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26 Oktober 2006

Drastic variability in very-high-energy gamma rays from M 87 shows that these come from nearby a super-massive black hole

H.E.S.S. detects variable emission from the central engine of the giant elliptical galaxy M 87

H.E.S.S. weist variable Emission aus dem Zentralbereich der riesigen Radiogalaxie M 87 nach
Image of radio galaxy M 87 seen in visible light. The central region, from which the VHE gamma rays are seen, is located in the upper left part of the image and the relativistic plasma jet extends to the bottom right. Image: Hubble Space Telescope (HST)

The astrophysicists of the international H.E.S.S. collaboration report, in the current edition of the journal "Science", the discovery of fast variability in very-high-energy (VHE) gamma rays from the giant elliptical galaxy M 87. The detection of these gamma-ray photons — with energies more than a million million times the energy of visible light — from one of the most famous extragalactic objects on the sky is remarkable, though long-expected given the many potential sites of particle acceleration (and thus gamma-ray production) within M 87.

Much more surprising was the discovery of drastic gamma-ray flux variations on time-scales of days. This result, for the first time, excludes all possible options for sites of gamma-ray production, except for the most exciting and extraordinary one: the immediate vicinity of the super-massive black hole which is located in the centre of M 87.

The H.E.S.S. collaboration is an international team of astrophysicists and particle physicists, with mostly German and French participation. From Heidelberg, the Landessternwarte (state observatory) at Königstuhl and the Max-Planck-Institut für Kernphysik (Max-Planck-Institute for Nuclear Physics) are part of the team. Since last year, the Landessternwarte belongs to the newly founded "Zentrum für Astronomie der Universität Heidelberg" (Center for Astronomy at Heidelberg University).

Gamma rays: Gamma rays resemble normal light or X-rays, but are much more energetic. Visible light has an energy of about one electronVolt (1 eV) of energy in physicist's terms. X-rays are thousands to millions of eV. H.E.S.S. detects very-high energy gamma-ray photons with an energy of a million million eVs, or Tera-electronVolt energies (TeV). These high energy gamma rays are quite rare; even for relatively strong astrophysical sources, only about one gamma ray per month hits a square metre at the top of the Earth's atmosphere.

The collaboration has built and operates in Namibia a detection system, known as Cherenkov telescopes, which permits to detect gamma-rays from ground level. Pointing this system at a nearby galaxy, M 87, the team has detected very-high-energy (VHE) gamma-rays over the past four years. This discovery was now reported in Science. The real surprise is, however, that the intensity of the emission can be seen to change drastically within a few days on occasion.

The giant radio galaxy M 87

This galaxy, located 50 million light-years away in the constellation Virgo, harbours a super-massive black hole of 3 thousand million solar masses from which a jet of particles and magnetic fields emanates. However, unlike for previously-seen extragalactic sources of VHE gamma rays — known as Blazars — the jet in M 87 is not pointing towards the Earth but is seen at an angle of about 30°. In Blazars, gamma rays are believed to be emitted in the jet, collimated around the jet direction and boosted in their energy and intensity by the relativistic motion of jet particles. M 87 therefore represents a new type of extragalactic gamma-ray source.

A first indication of VHE gamma-ray emission from M 87 was seen in 1998 with the HEGRA Cherenkov telescopes (one of the precursor experiments to H.E.S.S.). With the H.E.S.S. results these indications are now confirmed with high confidence. The flux of VHE gamma rays from M 87 is quite faint; no other radio galaxy was so far seen in VHE gamma rays, probably because most are more distant than the relatively nearby M 87.

Extragalactic VHE gamma ray sources: Many galaxies are thought to harbour a huge black hole in their centres, several millions to billions times the mass of the Sun. If this black hole is swallowing the matter around it, the galaxy may become "active", and produce jets of particles travelling near the speed of light, known as "relativistic jets" in a manner which is not yet fully understood. In the case that this jet is pointing towards the Earth, such a galaxy is called a "blazar". Relativistic effects in the jet (Doppler boosting) lead to a dramatic - by orders of magnitude - increase in intensity of the jet when it points towards us. This makes the gamma-rays from these objects, the nearest of which is located 500 million light-years away, detectable by current techniques. So, it is not a surprise that blazars are the only type of active galaxy seen in VHE gamma rays, with the much closer radio galaxy M 87 being currently the only exception.

What short time-scale variability tells us

The time-scale of variability tells us about the maximum size of the emission region. Since gamma rays from the rear end of the emission region travel longer until they reach us, variability time scales cannot be much shorter than the time gamma rays require to cross the emission region. Such variability measurements are frequently used to constrain the size of the emission site in distant objects, often to greater accuracy than by measuring the object´s size based on the angular extension on the sky. The few-days variability time-scale seen by H.E.S.S. in M 87 is extremely short, shorter than detected at any other wavelength. This tells us that the size of the region producing the VHE gamma rays is just about the size of our Solar system (1013 m, only about 0.000001 % of the size of the whole radio galaxy M 87). "This is not much larger than the event horizon of the super-massive black hole in the centre of M 87" says Matthias Beilicke, a H.E.S.S. scientist working at the University of Hamburg.

