VHE Observation of CTA 1 with VERITAS

Archival X-ray observations of CTA 1 in the 5-10 keV band show non-thermal diffuse emission of low surface brightness in the center of the remnant, likely corresponding to a pulsar wind nebula (PWN) driven by a young pulsar [5]. A faint point source, RX J0007.0+7303, is located at the brightest part of the synchrotron emission, and was suggested as a pulsar candidate by Seward et al. [6]. A Chandra image of this object provided further evidence of an energetic, rotation-powered pulsar, resolving a central point source, a compact nebula, and a bent jet [7].

The earliest association of gamma-ray emission with CTA 1 comes from the detection of the source 3EG J0010+7309 by the EGRET instrument, with a relatively small 95% error circle of 28 [8]. Brazier et al. [9] proposed that the gamma-ray emission could originate from a young Geminga-like pulsar, based upon the coincidence with CTA 1, hard spectral index (Γ = 1.58 ± 0.18 between 70 MeV and 2 GeV), and lack of flux variability. Confirmation of this association came recently when the Fermi Gamma-Ray Space Telescope discovered the radio-quiet, 316.86 ms gamma-ray pulsar PSR J0007+7303 in a blind search, using 0.14 years of data [10]. Subsequent observations by XMM-Newton resulted in the detection of pulsed X-ray emission out of phase with the gamma-ray pulsation [11], [12]. The spin-down power of the pul-sar ( Ė = 4.5 × 10 35 erg s -1 ) and characteristic age (τ = 1.39 × 10 4 yrs) confirmed estimates based on previous observations observations [10].

Prompted by the discovery of PSR J0007+7303 by Fermi, Zhang et al. [13] modeled the pulsed and unpulsed spectral components of the pulsar magnetosphere and PWN. The pulsed high-energy spectrum was calculated with an outer-gap model and fit to the EGRET spectrum of Brazier et al. [9]. The unpulsed spectrum of the PWN was calculated with a timedependent, broken power law injection model with non-thermal emission from synchrotron radiation and inverse Compton scattering of cosmic microwave background (CMB) and ambient infrared (IR) photons. These calculations predict that the PWN should be detectable in the very-high-energy (VHE) gamma-ray band by VERITAS.

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four 12-meter imaging atmospheric Cherenkov telescapes (IACTs) located at the base camp of the Fred Lawrence Whipple Observatory in southern Arizona. Each telescope consists of a Davies-Cotton design optical reflector which focuses the Cherenkov light from atmospheric showers onto a camera consisting of 499 photomulitplier tubes and light concentrators with a total FOV of 3 • . VERITAS is able to detect a point source with the strength of 1% of the Crab Nebula flux at a statistical significance of 5 standard deviation (5σ) level in approximately 26 hours of observations. VERITAS is sensitive to gamma rays over a wide range of energies (100 GeV to tens of TeV) with an energy resolution of 15-20%. VERITAS observed CTA 1 between September 2010 to January 2011 with a total livetime of approximately 26 hours, after selection for good weather conditions and hardware status. Observations were taken in “wobble” mode [14], in which the telescope pointing is offset from the source position by some angular distance. An offset distance of 0.7 • was used to accommodate the large size of the remnant and the extension of the PWN as seen in X-rays. Two sets of a priori defined gamma-ray/hadronic shower separation cuts, optimized for weak sources of moderate and hard spectra, were applied to the data. Background estimated using the ring background model (see, for example, [15]), with squared angular integration radii of 0.01 deg 2 and 0.055 deg 2 used for point-source and extended-source searches, respectively. The statistical significance of the excess is calculated using Equation (17) from Li & Ma [16].

Figure 1 shows the map of excess events in the region around CTA 1 as measured by VERITAS. The hard-spectrum, extended-source analysis produced an excess with a pre-trial significance of 7.3σ, in a blind search region of radius 0.4 • around the pulsar PSR J0007+7303, within the radio shell of the SNR CTA 1. Accounting for the sets of cuts and integration radii, and implementing a trails factor for the search region by tiling it with 0.04 • square bins [17], we conservatively estimate a post-trials significance of detection of 6.0σ.

The TeV gamma-ray emission region exceeds the point-spre

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