J. Zweerink

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.

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

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

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

Multiwavelength observations of a flare from Markarian 501

--> 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 During a TeV/X-Ray Flare

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

Gamma-Ray Variability of the BL Lacertae Object Markarian 421

We present multiwavelength observations of the BL Lacertae object Markarian 501 (Mrk 501) in 1997 between April 8 and April 19. Evidence of correlated variability is seen in very high energy (VHE; E * 350 GeV) gray observations taken with the Whipple Observatory g-ray telescope, data from the Oriented Scintillation Spectrometer Experiment of the Compton Gamma Ray Observatory , and quick-look results from the All-Sky Monitor of the Rossi X-Ray Timing Explorer , while EGRET did not detect Mrk 501. Short-term optical correlations are not conclusive, but the U-band flux observed with the 1.2 m telescope of the Whipple Observatory was 10% higher than in March. The average energy output of Mrk 501 appears to peak in the 2‐100 keV range, which suggests an extension of the synchrotron emission to at least 100 keV, the highest observed in a blazar and »100 times higher than that seen in the other TeV-emitting BL Lac object, Mrk 421. The VHE g-ray flux observed during this period is the highest ever detected from this object. The VHE g-ray energy output is somewhat lower than the 2‐100 keV range, but the variability amplitude is larger. The correlations seen here do not require relativistic beaming of the emission unless the VHE spectrum extends to *5 TeV.

Measurement of TeV gamma-ray spectra with the Cherenkov imaging technique

A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with the first ever simultaneous or nearly simultaneous observations at GeV gamma-ray, UV, IR, mm, and radio energies for this nearest BL Lac object. While the GeV gamma-ray flux increased slightly, there is little evidence for variability comparable to that seen at TeV and X-ray energies. Other wavelengths show even less variability. This provides important constraints on the emission mechanisms at work. We present the multiwavelength spectrum of this gamma-ray blazar for both quiescent and flaring states and discuss the data in terms of current models of blazar emission.

New Limits to the Infrared Background: Bounds on Radiative Neutrino Decay and on Contributions of Very Massive Objects to the Dark Matter Problem

We report on the γ-ray variability of Mrk 421 at Eγ > 300 GeV during the 1995 season, and concentrate on the results of an intense multiwavelength observing campaign in the period April 20 to May 5, which included >100 MeV γ-ray, X-ray, extreme-ultraviolet, optical, and radio observations, some of which show evidence for correlated behavior. Rapid variations in the TeV γ-ray light curve with doubling and decay times of 1 day require a compact emission region and significant Doppler boosting. The TeV data reveal that the γ-ray emission is best characterized by a succession of rapid flares with a relatively low baseline level of steady emission.

The TeV gamma-ray spectrum of markarian 421 during an intense flare

Abstract In this paper, we seek to establish reliable methods for extracting energy spectra for TeV gamma-ray sources observed using the atmospheric Cherenkov Imaging Technique. Careful attention has been paid to the calculation of the telescope gain, and we obtain good agreement between direct measurements, with a statistical error of about 10%, and an absolute calibration from the background cosmic-ray trigger rate that has an overall error of 18%. Two independent analyses that are based on different Monte Carlo shower simulations, employ different selection criteria in order to retain a large fraction of gamma-ray events, and use different approaches to spectral estimation are presented here. The first is a fairly traditional method that builds on established image selection techniques and calculates the detector collection area and an energy estimation function. The error in measuring the enrgy of a single event is estimated at 36%, and we try to compensate for this poor energy resolution. The second analysis uses more elegant gamma-ray selection criteria and implicity incorporates the properties of the detector into the simulations that are then compared with the data in order to obtain source spectra. The two simulations are compared to each other and to the data, with the aim of establishing that each method is robust and insensitive to simulation details. Finally, we consider the main sources of systematic errors, the largest of which is in the telescope gain calibration, arising from an incomplete knowledge of the relevant factors, and is estimated to be 16%. The effect of possible errors in the simulations is also considered. Both methods have been applied to a part of the Whipple observatory database on the Crab Nebula for the 1988/89 observing season, while the first method has also been applied to data taken in 1995/96. The statistical error in the flux constant is about 8% and that in the spectral index is about 5%, while the corresponding systematic errors are estimated to be 18% and 2%, respectively. The results presented here show good agreement between the two methods as well as between the two seasons. However, a comprehensive consideration of the implications of the derived spectra and a comparison to other work is addressed in another paper.

Outburst of TeV photons from Markarian 421

From considering the effect of γ-γ interactions on recently observed TeV gamma-ray spectra, improved limits are set to the density of extragalactic infrared photons which are robust and essentially model independent. The resulting limits are more than an order of magnitude more restrictive than direct observations in the 0.025–0.3 eV regime. These limits are used to improve constraints on radiative neutrino decay in the mass range above 0.05 eV and to rule out very massive objects as providing the dark matter needed to explain galaxy rotation curves. Lower bounds on the maximum distance which TeV gamma rays may probe are also derived.