1 A. Zech, Instrumentation in High Energy Astrophysics (5.2) Ground based observation: H.E.S.S. ● The "High Energy Stereoscopic System" (H.E.S.S.) consists of 4 Cherenkov telescopes, situated in Namibia, near the Gamsberg, at 1800 m above sea level. ● Observation of Extensive Air Showers from gamma rays with energies above 100 GeV on clear nights. ● H.E.S.S. is sensitive enough to detect sources with a flux a few thousandths of the flux from the Crab nebula. ● Project has been running since December 2003.

2 A. Zech, Instrumentation in High Energy Astrophysics Location of the H.E.S.S. site

3 A. Zech, Instrumentation in High Energy Astrophysics Overview of Existing Cherenkov Telescopes HESS MAGIC VERITAS CANGAROO

4 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation – Telescopes & Mirrors ● H.E.S.S consists currently of 4 telescopes. They are arranged in a square with 120 m side length and provide multiple stereoscopic view of air showers. ● Each telescope consists of a dish with an effective area of 107 m 2 and a camera. The mirrors collect Cherenkov light from air showers and focus it onto the camera. ● A computer controlled motor moves the telescope around a vertical axis and the dish around an elevation axis. Maximum slewing speed: 100 o / min. ● The Davies Cotton telescopes have a focal length of 15 m and a reflectivity of ~80 %.

5 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation – Mirror Alignment ● Each dish is segmented into 382 round mirror facets of 60 cm diameter. The facets are made of aluminised glass and coated with quartz. ● Each facet can be adjusted by computer controlled motors. To align the facets, the image of a star is observed with a CCD camera in the focus. All the images given by the different facets are aligned automatically onto one spot. The mirror pointing precision is 0.38 mrad on axis. ● Over most of the field of view, the spot is well contained within a single pixel (one PMT). The point spread function is better than 0.5 mrad on axis and 1 mrad off axis (RMS width of the distribution of the image of a point source). alignment of facets

6 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation - Cameras & Trigger ● field of view: 5 o, diameter ~ 1.6 m. ● each camera contains 960 PMTs (pixels) with fields of view of 0.16 o (~ 2.8 mrad). ● A camera is triggered by a coincidence of signals (> ~ 5 photoelectrons) in 3 to 5 pixels that are near each other within a 1.5 ns time-window. -> Efficient background rejection ● When a camera is triggered by an air shower image, it alerts a central trigger station. If two or more cameras trigger simultaneously, all the camera signals are read out and processed. ● If only a single camera is triggered, the recorded signals are deleted after a few microseconds.

7 A. Zech, Instrumentation in High Energy Astrophysics The Photomultiplier Tube (Phototube, PMT) (1) PMTs are very sensitive detectors of light in the visible, IR and UV range. A PMT consists of a glass vacuum tube with a window at one end. A high voltage supply (~1 kV) sets a cathode behind the window at a negative voltage, whereas the anode at the bottom of the PMT is grounded. A set of electrodes between the cathode and the anode, called "dynodes" are set to increasingly positive voltages, providing a gradually increasing electric field between top and bottom of the PMT.

8 A. Zech, Instrumentation in High Energy Astrophysics The Photomultiplier Tube (Phototube, PMT) (2) Incident photons free "photoelectrons" from a semitransparent cathode behind the window (photoelectric effect). The "quantum efficiency", i.e. the efficiency of this process, is of the order of 20 %, depending on the wavelength. The photoelectrons are accelerated towards the electron-multiplier by a large voltage difference between cathode and anode (at the bottom of the PMT). The electron-multiplier consists of a number of electrodes, called "dynodes", at increasingly positive voltages towards the bottom of the PMT. The photoelectron knocks secondary electrons out of the 1 st dynode, which are accelerated towards the 2 nd dynode and knock out electrons, which are accelerated towards the 3 rd dynode... The avalanche of secondary electrons can multiply the initial signal by a factor of up to ~ 10 8, the "gain" of the PMT.

9 A. Zech, Instrumentation in High Energy Astrophysics The Photomultiplier Tube (Phototube, PMT) (3) ● The bottom of the PMT is connected to readout electronics. The accumulation of charge from the electron avalanche leads to a current pulse. ● This analogue signal is then usually amplified by a pre- amplifier and digitised using an Analogue-Digital-Converter (ADC). ● Once a PMT has been calibrated against a known light source, the recorded pulse area can be converted into a count of photoelectrons and then into a flux of photons. Due to their great sensitivity, PMTs are used for detection of light of very low intensity.

10 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation – Phototubes ● The PMTs used in H.E.S.S. have a diameter of 29 mm and borosilicate windows. ● They use a high voltage DC power supply to provide a gain of 2x10 5. ● The sampling frequency is 1 GHz, i.e. time-slices of 1ns are recorded. ● The temporary storage keeps a signal history of 128 ns. During this time either a trigger is formed and the event is read out or it is rejected. Differences in PMT gains are adjusted with use of a laser in the centre of the dish that illuminates the PMT cluster uniformly ("flat-fielding").

