THE FIRM REDSHIFT LOWER LIMIT OF THE MOST DISTANT TeV-DETECTED BLAZAR PKS 1424+240
- Santa Cruz Institute of Particle Physics, and Department of Physics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 (United States)
- CASA, Department of Astrophysical and Planetary Sciences, University of Colorado, 389-UCB, Boulder, CO 80309 (United States)
- Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101 (United States)
- Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California, 1156 High Street, Santa Cruz, CA 95064 (United States)
- Department of Astronomy, Yale University, New Haven, CT 06520-8120 (United States)
- Department of Astronomy, University of California, Berkeley, CA 94720-3411 (United States)
- NF/Observatory, Silver City, NM 88041 (United States)
We present the redshift lower limit of z {>=} 0.6035 for the very high energy (VHE; E {>=} 100 GeV) emitting blazar PKS 1424+240 (PG 1424+240). This limit is inferred from Ly{beta} and Ly{gamma} absorption observed in the far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph. No VHE-detected blazar has shown solid spectroscopic evidence of being more distant. At this distance, VHE observations by VERITAS are shown to sample historically large gamma-ray opacity values at 500 GeV, extending beyond {tau} = 4 for low-level models of the extragalactic background light (EBL) and beyond {tau} = 5 for high levels. The majority of the z = 0.6035 absorption-corrected VHE spectrum appears to exhibit a lower flux than an extrapolation of the contemporaneous Large Area Telescope power-law fit beyond 100 GeV. However, the highest energy VERITAS point is the only point showing agreement with this extrapolation, possibly implying the overestimation of the gamma-ray opacity or the onset of an unexpected VHE spectral feature. A curved log parabola is favored when fitting the full range of gamma-ray data (0.5-500 GeV). While fitting the absorption-corrected VHE data alone results in a harder differential power law than that from the full range, the indices derived using three EBL models are consistent with the physically motivated limit set by Fermi acceleration processes.
- OSTI ID:
- 22130671
- Journal Information:
- Astrophysical Journal Letters, Vol. 768, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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