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## A First Look At VERITAS Data

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**A First Look At VERITAS Data**Stephen FeganVladimir VassilievUCLA**Data Analysis Flowchart**• Image conditioning. • Geometry reconstruction. • Event parameterization. • Energy reconstruction. • Data Selection. • Independent telescope parameters. • Array. • Results. Standard approach. Picture/boundary cleaning. Has not been optimized. Cuts based on individual telescope images using parameters from memo. Simple “box” shaped cuts and multi-dimensional cuts using “NSpace” Methods 1&2 from reconstruction memo of December. Parameterization from memo also. Has not been investigated. Array cuts, based on combining the various parameters from all images have not been investigated. To be investigated… A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Geometry reconstruction**Method 2: Fit single shower axis to all images simultaneously. This axis is projected onto focal plane of each telescope. Methods 1 and 2 give comparable results with two telescopes. Method 3 has not been investigated. Method1: fit arrival direction and core from individual image axes, weighted “appropriately”. Method 3: Fit single shower axis to all images simultaneously. This axis is defined in real 3D space. A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Event parameterization**Once the shower axis has been reconstructed each image can be mapped into “physical space”, giving: • Mean emission height and track length. • Depths of mean emission in atmosphere in g/cm2. • Mean angle of emission with respect to trajectory. • Physical width of emission region. • Dispersions in photon arrival times assuming emission along primary trajectory. A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Data selection**• Simple cuts on parameters in Dec. memo: • Log10(size) > 2.5 • Emission region width < 15m • Emission height > 5000m • Emission length > 750m • Simulations suggest cuts should be energy dependent. Studies using n-dimensional cutting system NSpace1 is ongoing. Preliminary results discussed here. 1 See T. Nagai’s thesis for write up of NSpace methodology A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: Mrk 421 – 9 pairs**(SIMPLE CUTS) Significance 29σ, rate 5.76/min Background rate of 16.4/min/deg2 based on θ<0.2° A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: table of Mrk421 pairs**(SIMPLE CUTS) A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: Mrk 421 – 10 wobble at 0.3°**(SIMPLE CUTS) Theta2 distribution with respect to source position Theta2 distribution with respect to symmetric wobble position Source counts as they appear from off source location. No obvious spill over to theta=0 area. Wobble offset of 0.3º seems to be sufficient. A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: table of Mrk421 wobble runs**(SIMPLE CUTS) A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: strong source optimization**(SIMPLE CUTS) Photon dominated regime: For strong source, optimize theta cut for significance. Peak is quite broad. Theta cut from 0.15° to 0.25° gives high significance. We chose 0.2° for high rate. 9 Mrk421 pairs A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: weak source optimization**(SIMPLE CUTS) Background dominated regime: For weak source optimize theta cut for excess/sqrt(background) – Q-value. Shown is Q-value, normalized to 1 at theta=1. Peak is much narrower. Theta cut from 0.12° to 0.16° gives best significance on weak sources. 9 Mrk421 pairs A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: Mrk421comparison with Whipple**(SIMPLE CUTS) 1 Whipple results from J. Kildea 2 VERITAS and Whipple run numbers are similar after 40 years of Whipple operation A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: Mrk501comparison with Whipple**(SIMPLE CUTS) 1 Whipple results from J. Kildea A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**Results: aperture function**(SIMPLE CUTS) Cosmic-ray acceptance as a function of off-axis angle, after cuts. Quite flat to 0.8 degrees. These events are the most gamma-ray like CRs. Gamma-ray response will likely be somewhat similar. Fast decline at 1.75 degrees A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: basic ideas**• Form multi-dimensional histogram in space defined by event parameters. • Fill two histograms: from ON & OFF runs or from SIM & OFF data. • Order cells “according to significance (likelihood,…)” (many ordering schemes possible). • Make “filter” by picking cells from ordering to maximize total significance (likelihood,…). • Repeat all steps above varying bin sizes of the histogram until significance (likelihood,…) is maximized. Bin sizes and histogram dimension are your optimization parameters. Final filter is an effective volume in multidimensional space. • Cut ON events according to whether they are inside or outside the filter. A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: example in two dimensions**ON counts OFF counts Highest single cell significance(4.7 sigma) Two cell set with highest overall significance(6.5 sigma) Seven cell set with highest overall significance (9.9 sigma). Thereafter adding any more cells decreases significance. A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: single telescope space**• Space based on three single telescope parameters: • Log10(λc)* 12 binsno 4 – 8 • Emission width 20 binsno 0 – 40 m • Emission depth 10 binsno 0 – 1000 g/cm2 • ON histogram: 9 Mrk421 runs – theta<0.2OFF histogram: 9 Mrk421 runs – theta<1.0 • Optimize filter on T1 parameters • Apply to both T1 & T2 when cutting * λc : density of emitters in coherent regime [m-1], see December memo for details A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: optimization results**For comparison, straight cuts have Qmax=6 Filter with 70 cells gives close to the peak Q-value and gives good rate (with theta cut of 0.16°) A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: optimization results**For comparison, straight cuts have Qmax=6 Filter with 70 cells gives close to the peak Q-value and gives good rate (with theta cut of 0.16°) A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: comparison with straight cuts**Bright source significance is approximately equal with this NSpace filter but rate is lower. A theta cut of 0.2 gives better significance and rate for both sets of cuts. Faint source improvement not as large as indicated from optimization (1.332=1.76 vs 1.232=1.51). This reflects some degree of “tuning” of space to specific runs used in optimization. 1 Optimization done on these runs (NSpace with theta cut of 0.15°) (Straight with theta cut of 0.14°) A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: 10 Mrk421 wobble runs**Background rate of 5.0/min/deg2 A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**A First Look At VERITAS Data Stephen Fegan & Vladimir**Vassiliev**A First Look At VERITAS Data Stephen Fegan & Vladimir**Vassiliev**NSpace: results for Mrk501 wobble**If Crab is 5g/min then 4% Crab in 20hours/wobble @ 5s or 18% Crab in 1hour/wobble @ 5s HESS claimed 11% Crab in 1hour/wobble @ 5s 1 Using traditional Gaussian significance formula (somewhat inaccurately)**NSpace: 501 comparison with Whipple**(NSpace) 1 Whipple results from J. Kildea A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev**NSpace: future work**• Preliminary optimization done quickly for meeting, need to look in detail at: • Ordering strategy • Array/Energy parameters • Bin size optimization • Parameter choice/Dimension Optimization • 9 runs are not enough to do filter selection properly. Need lots of CRAB data and/or simulations. • NSpace provides flexible and robust frame of work for data analysis including detailed spectrum analysis. • Optimistic about success of this method! A First Look At VERITAS Data Stephen Fegan & Vladimir Vassiliev