Invited review at the Frascati Workshop 2007, Vulcano, Italy, May 28 - June 2 on "Multifrequency Behaviour of High Energy Cosmic Sources". ChJAA, in press. 12 pages, 3 figures
The window of TeV Gamma-Ray Astrophysics was opened less than two decades ago, when the Crab Nebula was detected for the first time. After several years of development, the technique used by imaging atmospheric Cherenkov telescopes like HESS, MAGIC or VERITAS, is now allowing to conduct sensitive observations in the TeV regime. Water Cherenkov instruments like Milagro are also providing the first results after years of integration time. Different types of extragalactic and galactic sources have been detected, showing a variety of interesting phenomena that are boosting theory in very high energy gamma-ray astrophysics. Here I review some of the most interesting results obtained up to now, making special emphasis in the field of X-ray/gamma-ray binaries.
The possibility of detecting Gamma-Ray Bursts (GRB) in TeV energy range using large area muon detectors like AMANDA and Lake Baikal detector is examined. These detectors can detect TeV energy photons by detecting the secondary muons created by the TeV photons in the Earth's atmosphere. We calculate the expected number of muons and the signal to square root of noise ratios in these detectors due to TeV gamma-rays from individual GRBs for various assumption on their luminosity, distance from the observer (redshift), gamma-ray integral spectral index, maximum energy cutoff of the photon spectrum …
A unifying theme of this conference was the use of different approaches to understand astrophysical sources of energetic particles in the TeV range and above. In this summary I review how gamma-ray astronomy, neutrino astronomy and (to some extent) gravitational wave astronomy provide complementary avenues to understanding the origin and role of high-energy particles in energetic astrophysical sources.
We have examined the directional cross-correlation of statistical `hot-spots' between a Northern Sky TeV Gamma Ray Survey by the Milagro Observatory and a similar survey by the Tibet Array. We find the directions of these hot-spots are angularly uncorrelated between the two surveys for large angular separations (Delta theta > 4 degrees), but there appears to be a statistically significant correlation between hot-spot directions for Delta theta < 1.5 degrees. Independent simulations indicate the chance probability for the occurrence of this correlation is approximately 10^-4, implying the exist…
We discuss an effect of the intergalactic absorption of the TeV gamma-rays in time-averaged spectrum of Mkn 501 measured by the HEGRA Collaboration. Analysis of the spectral behavior, variability time scale and relevant calculations of TeV gamma-ray emission allow to conclude the presence of a noticeable absorption of the TeV gamma-rays in the Mkn 501 energy spectrum.
We have investigated TeV gamma-ray emissions from nearby X-ray selected BL Lac objects using the Utah Seven Telescope Array for more than two years. These objects can be considered as potential sources of TeV gamma rays with inverse compton model. The gamma-ray flares from Mrk421 and Mrk501 found in these observation will be reported.
Recent results of GeV and TeV observations of gamma-rays from the Universe are briefly reviewed. Topics include observational technique, diffuse gamma-rays, pulsars, unidentified sources, plerions, supernova remnants and AGNs.
The recent detection of gamma radiation from Mkn 501 at energies as high as 25 TeV suggests stringent upper bounds on the diffuse, far infrared, extragalactic radiation density. The production of electron-positron pairs through photon-photon collisions would prevent gamma photons of substantially higher energies from reaching us across distances of order 100 Mpc. However, coherently arriving TeV or sub-TeV gammas - Bose-Einstein condensations of photons at these energies - could mimic the Cherenkov shower signatures of extremely energetic gammas. To better understand such events, we describe t…
Limitations on the multi-dimensional TeV-scale quantum gravity model by Arkani-Hamed, Dimopoulos and Dvali (1998) and on model by Dvali, Gabadadze, Porrati (2000) are obtained from an analysis of gamma-ray bursts at cosmological distances (relativistic fireball model).
