Fermi LAT detection of increasing GeV gamma-ray activity from the high-energy peaked BL Lac object 1ES 1959+650

ATel #8193; Stefano Ciprini (ASDC Rome & INFN Perugia, Italy), on behalf of the Fermi Large Area Telescope Collaboration.
on 20 Oct 2015; 09:20 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)

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The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray emission from a source positionally consistent with the very-high energy peaked BL Lac object 1ES 1959+650 (also known as TXS 1959+650 and 3FGL J2000.0+6509, 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 1ES 1959+650 has been brightening in GeV gamma rays since July 2015. The slow and increasing trend is seen by the increasing frequency of daily detections and by the averaged weekly gamma-ray flux (E>100MeV) that has risen from (0.02+/-0.01) X 10^-6 photons cm^-2 s^-1, measured in the week of July 13-20, 2015, to (0.21+/-0.04) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) measured in the week of October 12-18, 2015. The weekly-averaged photon indexes ranged roughly from 2.1 to 1.5 in this 3-month time range.

In recent days, October 16, 17, 18 and 19, 2015 the daily gamma-ray flux (E>100MeV) of 1ES 1959+650 was respectively (0.25+/-0.07) X 10^-6 photons cm^-2 s^-1, (0.4+/-0.1) X 10^-6 photons cm^-2 s^-1, (0.3+/-0.1) X 10^-6 photons cm^-2 s^-1 and (0.24+/-0.09) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). The corresponding daily spectral photon indices (E>100MeV) of respectively 1.7+/-0.3, 1.8+/-0.2, 1.7+/-0.2, 1.9+/-0.2 are consistent with the average index of 1.883+/-0.022 reported in the Fermi LAT 3rd source catalog (3FGL).

This represents the first increasing gamma-ray flux trend for this source announced by the Fermi LAT Collaboration after more than 7 years of Fermi LAT all-sky survey observations. 1ES 1959+650 is one of the "LAT Monitored Sources" and consequently preliminary estimations of the daily and weekly gamma-ray flux observed by Fermi LAT is publicly available (fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/1ES_1959p650). Recent X-ray and VHE gamma-ray flaring activity for this blazar was reported in ATel #7211, ATel#8014, ATel#8148, ATel#8165.

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is David Paneque (dpanequeslac.stanford.edu).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.