Astrophysics

A Gamma-Ray Source Pinpointed

An international team of astrophysicists working with the High Energy Stereoscopic System (HESS) array of telescopes in Namibia recently announced the discovery of gamma-ray flux coming from the nearby giant elliptical galaxy M87. Surprisingly, dramatic variations in flux appear on time-scales of days, suggesting that the gamma rays are produced in the immediate vicinity of the black hole located at the center of M87.

Gamma rays, the most energetic form of light (more than a million million times the energy of visible light), emit short flashes of blue light when entering Earth's atmosphere. Although the flashes only last a few billionths of a second, Imaging Atmospheric Cherenkov Telescopes (IACT) can capture them from the ground. HESS, is the newest and the most sensitive of such instruments, with a field of view ten times the Moon's diameter. Astrophysicists from Europe¹ and Africa have been using HESS since 2003 to measure gamma rays stemming from the giant elliptical galaxy M87, in the nearby Virgo cluster of galaxies. In the center of M87 is a supermassive black hole with a mass of three billion solar masses.

This analysis revealed variability in intensity of gamma-ray flux emissions, on time-scales of days. The time-scale of variability is an indicator of the maximum size of the emission region. These variability measurements proved extremely useful in determining the size and region of the ray source. The results, published in Science,² challenge most scenarios of gamma-ray production in extragalactic sources. Indeed, this is the first time that most potential sites of gamma-ray production can be excluded, leaving almost just one possibility, the nucleus of M87 itself. Now the researchers will focus on exactly how the production processes occur, and HESS, combined with instruments at other wavelengths, should prove very helpful once more.

Marion Girault-Rime