Gamma Rays from the Distant BL Lac 1ES 0347-121

October 2007

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To disentangle the processes at work in gamma-ray sources, multi-wavelength observations over a wide range of frequencies are required. In the investigation of the active galaxy  1ES 0347-121, contemporaneous observations with the  SWIFT satellite were used to complement the H.E.S.S. very high energy gamma-ray data. Primarily launched for the investigation of gamma-ray bursts, Swift combines a Burst Alert Telescope(BAT), an X-Ray Telescope (XRT) and an UltraViolet/Optical Telescope (UVOT); the latter two were used in the analysis of 1ES 0347-121.

The active galaxy 1ES 0347-121 was discovered as an X-ray source in the Einstein slew survey (Elvis et al. 1992) and was later classified as a BL Lac type object (Schachter et al. 1993), characterized by a largely featureless non-thermal spectrum and rapid intensity variations. Emission from such objects is dominated by a relativistic particle jet pointed towards the observer (see also SOM 9/2007). Located at a redshift of z=0.188, the elliptical host galaxy harbors a supermassive black hole of about 100 million solar masses (Woo et al. 2005). Based on its X-ray emission, 1ES 0347-121 - fairly inconspicuous in optical images (Fig. 1) - was early recognized as a candidate VHE gamma-ray source (Stecker et al. 1996). However, observations with the HEGRA telescopes failed to detect the object (Aharonian et al. 2004). Based on 25 h of data taken with H.E.S.S. in August and December 2006, finally a clear gamma-ray signal with more than 10 standard deviations significance was detected (Fig. 2). The flux above 250 GeV corresponds to about 2% of the flux from the Crab Nebula; the spectrum drops off rapidly towards higher energy, with a photon spectral index of about 3.1.

Simultaneous observations of 1ES 0347-121 were conducted in October 2006 with the SWIFT satellite (top image) and with the ATOM optical telescope on the H.E.S.S. site. The resulting spectra are shown in Fig. 3 as black points, together with the VHE gamma ray data. The optical and X-ray data trace a synchrotron spectrum, which peaks in the SWIFT X-ray range; the gamma rays can be explained as Inverse Compton upscattering of synchrotron photons by the electrons, assuming a highly relativistic jet with a Doppler boost of about 20 and a magnetic field in the emission region of 0.035 G. The steepness of the gamma-ray spectrum can largely be attributed to intergalactic absorption due to electron-positron pair production in interactions with infrared background photons. Since the increase in gamma-ray spectral index is directly related to the intensity of IR background radiation fields, and assuming that the gamma-ray spectral index at the source can hardly be less than 1.5, one can derive an upper limit on the intensity of extragalactic IR photons in the micrometer wavelength region, confirming earlier indications of a rather low level of extragalactic background light (SOM 12/2005). More details about the spectrum of IR photons can be derived from data of another BL Lac, 1ES 0229+200, which will be featured next month.


Discovery of VHE gamma-rays from the distant BL Lacertae 1ES 0347-121",   Aharonian et al., Astronomy & Astrophysics 473 (2007) L25 (2007)

Fig. 1: Finding chart for locating 1ES 0437-121 (Source: Landessternwarte Heidelberg)
Fig. 2: Number of excess gamma ray candidates versus angular distance to the nominal source position of 1ES 0347-121. The green area shows the expected distribution for a point source, according to simulations of the instrument response.The excess of 327 gamma-ray events corresponds to a statistical significance of more than 10 standard deviations.
Fig. 3: Spectral energy distribution of 1ES 0347-121. Red points show the measured gamma-ray spectrum, black circles the SWIFT X-ray and UV/optical spectra, and black triangles the measurements with ATOM optical telescope on the H.E.S.S. site. Grey symbols represent archival data, not contemporaneous with the gamma-ray data. The red line and the blue dashed line show the gamma-ray spectrum corrected for intergalactic absorption by electron-positron pair production, for different assumptions on the density of IR target photons. The black line illustrates the synchrotron and Inverse Compton spectra expected from an electron population moving in a beamed relativistic jet.