Exploiting VERITAS Timing Information J. Holder a for the VERITAS Collaboration b

1. The PMT cameras of the VERITAS telescopes [1] are instrumented with 500 MHz (2 ns) sampling FADCs, with a memory depth of 32 s and 8 bit dynamic range [2]. The readout window is programmable – typically 24 samples per channel are recorded. The pulse arrival time in each channel, T0, is defined as the half-height position on the pulse leading edge This figure shows a cosmic ray initiated event with significant time structure. The colour scale corresponds to the pulse arrival time (in 2 ns FADC samples) Each channel has a fixed time offset which is measured and corrected for by uniformly illuminating the PMT camera with a brief laser pulse. Laser flashes of varying intensity are also used to measure the time resolution as a function of pulse size for each channel. The time gradient along the long axis of the image, Tgradx, is measured for each event. The uncertainty on each point is given by the pulse size dependent time resolution function illustrated to the left. For gamma-ray showers, the arrival time gradient along the long axis of the image is expected to be a function of the shower core distance from the telescope [3]. For gamma-rays from a point source located at the centre of the field of view, the core distance is proportional to the angular distance in the PMT camera. This figure shows Tgradx as a function of distance for simulated gamma rays (black points) and for gamma rays observed from the Crab Nebula and Markarian 421 using the first VERITAS telescope (red triangles). We have made a preliminary examination of the possibility of using this characteristic to discriminate between gamma-ray and hadronic showers. The dashed lines delimit the best gamma-ray selection region. Applying this cut after all geometrical cuts have been applied increases the significance of the gamma-ray excess for these observations by 5%. Exploiting VERITAS Timing Information J. Holdera for the VERITAS Collaborationb a) School of Physics and Astronomy, University of Leeds, UK b) For full author list see “Status and Performance of the first VERITAS Telescope” from these proceedings Alternatively, the FADC timing information can be used to optimize the signal-to-noise ratio for individual channels, thus lowering the effective energy threshold of the telescope. A number of signal processing techniques are being investigated (digital filters, trace fitting etc.). The figure to the left shows the effect of measuring the signal-to-noise ratio for the standard pulse shape as a function of the FADC integration window size. The optimum window size is found to be 5 samples (10 ns), illustrated by the shaded box in the figure to the right. The measured time gradient along the image is used to place this integration window in the correct position for each channel. Preliminary results indicate a sensitivity improvement of ~10% with this method. Approaches such as this are likely to become more important with a stereo system, when the analysis threshold is no longer defined by the presence of local muon arcs. • REFERENCES • J. Holder et al., “Status and Performance of the First VERITAS Telescope”, These proceedings. • J. H. Buckley et al., 28th ICRC, Tsukuba (2003) • M. Hess et al., Astropart. Phys. 11, 363, (1999)