Orbit Mode observations of Crab and Mrk 421

arXiv:1110.5974v1 [astro-ph.IM] 27 Oct 2011 32ND INTERNATIONAL COSMIC RAY CONFERENCE, BEIJING 2011 Orbit Mode observations of Crab and Mrk 421 D.B. KIEDA1, FOR THE VERITAS COLLABORATION2 1Department of Physics and Astronomy, University of Utah, 115 S 1400 E, Salt Lake City, UT 84112 USA 2see Holder et al (these proceedings) kieda@physics.utah.edu Abstract: 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 Obser- vatory 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. Keywords: new IACT observation technique: general gamma-ray 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 Imaging Atmospheric Cherenkov Telescopes (IACT). VERITAS 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 wobble mode[3]. During an observation using wobble mode, the center of the camera is held at a fixed position in right as- cension and declination offset from the intended targeted source [See Figure 1]. In orbit mode the center of the cam- era circumscribes the source in right ascension and declina- tion with an angular velocity and radial offset dependent on the type of source (point-like, extended, or a GRB). Typ- ical values for a point-like source are one revolution per 20 to 80 minutes and a 0.5 degree radial offset [See Fig- ure 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 elevation and az- imuth of the telescopes, the reconstructed direction can be found [See Figure 4]. Figures 5 and 6 show the pointed position (in right ascension and declination 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 Figures 7 and 8]. 3 Discussion The orbit mode technique was developed to help eliminate dead-time during transitions between wobble 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 successfully done with a run during the daytime with the charge injection system of the telescopes, and later on sin- gle eighty minute data run of Mrk 421 [See Figure 9]. The typical time between data runs for slewing of the telescopes can last one to two minutes. If implemented 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 back- ground estimation using the reflected regions method[4] or ring background method[5] would be more uniform and therefore increasing the sensitivity of the analysis. Prelim- KIEDA et al.ORBIT MODE OBSERVATIONS inary results of the orbit mode analysis on the Crab Neb- ula 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 back- ground level in small regions of the sky. With the stars more rapidly rotating in the field of view during orbit mode observations than during wobble mode observations, this may eliminate some of the background noise in the small regions of the sky effected by the stars. GRB alerts have a large error associated with the position. A GRB alert from the Fermi[6] LAT or Fermi GBM[6

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