We observed the first known very high energy (VHE) gamma-ray emitting unidentified source, TeV J2032+4130, for 94 hours with the MAGIC telescope. The source was detected with a significance of 5.6 sigma. The flux, position, and angular extension are compatible with the previous ones measured by the HEGRA telescope system five years ago. The integral flux amounts to (4.5+-0.3stat+-0.35sys)x10^{-13} ph cm$^{-2}$ s$^{-1}$ above 1 TeV. The source energy spectrum, obtained with the lowest energy threshold to date, is compatible with a single power law with a hard photon index of Gamma=-2.0+-0.3stat…
The idea that the TeV air showers, thought to be produced by >10 TeV gamma rays from Mrk 501, can be mimicked by coherent bunches of sub-TeV photons is reexamined, focusing on fundamental considerations. In particular, it is shown that the minimum spot size of the beam of pulsed TeV photons arriving at Earth is on the order of a few kilometers, unless a lens with certain characteristics is placed between the TeV laser and Earth. The viability of the laser production mechanism proposed by Harwit et al. (2000) is also reassessed.
We have detected gamma-rays from RXJ1713$-$39 in the energy range between 400 GeV and 5 TeV using a new kind of analysis: likelihood analysis. The statistical significance of the measurement is greater than 8 $σ$. The details of this analysis method are presented.
The field of TeV gamma-ray astronomy is reviewed with emphasis on its relation to the origin of cosmic rays. The discovery of TeV photons from supernova remnants and active galaxies has provided the first direct observational link between specific astrophysical objects and particle production at the TeV scale. TeV gamma-ray observations constrain the high end of the electromagnetic spectrum, a regime most sensitive for testing particle acceleration and emission models. TeV telescopes have made important contributions to the understanding of blazars and supernova remnants, however, it will take…
We present the results of a search for pulsed TeV emission from the Crab pulsar using 97 hours of data recorded with the high-resolution GRANITE III camera of the Whipple 10 m gamma-ray telescope.
One of remarkable features of the gamma ray blazar Markarian 501 is the reported shape of the TeV spectrum, which during strong flares of the source remains essentially stable despite dramatic variations of the absolute gamma-ray flux. I argue that this (to a large extent unexpected) behavior of the source could be explained assuming that the TeV gamma-ray emission is a result of synchrotron radiation of extremely high energy protons in highly magnetized compact regions of the jet.
In this paper we present the results of simultaneous observations of the TeV blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively. The source was monitored from February 2nd to February 16th and from May 3rd to May 8th, 2000. We discuss in detail the temporal and spectral properties of the source. Remarkably, the TeV observations of February 7th/8th showed statistically significant evidence for substantial TeV flux var…
Several strong TeV gamma-ray flares were detected from Markarian 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 sigma. Markarian 421 was unprecedentedly active at X-ray and TeV gamma-ray energies during this period, and a positive correlation was found between the change of the ASM/RXTE X-ray flux and the Tibet TeV gamma-ray flux. When a power-law energy spectrum for gamma-rays from this source is assumed, the spectral index is calculated to be -3.24+-0.69 at the most active phase in 2001. The spectral index observed by the Tibet …
We will report the observations of TeV gamma ray flares from Markarian 501 using Telescope Array Prototype. The observation were carried out continuously from the end of March to the end of July in 1997. The energy spectrum, and the time variation of the gamma ray intensities are shown. The intensity has been changed by the order of magnitude in this period and the possible quasi periodic oscillation of 12.7days were discovered.
The Milagro telescope monitors the northern sky for 100 GeV - 100 TeV transient emission through continuous very high energy wide-field observations. The large effective area and low energy threshold of Milagro allow it to detect very high energy gamma-ray burst emission with much higher sensitivity than previous instruments, and a fluence sensitivity at TeV energies comparable to dedicated gamma-ray burst satellites at keV-MeV energies. Observation of gamma-ray burst emission at TeV energies could place important constraints on gamma-ray burst progenitor and emission models. This study detail…
The detection of fast variations of the TeV (10^12 eV) gamma-ray flux, on time-scales of days, from the nearby radio galaxy M 87 is reported. These variations are ~10 times faster than that observed in any other waveband and imply a very compact emission region with a dimension similar to the Schwarzschild radius of the central black hole. We thus can exclude several other sites and processes of the gamma-ray production. The observations confirm that TeV gamma-rays are emitted by extragalactic sources other than blazars, where jets are not relativistically beamed towards the observer.
