D. A. Carter-Lewis

Detection of Gamma Rays with E > 300 GeV from Markarian 501

The detection of gamma rays of energy greater than 300 GeV from the BL Lacertae object Mrk 501 demonstrates that extragalactic TeV emission is not unique to Mrk 421. During 66 hr of observations between 1995 March and July we measured an average flux of 8.1 ± 1.4 × 10-12 cm-2 s-1 above 300 GeV, a flux that is only 20% of the average Mrk 421 flux. The new gamma-ray source has not been reported by the Compton Gamma Ray Observatory as an emitter of gamma rays at lower energies. There is evidence for variability on timescales of days.

VERITAS: The Very Energetic Radiation Imaging Telescope Array System

Abstract A next generation atmospheric Cherenkov observatory is described which uses the Whipple Observatory gamma-ray telescope as a prototype. An array of seven imaging telescopes will be deployed such that they will permit the maximum versatility and will give the highest sensitivity in the 50 GeV-50 TeV band (with maximum sensitivity from 100 GeV to 10 TeV). In this band critical measurements of natures most powerful accelerators will be made.The Very Energetic Radiation Imaging Telescope Array System (VERITAS) represents an important step forward in the study of extreme astrophysical processes in the universe. It combines the power of the atmospheric Cherenkov imaging technique using a large optical reflector with the power of stereoscopic observatories using arrays of separated telescopes looking at the same shower. The seven identical telescopes in VERITAS, each of aperture 10 m, will be deployed in a filled hexagonal pattern of side 80 m; each telescope will have a camera consisting of 499 pixels with a field of view of 3.5 deg VERITAS will substantially increase the catalog of very high energy (E>100GeV) gamma-ray sources and greatly improve measurements of established sources.

The Spectrum of TeV Gamma Rays from the Crab Nebula

The spectrum of gamma rays from the Crab Nebula has been measured in the energy range 500 GeV-8 TeV at the Whipple Observatory by the atmospheric Cerenkov technique. Two methods of analysis that were used to derive spectra, in order to reduce the chance of calibration errors, gave good agreement, as did analysis of observations made with changed equipment several years apart. It is concluded that stable and reliable energy spectra can now be made in the TeV range. The spectrum can be represented in this energy range by the power-law fit, J = (3.20 ? 0.17 ? 0.6) ? 10-7 ? (E/1 TeV)-2.49?0.06?0.04 m-2 s-1 TeV-1, or by the following form, which extends much better to the GeV domain: J=(3.25?0.14?0.6)?10 -->?7E

A MULTIWAVELENGTH VIEW OF THE TeV BLAZAR MARKARIAN 421: CORRELATED VARIABILITY, FLARING, AND SPECTRAL EVOLUTION

^{ − 2.44{±}0.06{±}0.04 − 0.151 {r log}10 E}

Discovery of Gamma-Ray Emission above 350 GeV from the BL Lacertae Object 1ES 2344+514

--> m-2 s-1 TeV-1 (E in TeV). The integral flux above 1 TeV is (2.1 ? 0.2 ? 0.3) ? 10-7 m-2 s-1. Using the complete spectrum of the Crab Nebula, the spectrum of relativistic electrons is deduced, and the spectrum of the inverse Compton emission that they would generate is in good agreement with the observed gamma-ray flux from 1 GeV to many TeV, if the magnetic field in the region where these scattered photons originate (essentially the X-ray-emitting region, around 0.4 pc from the pulsar) is ~16 nT (160 ?G), in reasonable agreement with the field deduced by Aharonian & Atoyan. If the same field strength were present throughout the nebula, there would be no clear need for an additional radiation source in the GeV domain such as has recently been suggested; the results give an indication that the magnetic field is well below the often-assumed equipartition strength (35-60 nT). Further accurate gamma-ray spectral measurements over the range from 1 GeV to tens of TeV have the potential to probe the growth in the magnetic field in the inner region of the nebula.

Multiwavelength Observations of Markarian 421 in 2001 March: An Unprecedented View on the X-Ray/TeV Correlated Variability

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of 2003-2004. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide range of flux. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales at both X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux did during a giant flare (or outburst) in 2004 (with the peak flux reaching ~135 mcrab in X-rays, as seen by the RXTE ASM, and ~3 crab in gamma rays). Such a difference in the development of the flare presents a further challenge to both the leptonic and hadronic emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in the optical seems to be comparable to that in the radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured spectral energy distributions (SEDs) with a one-zone synchrotron self-Compton model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

Limits to Quantum Gravity Effects on Energy Dependence of the Speed of Light from Observations of TeV Flares in Active Galaxies

