The TeV spectrum of H1426+428

The BL Lac object H1426+428 was recently detected as a high energy gamma-ray source by the VERITAS collaboration (Horan et al. 2002). We have reanalyzed the 2001 portion of the data used in the detection in order to examine the spectrum of H1426+428 above 250 GeV. We find that the time-averaged spectrum agrees with a power law of the shape dF/dE = 10^(-7.31 +- 0.15(stat) +- 0.16(syst)) x E^(-3.50 +- 0.35(stat) +- 0.05(syst)) m^(-2)s^(-1)TeV^(-1) The statistical evidence from our data for emission above 2.5 TeV is 2.6 sigma. With 95% c.l., the integral flux of H1426+428 above 2.5 TeV is larger than 3% of the corresponding flux from the Crab Nebula. The spectrum is consistent with the (non-contemporaneous) measurement by Aharonian et al. (2002) both in shape and in normalization. Below 800 GeV, the data clearly favours a spectrum steeper than that of any other TeV Blazar observed so far indicating a difference in the processes involved either at the source or in the intervening space.Comment: LaTeX, 8 pages, 4 figures, accepted for publication in Ap

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

Abstract: In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass ∼6.5 × 109 M ⊙. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87’s spectrum. We can exclude that the simultaneous γ-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the γ-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded

Cutoff in the t[clc]e[/clc]v energy spectrum of markarian 421 during strong flares in 2001

Exceptionally strong and long- lasting flaring activity of the blazar Mrk 421 occurred between 2001 January and March. Based on the excellent signal- to- noise ratio of the data, we derive the energy spectrum between 260 GeV and 17 TeV with unprecedented statistical precision. The spectrum is not well described by a simple power law even with a curvature term. Instead, the data can be described by a power law with exponential cutoff: dN/dE proportional to E(-2.14 +/-0.03 stat)e(-E/E0) m(-2) s(-1) TeV-1 with E0=4.3 +/-0.3(stat) TeV. Mrk 421 is the second gamma -ray blazar that unambiguously exhibits an absorption- like feature in its spectral energy distribution at 3-6 TeV

VERITAS: the Very Energetic Radiation Imaging Telescope Array System

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is the major next generation imaging atmospheric Cherenkov gamma-ray telescope in the western hemisphere and will be located in southern Arizona nearby Kitt Peak National Observatory. The VERITAS observatory will provide unprecedented sensitivity to photon energies between 50 GeV and 50 TeV. The first stage is an array of four telescopes to be fully operational in early 2006, with an expansion to seven telescopes envisioned for 2008. The construction of a prototype telescope is underway, for which first light is expected in Fall 2003. The technical concept is outlined and a progress report is given. (C) 2004 Published by Elsevier B.V