11 A. Zech, Instrumentation in High Energy Astrophysics Instrumentation – Winston Cones Winston cones are used to improve the light collection. The cones are connected to the top of each PMT and increase the light collecting surface of the PMT. The cones are made from polycarbonate, aluminized and protected with a quartz coating. They are mounted into a plate that fits on the camera.

12 A. Zech, Instrumentation in High Energy Astrophysics Atmospheric Monitoring The atmosphere plays a double role in air shower experiments: ● It is the medium in which the air shower develops. ● Light from the air shower is transmitted through the atmosphere before reaching the detector. It is therefore important to monitor atmospheric parameters and optical transmission. This is even more important for ultra- energy cosmic ray detectors, since air showers are observed at greater distances (see next Chapter). In H.E.S.S., several methods of monitoring the atmosphere are being used or under construction: ● Infrared radiometers measure the temperature of the sky in the field of view. Clouds are seen at higher temperatures than the background. ● A ceilometer, an active cloud sensor scans the sky with a laser and detecting backscattered ligth from clouds. ● An optical telescope measures transmission of starlight. ● A weather station

13 A. Zech, Instrumentation in High Energy Astrophysics Observing an air shower with a single telescope Air showers from gamma rays with E >100 GeV develop at a height of about 10 km. A pool of Cherenkov light from the shower with a radius of ~120 m reaches the ground. The image of the shower can be seen as a single track with the camera of one telescope.

14 A. Zech, Instrumentation in High Energy Astrophysics Stereoscopic Observation of an Air Shower With several telescopes, a stereoscopic (or multiscopic) view of a single shower is possible. This allows to reconstruct the shower geometry and to reject background signals.

15 A. Zech, Instrumentation in High Energy Astrophysics Air Shower Image Projection The image of the air shower that is projected onto the camera has the form of an ellipse. In the reconstruction of the air shower, one fits an elliptical form to the image to extract the "Hillas-parameters" that characterize the air shower. Two important parameters are the width and the length of the ellipse. One also takes into account the distribution of intensities over the PMTs that are part of the image. In the image shown here, the red pixel has the largest number of photoelectrons. It indicates the direction of the shower core. (figures taken from the Ph.D. thesis by Oliver Bolz, Ludwigshafen 2004)

16 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Direction of the Air Shower (1) The stereoscopic observation provides information on the direction of the air shower. All telescopes point at the same direction in the sky, so we can superpose the images from the air shower seen in different cameras.

17 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Direction of the Air Shower (2) In this case, the air shower came directly from the direction the telescopes are pointing at. If they are pointing at a known source, one would identify the shower with a photon from that source. The angular resolution of H.E.S.S. is a few arc minutes.

18 A. Zech, Instrumentation in High Energy Astrophysics Shower origin Image centre of gravity Reconstruction of the Shower Impact Point Geometrical determination of the shower impact point on the ground provides a better understanding of the shower geometry. This is very useful for the energy reconstruction of the event.

19 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Shower Energy (1) The energy of the primary particle, i.e. the γ-ray, is determined from the total recorded signal size, which can be converted into a flux of Cherenkov photons. Once the geometry of the air shower – i.e. the inclination of the shower axis and the impact point – has been determined, one compares the recorded signal to lookup tables. These lookup tables are generated with Monte Carlo simulations of γ-ray induced air showers at different energies and geometries. They contain lateral distributions of Cherenkov photon densities for each simulated shower. A comparison of the recorded signal size and the simulated photon fluxes provides the energy of the observed shower. The energy resolution of H.E.S.S. is on the order of 15 %.

20 A. Zech, Instrumentation in High Energy Astrophysics Reconstruction of the Shower Energy (2) Lateral distribution of Cherenkov light from γ-ray induced showers at sea level.

21 A. Zech, Instrumentation in High Energy Astrophysics From Photons to Photoelectrons and back taken from the Ph.D. thesis by Oliver Bolz, Ludwigshafen 2004

22 A. Zech, Instrumentation in High Energy Astrophysics Calibration (1) 80 ADC counts = 1 pe 1.) ADC counts -> p.e. First, the conversion factor of the recorded ADC counts to the p.e. number has to be determined. An LED illuminates the camera with a very low flux of photons. The signal from one PMT is shown on the right: The first peak is the electronic background ("pedestal"). The second peak is the response from a single p.e. It is (on the average) 80 ADC counts above the pedestal. Thus, 80 ADC counts = 1p.e. The calibration of the telescopes is a crucial step to allow a conversion of the measured pulses (in digitised ADC counts) into a photon flux. 2.) p.e. -> photon at PMT Then, the quantum efficiency of the PMTs has to be known to relate the measured photo-electrons (p.e.) to the incoming photon flux.

23 A. Zech, Instrumentation in High Energy Astrophysics Calibration (2) R X,Y 3.) photon at PMT -> photon at mirror A different method to relate the measured p.e. to the photon flux makes use of the recorded muons. This method also accounts for the optics of the mirrors. A muon that hits the mirror can be seen as a ring of illuminated PMTs. The radius of the ring is a direct measure of the Cherenkov cone opening angle and thus of the velocity of the muon: With the known muon velocity, one can calculate its energy deposit, i.e. the intensity of the emitted Cherenkov light. A comparison of the recorded number of photons (or p.e.) in the ring with the expected energy deposit provides a conversion factor between the photon flux at the PMT and the photon flux at the mirrors. Losses due to the mirror and camera optics can be calculated.

