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Fermi-LAT, FACT, MAGIC and VERITAS detection of increasing gamma-ray activity from the high-energy peaked BL Lac object 1ES 1959+650

ATel #9010; S. Buson (NASA-GSFC), J. D. Magill (UMD/GSFC), D. Dorner (University of Wurzburg, FAU Erlangen), A. Biland (ETH Zurich), R. Mirzoyan (Max-Planck-Institute for Physics), R. Mukherjee (Barnard College, Columbia University) on behalf of the Fermi-LAT, FACT, MAGIC and VERITAS collaborations
on 30 Apr 2016; 00:54 UT
Credential Certification: Sara Buson (sara.buson@nasa.gov)

Subjects: X-ray, Gamma Ray, >GeV, TeV, VHE, AGN, Blazar

Referred to by ATel #: 9070, 9139, 9148, 9203, 9239

The Fermi-LAT, FACT, MAGIC and VERITAS collaborations report the detection of enhanced gamma-ray activity from a source positionally consistent with the very-high-energy peaked BL Lac object 1ES 1959+650 (a.k.a 3FGL J2000.0+6509, in the 3rd LAT source catalog, 3FGL, Acero et al. 2015, ApJS 218, 23) with radio coordinates (J2000) R.A.: 299.999384 deg, Dec.: 65.148514 deg (Beasley et al. 2002, ApJS, 141, 13). This source has a redshift z=0.047 (Schachter et al. 1993, ApJ, 412, 541).

Preliminary analysis indicates that on 2016 April 27, 1ES 1959+650 was significantly detected by the LAT with a daily gamma-ray flux (E>100MeV) of (0.2+/-0.1) x 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). The corresponding daily average photon index is 1.5+/-0.2 which is smaller than the average index of 1.88+/-0.02 reported in the 3FGL.

The MeV-GeV brightening coincides with the detection of the source in an enhanced state in the TeV band as observed by the FACT, MAGIC and VERITAS telescopes. Due to bright and varying moonlight conditions, the energy threshold of Cherenkov telescopes is increased (and their sensitivity decreased). All quoted results are based on online analyses and should be treated as very preliminary.

FACT observations on April 29, from 04:30 UTC until 05:20 UTC showed an average flux corresponding to about 1 Crab Unit, with a hint of brightening over time. The preliminary results can be seen at http://www.fact-project.org/monitoring/index.php?y=2016&m=04&d=28&source=7&timebin=3&plot=night

MAGIC observed the source for 1hr on 29 April 2016, 4:10-5:10 UTC. The preliminary analysis of the MAGIC data indicates a flux of at least 80% of the flux from the Crab Nebula (CN) above 1TeV. The flux is reported as a lower limit as it was not corrected for the effect of the observations under moonlight conditions. It implies an increase by a factor of at least 4 with respect to the 20% of the CN flux above 180GeV reported previously in Albert et al. 2006 ApJ, 639, 761.

VERITAS observed the source for about 2hrs under bright moonlight conditions on 2016 April 29, between 9:42 and 11:29 UTC. A preliminary analysis of the data shows a strong detection of the source with a statistical significance of ~40 standard deviations, and an average flux above 350GeV of approximately 1 x 10^-10 cm^-2 s^-1 (i.e. about the CN flux). A power-law fit to the gamma-ray spectrum between 0.35-5TeV is consistent with a spectral index of approximately 2.5, harder than the archival low spectral state (e.g. Aliu et al. 2014, ApJ, 797, 89).

Increasing gamma-ray flux from this source was previously reported by the Fermi-LAT and VERITAS Collaborations in 2015 October (see ATel #8148, ATel #8193). 1ES 1959+650 is one of the "LAT Monitored Sources" and consequently preliminary estimates of the daily and weekly gamma-ray flux observed by Fermi LAT are publicly available (fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/1ES_1959p650). Recently, the source reached its highest historical X-ray brightness as reported in ATel #8468. 1ES 1959+650 is regularly monitored by FACT, but due to high zenith angle, it is currently outside the standard observation parameters. The results from past online analyses are publicly available at http://www.fact-project.org/monitoring/index.php?y=2016&m=04&d=28&source=7&timebin=12&plot=all

The Fermi Gamma-ray Space Telescope began a Target of Opportunity observation on 29 April. Coordinated observations among Fermi-LAT, FACT, MAGIC, VERITAS and Swift are ongoing. In consideration of the current activity of this source, we encourage multiwavelength observations. For this source, the Fermi LAT contact person is S. Buson (sara.buson@nasa.gov), for FACT is D. Dorner (dorner@astro.uni-wuerzburg.de), for MAGIC is R. Mirzoyan (Razmik.Mirzoyan@mpp.mpg.de), for VERITAS is R. Mukherjee (muk@astro.columbia.edu).