T. C. Weekes

TeV Gamma-Ray Observations of the Galactic Center

We report a possible detection of TeV gamma rays from the Galactic center by the Whipple 10 m gamma-ray telescope. Twenty-six hours of data were taken over an extended period from 1995 through 2003 resulting in a total significance of 3.7 σ. The measured excess corresponds to an integral flux of 1.6 × 10-8 ± 0.5 × 10-8 (stat) ± 0.3 × 10-8 (sys) photons m-2 s-1 above an energy of 2.8 TeV, roughly 40% of the flux from the Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15 and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence of variability.

A search for TeV emission form Cygnus X-3

Cygnus X-3 has been observed during 1988-1990 by the Whipple Observatory collaboration using the 109 pixel high-resolution γ-ray imaging telescope. The data recorded has been analyzed to search for (a) steady emission of TeV γ-rays, (b) 4.8 hr modulation of the γ-ray light curve, and (c) periodic emission at 12.59 ms. No evidence is found for emission based on these searches. The upper limit (3 σ) for long-term steady emission is 3.5 × 10 -12 photons cm -2 s -1 , a factor of 10 below fluxes reported a decade ago for this energy

Calibration of the Whipple atmospheric Čerenkov telescope

Cerenkov ring images from single muons have been used to calibrate the Whipple Observatory 10 m imaging telescope. This approach tests the total throughput of the telescope and uses a known atmospheric Cerenkov light signal that closely matches the spectrum of the atmospheric Cerenkov signal from an air-shower, The absolute calibration is derived by matching the observed ring images with those predicted by a simple geometrical and physical model; a value of 1.25 +/- 0.13 photoelectrons equivalent to 1 digital count was found. Using this value simulations indicate that the telescope had an energy threshold of 300 GeV when this calibration was made.

Measurement of the angular resolution of an extensive air shower array using a Cherenkov light detector

Abstract We describe a method which has been used to evaluate the pointing accuracy and angular resolution of the South Pole air shower array. It makes use of coincidences between the air shower array and a very simple air Cherenkov detector of small aperture. The alignment of the array is shown to be known to be within ±0.2° for the zenith direction and ±0.5° for the azimuth direction. Additionally the angular resolution has been measured at energies below 200 TeV, lower than those at which the subarray comparison technique can be applied. At higher energies conclusions drawn previously from subarray comparisons are confirmed: the angular resolution, as described by the root mean square uncertainty in zenith angle, of showers produced by primaries of 200 TeV is found to be 0.8° for showers incident at about 20° from the vertical.

Pulsed 1 eV gamma rays detected from Hercules X-1 during X-ray source eclipse

On a detecte des pulsations distinctes, a 0,808 Hz, de Her X-1, en 1985. Les pulsations ont persiste 1 heure apres le debut de leclipse de ∼6 heures. On conclut que le site de production des rayons γ ne coincide pas, dans ce cas, avec la source RX

VERITAS: Status c.2005

VERITAS is a ground-based gamma-ray observatory that uses the imaging atmospheric Cherenkov technique and operates in the very high-energy (VHE) region of the gamma- ray spectrum from 100 GeV to 50 TeV. The observatory consists of an array of four 12m-diameter imaging atmospheric Cherenkov telescopes located in southern Arizona, USA. The four-telescope array has been fully operational since September 2007, and over the last two years, VERITAS has been operating with high reliability and sensitivity. It is currently one of the most sensitive VHE observatories. This paper summarizes the status of VERITAS as of October, 2009, and describes the detection of several new VHE gamma-ray sources.

Recent Observations of IC443 with the Whipple 10m Telescope

We present here the results of recent observations made with the Whipple 10m imaging Cherenkov telescope of the region of the supernova remnant IC443. No evidence for gamma‐ray emission was found, and we obtain an upper limit above 500 GeV (99.9% confidence) of 0.6 × 10−7 ph m−2 s−1 (0.11 Crab) at the location of the recently identified X‐ray plerion nebula and 0.8 × 10−7 ph m−2 s−1 (0.14 Crab) at the site of the OH maser at the densest part of the molecular cloud.

The VERITAS Prototype and the Upcoming VERITAS Array

The prototype for the VERITAS imaging atmospheric Cherenkov telescope array was successfully operated in southern Arizona between September 2003 and April 2004. The prototype consisted of 86 mirror facets mounted centrally on a 12‐meter dish, which was built to accommodate up to 350 facets when converted to a complete VERITAS telescope. The camera consisted of half of the full 499 pixel camera. The signal and trigger electronics were nearly identical to those that will be used for the individual VERITAS array telescopes. By observing the Crab and Mrk421, as well as performing a variety of tests, the characteristics of the instrument were evaluated. The prototype met all performance expectations and served as a valuable test bed for the current design, as well as for the construction and operation of VERITAS. This prototype instrument is now being upgraded to a complete VERITAS telescope that will be operated during the construction of the full VERITAS array. The array is expected to be operational by Nove...

The very high energy gamma ray spectra of IES 1959+650 and Mrk 421 as measured with the Whipple 10 m telescope

In observations made with the Whipple 10 m telescope, 1ES 1959+650 (z = 0.048) was caught in a high flaring state in May 2002, concurrent with a high X‐ray state, and in June 2002, for which there was no corresponding X‐ray flare. The spectra for both of those occasions are well fitted by a power law of differential spectral index ∼ −2.8. The relative stability of the spectral index for those flares argues strongly in favour of a two‐component model as to the emission zones for the two radiation regimes.Markarian 421 (z = 0.031) was observed to be in a high flaring state, at levels of ⩾ 3 Crab, during March and April 2004. The average spectrum over this time period shows evidence for a cut‐off in the spectrum at ∼ 5 TeV, similar to a cut‐off seen during an equivalently strong episode of flaring activity in 2001. The continued appearance of this feature indicates a long term stability, either in the physical conditions at the source, or in the intervening medium (such as attenuation on the extra‐galactic i...

Very High Energy Observations of Gamma Ray Bursts with the Whipple/VERITAS Telescopes

Gamma‐ray Burst (GRB) observations at Very High Energies (VHE, E > 100 GeV) can impose tight constraints on some GRB emission models. Many GRB after‐glow models predict a VHE component similar to that seen in blazars and supernova remnants, in which the GRB spectral energy distribution has a double‐peaked shape extending into the VHE regime. Consistent with this afterglow scenario, EGRET detected delayed high energy emission from all five bright BATSE GRBs that occurred within its field of view. GRB observations have had high priority in the observing program at the Whipple 10m Telescope and will continue to be high priority targets when the next generation observatory VERITAS comes online. Upper limits on the VHE emission from ten GRBs observed with the Whipple Telescope are reported here.