We study the emission from an old supernova remnant (SNR) with an age of around 10^5 yrs and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10^5 yrs, proton acceleration is efficient enough to emit TeV gamma-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV gamma-rays and X-rays are dominated by hadronic processes, pi^0-decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10^5 yrs, there are few high-energy part…
Using data from the HEGRA air shower array, taken in the period from April 1998 to March 2000, upper limits on the ratio I_gamma /I_CR of the diffuse photon flux I_gamma to the hadronic cosmic ray flux I_CR are determined for the energy region 20 TeV to 100 TeV. The analysis uses a gamma-hadron discrimination which is based on differences in the development of photon- and hadron-induced air showers after the shower maximum. A method which is sensitive only to the non-isotropic component of the diffuse photon flux yields an upper limit of I_gamma /I_CR (at 54 TeV) < 2.0*10^-3 (at the 90% confid…
Milagro is a gamma-ray observatory employing a water Cherenkov detector to observe extensive air showers produced by high-energy particles impacting in the Earth's atmosphere. We discuss the first detection of TeV gamma-rays from the Galactic plane and report the detection of an extended TeV source coincident with the EGRET source 3EG J0520+2556, and the observation of TeV emission from the Cygnus region of our Galaxy. We also summarize the status of our search for Very High Energy (VHE) emission from satellite-triggered Gamma Ray Bursts (GRBs) and discuss plans for the next generation water C…
We report the TeV gamma-ray observations of the nearby normal spiral galaxy NGC 253. At a distance of $\sim$2.5 Mpc, NGC 253 is one of the nearest starburst galaxies. This relative closeness, coupled with the high star formation rate in the galaxy, make it a good candidate TeV gamma-ray source. Observations were carried out in 2000 and 2001 with the CANGAROO-II 10 m imaging atmospheric Cerenkov telescope. TeV gamma-ray emission is detected at the $\sim 11σ$ level with a flux of $(7.8 \pm 2.5)\times 10^{-12} {\rm cm}^{-2} {\rm sec}^{-1}$ at energies $>$0.5 TeV. The data indicate that the emissi…
The origin of Galactic CRs up the knee energy remains unanswered and provides strong motivation for the study of gamma-ray sources at energies above 10 TeV. We discuss recent results from ground-based gamma-ray Cherenkov imaging systems at these energies as well as future observational efforts in this direction. The exciting results of H.E.S.S. give clues as to the nature of Galactic CR accelerators, and suggest that there is a population of Galactic gamma-ray sources with emission extending beyond 10 TeV. A dedicated system of Cherenkov imaging telescopes optimised for higher energies appears…
The Atmospheric Cherenkov Imaging Technique has opened up the gamma-ray spectrum from 100 GeV to 50 TeV to astrophysical exploration. The development of the technique (with emphasis on the early days) is described as are the basic principles underlying its application to gamma-ray astronomy. The current generation of arrays of telescopes, in particular, VERITAS is briefly described.
We obtained new upper limits on the diffuse gamma rays from the inner Galactic (IG) and outer Galactic (OG) planes in 3.10 TeV region, using the Tibet air shower data and new Monte Carlo simulation results. A difference of the effective area of the air-shower array for observing gamma rays and cosmic rays was carefully taken into account in this analysis, resulting in that the flux upper limits of the diffuse TeV gamma rays were reduced by factors of 4.0.3.7 for 3.10 TeV than those in our previous results (Amenomori, M., Ayabe, S., Cui, S.W., et al. Observation of multi-TeV diffuse gamma rays …
We searched for TeV gamma-rays from the remnant of SN 1987A around 5400 days after the supernova. The observations were carried out in 2001, from November 16 to December 11, using the CANGAROO-II Imaging Atmospheric Cherenkov Telescope. In total, 708 minutes of ON- and 1019 minutes of OFF-source data were obtained under good conditions. The detection threshold was estimated to be 1 TeV, due to the mean zenith angle of 39$^\circ$. The upper limits for the gamma-ray flux were obtained and compared with the previous observations and theoretical models. The observations indicate that the gamma-ray…
We searched for pulsed gamma-ray emissions from the Crab pulsar using data of the Tibet-III air shower array from November 1999 through November 2005. No evidence for the pulsed emissions was found in our analysis. Upper limits at different energies were calculated for a $3 σ$ confidence level in the energy range of multi-TeV to several hundred TeV.
