R. C. Lamb

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}

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

--> 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 a flare from Markarian 501

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

Point sources of GeV gamma rays

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

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

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 THE MULTI-TeV GAMMA-RAY FLARE SPECTRA OF MARKARIAN 421 AND MARKARIAN 501

A catalog of c-ray sources based on photons with energies greater than 1 GeV has been developed nfrom observations taken by the EGRET instrument of the Compton Gamma Ray Observatory. The data nare taken from the 4.5 yr of observation available in the public data archives. We emphasize sources that nare detected using the entire database, without regard to any possible transient or variable behavior. Ten nof the 57 sources reported here have not previously been reported in the catalogs developed using nphotons above 100 MeV in energy. Twenty-seven sources have identiÐcations with objects seen at other nwavelengths: the Large Magellanic Cloud, Ðve pulsars, and 21 blazars. The remaining 30 sources are nclassiÐed as unidentiÐed; however, seven may be associated with Galactic supernova remnants and one nsource may be a Galactic X-ray binary (LSI 61 303). The 30 unidentiÐed sources are distributed nearly nuniformly along the Galactic plane and are symmetric about it. Only one of the unidentiÐed sources has na Galactic latitude in excess of 30i, whereas, if the sources were distributed uniformly, D12 would be nexpected on the basis of the combined EGRET exposure. A scatter plot of the Nux from the unidentiÐed nsources versus Galactic latitude reveals two rather distinct categories of source: ““ bright II sources with nNuxes greater than or equal to 4.0]10~8 photons cm~2 s~1 and ““ dim II sources with Nuxes of less than n4.0]10~8 photons cm~2 s~1. The absence of high-latitude bright sources is striking. The bright nunidentiÐed sources have an average Galactic latitude of 2i.7, which is consistent with a Population I ndistribution at distances of 1E5 kpc. The dim unidentiÐed sources have a broader latitude distribution nwith an average o b o13i.8, indicating that if they are at the same average distance from the Galactic nplane as the bright sources, they are paradoxically approximately 5 times closer than the bright objects non average and therefore roughly 2 orders of magnitude less luminou

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

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.

The TeV Spectrum of Markarian 501

The energy spectrum of Markarian 421 in flaring states has been measured from 0.3 to 10 TeV using both small and large zenith angle observations with the Whipple Observatory 10 m imaging telescope. The large zenith angle technique is useful for extending spectra to high energies, and the extraction of spectra with this technique is discussed. The resulting spectrum of Markarian 421 is fitted reasonably well by a simple power law: J(E)=E−2.54 ± 0.03 ± 0.10 photons m-1 s-1 TeV-1, where the first set of errors is statistical and the second set is systematic. This is in contrast to our recently reported spectrum of Markarian 501, which over a similar energy range has substantial curvature. The differences in TeV energy spectra of gamma-ray blazars reflect both the physics of the gamma-ray production mechanism and possibly differential absorption effects at the source or in the intergalactic medium. Since Markarian 421 and Markarian 501 have almost the same redshift (0.031 and 0.033, respectively), the difference in their energy spectra must be intrinsic to the sources and not due to intergalactic absorption, assuming the intergalactic infrared background is uniform.

X-Ray Pulsars in the Small Magellanic Cloud

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.