This observation make the immediate vicinity of the central black hole of M 87 the most likely place for the production of VHE gamma rays; other structures in the jet of M 87 tend to have larger scales. The physics of the production processes have yet to be determined, and completely novel mechanisms can be invoked due to the proximity of the black hole which this discovery by the H.E.S.S. team has demonstrated. It is likely that we are dealing with a different production mechanism than for the blazars, whose jets point towards us. In this region near the black hole, the matter which is accreted form the black hole is also creating the relativistic plasma jet — a process which is not fully understood in general. That gamma rays can escape from the violent region may appear surprising, but is possible since the black hole in M 87 is accreting matter at a relatively low rate, compared to other black holes. Also one cannot exclude that relativistic effects such as those taking place in other extragalactic sources contribute at some level, but given that the jet is not pointing towards us, large relativistic effects are unlikely.

Flussvariationen der hochenergetischen Gamma-Strahlung von M 87.
The variation in VHE gamma rays from M 87: Lower panel: Year-by-year intensity as a function of time, as seen by H.E.S.S. (and HEGRA). Upper panel: Night-by-night variations with bursts on time-scales of a few days, as measured in 2005 from M 87.

H.E.S.S. leading the way

With this and preceding discoveries of extragalactic sources H.E.S.S. will lead the way in the comprehension of the processes involved in the production of these extraordinarily energetic photons. The radio galaxy M 87 is an excellent laboratory for studying the core of these galaxies, with their super-massive black holes which act as engines to accelerate particles to extremely high energies, giving out VHE gamma rays in the process. This object can be studied, and compared to the more numerous, but more distant blazars where the jet obscures our view of the central source. For M 87, we now know that we have an clear view of the central engine with H.E.S.S., so leading to a better understanding of all extragalactic VHE gamma-ray sources.

Die Radio-Galaxy M 87 Die Radio-Galaxy M 87
The radio galaxy M 87 seen at very high energies by H.E.S.S. (left panel, colour scale). The emission seems extended which is, however, explained by the measurement accuracy of the telescopes. A much stronger constraint on the size of the emission region can be derived from the measured variability of the VHE gamma rays (see text). The black lines reflect the structure of M 87 at radio wave-lengths. Right: Zoom to dotted square in left panel. The radio galaxy M 87 seen at radio wave lengths, corresponding to energies which are 19 orders of magnitude lower than the VHE gamma rays. The position of the maximum emission of the VHE gamma rays is also given (cross). (Radio data adapted from F.N. Owen et al.).

Notes on H.E.S.S.

The collaboration: The High Energy Stereoscopic System (H.E.S.S.) team consists of scientists from Germany, France, the UK, the Czech Republic, Ireland, Armenia, South Africa and Namibia.

The detector: The results were obtained using the High Energy Stereoscopic System (H.E.S.S.) telescopes in Namibia, in South-West Africa. This system of four 13 m diameter telescopes is currently the most sensitive detector of very high energy gamma rays. These are absorbed in the atmosphere, where they give a short-lived shower of particles. The H.E.S.S. telescopes detect the faint, short flashes of blueish light which these particles emit (named Cherenkov light, lasting a few billionths of a second), collecting the light with big mirrors which reflect onto extremely sensitive cameras. Each image gives the position on the sky of a single gamma-ray photon, and the amount of light collected gives the energy of the initial gamma ray. Building up the images photon by photon allows H.E.S.S. to create maps of astronomical objects as they appear in gamma rays.

The H.E.S.S. telescope array represent a multi-year construction effort by an international team of more than 100 scientists and engineers. The instrument was inaugurated in September 2004 by the Namibian Prime Minister, Theo-Ben Guirab, and its first data have already resulted in a number of important discoveries, including the first astronomical image of a supernova shock wave at the highest gamma-ray energies.

Future plans: The scientists involved with H.E.S.S. are continuing to upgrade and improve the system of telescopes. Construction of a central telescope — a behemoth 30 m in diameter — is under way, including new partner countries such as Poland. The improved system, known as H.E.S.S.-II, will be more sensitive and will cover an increased range of gamma-ray energies, so enabling the H.E.S.S. team to increase the gamma-ray source catalogue and to make new discoveries.

Dr. Matthias Beilicke &
Prof. Dr. Götz Heinzelmann
Institut für Experimentalphysik
Universität Hamburg
Luruper Chaussee 149
22761 Hamburg, GERMANY
Tel +49 40 8998 2202 &
+49 40 8998 2214

Dr. Felix Aharonian &
Prof. Dr. Werner Hofmann
Max-Planck-Institut für Kernphysik
Saupfercheckweg 1
69117 Heidelberg, GERMANY
Tel +49 6221 516 485 &
< +49 6221 516 330

Dr. Gerd Pühlhofer &
Prof. Dr. Stefan Wagner
ZAH Landessternwarte
Königstuhl 12
69117 Heidelberg, GERMANY
Tel +49 6221 54 1719 &
+49 6221 54 1712

Dr. Michael Schwarz
Press Officer of the University of Heidelberg
phone: 06221/542310, fax: 54317

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