Marcos López­Moya1 and the MAGIC Collaboration2
Dpto. Física Atómica, Facultad Ciencias Físicas, Universidad Complutense, 28040, Madrid, Spain
1
Updated collaborator list at http://hegra1.mppmu.mpg.de/MAGICWeb/collaborators.html 2
Abstract
MAGIC is a new generation Cherenkov telescope, with a reflector of 17m (the biggest of all the existing Cherenkov telescopes around the world), aims to detect cosmic γ­rays at energies above 30 GeV, closing the last unexplored window in the electromagnetic spectrum between 30 and 300 GeV. New technologies have been used in MAGIC to reduce the energy threshold and to increase the sensitivity for gamma detection. The analysis of the first data taken with the MAGIC Telescope during this year are presented in this poster.
γ­Ray Astronomy from ground
The MAGIC Telescope
The MAGIC (Major Atmospheric Gamma­ray Imaging Cherenkov) telescope[1] is a new and innovative instrument to detect gamma­rays from astrophysical sources. With an energy threshold of 30 GeV, MAGIC has the main goal of covering the gap between satellites and former ground­based γ­ray telescopes. The telescope is located at the “Roque de los Muchachos” Observatory, on the Canary Island of La Palma (Spain), and has been developed by an international collaboration of 18 institutions from 9 countries.
MAGIC innovates in several key aspects: • a 17 m diameter tessellated mirror mounted on an extremely light carbon­fibre frame (<10 tons), with active mirror control. • a high­efficiency camera composed by an array of 577 fast photomultipliers (PMTs), with a 3.6° field of view. • analogue signal transmission through optical fibre. • digitalisation of the analogue signals performed by 300 MHz FlashADCs, and a high data acquisition rate of up to 1 kHz.
The MAGIC camera with its 577 PMTs, placed on the camera access tower.
The MAGIC telescope
With all these innovations MAGIC is the Cherenkov telescope with the lowest energy threshold (30 GeV) attempted so far. These developments allow MAGIC to provide vital information on several established gamma­
ray sources, like Active Galactic Nuclei, Supernova Remnants, Gamma Ray Bursts and Pulsars.
Current Status
The official inauguration of MAGIC was held in October 2003, and since then, a commissioning phase started, testing the telescope performance and taking the first data. During this summer, the last mirrors have been mounted, and now the commissioning phase is over.
MAGIC started data taking in a commissioning phase in October 2003.
The Data Analysis Chain:
• An event recorded by MAGIC consists of the digitalized signals from the 577 PMTs of the camera, and this represents all
the information available for the analysis.
• The first step is to obtain the number of Cherenkov photons from the atmospheric shower which produced the recorded PMT
signals. For that, dedicated Pedestal and Calibration runs are taken at 5 min intervals, in order to subtract the noise introduced by the electronics chain and the night sky background (NSB), and to obtain the conversion factor between FADC counts and Cherenkov pulse digitalized by one of the 577 number of photons. The calibration is performed by illuminating channels of MAGIC, with the low (black) and high gains. Each FADC sample lasts 3.3 ns.
the camera with short light pulses (4
ns) from a set of ultra­fast LEDs of different colours housed in the centre of the reflector. • An image cleaning algorithm follows, removing for the subsequent analysis those pixels with signals likely due to the NSB.
γ­like event
proton­like event
Muon ring
Although high energy γ­rays get absorbed in the upper atmosphere, they can be detected indirectly from ground. When a γ photon coming from an astrophysical source plunges into the Earth’s atmosphere, it interacts with air nuclei, giving rise to a shower of secondary particles called Extensive Air Shower (EAS). As these secondary particles travel downwards with speed greater than the speed of light in the air, Cherenkov radiation is emitted in very short pulses of a few ns. Thus, γ­rays can be detected by collecting the Cherenkov light with the so­called Cherenkov telescopes. These detectors have the advantage of their huge effective collection areas, which exceed 105 m2, due to the big extension of an EAS in the ground. However, their main limitation is the rejection of the enormous cosmic ray background, since more than 99% of the atmospheric showers are initiated by hadronic cosmic rays. This problem was overcome with the development of the so­called Imaging Cherenkov Technique.
In this technique, a camera of small photomultipliers (PMTs) placed at the telescope focus obtains an image of the air shower. The different physical processes that γ­ and hadron­initiated showers undergo in the atmosphere, led to differences in the shape and orientation of the images left in the camera by both kind of showers. Cherenkov images formed by γ­ray showers are usually elliptical in shape, with their major axis pointing to the source. Conversely, cosmic­ray initiated showers are more irregular in shape and arrive randomly distributed. It is therefore possible to distinguish the γ­rays from the background by analyzing the shape of the Detection of γ ­rays with Cherenkov telescopes.
images.
