The Geminga gamma ray source was first detected by the SAS-2 gamma-ray satellite observatory and the COS-B x-ray satellite observatory, and has been identified as a radio-quiet pulsar associated with a 300,000 year old supernova remnant. Geminga is one of the brightest GeV sources as seen by the Large Area Telescope on board the Fermi gamma-ray satellite observatory. A ground based detection was accomplished by the Milagro extensive air shower array at energies greater than 20 TeV. During 2007 VERITAS performed observations to search for TeV gamma ray emission from the Geminga pulsar and the region near Geminga. In this paper, we describe these measurements and the new analysis of these observations.
Deep Dive into Search for TeV Emission from Geminga by VERITAS.
The Geminga gamma ray source was first detected by the SAS-2 gamma-ray satellite observatory and the COS-B x-ray satellite observatory, and has been identified as a radio-quiet pulsar associated with a 300,000 year old supernova remnant. Geminga is one of the brightest GeV sources as seen by the Large Area Telescope on board the Fermi gamma-ray satellite observatory. A ground based detection was accomplished by the Milagro extensive air shower array at energies greater than 20 TeV. During 2007 VERITAS performed observations to search for TeV gamma ray emission from the Geminga pulsar and the region near Geminga. In this paper, we describe these measurements and the new analysis of these observations.
Geminga is a relatively close pulsar approximately 200 parsecs away (Caraveo et al. 1996) (Faherty et al. 2007). It was first detected in high-energy (> 20M eV ) gamma rays by the SAS-2 satellite (Fichtel et al. 1975). In 1977 the COS-B x-ray satellite was launched. Hermsen et al. (1977) reported a detection of x-ray radiation from the COS-B data of the same region as Fichtel et al. (1975). Bignami et al. (1983) were able to determine that Geminga was a neutron star from the COS-B data. No pulsations from the star were detected in the SAS-2 or COS-B data. A 237 millisecond period of pulsed gamma rays was first detected by the EGRET telescope aboard the Compton Gamma Ray Observatory satellite (Halpern & Holt 1992). Shortly after the report of Halpern and Holt the Egret gamma-ray observatory measured a period derivative of 1.9521712×10 -13 [Hz s -1 ] ( Bertsch et al. 1992), allowing an extrapolation of Geminga's ephemeris back to the era of the SAS-2 observations. Subsequently 237 millisecond pulses were now detected in the archival SAS-2 and COS-B data. From the pulsed analysis of Geminga a characteristic age was calculated to be ∼ 300, 000 years old. The only pulsed radio emission detection of Geminga has come from the Pushchino Radio Astronomy Observatory at 102.5 MHz from three groups, Malofeev & Malov (1997), Kuzmin &Losovsky (1997), andShitov &Pugachev (1998). No other radio frequencies have been detected from Geminga. More recently the Large Area Telescope on the Fermi gammaray space satellite observatory (Fermi LAT) has also detected high energy gamma rays from Geminga (Abdo et al. 2010). Very high energy gamma rays (TeV) have been detected by the Milagro extensive air shower array that are positionally coincident with Geminga at energies greater than 20 TeV (Abdo et al. 2009). This leaves a gap in the energy spectrum of Geminga from the GeV range to 20 TeV, see figure 1. Imaging atmospheric Cherenkov telescopes (IACTs) such as VERITAS have a sensitivity from 100 GeV to greater than 30 TeV that will cover the energy gap between space-based detectors and the Milagro ground-based extensive air shower array.
The VERITAS IACT array is located south of Tucson, Arizona, at the Fred Lawrence Whipple Observatory.
Each telescope is equipped with a 499 photomultiplier tube camera with a 500 mega-samples per second flash ADC readout system. The optical reflector of each telescope is twelve meters in diameter and uses a Davies-Cotton design. VERITAS IACTs have an energy range of 100 GeV to greater than 30 TeV with and energy resolution of 15% at 1 TeV. The peak effective photon collection area is approximately 100,000 square meters. The angular resolution is 0.1 degrees at 1 TeV with a location accuracy less than 50 arc seconds. VERITAS is capable of detecting the Crab nebula (the standard candle for TeV astronomy) in two minutes and a source with a flux of 1% of the Crab in less than 50 hours. VERITAS operates September through July, with an average yield of 750 hours per year of time when the Moon is set, and 100 hours per year when the Moon is above the horizon.
VERITAS detects the Cherenkov light emitted by an extensive air showers (EASs) that is created when a gamma ray or cosmic ray enters and interacts with the atmosphere. The image parameters of the EAS are then compared with image parameters that are derived from Monte Carlo simulations and comparisons are made to separate gammaray showers from cosmic-ray/hadronic showers. Geometric parameters are applied to each event to calculate the arrival direction and the impact distance on the ground of the EAS. For each EAS the number of photo electrons from the photomultiplier tubes is counted. By comparing the shape and impact parameters and the photo electrons produced by the EAS to simulations, the energy of the gamma ray is estimated.
During November and December of 2007, VERITAS collected 15 hours of data. After quality weather selection 10.4 hours of observations were selected to analyze. All observations were taken in wobble mode, where the source is offset 0.5 • from the camera center. The average observation was performed at an elevation of 71.4 • .
The data from the wobble mode observations were analyzed using a point source analysis using the reflected region background method (Aharonian et al. 2001) and the ring background method (Berge et al. 2007). A significance is determined using the likelihood ratio method from Li & Ma (1983). This type of analysis was done on the Crab nebula which resulted in a significant detection of ∼ 8 gamma rays per minute and a differential flux at 1 TeV of (3.63 ± 0.15) × 10 -12 photons T eV -1 cm -2 s -1 .
A steady/point source analysis was first done on the Geminga data. Figure 2 shows a histogram of the squared angular distribution of the ON (signal) and OFF (background) events recorded around the Geminga source location. The ON source data points of figure 2 are consistent with the OFF region (shaded area), a
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