The VERITAS Survey of the Cygnus Region of the Galactic Plane

📝 Abstract

The VERITAS IACT observatory has carried out an extensive survey of the Cygnus region between 67 and 82 degrees in galactic longitude and between -1 and 4 degrees in galactic latitude. This region is a natural choice for a Very High Energy (VHE) gamma-ray survey in the Northern Hemisphere, as it contains a substantial number of potential VHE gamma-ray emitters such as supernova remnants, pulsar wind nebulae, high-mass X-ray binaries, and massive star clusters, in addition to a few previously detected VHE gamma-ray sources. It is also home to a number of GeV gamma-ray sources, including no less than four new high-significance sources detected in the first six months of Fermi data. The VERITAS survey, comprising more than 140 hours of observations, reaches an average VHE point-source flux sensitivity of better than 4% of the Crab Nebula flux at energies above 200 GeV. Here we report on preliminary results from this survey, including two source detections, and discuss the prospects for further studies that would exploit the joint coverage provided by VERITAS and Fermi data in this region.

💡 Analysis

The VERITAS IACT observatory has carried out an extensive survey of the Cygnus region between 67 and 82 degrees in galactic longitude and between -1 and 4 degrees in galactic latitude. This region is a natural choice for a Very High Energy (VHE) gamma-ray survey in the Northern Hemisphere, as it contains a substantial number of potential VHE gamma-ray emitters such as supernova remnants, pulsar wind nebulae, high-mass X-ray binaries, and massive star clusters, in addition to a few previously detected VHE gamma-ray sources. It is also home to a number of GeV gamma-ray sources, including no less than four new high-significance sources detected in the first six months of Fermi data. The VERITAS survey, comprising more than 140 hours of observations, reaches an average VHE point-source flux sensitivity of better than 4% of the Crab Nebula flux at energies above 200 GeV. Here we report on preliminary results from this survey, including two source detections, and discuss the prospects for further studies that would exploit the joint coverage provided by VERITAS and Fermi data in this region.

📄 Content

arXiv:0912.4492v1 [astro-ph.HE] 22 Dec 2009 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 1 The VERITAS Survey of the Cygnus Region of the Galactic Plane Amanda Weinstein for the VERITAS Collaboration University of California Los Angeles, 475 Portola Plaza, Los Angeles, CA 90095 USA The VERITAS IACT observatory has carried out an extensive survey of the Cygnus region between 67 and 82 degrees in galactic longitude and between -1 and 4 degrees in galactic latitude. This region is a natural choice for a Very High Energy (VHE) gamma-ray survey in the Northern Hemisphere, as it contains a substantial number of potential VHE gamma-ray emitters such as supernova remnants, pulsar wind nebulae, high-mass X-ray binaries, and massive star clusters, in addition to a few previously detected VHE gamma-ray sources. It is also home to a number of GeV gamma-ray sources, including no less than four new high-significance sources detected in the first six months of Fermi data. The VERITAS survey, comprising more than 140 hours of observations, reaches an average VHE point-source flux sensitivity of better than 4% of the Crab Nebula flux at energies above 200 GeV. Here we report on preliminary results from this survey, including two source detections, and discuss the prospects for further studies that would exploit the joint coverage provided by VERITAS and Fermi data in this region.

1. Introduction To date, only a few moderate-scale surveys have been performed in gamma rays between 100 GeV and 10 TeV: the HESS scan of the central region of the Galactic Plane[9], the much less sensitive HEGRA sur- vey of the quarter of the Galactic Plane between −2◦ and 85◦in galactic longitude[1], and the VERITAS scan of the Cygnus region under discussion here. How- ever, there is a strong motivation for performing such surveys; an unbiased search of a substantial region of sky is less subject to the experimental and theoretical prejudices that guide most VHE gamma-ray observa- tions, and therefore (as demonstrated by HESS) offers greater scope for serendipitous discoveries. An unbi- ased search also allows for more quantitative state- ments to be made about the source population in the region surveyed. The stereoscopic imaging atmospheric Cherenkov telescope (IACT) array VERITAS has just completed a two-year survey of a 5 by 15 degree portion of the Cygnus region of the Galactic Plane. The Cygnus re- gion was a natural target for survey observations, as it is already known to contain a significant number of potential TeV gamma-ray emitters. In the GeV (20MeV −300 GeV) energy band, it is home to a number of sources or potential sources, including no less than 4 distinct Fermi sources [7]. Moreover, both Fermi and its predecessor EGRET have detected dif- fuse emission from this region that is greater in flux than all of the currently resolved sources taken to- gether. Viewed in the energy range between 1TeV-50 TeV, it contains a pair of unidentified TeV sources (MGRO J2031+41 and MGRO 2019+37) detected by the Milagro Gamma Ray Observatory, a water Cherenkov extensive air shower array that is sensitive to TeV sources at a median energy of 20 TeV over the entire sky [8], as well as the unidentified source TeV J2032+4130 (first detected by the HEGRA IACT array[13]), that is spatially coincident with MGRO J2031+41. The exact nature of these sources is cur- rently unknown. There is also a significant catalog of objects detected at other wavelengths, including SNRs, pulsar wind nebulae (PWNe), high-mass x-ray binaries (HMXBs) and massive star clusters, that are considered potential TeV sources.
2. Survey Observations Survey observations, which began in April 2007 and were completed in November 2008, cover the field be- tween galactic longitudes 67 ◦and 82 ◦and galactic latitudes −1 ◦and 4 ◦with a grid of pointed observa- tions. Grid points have 0.8 ◦separation in galactic lon- gitude and 1 ◦separation in galactic latitude, allowing for substantial overlap in the fields of view for observa- tions at nearby grid points. Approximately one hour of observing time was taken at every grid point (gen- erally within a 1-3 day period), with that hour broken into 20-minute observation periods. Figure 1 shows both a schematic of the survey observation strategy and the acceptance-corrected (“effective”) exposure time for the “base” survey (that is, the survey data taken prior to follow-up observations) over the entire survey field. The base survey achieves a relatively uni- form effective exposure of ∼6 −7 hours. The full sur- vey dataset (> 140 hours of good-quality observation time) has regions of enhanced exposure due to follow- up observations taken in Fall 2008, Spring 2009, and Fall 2009. Survey data have been quality-selected to remove runs with unstable trigger rates, poor weather condi- tions, and known hardware problems. Almost all sur- vey data were taken on moon-less nights, with a few percent taken under slight to moderate moonlight con- ditions. Since survey

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