Journal Abstracts 1

Abstract Guidelines 3

PKS 0537-441: extended [O II] emission and a binary QSO?

Jochen Heidt¹, Klaus Jäger², Kari Nilsson³, Ulrich Hopp⁴, Josef W. Fried⁵  and Eckhard Sutorius⁶

¹ Landessternwarte Heidelberg, Königstuhl, D-69117 Heidelberg, Germany
² Universitätssternwarte Göttingen, Geismarlandstr. 11, D-37083 Göttingen, Germany
³ Tuorla Observatory, FIN-21500 Piikkiö, Finland
⁴ Universitätssternwarte München, Scheinerstr. 1, D-81679 München, Germany
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁶ Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, United Kingdom

We present high-resolution imaging and low-resolution spectroscopy of the BL Lac object PKS 0537-441 (z = 0.893) and its environment carried out with the ESO-NTT and VLT telescopes. The observations were designed to clarify, whether the properties of PKS 0537-441 are affected by gravitational microlensing due to the claimed detection of a galaxy along the line-of-sight to the BL Lac, or whether PKS 0537-441 and its environment act as a lensing system itself, as suggested by the detection of several closeby companion galaxies with similar morphologies close to PKS 0537-441.
Our observations show that neither case seems to be likely. Within our images we did not find a galaxy along the line-of-sight to the BL Lac as claimed previously. In addition, our spectroscopy shows that none of the four closest companion galaxies (including one new detection by us) is at high redshift. Instead, two of the four nearby companion galaxies to PKS 0537-441 are within 200 km/s of the systemic velocity of the BL Lac (z = 0.892 and 0.895, respectively). The third companion galaxy is at higher redshift (z = 0.947). The fourth companion galaxy shows evidence of Mg II absorption redwards of its systemic velocity and is perhaps a mini low-ionization BAL QSO at z = 0.885. If the latter can be confirmed, PKS 0537-441 is the first BL Lacertae object being a member of a binary Quasar.
While we do not find (micro)lensing effects being important for this source or its environment, our observations revealed a highly interesting feature. We detected extended [O II] emission in the off-nuclear spectrum of PKS 0537-441, which is most likely due to photoionization from the active nucleus, although we can not rule out the possibility that the extended emission is due to jet-cloud interaction with the counterjet of PKS 0537-441.
According to our analysis of the photometric data, PKS 0537-441 seems to be located in a cluster environment as rich as Abell type 0-1. This is supported by the detection of four galaxies in the field with similar redshifts as the BL Lac (D z < 0.002). However, we found serendipitously even more galaxies at somewhat higher redshifts (z = 0.9-1). Thus, PKS 0537-441 might be located in front of a galaxy cluster at somewhat higher redshift or even be part of a large-scale structure with an extension towards the BL Lac.

Accepted by A&A

For preprints contact: jheidt@lsw.uni-heidelberg.de

For preprints via ftp or WWW: http://xxx.lanl.gov/abs/astro-ph?0305401

Detection of TeV Gamma-Rays from the BL Lac 1ES1959+650 in its low states and during a major outburst in 2002

The HEGRA Collaboration (http://www-hegra.desy.de/hegra):

F. Aharonian¹, A. Akhperjanian⁷, M. Beilicke⁴, K. Bernlöhr¹, H.-G. Börst⁵, H. Bojahr⁶, O. Bolz¹, T. Coarasa², J.L. Contreras³, J. Cortina¹0, S. Denninghoff², M.V. Fonseca³, M. Girma¹, N. Götting⁴, G. Heinzelmann⁴, G. Hermann¹, A. Heusler¹, W. Hofmann¹, D. Horns¹, I. Jung¹, R. Kankanyan¹, M. Kestel², A. Kohnle¹, A. Konopelko¹, H. Kornmeyer², D. Kranich², H. Lampeitl⁴, M. Lopez³, E. Lorenz², F. Lucarelli³, O. Mang⁵, H. Meyer⁶, R. Mirzoyan², A. Moralejo³, E. Ona-Wilhelmi³, M. Panter¹, A. Plyasheshnikov¹,8, G. Pühlhofer¹, R. de los Reyes³, W. Rhode⁶, J. Ripken⁴, J. Robrade⁴, G. Rowell¹, V. Sahakian⁷, M. Samorski⁵, M. Schilling⁵, M. Siems⁵, D. Sobzynska²,9, W. Stamm⁵, M. Tluczykont⁴, V. Vitale², H.J. Völk¹, C. A. Wiedner¹, W. Wittek²

¹Max-Planck-Institut für Kernphysik, Postfach 103980, D-69029 Heidelberg, Germany
² Max-Planck-Institut für Physik, Föhringer Ring 6, D-80805 München, Germany
³ Universidad Complutense, Facultad de Ciencias Físicas, Ciudad Universitaria, E-28040 Madrid, Spain
⁴ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, D-22761 Hamburg, Germany
⁵ Universität Kiel, Institut für Experimentelle und Angewandte Physik, Leibnizstraße 15-19, D-24118 Kiel, Germany
⁶ Universität Wuppertal, Fachbereich Physik, Gaußstr.20, D-42097 Wuppertal, Germany
⁷ Yerevan Physics Institute, Alikhanian Br. 2, 375036 Yerevan, Armenia
⁸ On leave from Altai State University, Dimitrov Street 66, 656099 Barnaul, Russia
⁹ Home institute: University Lodz, Poland
¹⁰ Now at Institut de Física d'Altes Energies, UAB, Edifici Cn, E-08193, Bellaterra (Barcelona), Spain

TeV g-rays from the BL Lac object 1ES1959+650 have been measured during the years 2000 and 2001 with a significance of 5.2 s at a value of 5.3 % of the Crab flux and in May 2002 during strong outbursts with > 23 s at a flux level of up to 2.2 Crab, making 1ES1959+650 the TeV Blazar with the third best event statistics. The deep observation of 197.4 h has been performed with the HEGRA stereoscopic system of 5 imaging atmospheric Cherenkov telescopes (IACT system). 1ES1959+650 is located at a redshift of z = 0.047, providing an intermediate distance between the nearby Blazars Mkn 421 and Mkn 501, and the much more distant object H1426+428. This makes 1ES1959+650 an important member of the class of TeV Blazars in view of the absorption of TeV photons by the diffuse extragalactic background radiation (DEBRA). The differential energy spectrum of 1ES1959+650 during the flares can be fitted by a power law with a spectral index of 2.83 ±0.14_stat ±0.08_sys or by a power law with an exponential cut-off at (4.2^+0.8_-0.6 stat ±0.9_sys) TeV and a spectral index of 1.83 ±0.15_stat ±0.08_sys. The low state differential energy spectrum obtained with lower statistics can be described by a pure power law with a spectral index of 3.18 ±0.17_stat ±0.08_sys.

Accepted by A & A Letters

For preprints contact: Niels.Goetting@desy.de

For preprints via WWW: http://arXiv.org/abs/astro-ph/0305275

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