Fermi-LAT detection of enhanced gamma-ray activity and hard spectrum of TXS 0506+056, located inside the IceCube-170922A error region

ATel #11419; Roopesh Ojha (NASA/GSFC/UMBC), and Janeth Valverde (LLR/Ecole Polytechnique) on behalf of the Fermi Large Area Telescope Collaboration
on 14 Mar 2018; 20:16 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

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The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope has observed enhanced gamma-ray flux and an unusually hard spectrum from a source positionally consistent with the BL Lacertae object TXS 0505+056 also known as 3FGL J0509.4+0541 (Acero et al. 2015, ApJS, 218, 23). This source is also included in the 3FHL catalog (Ajello et al. 2017, ApJS, 232, 18, as 3FHL J0509.4+0542). Its coordinates are R.A.: 05h09m25.9644s, Dec: 05d41m35.333s (J2000; Beasley et al. 2002, ApJS, 141, 13) and it has a redshift z = 0.3365 (Paiano et al. 2018, ApJ, 854, 32).

Preliminary analysis indicates that on 13 March 2018 this source was in a high-flux state, with a daily averaged gamma-ray flux (E>100MeV) of (0.4+/-0.1) X 10^-6 photons cm^-2 s^-1 (statistical uncertainty only). This corresponds to a flux increase of a factor of about 5.5 relative to the average flux reported in the third Fermi LAT catalog (3FGL) and is the highest daily averaged flux detected from this object. The corresponding photon spectral index of 1.8+/-0.1 is significantly harder than the 3FGL value 2.04+/-0.03. Some high energy (E > 10 GeV) photons have also been detected in the past few days.

This source lies in the error region of the extremely high-energy (EHE) IceCube-170922A neutrino event (https://gcn.gsfc.nasa.gov/gcn3/21916.gcn3, ATel#10773, #10787) and the LAT detected a flaring state close to that neutrino event (ATel#10791).

Because Fermi operates in an all-sky scanning mode, regular gamma-ray monitoring of this source region will continue. In consideration of the ongoing activity of this source we encourage multiwavelength observations. For this source the Fermi-LAT contact person is Sara Buson (sara.buson@gmail.com).

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.