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Published February 1, 1981 | Published
Journal Article Open

Quasi-simultaneous observations of BL Lac object MRK 501 in X-ray, UV, visible, IR, and radio frequencies


Quasi-simultaneous observations of the BL Lac object Mrk 501 were performed for the first time at X-ray, ultraviolet, visible infrared, and radio frequencies. As the BL Lac objects are known to vary in their flux, such a "quasi-instantaneous" spectral energy profile is necessary in order to describe properly the energy generation mechanism. The observed spectral slope from the X-ray to UV regions is positive and continuous, but that from the mid-UV to visible light region becomes gradually flat and possibly turns down toward lower frequencies; the optical-radio emission cannot be accounted for by a single power law. Several theoretical models have been considered for the emission mechanism. In some cases quantitative comparison with the data is not practical. However, most of the models are, at least, not inconsistent with the observations. A quantitative comparison has been performed with the synchroton self-Compton model; the total spectrum is found consistent with this model. The spectrum from visible light to X-ray is consistent with synchrotron radiation or with inverse-Compton scattering by a hot thermal cloud of electrons. The continuity of the spectral slope from X-ray to UV implied by the current data suggests that the previous estimates of the total luminosity of this BL Lac object has been underestimated by a factor of about 3 or 4.

Additional Information

© 1981 American Astronomical Society. Received 1980 March 31; accepted 1980 August 8. The HEAO A2 experiment is a collaborative effort led by E. Boldt of GSFC and G. Garmire of CIT with collaborators at GSFC, CIT, JPL, and UCB. Thanks are due to Dr. A. Boggess and the I U E Observatory staff at GSFC for their competent assistance in obtaining the data. Y.K., R.F.M., R.L.H., and K.R.H.H. are guest observers on the I U E. R.L.H. and K.R.H.H. gratefully acknowledge the assistance of W. Kinzel in the reduction of the JUE data and funding by NASA grant NSG 5238 and the WKU Faculty Research Grant Committee. G.N. and K.M. acknowledge support by NASA and NSF. We thank A.D. