PKS 2005-489 - the first new AGN discovered by H.E.S.S.

November 2005

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An artist's view of the central region of an active galaxy shows a supermassive black hole surrounded by a disk of hot gas, and a large doughnut or torus of cooler gas and dust. The light blue ring on the back of the torus is due to the fluorescence of iron atoms excited by X-rays from the hot gas disk. Jets of high energy particles (white) are propelled away from the vicinity of the black hole by intense electric and magnetic fields. Presumable in shock waves in these jets, electrons are accelerated to high energies and, when colliding with photons, create high energy gamma rays. The  gamma ray emission is collimated along the axis of the jet, and can usually only be observed when the jet points towards Earth. (Source: NASA/CXC/M.Weiss)

The winter months are the main observing season for extragalactic objects for H.E.S.S., hence we will here and in the next SOM's concentrate on extragalactic sources of TeV gamma rays. PKS 2005-489 is in a certain sense of historical interest, being the first active galactic nucleus (AGN) newly discovered by H.E.S.S., based on data partly taken during the commissioning phase with two and three telescopes. The object was selected as an interesting candidate for observation on the basis of its high flux in X-rays and at radio wavelengths (Fig. 1), both tracers of high-energy electrons. H.E.S.S. observations resulted in a detection with a significance of 6 standard deviations (Fig. 2), sufficiently strong to derive an energy spectrum (Fig. 3). Attemps to detect the object with previous instruments had failed (Roberts et al. 1999, Nishijima et al. 2002, Chadwick et al. 2000), which is not surprising given its small gamma ray flux, of only 2.5% of the Crab Nebula flux. During the H.E.S.S. observations, X-ray activity of PKS 2005-489 was rather low in comparison to earlier times (e.g. 1998), implying that the AGN was in a low-emission state. Given its relatively modest redshift of 0.071, the steep energy spectrum appears to be an intrinsic feature of the source - the acceleration engine running out of steam - opposed to effects caused by gamma-ray absorption due to interactions with photons in extragalactic space, which preferentially remove high-energy particles.

The object PKS 2005-489 was observed again with H.E.S.S.  in 2005, resulting in a strong detection and confirming the earlier results. A detailed analysis of this data and of simultaneous measurements at other wavelengths is still in progress.

Discovery of VHE gamma rays from PKS 2005-489, F. Aharonian et al., Astron. Astrophys. 436 (2005) L17-L20


Fig. 1: The selection of extragalactic observation targets relies strongly on this plot of X-ray intensity versus radio intensity for a special type of active galactic nuclei (AGN), the BL Lac objects (Costamante & Ghisellini, 2002). These objects have their jet pointing towards us. Virtually all active galaxies detected previously at TeV energies (full black points) belong to this class and are characterized by high X-ray and radio flux, indicative of the presence of high-energy electrons in the source. Scattering off ambient or synchrotron photons, these electrons produce TeV gamma rays. PKS 2005-489 is indicated by the filled red circle, other new H.E.S.S. AGN sources - to be presented later in this series - by the open red circles.
Fig. 2: Angular distribution of gamma-ray candidates relative to the direction towards PKS 2005-489. The shaded area shows the background estimate. The significance of the peak at the source is more than 6 standard deviations.
Fig. 3: Energy spectrum of gamma rays from PKS 2005-489. The spectrum is well represented by a power law with index Γ=4.