Background Rejection Mispointing Correction
• Once the image of the shower has been obtained, it Along with the pulsed signal of the PMTs, the camera of the MAGIC telescope records the DC currents of each PMT by a dedicated electronics branch. Those pixels illuminated by stars record higher DC currents, which allow to identify the position of the stars in the camera and obtain the real pointing coordinates of the telescope.
is parameterized in terms of the so­called Hillas parameters[2]. The differences between γ­ and hadronic­showers are used to reduce the background
by applying cuts in these parameters[3]. One of the most important are Length and Width, which describe the size of the image along its major and minor axis.
DC currents for the Crab Nebula. The red cross marks the reconstructed pointing position. Distribution of the parameters Length and Width for Monte Carlo gammas and for real background events. • The fact that γ­showers point to the camera centre Three different kinds of events recorded by MAGIC. The ellipse represents the RMS of the light distribution.
while background showers arrive isotropically, is used to estimate the background and calculate the significance of the detection by filling the so­called Alpha­Plot[4], in which the angle Alpha between the shower axis and the camera centre is drawn. A signal appears as an excess at low Alpha values. ∀γ/hadron separation based on Hillas parameters is working well in MAGIC up to energies of ∼100 GeV. Below, γ and hadron showers are too similar, and new methods have to be developed.
The Crab Nebula seen in X­ray and in optical wavelengths.
The Crab Nebula is the remnant of a supernova explosion occurred in 1054 a.d. In the centre of the remnant there is a pulsar with a period of 33 ms, which injects relativistic electrons into the nebula. The continuous emission of the Crab nebula is predominantly produced by non­thermal processes, covering the huge energy range from radio to hundreds of TeV. At γ­ray energies the Crab is seen as a stable source, and for that reason it is considered the standard candle of the Gamma­Ray Astronomy. In contrast to the continuous emission of the nebula, the pulsed radiation coming from the pulsar has not been detected up to now by any former ground­based Cherenkov telescope, suggesting the existence of a sharp cutoff at tens of GeV. If a signal has been detected after the background rejection, the final goal is to obtain the spectra of the source under observation. For that, the energy of the primary γ­rays which originated the atmospheric showers has to be obtained, being the energy proportional to the light content of the showers. One of the methods to estimate the energy is to use Monte Carlo simulations to find a semi­empirical relationship between the energy and the measured image parameters[4]. The energy resolution achieved is around 30%.
Mkn 421
Conclusions
Mkn 421 is the nearest known blazar (redshift z=0.03), and the first extragalactic source detected at TeV energies. It has been extensively observed at all wavelengths, showing large flux variations in short time scales. Previous observation of Mkn 421 with Cherenkov telescopes revealed a good correlation between flares in X and TeV γ­
rays. Mkn 421 has been observed with MAGIC since January until April 2004, collecting more than 40 hours of data. The source showed to be in a very high state in April, with an average flux level of three times the flux observed from the Crab Nebula. During this flare, MAGIC detected Mkn 421 at a significance level of more than 25σ per night. The analysis presented here, of the data taken by the MAGIC telescope during its commissioning phase, shows the capabilities of this new detector to explore the Universe in Gamma­Rays. The Crab Nebula has been detected since January, providing very useful data for the understanding of the telescope behaviour. The observations of Mkn 421 revealed a high activity of this source in γ­rays in April.
Apart from the Crab Nebula and Mkn421, MAGIC has been observing many other sources during this year, like the AGNs 1ES­1959 and 3c279, pulsars like PSR B1957+20, or unidentified EGRET sources, whose analysis is still on going.
The commissioning phase is over, and now the efforts concentrate on lowering the energy threshold of the telescope to 30 GeV, and in developing new methods for the analysis of low energy showers.
Crab Nebula: The standard candle
The Crab nebula has been one of the primary targets of the MAGIC Telescope during its commissioning phase. The Crab has been observed regularly since January 2004 until March 2004 for calibration studies. In total more than 15 hours have been taken. Detections at the significance level of 10 σ per hour (using simple cuts and selecting only high energy showers) were typically achieved. During this commissioning phase, the estimated trigger threshold of the telescope was about 70 GeV. Energy reconstruction Non = 1034
Nex = 841
Nbg = 193
Significance = 29σ
References
1) Lorenz, E., New Astron. Rev. vol. 48, 339­344 (2004) 2) Hillas, A.M. Proc 19th ICRC (1985)
3) Fegan, D.J., Nucl. Part. Phys. Vol 23, 1013 (1997)
4) Kranich, D., PhD. Thesis, 2001
Excess of events coming from the Crab Nebula direction as detected by the MAGIC telescope in March 04 during 1 hour.
Left: Signal of Mkn421 after 1 hour of data taken in April 04. The blue and red curves are the fit to the data and background respectively.
Right: Distribution of the excess events arrival directions in the camera plane.
MAGIC Webpage @ http:/ /magic.mppmu.mpg.de
Background picture by R.Wagner