24 A. Zech, Instrumentation in High Energy Astrophysics Background - Muons Muons that hit the telescope leave a ring-shaped Cherenkov light signal and are easily identifiable. Muons that pass the telescope at some (not too large) distance can leave a signature that is not easy to distinguish from the image of an air shower. Due to the large muon flux in the atmosphere, this is a considerable source of background. Muons can however be rejected by requiring at least two telescopes to be triggered simultaneously.

25 A. Zech, Instrumentation in High Energy Astrophysics More Background – Hadronic Showers Air showers from cosmic rays (hadrons) constitute an important background in the search for γ-ray events. Images of hadronic showers can be distinguished from γ-ray showers in two ways: Hadronic showers do not leave a clear track. They look more like a "blob". When fitting an ellipse to the image, the width of the ellipse is usually larger than in the case of a γ-ray shower. One rejects hadronic showers by applying a cut on the observed width. When observing a source, showers initiated by γ-rays from that source should point back to it. If the telescope points at the source, the γ-ray showers should point to the center of the telescope. Hadronic showers point in deliberate directions. a γ-ray shower a hadronic shower

26 A. Zech, Instrumentation in High Energy Astrophysics some Results - Galactic Plane Survey (1) Observations in the Very High Energy Gamma Ray band ( ~100 GeV-TeV) open another window on our Galaxy.

27 A. Zech, Instrumentation in High Energy Astrophysics some Results – Galactic Plane Survey (2)

28 A. Zech, Instrumentation in High Energy Astrophysics some Results - Acceleration of cosmic rays in the GC H.E.S.S. has found a bright unknown source at the Galactic Center in VHE gamma-rays. After subtracting the bright source from the image, gamma-ray emission can be seen from giant clouds of hydrogen gas. This seems to be the first direct proof of generation of gamma-rays by cosmic rays in gas clouds. The cosmic ray accelerator could be a supernova remnant close to the central black hole of our galaxy.

29 A. Zech, Instrumentation in High Energy Astrophysics some Results – Resolved Supernova Remnants RX J ● the coloured map shows the gamma-ray counts observed by H.E.S.S. ● the contours show the X-ray surface brightness seen by ASCA in the range of 1-3 keV differential flux F(E) with fit E 2 F(E) plot of gamma-ray and X-ray data with several models fit to the data points.

30 A. Zech, Instrumentation in High Energy Astrophysics some Results - flux variability of M87 radio galaxy M87 seen in the optical by the Hubble Space Telescope (source & jet) These lightcurves show a very high variability in the gamma-ray flux observed by H.E.S.S. in The time scale of flux variations is used as an indicator of the maximum size of the emission region. Signals from different ends of the emission region have a time delay that depends on the size of the region. Flux variations on a smaller time scale cannot be observed, since signals from different ends would interfere and wash out any variation. The very fast variations observed by H.E.S.S. indicate a very small emission region (size of our solar system). The emission region seems to be the direct surroundings of the central black hole!

31 A. Zech, Instrumentation in High Energy Astrophysics All sky telescopes – Milagro (1) ● located near Los Alamos, NM, USA; altitude 2650 m ● a pond of size 80m x 60m x 8 m filled with pure water ● 175 tanks in a larger array ● 2 layers of PMTs (723 in total) observe Cherenkov light from air shower particles ● upper layer: electrons, positrons lower layer: muons

32 A. Zech, Instrumentation in High Energy Astrophysics All sky telescopes: Milagro (2) ● 100% duty cycle, very large field of view (~ 1 sr), good sensitivity at TeV energies => ideal for all (northern) sky survey of gamma-ray sources ● only 0.8 degree angular resolution, higher energy threshold than IACTs => complementary method to IACTs and satellites; similar method used by ARGO (Tibet)

33 A. Zech, Instrumentation in High Energy Astrophysics Future Projects on the Ground H.E.S.S. II: A 5th telescope is currently being added to H.E.S.S. mirror area: 600 m 2 # of pixels: 2048 field of view: 3.2 o lower energy threshold: ~50 GeV The lower energy threshold will allow a study of AGN at greater redshifts, since at lower energies gamma-rays are less absorbed in the extragalactic background light. The large mirror area will yield a better sensitivity to faint sources. Cherenkov Telescope Array (CTA): A joint project of the gamma-ray groups (HESS, MAGIC, etc.). The idea is to have a large array with many telescopes to increase sensitivity. H.E.S.S. II: an artist's impression High Altitude Water Cherenkov array (HAWC): next generation of the Milagro style detectors, larger effective area, higher altitude (lower E threshold)

34 A. Zech, Instrumentation in High Energy Astrophysics More Information Compton Gamma Ray Observatory: INTEGRAL: SWIFT: The H.E.S.S. project: (en français: ) The CTA project: (en français: ) The MAGIC project: The VERITAS project: The MILAGRO project: