Gamma-Ray Astronomy at MPE

The gamma-ray astronomy emerged as an interesting field of research already a few years after the foundation of MPE (in 1963), originally with an emphasis on balloon experiments. Over the years, the gamma-ray group participated in major satellite missions of ESA (COS-B, INTEGRAL) and NASA (CGRO, Fermi), and took leading roles in the development and scientific use of instruments like COMPTEL, EGRET, INTEGRAL/SPI and Fermi/GBM. In 2010, the gamma-ray group has been merged with the X-ray group to form the High-Energy Astrophysics division of MPE.
During the last decade of existence, the gamma-group involved about 30 scientists (incl. PhD students). THe research focused on gamma-ray line studies, diffuse galactic continuum emission and gamma-ray bursts. Additional interests were in the areas of gamma-ray pulsars, AGN and microquasars. Prime resources were data obtained by ESA's INTEGRAL mission, operational since 2002, NASA's Swift mission, launched in 2004, and NASA's Fermi mission with the GBM and LAT instruments, operating since June 2008. Also, analysis of COMPTEL and EGRET data taken during the 1991-2000 with the Compton Gamma-Ray Observatory (CGRO) were still continuing. In addition, the group has built and operated two ground-based instruments: (1) an optical high-time resolution camera (OPTIMA) temporarily mounted on various ground-based telescopes to observe pulsars and short-period CVs, and (2) GROND, a 7-channel optical/near-infrared camera, operating since April 2007 on the MPG/ESO 2.2 m telescope in Chile, for observations of gamma-ray bursts afterglows.

A more detailed review of the gamma-ray astrophysics history at MPE is available in the European Physical Journal H (History) 46 (2021) 27.

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Gamma-Ray Astronomy

Gamma-rays are the most energetic form of electromagnetic radiation, with over 10,000 times more energy than visible light photons. If you could see gamma-rays, the night sky would look strange and unfamiliar. The familiar sights of constantly shining stars and galaxies would be replaced by something ever-changing. Your gamma-ray vision would peer into the hearts of solar flares, supernovae, neutron stars, black holes, and active galaxies. Gamma-ray astronomy presents unique opportunities to explore these exotic objects and the most energetic phenomena they produce. By exploring the Universe at these high energies, scientists can search for new phenomena, testing theories and performing experiments which are not possible in Earth-bound laboratories.
The energy band of gamma-ray astronomy extends over more than seven orders of magnitude, from typically 500 keV to more than 1 TeV. It is thus not surprising that a wide variety of detectors are used to study smaller sub-ranges. Most gamma-rays are absorbed by the Earth's atmosphere. Thus, cosmic gamma-rays are typically observed from high-altitude balloons and satellites. Only above energies in the TeV range, gamma-rays can be measured from the ground via their interaction processes (and products) in the atmosphere.

A short introduction to Gamma-Rays and Gamma-Ray Astronomy can be found in the Wikipedia article.

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Research

• gamma-ray lines
  Gamma-ray lines are our window into nuclear physics processes in the universe. We can measure radioactive decay gamma-rays from isotopes which are freshly-produced in cosmic element synthesis events such as supernovae, and high-energy collisions among atoms at cosmic-ray energies lead to de-excitation gamma-rays, such as have been observed in solar flares. For the brightest lines, our previous-generation experiment COMPTEL  was the base of pioneering maps and discoveries due to its long mission duration. With INTEGRAL's spectrometer, we now have an instrument which has adequate spectral resolution for spectroscopy of such lines. We collaborate with other institutes to prepare future-generation improved imaging telescopes for this purpose.
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• cosmic-ray produced gamma-rays
  Cosmic ray interactions with the interstellar medium and radiation result in diffuse emission of a gamma-ray continuum spectrum. With CGRO, we have obtained maps of the Galaxy. Contributing processes are pion decay, bremsstrahlung, inverse-Compton emission, superimposed on the contribution of sources in the Galaxy with their spectral variety. With INTEGRAL and FERMI we study the relation between sources and such diffuse emission, specifically in the inner Galaxy).
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• gamma-ray bursts
  The study of GRBs at MPE has many facettes: it ranges from operating GRB detectors on FERMI and INTEGRAL, to performing optical and near-infrared follow-up observations of the afterglow emission of GRBs as well as studying the host galaxies of GRBs with GROND at the 2.2m MPI/ESO telescope or the ESO/VLT. We are also actively participating in the Swift mission data analysis with its BAT detector, and share a 50% load with MSFC in the operation of FERMI's GBM via so-called Burst-Advocates.
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• AGN
  Active galactic nuclei are remarkably different cores of galaxies, which are particularly intense sources of radiation, from radio to gamma-rays. Their radiation is probably caused by matter falling into the gravitational well of a central supermassive black hole. This accretion flow leads to an intense and narrow plasma jet, being oriented perpendicular to the accretion flow, and causing the high-energy emission we observe. Jet-internal shocks accelerate particles to relativistic energies, and their interaction with jet plasma and surrounding interstellar material is the subject of our study.
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• microquasars
  The processes responsible for high-energy emission in active galactic nuclei also operate on a smaller level in matter accretion onto compact objects in our galaxy. Those "microquasars" are therefore objects of similar studies.
  

