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Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample

Stevens, J. A. and Gear, Walter Kieran ORCID: 1999. Variations in the broad-band spectra of BL Lac objects: millimetre observations of an X-ray-selected sample. Monthly Notices of the Royal Astronomical Society 307 (2) , pp. 403-412. 10.1046/j.1365-8711.1999.02575.x

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Observations at millimetre wavelengths are presented for a representative sample of 22 X-ray-selected BL Lac objects (XBLs). This sample comprises 19 high-energy cut-off BL Lac objects (HBLs), 1 low-energy cut-off BL Lac object (LBL) and 2 ‘intermediate’ sources. Data for LBLs, which are mostly radio-selected BL Lac objects (RBLs), are taken from the literature. It is shown that the radio–millimetre spectral indices of HBLs inline image are slightly steeper than those of the LBLs inline image. A correlation exists between α5–230 and 230 GHz luminosity. While this correlation could be an artefact of comparing two populations of BL Lac objects with intrinsically different radio properties, it is also consistent with the predictions of existing unified schemes that relate BL Lac objects to Fanaroff–Riley class I radio galaxies. The HBLs have significantly flatter submillimetre–X-ray spectral indices inline image than the LBLs inline image although the two intermediate sources also have intermediate values of α230–X∼−0.9. It is argued that this difference cannot be explained entirely by the viewing-angle hypothesis and requires a difference in physical-source parameters. The α230–X values for the HBLs are close to the canonical value found for large samples of radio sources and thus suggest that synchrotron radiation is the mechanism that produces the X-ray emission. As suggested by Padovani & Giommi, the inverse-Compton mechanism is likely to dominate in the LBLs requiring the synchrotron spectra of these sources to steepen or cut off at lower frequencies than those of the HBLs.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Physics and Astronomy
Subjects: Q Science > QB Astronomy
Uncontrolled Keywords: radiation mechanisms: non-thermal; galaxies: active; BL Lacertae objects: general; radio continuum: galaxies
Publisher: John Wiley
ISSN: 0035-8711
Last Modified: 21 Oct 2022 09:17

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