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Astron. Astrophys. 364, 43-52 (2000)

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ASCA and other contemporaneous observations of the blazar B2 1308+326

D. Watson 1, N. Smith 2, L. Hanlon 1, B. McBreen 1, F. Quilligan 1, M. Tashiro 3, L. Metcalfe 4, P. Doyle 5, H. Teräsranta 6, A. Carramiñana 7 and J. Guichard 7

1 Department of Experimental Physics, University College Dublin, Belfield, Dublin 4, Ireland
2 Department of Applied Physics and Instrumentation, Cork Institute of Technology, Cork, Ireland
3 Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
4 ISO Data Centre, Astrophysics Division, Space Science Department of ESA, Villafranca del Castillo, P.O. Box 50727, 28080 Madrid, Spain
5 Physics Department, National University of Ireland, Cork, Ireland
6 Metsähovi Radio Observatory, 02540 Kylmälä, Finland
7 Instituto Nacional de Astrofísica Optica y Electrónica, Luis Enrique Erro 1, Tonantzintla, Puebla 72840, México

Received 6 March 2000 / Accepted 11 September 2000

Abstract

The high redshift ([FORMULA]) blazar B2 1308+326 was observed contemporaneously at x-ray, optical and radio wavelengths in June 1996. The x-ray observations were performed with ASCA. The ASCA results were found to be consistent with reanalysed data from two earlier ROSAT observations. The combined ASCA and ROSAT data reveal an x-ray spectrum that is best fit by a broken power law with absorber model with photon spectral indices of [FORMULA] and [FORMULA] and a break energy at [FORMULA] keV in the rest-frame of the blazar. The break in the x-ray spectrum is interpreted, from the shape of the simultaneous broadband spectral energy distribution, to be the emerging importance of inverse Compton (IC) emission which dominates the ASCA spectrum. The faint optical state reported for these observations ([FORMULA]) is incompatible with the high synchrotron flux previously detected by ROSAT. The IC emission detected by both ROSAT and ASCA was not significantly affected by the large change in the synchrotron component.

MgII emission was detected with an equivalent width ([FORMULA]) of [FORMULA] 15 Å, significantly different from previously reported values. The small and variable [FORMULA] in B2 1308+326 may be due to the highly variable continuum and not intrinsically weak lines in the source. A lower limit on the Doppler boost factor calculated from the contemporaneous data is consistent with expectations for highly polarised quasars and higher than expected for BL Lacs. Absorption at a level of [FORMULA] cm-2 was detected which is in excess of the Galactic value of [FORMULA] cm-2, indicating the possible presence of a foreground absorber. A gravitational microlensing scenario cannot therefore be ruled out for this blazar. No significant variability on timescales of hours was detected in the optical or x-ray data.

B2 1308+326 could be a typical radio-selected BL Lac in terms of peak synchrotron frequency and optical and radio variability but its high bolometric luminosity, variable line emission and high Doppler boost factor make it appear more like a quasar than a BL Lac. It is suggested that B2 1308+326 be considered as the prototype of this class of composite source.

Key words: galaxies: BL Lacertae objects: individual: B2 1308+326 – galaxies: quasars: individual: B2 1308+326 – X-rays: galaxies – radio continuum: galaxies – cosmology: gravitational lensing

Send offprint requests to: D. Watson (dwatson@bermuda.ucd.ie)

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© European Southern Observatory (ESO) 2000

Online publication: December 15, 2000
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