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Experimental projects

1. Active experiments / missions

   INTEGRAL is an ESA gamma-ray observatory mission, launched in October 2002, and planned to operate till 2012 or beyond. The gamma-ray group of MPE is involved in its spectrometer instrument SPI. Results on positron annihilation, diffuse nucleosynthesis lines from 26Al and 60Fe and 44Ti gamma-rays from young supernova remnants, on transients in the Galactic Centre Region, on interstellar diffuse emission, on non-thermal emission from compact binary systems in the Galaxy and spectacular transients such as magnetars, the diffuse cosmic X-ray background, active galaxies, and gamma-ray bursts have been obtained. (first results see special Astronomy & Astrophysics issue Vol. 411).

   OPTIMA is a high-speed photo-polarimeter which is used to perform optical measurements with high time resolution on gamma-ray source objects such as pulsars and accreting binaries. OPTIMA has been installed on several major observatories as a guest instrument and will be specifically used for rapid response gamma-ray burst afterglow measurements in 2005. A new project for high-time resolution astronomy is being funded by the European FP6 programme as part of the OPTICON consortium: we started to design a fast, single-photon sensitive PN-CCD detector in collaboration with the MPE HLL.

   GROND is an imaging instrument to investigate Gamma-Ray Burst Afterglows and other transients simultaneously in seven filter bands. Several dichroic beamsplitters feed light into three NIR channels and four visual channels, each equipped with its own detector. GROND is mounted at the MPI/ESO 2.2m telescope on La Silla (Chile), and is operational since May 2007.

   Swift is a NASA mission launched in 2004 for Gamma-ray Burst investigations. It features two X-ray telescopes (one wide field-of view Burst Alert Telescope, the other for immediate localization and afterglow observations (XRT), plus a UV/Optical telescope. Swift also observes the high-energy X-ray sky up to ~150 keV. More information: US Swift website
   

FERMI/GBM, the Gamma-Ray Burst Monitor, is the secondary instrument of the FERMI (former GLAST) mission of NASA. It has been built by a collaboration between the MPE and NASA's Marshall Space Flight center and the University of Huntsville, AL. FERMI was launched in June 2008. With GBM, gamma-ray bursts are recognized and localised on the sky, alerting the community and the LAT instrument on FERMI. In addition, their spectra in the energy range 10 keV to 25 MeV are measured with high time resolution.

2. Missions under study

   GRIPS, the Gamma-Ray Burst Investigation with Polarimetry and Spectroscopy experiment has been a proposal for ESA's Cosmic Vision program in 2007 (published version in Exp. Astron. 23, 91). It features a Compton and Pair Tracking instrument with a large field of view for the 200 keV to 50 MeV energy range, and an X-ray monitor with a 3-degree field of view. The anticipated mission would be a continuous scan of the sky, with X-ray post-observations of gamma-ray burst of high redshifts, as identified instantaneously with the gamma-ray instrument. The gamma-ray instrument would be capable of polarimetry in the 100-1000 keV range. Besides GRBs at redshifts above 10-15, the mission also addresses SNIa and other nuclear gamma-ray line sources, and relativistic particle acceleration sources near compact stars, interstellar shocks, and in galaxies or galaxy clusters. MPE leads the GRIPS collaboration consisting of ~18 institutions.

   GRI, the Gamma-Ray Imager experiment has also been a proposal for ESA's Cosmic Vision program in 2007. It features a Laue Lens instrument designed for sensitive explorations of the 511 keV and 847 keV lines within a 5 arcmin field of view, and a multilayer X-ray mirror telescope for the 10-250 keV band. The two-spacecraft formation-flying mission would target SNIa and 511 keV point sources, as well as other high-energy sources with emission in these bands. MPE is a consortium member, no hardware contributions were foreseen.

   AGaDe, is our project name for the development of future Advanced Gamma-ray Detectors, making use of newest technologies in scintillation detectors and semiconductor devices.

3. De-commissioned experiments / Data analysis

   CGRO (Compton Gamma-Ray Observatory) was a pioneering gamma-ray observatory mission of NASA and had been in orbit for more than 9 years (1991-2000). It was destroyed by a controlled re-entry, a very much debated decision by NASA.
   The MPE has been involved in the project in two experiments: COMPTEL and EGRET.
   Many of the results obtained from CGRO data are exciting and gained much attention in the astronomical commmunity. CGRO established gamma-ray astronomy as an important part of astronomy and astrophysics. We exploit its databases for reference studies, to combine and to compare with more specific observations performed with current instruments (such as INTEGRAL).

   MEGA had been an in-house development (<2006) for low-energy gamma-ray astronomy (0.5 - 50 MeV) based on advanced, highly integrated solid-state detector technology. A scaled prototype telescope was developed and successfully calibrated with radioactive sources and gamma-ray beams from an accelerator.

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