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Title: Fermi Large Area Telescope detection of two very-high-energy (E > 100 GeV) γ-ray photons from the z = 1.1 blazar PKS 0426–380

Journal Article · · The Astrophysical Journal. Letters
 [1];  [2];  [3];  [4];  [5];  [2];  [6];  [7];  [1];  [1];  [1];  [8];  [9]
  1. Hiroshima Univ., Hiroshima (Japan)
  2. Naval Research Lab., Washington, D.C. (United States)
  3. Stanford Univ., Stanford, CA (United States). SLAC National Accelerator Lab.
  4. Institute of Space and Astronautical Science, Kanagawa (Japan); Jagiellonian Univ., Krakow (Poland)
  5. Univ. of California, Berkeley, CA (United States)
  6. Praxis Inc., Alexandria, VA (United States); Naval Research Lab., Washington, D.C. (United States)
  7. George Mason Univ., Fairfax, VA (United States); Naval Research Lab., Washington, D.C. (United States)
  8. Max-Planck-Institut fur Physik, Munchen (Germany); Stanford Univ., Stanford, CA (United States). SLAC National Accelerator Lab.
  9. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States)
+ Show Author Affiliations

Here, we report the Fermi Large Area Telescope (LAT) detection of two very-high-energy (VHE, E > 100 GeV) γ-ray photons from the directional vicinity of the distant (redshift, z = 1.1) blazar PKS 0426–380. The null hypothesis that both the 134 and 122 GeV photons originate from unrelated sources can be rejected at the 5.5σ confidence level. We therefore claim that at least one of the two VHE photons is securely associated with PKS 0426–380, making it the most distant VHE emitter known to date. The results are in agreement with recent Fermi-LAT constraints on the extragalactic background light (EBL) intensity, which imply a z sime 1 horizon for sime 100 GeV photons. The LAT detection of the two VHE γ-rays coincided roughly with flaring states of the source, although we did not find an exact correspondence between the VHE photon arrival times and the flux maxima at lower γ-ray energies. Modeling the γ-ray continuum of PKS 0426–380 with daily bins revealed a significant spectral hardening around the time of the first VHE event detection (LAT photon index Γ sime 1.4) but on the other hand no pronounced spectral changes near the detection time of the second one. This combination implies a rather complex variability pattern of the source in γ-rays during the flaring epochs. An additional flat component is possibly present above several tens of GeV in the EBL-corrected Fermi-LAT spectrum accumulated over the ~8 month high state.

Research Organization:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Organization:
USDOE
Grant/Contract Number:
AC02-76SF00515
OSTI ID:
1356553
Journal Information:
The Astrophysical Journal. Letters, Vol. 777, Issue 1; ISSN 2041-8205
Publisher:
Institute of Physics (IOP)Copyright Statement
Country of Publication:
United States
Language:
English
Citation Metrics:
Cited by: 19 works
Citation information provided by
Web of Science

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Cited By (3)

Fermi-LAT high-z active galactic nuclei and the extragalactic background light journal May 2017
Towards discrimination between galactic and intergalactic axion-photon mixing journal February 2016
Fermi -LAT Detection of a Transient γ -Ray Source in the Direction of a Distant Blazar B3 1428+422 at z = 4.72 journal September 2018

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Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
acceleration of particles
galaxies: active
galaxies: jets
gamma rays: galaxies
quasars: individual (PKS 0426–380)
radiation mechanisms: non-thermal