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Abeysekara, A. U. ; Archer, A. ; Benbow, Wystan ; Bird, Ralph ; Brose, Robert ; Buchovecky, M. ; Bugaev, V. ; Connolly, M. P. ; Cui, Wei ; Errando, Manel ; Falcone, A. ; Feng, Qi ; Finley, John P. ; Flinders, A. ; Fortson, L. ; Furniss, Amy ; Gillanders, Gerard H. ; Huetten, M. ; Hanna, David ; Hervet, O. ; Holder, J. ; Hughes, G. ; Humensky, T. B. ; Johnson, Caitlin A. ; Kaaret, Philip ; Kar, P. ; Kelley-Hoskins, N. ; Kertzman, M. ; Kieda, David ; Krause, Maria ; Krennrich, F. ; Lang, M. J. ; Lin, T. T. Y. ; Maier, Gernot ; McArthur, S. ; Moriarty, P. ; Mukherjee, Reshmi ; Ong, R. A. ; Park, N. ; Perkins, Jeremy S. ; Petrashyk, A. ; Pohl, Martin ; Popkow, Alexis ; Pueschel, Elisa ; Quinn, J. ; Ragan, K. ; Reynolds, P. T. ; Richards, Gregory T. ; Roache, E. ; Rulten, C. ; Sadeh, I. ; Santander, M. ; Sembroski, G. H. ; Shahinyan, Karlen ; Tyler, J. ; Wakely, S. P. ; Weiner, O. M. ; Weinstein, A. ; Wells, R. M. ; Wilcox, P. ; Wilhelm, Alina ; Williams, David A. ; Zitzer, B. ; Vurm, Indrek ; Beloborodov, Andrei
On 2015 March 23, the Very Energetic Radiation Imaging Telescope Array System (VERITAS) responded to a Swift-Burst Alert Telescope (BAT) detection of a gamma-ray burst, with observations beginning 270 s after the onset of BAT emission, and only 135 s after the main BAT emission peak. No statistically significant signal is detected above 140 GeV. The VERITAS upper limit on the fluence in a 40-minute integration corresponds to about 1% of the prompt fluence. Our limit is particularly significant because the very-high-energy (VHE) observation started only similar to 2 minutes after the prompt emission peaked, and Fermi-Large Area Telescope observations of numerous other bursts have revealed that the high-energy emission is typically delayed relative to the prompt radiation and lasts significantly longer. Also, the proximity of GRB 150323A (z = 0.593) limits the attenuation by the extragalactic background light to similar to 50% at 100-200 GeV. We conclude that GRB 150323A had an intrinsically very weak high-energy afterglow, or that the GeV spectrum had a turnover below similar to 100 GeV. If the GRB exploded into the stellar wind of a massive progenitor, the VHE non-detection constrains the wind density parameter to be A greater than or similar to 3 x 10(11) g . cm(-1), consistent with a standard Wolf-Rayet progenitor. Alternatively, the VHE emission from the blast wave would be weak in a very tenuous medium such as the interstellar medium, which therefore cannot be ruled out as the environment of GRB 150323A.