The recent advances in TeV gamma-ray astronomy are largely the result of the ability to differentiate between extensive air showers generated by gamma rays and hadronic cosmic rays. Air Cherenkov telescopes have developed and perfected the "imaging" technique over the past several decades. However until now no background rejection method has been successfully used in an air shower array to detect a source of TeV gamma rays. We report on a method to differentiate hadronic air showers from electromagnetic air showers in the Milagro gamma ray observatory, based on the ability to detect the energe…
Although they were discovered more than 25 years ago, gamma-ray bursts are still a mystery. Even their characteristic distance is highly uncertain. All that we can be confident about is that they involve compact objects and relativistic plasma. Current ideas and prospects are briefly reviewed. There are, fortunately, several feasible types of observation that could soon clarify the issues.
Nuclear gamma-ray lines constitute the most genuine diagnostic tool of nuclear astrophysics, since they allow for an unambiguous identification of isotopic species. Continuous improvement in instrumentation led to the discovery of several radioactive species in the past 15 years (Al26, Co56, Co57, Ti44) in various astrophysical sites (SN1987A and SN1991T, Cas-A, the Milky Way disk). These discoveries boosted theoretical activity on the nucleosynthesis of these radioactive isotopes and on the refined modelling of the corresponding sites, improving our knowledge on stellar evolution, stellar exp…
We report a possible detection of TeV gamma-rays from the Galactic Center by the Whipple 10m 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 standard deviations. The measured excess corresponds to an integral flux of \Flux above an energy of $2.8 \mathrm{TeV}$, roughly 40% of the flux from the Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15 arcmin and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence for va…
We propose a method of measurement of extragalactic magnetic fields in observations of TeV gamma-rays from distant blazars. Multi-TeV gamma-rays are emitted by these sources in narrow jet with opening angle of few degrees. These primary gamma-rays interact with the infrared photon background producing secondary electrons and positrons, which can be just slightly deflected by extragalactic magnetic fields before they emit secondary gamma-rays via inverse Compton scattering of cosmic microwave background photons. Secondary gamma-rays emitted toward an observer on the Earth can be detected as ext…
The differential spectrum of TeV gamma rays between 1.5 TeV and 20 TeV from the north-east rim of SN1006 was obtained from the data observed in 1996 and 1997 using the 3.8m CANGAROO Čerenkov telescope. This spectrum matches the model calculated using the Inverse Compton (IC) process with 2.7k Cosmic Microwave Background (CMB). This enables us to estimate the absolute strength of the magnetic field around the shock and the maximum energy of accelerated electrons with the considerable accuracy: the obtained field strength and maximum electron energy are $4\pm1$ $μ$G and 50 TeV respectively. Also…
The detection of TeV gamma-rays from the blazar H 1426+428 at an integral flux level of (4 +- 2(stat) +- 1(syst)) 10^(-12) erg cm^(-2) s^(-1) above 1 TeV with the HEGRA imaging atmospheric Cherenkov telescope system is reported. H 1426+428 is located at a redshift of z=0.129, which makes it the most distant source detected in TeV gamma-rays so far. The TeV radiation is expected to be strongly absorbed by the diffuse extragalactic background radiation (DEBRA). The observed energy spectrum of TeV photons is in good agreement with an intrinsic power law spectrum of the source ~E^(-1.9) corrected …
The Whipple 10 meter atmospheric Cherenkov telescope has made observations of the region known as the Cygnus arm. This region has been recently reported by the Milagro experiment to contain a diffuse TeV gamma-ray source centered at RA=308 and Dec=42. We report upper limits (using the Whipple 10 m telescope) obtained during the Fall 2004 observing season centered on RA=310 and Dec=42.65.
We report on observations of the Perseus cluster of galaxies using the 10 m Whipple $γ$-ray telescope during the 2004-2005 observing season. We apply a two dimensional analysis technique to allow us to scrutinize the cluster for TeV emission. In this contribution we will first calculate flux upper limits on TeV gamma-ray sources within the cluster. Second, we derive an upper limit on the extended cluster emission. We then compare the flux upper limits with the EGRET upper limit at 100 MeV and theoretical models.
