LAT data, according to the prescription of Pacciani et al. ApJ 790 (2014) 45. The preliminary analysis of the first three nights of MAGIC data showed an excess with a statistical significance of ~5 standard deviations. The VHE flux of this detection was estimated to be (1.6+/-0.7)e-11 ph/cm2/s above 100 GeV, about 3% of the flux from the Crab nebula. The daily flux shows a marked enhancement on the night of 25 July up to (9.7+/-1.5)e-11 ph/cm2/s, ~15% of the Crab flux at E>100 GeV, corresponding to an excess with >7 sigma statistical significance. S2 0109+22 is located at the red shift z=0.265 (from CGRaBS spectroscopy, Healey et al., ApJ 175 (2008) 97). It is classified as an intermediate-synchrotron peaked BL Lac object in the 2LAC (Ackermann et al. ApJ 743 (2011) 171). The multi-year optical and radio historical coverage (Ciprini et al. MNRAS 348 (2004) 1379) showed optical variations with typical timescales from weeks to months. The Fermi/LAT flaring activity triggered a ToO observation with Swift, started on 2015/07/21. The preliminary Swift/XRT analysis showed an unabsorbed flux (7.0+-0.3)E-12 erg/cm2/s in the 0.3-10 keV range. From archival Swift/XRT data the brightest state previously observed had a flux (2.5+-0.3)E-12 erg/cm2/s. This corresponds to a 2-3 fold flux enhancement in X-rays. Instead the optical (UV) flux is ~50% (~30%) lower than at the brightest state observed with Swift/UVOT. MAGIC observations on S2 0109+22 will continue during the next days until July 28 and multi-wavelength observations are encouraged. The MAGIC contact persons for these observations are A. Stamerra (INAF-SNS, stamerra@oato.inaf.it) and R. Mirzoyan (MPI, Razmik.Mirzoyan@mpp.mpg.de) MAGIC is a system of two 17m-diameter Imaging Atmospheric Cherenkov Telescopes located at the Canary island of La Palma, Spain, and designed to perform gamma-ray astronomy in the energy range from 50 GeV to greater than 50 TeV.

Gamma Ray ; TeV ; VHE ; AGN ; Blazar

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