Fermi-LAT detection of hard spectrum and enhanced gamma-ray emission from the BL Lac object PKS 1717+177
ATel #8427; Stefano Ciprini (ASDC Rome & INFN Perugia, Italy), on behalf of the Fermi Large Area Telescope Collaboration
on 17 Dec 2015; 22:47 UT
Credential Certification: Stefano Ciprini (stefano.ciprini@asdc.asi.it)
Subjects: Gamma Ray, >GeV, Request for Observations, AGN, Blazar
The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has observed a gamma-ray flare from a source positionally consistent with the BL Lac object PKS 1717+177 (also known as S3 1717+17, OT 129, and 3FGL J1719.2+1744, Acero et al. 2015, ApJS, 218, 23), with radio coordinates (J2000.0), R.A.: 259.804368 deg, Dec.: 17.751788 deg (Johnston et al. 1995, AJ, 110, 880). This BL Lac object has a redshift of z=0.137 (Sowards-Emmerd et al. 2005, ApJ, 626, 95).
Preliminary analysis indicates that on 16 December 2015 the source was in a high-flux state, with a daily gamma-ray flux (E>100MeV) of (0.4+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) corresponding to a flux increase of a factor of about 20 with respect to the averaged flux reported in the Fermi LAT Third Source Catalog (3FGL). The corresponding spectral photon index (E>100MeV) of 1.7+/-0.2 is smaller than the average index of 2.043+/-0.057 in the 3FGL catalog. The source is also detected in the six-hour period (00:00-06:00 UT) with a gamma-ray flux of (1.6+/-0.4) X 10^-6 photons cm^-2 s^-1 and spectral photon index of 2.1+/-0.3.
PKS 1717+177 is also a member of the First Fermi-LAT Catalog of Sources above 10 GeV (1FHL catalog, Ackermann et al. 2013, ApJS, 209, 34). There are three HE (>10GeV) gamma-ray photons detected on 16 December and associated with the source.
This is the first time that Fermi is announcing increased gamma-ray activity from PKS 1717+177. Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity and hard-state of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is S. Ciprini (stefano.ciprini@asdc.asi.it).
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.