Using air showers induced by TeV gamma-rays from Mrk 501, the radial distribution of Cherenkov light is investigated. The shower geometry is reconstructed from the stereoscopic shower images obtained with the telescopes of the HEGRA IACT system. We observe a change in the shape of the light pool with shower energy, and with zenith angle. Data are well reproduced by Monte-Carlo air shower simulations.
CANGAROO-III is an Imaging Atmospheric Cherenkov Telescope (IACT) array of four 10 m telescopes for very high energy (sub-TeV) gamma-ray astronomy. A design study of the CANGAROO-III telescope system was carried out using the Monte Carlo technique in order to optimize the pixel size and the telescope spacing. Studies were also made of observations at low elevation angles.
The development of techniques whereby gamma rays of energy 100 GeV and above can be studied from the ground, using indirect, but sensitive, techniques has opened up a new area of high energy photon astronomy. The most exciting result that has come from these is the detection of highly variable fluxes of TeV gamma rays from the relativistic jets in nearby AGN. The recent detection of signals from a starburst galaxy and from a radio galaxy opens the possibility that the extragalactic emission of TeV gamma rays is a ubiquitous phenomenon. Here we attempt to summarize the properties of the sources…
Because of the limited size of the satellite-borne instruments, it has not been possible to observe the flux of gamma ray bursts (GRB) beyond GeV energy. We here show that it is possible to detect the GRB radiation of TeV energy and above, by detecting the muon secondaries produced when the gamma rays shower in the Earth's atmosphere. Observation is made possible by the recent commissioning of underground detectors (AMANDA, the Lake Baikal detector and MILAGRO) which combine a low muon threshold of a few hundred GeV or less, with a large effective area of 10^3 m^2 or more. Observations will no…
Using the HEGRA system of imaging atmospheric Cherenkov telescopes, the Andromeda galaxy (M31) was surveyed for TeV gamma ray emission. Given the large field of view of the HEGRA telescopes, three pointings were sufficient to cover all of M31, including also M32 and NGC205. No indications for point sources of TeV gamma rays were found. Upper limits are given at a level of a few percent of the Crab flux. A specific search for monoenergetic gamma-ray lines from annihilation of supersymmetric dark matter particles accumulating near the center of M31 resulted in flux limits in the 10^-13/cm^2s ran…
Historical TeV gamma-ray flares from Markarian 501 was observed using Utah Seven Telescope Array (7TA) from the end of March to the end of July of 1997. The Rossi X-ray Timing Explorer All Sky Monitor (RXTE ASM) has been observing X rays from Mrk 501 since Jan. 5, 1996. We find evidence for periodicities of $23.9^{+2.3}_{-2.0}$ days and $23.9^{+1.6}_{-2.9}$ days from TeV gamma-ray and X-ray light curves using 7TA and RXTE ASM. Their false-alarm probabilities are $8.2 \times 10^{-3}$ and $5.6 \times 10^{-5}$, respectively.
TeV gamma-rays were recently detected from the nearby normal spiral galaxy NGC 253 (Itoh et al., 2002). Observations to detect the Cherenkov light images initiated by gamma-rays from the direction of NGC 253 were carried out in 2000 and 2001 over a total period of $\sim$150 hours. The orientation of images in gamma-ray--like events is not consistent with emission from a point source, and the emission region corresponds to a size greater than 10 kpc in radius. Here, detailed descriptions of the analysis procedures and techniques are given.
