Performance estimation

In Figure 3.16 the gamma-ray detection efficiencies on trigger level are shown. One can clearly see an increase in effective area at low energies (< 300 GeV) with the high-quantum efficiency PMTs compared to the Photonis ones. This directly translates into a lower energy threshold at trigger level. It is about 95 GeV with the Photonis PMTs and reduces down to approximately 75 GeV with the Hamamatsu PMTs, as obtained from the peak in the differential counting rate. Thus, the installation of the new PMTs in the cameras enables the detection of gamma-ray showers of much lower energy as more Cherenkov photons can be recorded. On the other hand it should be noted, that the array trigger rate due to cosmic rays will also increase (from about 250 Hz up to approximately 500 Hz). Without any changes to the readout, this will introduce a dead time of the order of 20% to the system10.

In order to evaluate possible improvements in the overall performance after the camera up- grade, the simulations are analyzed with eventdisplay. To take advantage of the 40% shorter pulses after the upgrade the summation window in the trace analysis is reduced from 7 samples to 5 samples. This almost immediately cancels out the increased NSB contribution in the inte- grated charge per pixel. Further on, no cut optimization has been performed for the new PMTs and cuts used are similar to those of a standard point source analysis (as listed in Table 3.3), but without an image size cut.

The results of the analysis, i.e. the effective areas for gamma rays after cuts, are shown in Figure 3.17. At lowest energies, the lower energy threshold of the new PMTs leads to a clear increase in effective area while at higher energies the effective area is similar before and after the

10 _{In order to reduce the dead time of the system several possibilities have been tested during the camera}

commissioning phase in fall 2012. Those included a shorter trigger coincidence time window and a reduced readout window, resulting at a dead time of about 15% for trigger rates of ∼ 450 Hz.

3.3. The VERITAS camera upgrade log10(energy [TeV]) -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 ] 2 e ff e c ti v e a re a [ m 3 10 4 10 5 10 Before Upgrade After upgrade energy [TeV] 0.05 0.1 0.15 0.2 0.25 0.3 1 10

Differential trigger rate Before upgrade After upgrade

Figure 3.16.: Effective area and differential counting rate on trigger level assuming a Crab Nebula-like spectrum. Black squares are before the upgrade (XP2970), red tri- angles are after the upgrade (R10560).

upgrade. It should be noted that at energies above ∼ 30 TeV, the drop in effective area is related to the finite size of the VERITAS camera and the maximum distance cut of 250 m, applied to avoid nearly parallel images which worsen the geometrical reconstruction of the event.

Another way to look at the improvements of the camera upgrade is to determine the sensitivity of the array. Traditionally, the representation of the sensitivity is in terms of integral sensitivity, including all events reconstructed above a given energy within an observation time of 50 hours. This representation is highly dependent on the source spectrum and thus might be misleading. A different way to represent the sensitivity of VERITAS (and later as well of CTA) is in terms of differential sensitivity. The differential sensitivity represents the lowest flux in a given energy bin which results in a significant detection after a certain observation time t_obs. It is calculated in small energy bins, i.e. five bins per decade in energy are used within this thesis. An observation time of t_obs = 50 hours and a signal-free background region five times larger than the signal region (α = 0.2) are assumed in the following. The basic requirements for a significant detection per bin are a 5σ statistical significance, at least 10 events and the number of gamma rays is 5% above the background rate. They can be summarized as follows:

S ≥ 5 Nγ ≥ 10

Nγ/(α · NOFF) ≥ 0.05

(3.12)

(E/TeV) 10 log -1.5 -1 -0.5 0 0.5 1 1.5 2 ] 2 effective area [m 1 10 2 10 3 10 4 10 5 10

Effective Area for 80% point-source containment

Figure 3.17.: Effective area for gamma-ray simulations at 20◦ zenith angle after analysis cuts. Black squares are before the upgrade (XP2970), red triangles are after the upgrade (R10560).

following the HEGRA measurements while proton and electron spectra are weighted according to measurements as listed in Appendix A. The errors in the sensitivity calculation are derived from the uncertainties in the gamma-ray and cosmic-ray background effective areas. The resulting sensitivity curve, multiplied by E2, is shown in Figure 3.18. Even without a re-optimization of the cuts for the new PMTs, the sensitivity over the whole energy range is slightly increased for the high-quantum efficiency PMTs compared to the Photonis PMTs before the camera upgrade. At the lowest energies, the increase in effective area (shown in Figure 3.17) and the lower energy threshold for the new PMTs results in an additional low energy bin in the sensitivity curve. The main limiting factor in this energy regime are the fluctuations in the air-shower development and hence the background. However, for short duration phenomena like flares from AGN or pulsed emission from pulsars, the increase in effective area will enable the detection of more gamma-rays which will lead to better flux estimates at lowest energies.

At moderate energies (between 300 GeV and 2 TeV), no increase in gamma-ray effective area could be achieved with the new PMTs but an improved sensitivity can be seen. This can be understood in terms of background rejection power. Due to higher light yield of the new PMTs, also more Cherenkov photons from background events (i.e. protons) are recorded and the images surviving the image cleaning are longer and wider. This makes a background rejection with shape cuts more efficient. In this energy regime, further improvements are expected after a proper cut optimization.

3.3. The VERITAS camera upgrade energy [TeV] -2 10 10-1 1 10 ] -1 s -2

x Flux Sensitivity [erg cm

2 E -12 10 -11 10 100% Crab 10% Crab 1% Crab

Figure 3.18.: Differential sensitivity curve of the VERITAS array for 50 hours of observations using the standard requirements given in Eq. 3.12. Simulations at 20◦ zenith and background includes cosmic-ray protons and electrons; cuts are not optimized cuts for the new PMTs. Black curve corresponds to before the upgrade (XP2970), red curve to after the upgrade (R10560).

At high energies (above a few TeV), neither the sensitivity curves nor the effective areas before and after the upgrade show significant deviations from each other. In this energy range, the sensitivity of the array is photon statistics limited and improves inversely proportional to the observation time. A possible improvement in this energy regime from the high-quantum efficiency PMTs might arise from the detection of farther distant showers, but this effect has not been investigated as a cut on the maximum distance from the array of 250 m has always been applied.