A. U. Abeysekara, A. Archer, Wystan Benbow, Ralph Bird, Robert Brose, M. Buchovecky, V. Bugaev, M. P. Connolly, Wei Cui, Manel Errando, A. Falcone, Qi Feng, John P. Finley, A. Flinders, L. Fortson, Amy Furniss, Gerard H. Gillanders, M. Huetten, David Hanna, O. Hervet, J. Holder, G. Hughes, T. B. Humensky, Caitlin A. Johnson, Philip Kaaret, P. Kar, N. Kelley-Hoskins, M. Kertzman, David Kieda, Maria Krause, F. Krennrich, M. J. Lang, T. T. Y. Lin, Gernot Maier, S. McArthur, P. Moriarty, Reshmi Mukherjee, R. A. Ong, N. Park, Jeremy S. Perkins, A. Petrashyk, Martin Pohl, Alexis Popkow, Elisa Pueschel, J. Quinn, K. Ragan, P. T. Reynolds, Gregory T. Richards, E. Roache, C. Rulten, I. Sadeh, M. Santander, G. H. Sembroski, Karlen Shahinyan, J. Tyler, S. P. Wakely, O. M. Weiner, A. Weinstein, R. M. Wells, P. Wilcox, Alina Wilhelm, David A. Williams, B. Zitzer, Indrek Vurm, Andrei Beloborodov

• 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 theOn 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.…