High-energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge of the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high-energy gamma-rays. Observation of gamma-rays gives an exciting view of the high-energy universe thanks to satellite-based telescopes (AGILE, GLAST) and to ground-based detectors like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which recently discovered more than 60 new very-high-energy sources. The progress ac…
Observations of very-high-energy (VHE, E > 250 GeV) gamma-ray emission from several blazars at z > 0.1 have placed stringent constraints on the elusive spectrum and intensity of the intergalactic infrared background radiation (IIBR). Correcting their observed VHE spectrum for gamma-gamma absorption even by the lowest plausible level of the IIBR provided evidence for a very hard (photon spectral index Gamma_{ph} < 2) intrinsic source spectrum out to TeV energies. Such a hard VHE gamma-ray spectrum poses a serious challenge to the conventional synchrotron-self-Compton interpretation of the VHE e…
Cosmic gamma rays provide insight into some of the most dynamic processes in the Universe. At the dawn of a new generation of gamma-ray telescopes, this review summarizes results from the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory, the principal predecessor mission studying high-energy photons in the 100 MeV energy range. EGRET viewed a gamma-ray sky dominated by prominent emission from the Milky Way, but featuring an array of other sources, including quasars, pulsars, gamma-ray bursts, and many sources that remain unidentified. A central feature of t…
The Crab Nebula has been observed by the HEGRA stereoscopic system of 4 imaging air Cherenkov telescopes (IACTs) for about 100 hours from 1997 September to 1998 March. The recent detailed studies on the system performance give an energy threshold and an energy resolution for gamma-rays of 500 GeV and \~20 %, respectively. The Crab spectrum was measured with the IACTs system in a very broad energy range from 500 GeV to 25 TeV, using the observations at zenith angles up to 50 degree. The pure power-law spectrum in TeV gamma-rays constrains the physics parameters of the nebula environment as well…
Gamma-ray bursts are violent events occurring randomly in the sky. In this review, I will present the fireball model, proposed to explain the phenomenon of gamma-ray bursts. This model has important consequences for the production and observation at Earth of gravitational waves, high energy neutrinos, cosmic rays and high energy photons, and the second part of this review will be focused on these aspects. A last section will briefly discuss the topic of the use of gamma-ray bursts as standard candles and possible cosmological studies.
232 hours of data were accumulated from 1997 to 1999, using the HEGRA Stereoscopic Cherenkov Telescope System to observe the supernova remnant Cassiopeia A. TeV gamma ray emission was detected at the 5 sigma level, and a flux of (5.8 +- 1.2(stat) +- 1.2(syst)) 10^(-9) ph m^(-2) s^(-1) above 1 TeV was derived. The spectral distribution is consistent with a power law with a differential spectral index of -2.5 +- 0.4(stat) +- 0.1(syst) between 1 and 10 TeV. As this is the first report of the detection of a TeV gamma ray source on the "centi-Crab" scale, we present the analysis in some detail. Imp…
We report on a study of sub-TeV ($E_γ> 10$ GeV) gamma-ray-induced muon secondaries in coincidence with BATSE gamma ray bursts (GRBs). Each TeV gamma ray striking the atmosphere produces $\approx$0.2 muons whose identity and angle can be measured by the Project GRAND array. Eight GRB candidates were studied; seven were selected based upon the optimum product of (detected BATSE fluence) $\times$ (GRAND's acceptance). One candidate was added because it was reported as a detection by the Milagrito collaboration. Seven candidates show a positive, though not statistically significant, muon excess. T…
We report on observations of Markarian 421 made with the Whipple 10m Cerenkov telescope and the RXTE satellite during a multi-wavelength campaign in December 2002 and January 2003, initiated by a Whipple target of opportunity. The observations revealed several flares with flux levels between 1 and 2 times the flux of the Crab Nebula. We will discuss the temporal properties, including evidence of X-ray/TeV gamma-ray flux correlation.
Gamma-ray bursts (GRBs) and following afterglows are considered to be produced by dissipation of kinetic energy of a relativistic fireball and radiation process is widely believed as synchrotron radiation or inverse Compton scattering of electrons. We argue that the transfer of kinetic energy of ejecta into electrons may be inefficient process and hence the total energy released by a GRB event is much larger than that emitted in soft gamma-rays, by a factor of \sim (m_p/m_e). We show that, in this case, very strong emission of TeV gamma-rays is possible due to synchrotron radiation of protons …
TeV gamma rays from the blazar Markarian 501 have been detected with the University of Durham Mark 6 atmospheric Cerenkov telescope using the imaging technique at large zenith angles. Observations were made at zenith angles in the range 70 - 73 deg during 1997 July and August when Markarian 501 was undergoing a prolonged and strong flare.
