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Gamma-ray astronomers convene in Heidelberg

20 October 2008

High-energy, gamma-ray astronomy symposium

Over the past few years, the quality and diversity of data from modern imaging atmospheric Cherenkov telescopes (IACTs) has revolutionized gamma-ray astronomy. With ground-based instruments, detailed imaging of the gamma-ray sky at 100 GeV to 100 TeV has become a reality and a wealth of information is currently being gathered about the universe. The 4th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy (γ 2008) was a timely opportunity to review the status and perspectives of this young field of astroparticle physics.

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The Heidelberg Symposium is a well established series of conferences organized by the Max-Planck-Institute for Nuclear Physics (MPIK) in Heidelberg, a leading institute of the H.E.S.S. collaboration, which operates an array of four IACTs in Namibia. After fruitful meetings in 1996, 2000 and 2004, the 4th symposium, which took place in July this year, celebrated an important breakthrough in gamma-ray astronomy. More than 50 very-high-energy (VHE) gamma-ray sources – with energies above 100 GeV – have been discovered since the last symposium, when only about 20 such sources were known.

This tremendous progress was reflected in the high-quality contributions at γ 2008. Twenty-six invited speakers reviewed the status of the field and related disciplines, and discussed the perspectives for gamma-ray astronomy and astroparticle physics in general. In addition, 56 spoken contributions and some 200 poster presentations addressed a range of topics. The number of abstracts submitted to the conference was significantly higher than for the 2004 symposium, reflecting again the growing interest in gamma-ray astronomy round the world. Talks were given in plenary sessions, allowing for lively discussions among the 300 experts from different fields of astroparticle physics. A significant amount of time was also devoted to the poster sessions, which took place in the relaxing atmosphere of “coffee and cake”, a typical German tradition.

VHE gamma-ray astronomy is currently being driven by four large installations of Cherenkov telescopes: the MAGIC telescope (La Palma, Canary Islands) and the VERITAS telescope array (Arizona, US) in the northern hemisphere; and the H.E.S.S. (Khomas Highlands, Namibia) and CANGAROO-III (Woomera, Australia) arrays in the southern hemisphere. While the northern instruments focus mainly on the observation of extragalactic objects and transient phenomena such as gamma-ray bursts, the southern arrays provide an excellent view of the inner Milky Way and are therefore also devoted to observations of Galactic objects.

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As testified in short status reports at the symposium, MAGIC, VERITAS and H.E.S.S. are operating successfully. However, as Ryoji Enomoto of Tokyo University pointed out, the CANGAROO-III array is currently operating only two of its four telescopes, owing to severe mirror deterioration and lack of funding. There were also reports on results from joint observation campaigns on various targets, such as the nuclei of the active galaxies Mkn 421 and M 87. These campaigns provide a way of cross-calibrating the instruments and result in an enhanced energy coverage. Upgrades of MAGIC (with the installation of a second 17 m telescope) and H.E.S.S. (with the installation of a single 28 m telescope in the centre of the existing four 12 m telescopes) to increase their sensitivity are well underway, and first light is expected in late 2008 and 2009, respectively.

After almost a decade of successful operation, the Milagro experiment – a 2000 m2, large field-of-view water Cherenkov detector in New Mexico – has stopped data taking after mapping the northern gamma-ray sky at multi-tera-electron-volt energies. Compared to the Cherenkov telescopes that point to regions of the sky, Milagro’s wide field of view allowed it to monitor the sky continuously, albeit at a higher energy threshold and with rather worse angular resolution. Although energy estimation is difficult for Milagro, Petra Hüntemeyer of the Los Alamos National Laboratory reported on the experiment’s recent success in measuring the energy spectra of sources up to 100 TeV. Plans to replace the instrument by the High Altitude Water Cherenkov (HAWC) project, which would be 10 times more large and more sensitive, were presented in a special session dedicated to future instruments. This session also included discussion of the science issues related to the next generation of gamma-ray instruments.

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The key European future project in VHE gamma-ray astronomy is the Cherenkov Telescope Array (CTA). Several tens of medium-sized Cherenkov telescopes will form the core of the CTA observatory, providing a 10-fold boost in sensitivity in the tera-electron-volt energy range compared with current instruments, as well as improved angular resolution. Additional large telescopes at the centre of the array will extend the energy range down to some tens of giga–electron-volts and a widespread halo of telescopes should add enough detection area to reach well into the 100 TeV range. CTA sites in the northern and southern hemispheres should allow full-sky coverage. In this context, the symposium served to foster the already intense communication between CTA and the project for the Advanced Gamma-ray Imaging System in the US, which has similar goals.

Just a few weeks before the conference, the astrophysics community celebrated the successful launch of the Fermi Gamma-ray Space Telescope (formerly GLAST) satellite, a gamma-ray observatory that will provide data in the energy range of approximately 10 MeV to 10 GeV (Fermi Gamma-ray Space telescope sees first light). Together with future ground-based instruments, this instrument will enable gamma-ray observations over seven decades of energy and a direct cross-calibration of ground-based and space-borne instruments for the first time. The perspectives for joint observations between Fermi and the Cherenkov telescopes was an important topic at the meeting, which was discussed by Stefan Wagner of the Landessternwarte Königstuhl in Heidelberg and Stefan Funk of SLAC, among others.

Physics highlights at γ 2008 included the discovery by the H.E.S.S. collaboration of the remnant of the historical supernova SN 1006 in VHE gamma rays. After more than 100 hours of observing time, H.E.S.S. now sees the remains of a massive stellar explosion, which Chinese astronomers reported in 1006, with a statistical significance of six standard deviations above the background. As Melitta Naumann-Godo of the Laboratoire Leprince-Ringuet pointed out, the preliminary image of the object seen by H.E.S.S. resembles the morphology of non-thermal X-ray filaments in the north-west and south-east part of the supernova remnant shell (see figure 1). Because these filaments are produced by synchrotron radiation of electrons that have been accelerated to an energy of about 100 TeV, SN 1006 has long been a prime target for Cherenkov telescopes; it is only the improved sensitivity of the current instruments that has made its discovery possible.

The detection of pulsed emission from the Crab pulsar by the MAGIC collaboration provided another highlight at the symposium. Many of the VHE gamma-ray sources in our galaxy can be identified with pulsar wind nebulae, but no VHE gamma-ray emission had previously been observed from a pulsar itself. The search for pulsed emission – which is well established at energies up to the giga-electron-volt range – matches the continuous efforts to minimize the energy threshold of Cherenkov telescopes. Using a special trigger setup, the MAGIC collaboration succeeded in lowering the threshold of their telescope to 25 GeV, making the detection of pulsed emission possible. Thomas Schweizer of the Max-Planck-Institute for Physics in Munich presented a VHE gamma-ray phasogram from 22 hours of observations of the Crab pulsar, which shows two distinct peaks corresponding to the main pulse and the interpulse. The data are in phase with observations at lower energies and with simultaneous measurements in the optical waveband carried out by the MAGIC collaboration.

Overall, the symposium showed that the stage is set for a bright future in gamma-ray astronomy. As Felix Aharonian of MPIK said in his concluding remarks: “Gamma-ray astronomy has evolved into a new astronomical discipline. Our observations meet all the key features usually attributed to astronomy: imaging, energy spectra, light curves, surveys…”. The community is now looking forward to seeing many new results at the next symposium, which will take place around 2012.

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