Very-high-energy gamma-ray emission from M87 during 2007-2008 (H.E.S.S, MAGIC & VERITAS). The regular gaps in the light curve correspond to phases of full moon during which no observations are possible. The vertical grey box indicates the time period of the strong flaring activity. Lower panel: Radio flux as measured with the VLBA from a region very close to the supermassive black hole in M87.

γ-rays from the Edge of a Supermassive Black Hole

Reference:  V. A. Acciari et al. (The VERITAS Collaboration), Science, 325, 444, 2009

Abstract , Full text version

ArXiv version: ArXiV:0908.0511

Contact person: Matthias Beilicke

High-resolution radio and gamma-ray observations reveal the site of relativistic particle acceleration in the galaxy M87

An international collaboration of 390 scientists reports the discovery of an outburst of very-high-energy (VHE) gamma radiation from the giant radio galaxy Messier 87 (M87), accompanied by a strong rise of the radio flux measured from the direct vicinity of its super-massive black hole. The combined results give first experimental evidence that particles are accelerated to extremely high energies of tera electron Volt (one electron Volt is the energy an electron or proton gains when it is accelerated by a voltage of one Volt) in the immediate vicinity of a supermassive black hole and then emit the observed gamma rays. The gamma rays have energies a trillion times higher than the energy of visible light. The large collaborative effort involved three arrays of 12-meter to 17-meter telescopes (VERITAS, MAGIC and H.E.S.S. ) that detect very high-energy gamma rays and the Very Long Baseline Array (VLBA) that detects radio waves with high spatial precision. The results will appear in the July 2 Science Express, the advance online publication of the journal Science.

"We had scheduled gamma-ray observations of M87 in a close cooperative effort with the three major gamma-ray observatories VERITAS, H.E.S.S. and MAGIC, and we were lucky that an extraordinary gamma-ray flare happened just when the source was observed with the VLBA and its impressive spatial resolving power," says Beilicke, working with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). "Only combining the high-resolution radio observations with the VHE gamma-ray observations allowed us to locate the site of the gamma-ray production," says Craig Walker, staff scientist at the National Radio Astronomy Observatory in Socorro, New Mexico.

All figures from paper (click to get full size image): 

Figure 1: M87 at different photon frequencies and length scales. (A) Comparison of the different length scales. (B) 90-cm radio emission measured with the VLA. The jet outflows terminate in a halo that has a diameter of ~80 kpc (15'). The radio emission in the central region is saturated in this image. [Credit: F. N. Owen, J. A. Eilek, and N. E. Kassim (32), NRAO/Associated Universities Incorporated/NSF] (C) Zoomed image of the plasma jet with an extension of 2 kpc (20''), seen in different frequency bands: x-rays (Chandra, top), optical (V band, middle), and radio (6 cm, bottom). Individual knots in the jet and the nucleus can be seen in all three frequency bands. The innermost knot HST-1 is located at a projected distance of 0.86 arc sec (60 pc, {approx}10^5Rs) from the nucleus. [Credit: x-ray, NASA/Chandra X-Ray Observatory Science Center/Massachusetts Institute of Technology/H. Marshall et al.; radio, F. Zhou, F. Owen (NRAO), J. Biretta (Space Telescope Science Institute); optical, NASA/STScI/University of Maryland Baltimore County/E. Perlman et al. (8)] (D) An averaged, and hence smoothed, radio image based on 23 images from the VLBA monitoring project at 43 GHz. The color scale gives the logarithm of the flux density in units of 0.01 millijansky per beam. The indication of a counter-jet can be seen, emerging from the core toward the lower left side. mas, milli–arc seconds.

Figure 2: Combined M87 light curves from 2007 to 2008. (A) VHE {gamma}-ray fluxes (E > 0.35 TeV, nightly average), showing the H.E.S.S., MAGIC, and VERITAS data. The fluxes with statistical errors (1 SD) were calculated assuming a power-law spectral shape of dN/dE {propto} E–2.3. Monthly binned archival VERITAS data taken in 2007 are also shown (4). The systematic uncertainty in the flux calibration between the experiments was estimated to be on the order of 20%, based on Crab Nebula data. The regular gaps in the light curve correspond to phases of full moon during which no observations were possible. The inlay shows a zoomed version of the flaring activity in February 2008; the time span is indicated by the gray vertical box in all panels. (B) Chandra x-ray measurements (2 to 10 keV) of the nucleus and the knot HST-1 (28). (C) Flux densities from the 43-GHz VLBA observations are shown for (i) the nucleus (circular region with radius r = 1.2 milli–arc sec = 170Rs centered on the peak flux), (ii) the peak flux (VLBA resolution element), and (iii) the flux integrated along the jet between distances of r = 1.2 to 5.3 milli–arc sec (compare with Fig. 3). The error bars correspond to 5% of the flux. The shaded horizontal area indicates the range of fluxes from the nucleus before the 2008 flare. Whereas the flux of the outer regions of the jet does not change substantially, most of the flux increase results from the region around the nucleus.

Figure 3: VLBA images of M87 at 43 GHz. (A) Average (hence smoothed) of 11 images from data taken between January and August 2007, well before the VHE and radio flare. The contour levels start at 5, 10, 14.3, and 20 mJy per beam and increase from there by factors of Formula. The restoring beam used for all of the images is 0.21 x 0.43 milli–arc sec (30 x 60 Rs) elongated in position angle –16°, as shown by the ellipse in the upper left corner. (B) Image from 5 April 2008, with the same contours and colors as in (A). The linear scale in light days and Schwarzschild radii is also shown. (C to F) Difference images for observations during the period of the radio flare, showing its effects. These were made by subtracting the average image (A) from the individual epoch images. The contours are linear with 10 (white) at intervals of 7 mJy per beam, followed by the rest (black) at intervals of 70 mJy per beam; negative contours are indicated by dashed lines. At the time of the VHE flare, the core flux density was already above the average, but the region of the jet between –0.5 and –1.0 milli–arc sec RA offset was below average, suggesting that there had been a period of below-normal activity leading up to the flare and that the radio flare may have begun before the VHE flare. The sequence shows the substantial rise in the core flux density and the appearance of enhanced emission along the inner jet.