Observations of Mkn 421 in 2004 with HESS at large zenith angles
نویسندگان
چکیده
Mkn 421 was observed during a high flux state for nine nights in April and May 2004 with the fully operational High Energy Stereoscopic System (HESS) in Namibia. The observations were carried out at zenith angles of 60◦–65◦, which result in an average energy threshold of 1.5 TeV and a collection area reaching 2 km2 at 10 TeV. Roughly 7000 photons from Mkn 421 were accumulated with an average gamma-ray rate of 8 photons/min. The overall significance of the detection exceeds 100 standard deviations. The light-curve of integrated fluxes above 2 TeV shows changes of the diurnal flux up to a factor of 4.3. For nights of high flux, intra-night variability is detected with a decay time of less than 1 h. The time averaged energy spectrum is curved and is well described by a power-law with a photon index Γ = 2.1±0.1stat ±0.3sys and an exponential cutoff at Ec = 3.1(+0.5 − 0.4)stat ± 0.9sys TeV and an average integral flux above 2 TeV of 3 Crab flux units. Significant variations of the spectral shape are detected with a spectral hardening as the flux increases. Contemporaneous multi-wavelength observations at lower energies (X-rays and gamma-rays above ≈300 GeV) indicate smaller relative variability amplitudes than seen above 2 TeV during high flux state observed in April 2004.
منابع مشابه
Detection of Vhe -rays from Mkn 421 with the Hegra Cherenkov Telescopes
A detection of-rays from Mkn 421 at energies above 1 TeV is reported, based on observations made in December 1994-May 1995 with the rst two HEGRA Cherenkov telescopes. From the image analysis, 111 excess gamma candidates are obtained from the 26 h Telescope #1 (CT1) dataset (signiicance 4.0) and 218 from the 41 h Telescope #2 (CT2) dataset (signiicance 4.2) at zenith angles < 25. The combined s...
متن کاملLarge zenith angle observations with the high-resolution GRANITE III camera
The GRANITE III camera of the Whipple Cherenkov Telescope at the Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona (2300 m a.s.l.) has the highest angular resolution of all cameras used on this telescope so far. The central region of the camera has 379 pixels with an individual angular diameter of 0.12. This makes the instrument especially suitable for observations of gamma-induced ai...
متن کاملEffectiveness of TeV γ-ray observations at large zenith angles with a stereoscopic system of imaging atmospheric Čerenkov telescopes
The sensitivity of imaging atmospheric Čerenkov telescopes (IACTs) in TeV γ-ray observations reachs its maximum at small zenith angles (Θ ≤ 30◦) which provide the minimum attainable energy threshold of an instrument. However, for a specific telescope site a number of γ-ray sources, or source candidates, can only be observed at much larger zenith angles (Θ ≤ 60◦). Moreover the observations at la...
متن کاملMulti-wavelength Observations of the TeV Blazars Mkn 421, 1ES1959+650, and H1426+428 with the HEGRA Cherenkov Telescopes and the RXTE X-ray Satellite
Recent results obtained with the HEGRA system of imaging Cherenkov telescopes on the TeV emission of the Blazars Mkn 421 (z = 0.030), 1ES1959+650 (z = 0.047), and H1426+428 (z = 0.129) are reported. For Mkn 421, a close connection of the average flux level and spectral shape has been observed during the periods of increased activity in the years 2000 and 2001. Simultaneously taken data with the...
متن کاملSearch for gamma - rays above 10 TeV from Markarian 421 in a high state with the CANGAROO - II telescope
A preliminary result from Markarian 421 observations in the energy region above 10 TeV with the CANGAROO-II 10 m telescope is presented. In January 2001, the HEGRA group reported that Markarian 421 had become very active, with flux levels up to 4 times that of the Crab Nebula. As a result, we observed Mkn 421 during six nights from Jan-uary 24th to February 1st, and four nights from March 1st t...
متن کاملذخیره در منابع من
با ذخیره ی این منبع در منابع من، دسترسی به آن را برای استفاده های بعدی آسان تر کنید
عنوان ژورنال:
دوره شماره
صفحات -
تاریخ انتشار 2005