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Fermi LAT detection of a GeV gamma-ray flare from the BL Lac object AP Librae

ATel #9442; Stefano Ciprini (ASI ASDC Rome & INFN Perugia, Italy), on behalf of the Fermi Large Area Telescope Collaboration
on 31 Aug 2016; 21:57 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 the two instruments on the Fermi Gamma-ray Space Telescope, has observed increasing gamma-ray flux from a source positionally consistent with the BL Lac object AP Lib (also known as PKS 1514-24 and 3FGL J1517.6-2422), with radio counterpart position R.A.: 229.424221 deg, Dec.: -24.372076 deg, (J2000.0, Johnston et al. 1995, AJ, 110, 880). The initial redshift measurement is consistent with the most recent measurement from the 6dF galaxy survey (z = 0.0490+/-0.0001, Jones et al. 2009, MNRAS, 399, 683).

Preliminary analysis indicates that on 2016 August 30, AP Lib was in a high state with a daily averaged gamma-ray flux (E>100 MeV) of (1.0+/-0.2) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), about 15 times greater than its four-year average flux reported in the third Fermi-LAT source catalog (3FGL, Acero et al. 2015, ApJS, 218, 23). The corresponding daily averaged photon spectral index (E>100 MeV) of 2.2+/-0.2 (statistical uncertainty only) is compatible with the 3FGL catalog value of 2.11+/-0.03.

The peak 6-hour integrated gamma-ray flux (E>100 MeV) of (2.5+/-0.6) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only) was reached by AP Lib on 2016 August 30 in the interval 06:00-12:00 UT, equivalent to about 37 times the average flux reported in the 3FGL catalog. The corresponding photon spectral index (E>100 MeV) is 2.0+/-0.2. AP Lib is active also in the first two 6-hour intervals of 2016 August 31.

This is the second time that the Fermi-LAT Collaboration has announced flaring GeV gamma-ray activity from AP Lib; the first was in February 2011 (ATel#3187). AP Lib was also discovered to emit very high energy gamma rays, above 300 GeV, in July 2010 (ATel#2743).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. This source is a member of the "LAT Monitored Sources", with preliminary, uncalibrated estimation of the daily gamma-ray flux observed by Fermi LAT publicly available at http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/source/AP_Lib . In consideration of the ongoing activity of this source, we encourage multiwavelength observations. For this source the Fermi LAT contact person is P. Fortin (pafortin@cfa.harvard.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.