PKS 2155-304 - Photons from a Hotter Hell

September 2007

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This artist's concept depicts a supermassive black hole at the center of a galaxy. The active galaxy PKS 2155-304 hosts such a black hole of about 109 solar masses. The blue color represents radiation pouring out from material very close to the black hole. The grayish structure surrounding the black hole, called a torus, is made up of gas and dust. Beyond the torus, only the old red-colored stars that make up the galaxy can be seen. A jet of energetic particles is launched in the vicinity of the black hole. For blazars such as PKS 2155-304, the jet is pointed towards our Galaxy and is the source of the high-energy gamma rays. Based on NASA/JPL-Caltech.

"Photons from a hotter hell" is the title of T. Weekes recent commentary in Nature, concerning the recent detection of giant outbursts of active galaxies observed in high-energy gamma rays. At the core of an active galaxy, a supermassive black hole is accreting matter, frequently launching a jet of high energy particles (top figure), which are the source of the observed gamma rays. The detailed mechanisms which generate the jet and also the nature of the particles which make up the jet are not fully understood. Fluctuations in the lauching of the jet, or within the jet cause sources to flare. A most impressive outburst of the Southern active galaxy PKS 2155-304 was recently reported by H.E.S.S. (Aharonian et al., 2007). PKS 2155-304 was one of the first objects covered in SOM, but these recent data merit a second report. Within about 90 minutes of observations, close to 12000 gamma rays were detected, corresponding to a rate of more than 2 Hz. The measured light curve is shown in Fig. 1 in one-minute bins; it displays at least five distinct sub-flares. The peak flux reaches up to 15 times the flux of the Crab Nebula, and is almost a factor 100 higher than the minimal ("quiescent")  flux from PKS 2155-304. Given that this active galaxy is more than 200000 times more distant than the Crab Nebula - about 450 Mpc compared to 2 kpc - this implies an energy output in high-energy gamma rays which is about 12 orders of magnitude higher than for the Crab Nebula, whereas the emission region, governed by the scale of the black hole, is several orders of magnitude smaller than the size of the Crab Nebula. The fast rise times of the flares, in the range of 100 s, are among the fastest ever observed from such objects. This is puzzling since it is only about 1% of light-crossing time of the black hole, usually assumed to be the shortest time scale on which the jet can flare. Relativistic boosting can shorten time variation as viewed by an observer at rest, but to achieve a speed-up of a factor 100, the jet would have to move with an unprecedented Doppler factor around 100, or 99.995% of the speed of light.

A frequency decomposition of the light curve was made to find out which frequency components are most strongly represented in the flux variation (Fig. 2), with the surprising result that the signal somewhat resembles "red noise": all frequencies are present, with increasing intensity towards lower frequency. This result questions the assumption that the rise times of the individual sub-flares represent a single characteristic time scale which can a associated with a corresponding length scale of the emission region. Possibly, the emission region has a fractal nature where emission from features of many different scales is superimposed, with decreasing intensity from smaller-scale features, which cause the higher-frequency components.

In 70 hours of H.E.S.S. data taken in 2005 and 2006, a clear signal was detected from the blazar 1ES 0229+200, at a redshift of z=0.14, corresponding to a flux of about 2% of that of the Crab Nebula. What makes this observation particularly interesting is that the source exhibits a rather hard gamma-ray spectrum, with a spectral index of 2.5, with a hint of a flattening of the spectrum around energies of a few TeV (Fig. 3). Though statistically not very significant, indications of similar spectral features are seen in other blazars at redshift z>0.1 (Fig. 3). The hard spectrum constrains the shape of the EBL in the mid-infrared: assuming that the intrinsic gamma-ray spectra produced by blazars have a spectral index of 1.5 or larger, EBL spectra with significant energy in the mid-infrared and with corresponding strongly energy-dependent attenuation would steepen gamma-ray spectra beyond the observed index of 2.5 and are excluded, favoring EBL models with a pronounced mid-infrared dip. As emphasized by Stecker and Scully 2007, this conclusion depends, however, on the assumption concerning the intrinsic blazar spectrum. If 1ES 0229+200 produces gamma-rays with a spectral index of 1 to 1.5, as Stecker et al. (2007) predict for relativistic shock acceleration scenarios, but so far not observed for any of the nearby VHE blazars, the higher mid-infrared EBL can be accommodated. Either of these two explanations of the observed hard spectrum make 1ES 0229+200 a very interesting VHE gamma-ray source!


An Exceptional Very High Energy Gamma-Ray Flare of PKS 2155-304
Aharonian et al., Astrophys. Journal, 664 (2007) L71

Fig. 1: Integral flux above 200 GeV observed from PKS 2155-304 on July 28, 2006. The data are binned in 1 minute intervals. The horizontal dotted line represents the flux from the Crab Nebula.
Fig. 2: Fourier power spectrum of the light curve, indicating at which frequencies the emission is varying. The gray-shaded area corresponds to the estimated 90% confidence interval. The horizontal line indicates the average noise level.