We present the discovery of gamma-ray emission greater than 350 GeV from the BL Lacertae (BL Lac) object 1ES 2344+514 with the Whipple Observatory 10 m gamma-ray telescope. This is the third BL Lac object detected at very high energies (VHE, E > 300 GeV), the other two being Markarian 421 (Mrk 421) and Mrk 501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified with declination greater than 0?. The evidence for emission from 1ES 2344+514 comes mostly from an apparent flare on 1995 December 20 (UT) during which a 6 ? excess was detected with an average flux of I(>350 GeV) = 6.6 ? 1.9 ? 10-11 photons cm-2 s-1. This is approximately 63% of the VHE emission from the Crab Nebula, the standard candle in this field. Observations taken between 1995 October and 1996 January, excluding the night of the flare, yield a 4 ? detection indicating a flux level of I(>350 GeV) = 1.1 ? 0.4 ? 10-11 photons cm-2 s-1, or about 11% of the VHE Crab Nebula flux. Observations taken between 1996 September and 1997 January on this object did not yield a significant detection of a steady flux or any evidence of flaring activity. The 99.9% confidence level upper limit from these observations is I(>350 GeV) < 8.2 ? 10-12 photons cm-2 s-1, 8% of the Crab Nebula flux. The low baseline emission level and variation in the nightly and yearly flux of 1ES 2344+514 are the same as the VHE emission characteristics of Mrk 421 and Mrk 501.We present the discovery of >350 GeV gamma-ray emission from the BL Lacertae object 1ES 2344+514 with the Whipple Observatory 10m gamma-ray telescope. This is the third BL Lac object detected at gamma-ray energies above 300 Gev, the other two being Markarian 421 (Mrk 421) and Mrk501. These three active galactic nuclei are all X-ray selected and have the lowest known redshifts of any BL Lac objects currently identified. The evidence for emission derives primarily from an apparent flare on December 20, 1995 when a 6 sigma excess was detected with a flux approximately 63% of the very high energy gamma-ray emission from the Crab Nebula, the standard candle for TeV gamma-ray sources. Excluding the flare, observations between October 1995 and January 1996 yield a 4 sigma detection corresponding to 11% of the VHE Crab Nebula flux. Observations spanning September 1996 to January 1997 failed to yield a significant detection of a steady flux or any flaring. For this period, the 99.9% confidence level upper limit is <8% of the Crab Nebula. The low baseline emission level and variations in nightly and yearly flux of 1ES 22344+514 are the same as the VHE emission characteristics of Mrk 421 and Mrk 501

The first VERITAS telescope

We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2-60 keV X-rays (RXTE) and TeV γ-rays (Whipple and HEGRA) in 2001. Accompanying optical monitoring was performed with the Mt. Hopkins 48 inch telescope. The unprecedented quality of this data set enables us to establish the existence of the correlation between the TeV and X-ray luminosities, and also to start unveiling some of its characteristics, in particular its energy dependence and time variability. The source shows strong variations in both X-ray and γ-ray bands, which are highly correlated. No evidence of an X-ray/γ-ray interband lag τ is found on the full week data set, with τ 3 ks. A detailed analysis of the March 19 flare, however, reveals that data are not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band being simultaneous. We estimate a -->2.1 ± 0.7 ks TeV lag. The amplitudes of the X-ray and γ-ray variations are also highly correlated, and the TeV luminosity increases more than linearly with respect to the X-ray one. The high degree of correlation lends further support to the standard model in which a unique electron population produces the X-rays by synchrotron radiation and the γ-ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the γ-ray flux scales approximately quadratically with respect to the X-ray flux poses a serious challenge to emission models for TeV blazars, as it requires rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region. We briefly discuss the astrophysical consequences of these new findings in the context of the competing models for the jet emission in blazars.

Multiwavelength observations of a flare from Markarian 501

We have used data from the TeV γ-ray flare associated with the active galaxy Markarian 421 observed on 15 May 1996 to place bounds on the possible energy-dependence of the speed of light in the context of an effective quantum gravitational energy scale. The possibility of an observable time dispersion in high energy radiation has recently received attention in the literature, with some suggestions that the relevant energy scale could be less than the Planck mass and perhaps as low as 10GeV. The limits derived here indicate this energy scale to be in excess of 4 × 10GeV at the 95% confidence level. To the best of our knowledge, this constitutes the first convincing limit on such phenomena in this energy regime. Submitted to Physical Review Letters

Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare

Abstract The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic Radiation Imaging Telescope Array System) has been in operation since February 2005. We present here a technical description of the instrument and a summary of its performance. The calibration methods are described, along with the results of Monte Carlo simulations of the telescope and comparisons between real and simulated data. The analysis of TeV γ-ray observations of the Crab Nebula, including the reconstructed energy spectrum, is shown to give results consistent with earlier measurements. The telescope is operating as expected and has met or exceeded all design specifications.