Orbit Mode observation Technique Developed for VERITAS

arXiv:1111.0121v1 [astro-ph.HE] 1 Nov 2011 2011 Fermi Symposium, Roma., May. 9-12 1 Orbit Mode observation Technique Developed for VERITAS G. Finnegan∗ Department of Physics and Astronomy University of Utah 115 South 1400 East, Salt Lake City, UT 84112, USA for the VERITAS Collaboration http://veritas.sao.arizona.edu The canonical observation mode for IACT gamma-ray observations employs four discrete pointings in the cardinal directions (the ”wobble” mode). For the VERITAS Observatory, the target source is offset by 0.5-0.7 degrees from the camera center, and the observation lasts 20 minutes. During January/February of 2011, the VERITAS Observatory tested a new ”orbit” observation mode, where the target source is continuously rotated around the camera center at a fixed radial offset and constant angular velocity. This mode of observation may help better estimate the cosmic ray background across the field of view, and will also reduce detector dead-time between the discrete 20 minute runs. In winter 2011, orbit mode observations where taken on the Crab Nebula and Mrk 421. In this paper we present the analysis of these observations, and describe the potential applications of orbit mode observations for diffuse (extended) sources as well as GRBs. 1. The VERITAS Imaging Atmospheric Cherenkov Telescopes VERITAS [1][2], located at the Fred Lawrence Whipple Observatory (FLWO) in southern Arizona, USA, is an array of four 12 meter diameter Imag- ing Atmospheric Cherenkov Telescopes (IACT). VER- ITAS can detect gamma-rays with energies from 100 GeV to 30 TeV with a flux of one percent of the Crab Nebula in approximately 25 hours. VERITAS has an energy resolution of 15-25%, an angular resolution of 0.1 degrees (68% containment radius), and a pointing accuracy within 50 arc-seconds. 2. Source Locations Reconstruction VERITAS observations are normally taken in wob- ble mode[3]. During an observation using wobble mode, the center of the camera is held at a fixed posi- tion in right ascension and declination offset from the intended targeted source [See Figure 1]. In orbit mode the center of the camera circumscribes the source in right ascension and declination with an angular veloc- ity and radial offset dependent on the type of source (point-like, extended, or a GRB). Typical values for a point-like source are one revolution per 20 to 80 minutes and a 0.5 degree radial offset [See Figure 2]. Using orbit mode, prior to rotation corrections to the field of view, the source appears as a ring [See Figure 3]. For each event there is an elevation and azimuth angle recorded. With this information and the eleva- tion and azimuth of the telescopes, the reconstructed direction can be found [See Figure 3]. Figure 4 shows ∗E-mail: garyf@physics.utah.edu the pointed position (in right ascension and declina- tion as a function of time) of each telescope followed a smooth sine and cosine curve. During testing it has been shown that the angular velocity and radial offset are fairly constant [See Figure 5]. 3. Discussion The orbit mode technique was developed to help eliminate dead-time during transitions between wob- ble directions for data runs sets, to slightly increase the area of the field of view by maintaining azimuthal symmetry of the exposure around the source, and to produce an uniform background estimate. In order to minimize the dead-time between runs, we had to test whether the VERITAS data network could transfer file sizes of twenty to thirty gigabytes. This was suc- cessfully done with a run during the daytime with the charge injection system of the telescopes, and later on single eighty minute data run of Mrk 421 [See Figure 6]. The typical time between data runs for slewing of the telescopes can last one to two minutes. If imple- mented for regular data operations, orbit mode would add additional thirty to sixty minutes of observation time per night. Orbit mode has been developed to test whether the background estimation using the reflected regions method[4] or ring background method[5] would be more uniform and therefore increasing the sensitiv- ity of the analysis. Preliminary results of the orbit mode analysis on the Crab Nebula produced 10.0±0.6 gamma-rays a minute. A wobble mode analysis was also performed on the Crab Nebula with data taken the same night at a similar zenith angle produced 9.1 ± 0.7 gamma-rays per minute. Stars in the field of view can cause higher trigger rates in individual pixels, and therefore cause a higher eConf C110509 2 2011 Fermi Symposium, Roma., May. 9-12 Figure 1: Exposure area (left) of the wobble mode technique for four runs with East to West (or North to South) profile (right) Distance from the Source [Degrees] -3 -2 -1 0 1 2 3 Exposure [% time per run] 0 20 40 60 80 100 Orbit Mode Exposure Profile Figure 2: Exposure area (left) of the orbit mode technique with profile (right) background level in small regions of the sky. With the stars more rapidly rotating in the field of view during orbit mode ob

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