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Register a new userMeasurement of the Crab Flux Above 60 GeV with the CELESTE Cherenkov Telescope
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We have converted the former solar electrical plant THEMIS (French Pyrenees) into an atmospheric Cherenkov detector called CELESTE, which records gamma rays above 30 GeV (7E24 Hz). Here we present the first sub-100 GeV detection by a ground based telescope of a gamma ray source, the Crab nebula, in the energy region between satellite measurements and imaging atmospheric Cherenkov telescopes. At our analysis threshold energy of 60 +/- 20 GeV we measure a gamma ray rate of 6.1 +/- 0.8 per minute. Allowing for 30% systematic uncertainties and a 30% error on the energy scale yields an integral gamma ray flux of I(E>60 GeV) = 6.2^{+5.3}_{-2.3} E-6 photons m^-2 s^-1. The analysis methods used to obtain the gamma ray signal from the raw data are detailed. In addition, we determine the upper limit for pulsed emission to be <12% of the Crab flux at the 99% confidence level, in the same energy range. Our result indicates that if the power law observed by EGRET is attenuated by a cutoff of form e^{-E/E_0} then E_0 < 26 GeV. This is the lowest energy probed by a Cherenkov detector and leaves only a narrow range unexplored beyond the energy range studied by EGRET.
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The CELESTE atmospheric Cherenkov detector, running until June 2004 at the Themis solar facility, has taken data on compact sources such as pulsars and blazars. We will take stock of the experiment, in particular regarding the latest improvements of the detector simulation and data analysis. These changes provide us with a new analysis of old data with smaller uncertainties. We present here the evidence for a weak signal from Mrk 501 in 2000-2001.
The new analysis variable xi, shown to be powerful on the data taken with the final configuration of CELESTE, has been applied to data taken with previous detector configurations. First, the analysis is validated on Crab observations, and then the cuts for the blazar Mrk 501 are optimized using Mrk 421 data since the sources have similar declinations. Data from Mrk 501 was recorded in 2000 and 2001. The old analysis gave a 2.5 sigma excess. We obtain an excess of 2.9 sigma during this time and of 4.9 sigma during May and June 2000 that we interpret as a gamma-ray signal from Mrk 501, for which we calculate a flux of (6.9 +/- 2.2) * 10^{-7} photons m^{-2} s^{-1}. An upper limit from the other data with no signal is determined.
We present results from a NuSTAR observation of the Crab made at a large off-axis angle of 1.5degree. At these angles X-rays do not pass through the optics, but rather illuminate the detectors directly due to incomplete baffling. Due to the simplicity of the instrument response in this configuration and the good absolute calibration of the detectors, we are able to measure the absolute intrinsic flux of the Crab to better than 4%. We find the spectral parameters of the powerlaw to be Gamma=2.106 +/- 0.006, N=9.71 +/-0.16, in agreement with the values measured 42 years ago by Toor & Seward (1974). This suggests that the observed variability of the Crab is not part of a long term trend, but instead results from fluctuations around a steady mean. The NuSTAR observation also enabled improved measurement of the detector absorption parameters without the added complications of the mirror response.
Since 2009, several rapid and bright flares have been observed at high energies (>100 MeV) from the direction of the Crab Nebula. Several hypotheses have been put forward to explain this phenomenon, but the origin is still unclear. The detection of counterparts at higher energies with the next generation of Cherenkov telescopes will be determinant to constrain the underlying emission mechanisms. We aim at studying the capability of the Cherenkov Telescope Array (CTA) to explore the physics behind the flares, by performing simulations of the Crab Nebula spectral energy distribution, both in flaring and steady state, for different parameters related to the physical conditions in the nebula. In particular, we explore the data recorded by Fermi during two particular flares that occurred in 2011 and 2013. The expected GeV and TeV gamma-ray emission is derived using different radiation models. The resulting emission is convoluted with the CTA response and tested for detection, obtaining an exclusion region for the space of parameters that rule the different flare emission models. Our simulations show different scenarios that may be favourable for achieving the detection of the flares in Crab with CTA, in different regimes of energy. In particular, we find that observations with low sub-100 GeV energy threshold telescopes could provide the most model-constraining results.
The gamma-ray emission of the blazar Markarian 421 above 250 GeV has been observed by the CAT Cherenkov imaging telescope since December, 1996. We report here results on the source variability up to April, 1998, with emphasis on the 1998 campaign. For the flaring periods of this year, the energy spectrum was derived from 330 GeV up to 5.2 TeV: it is very well represented by a simple power law, with a differential spectral index of 2.96 +/- 0.13.
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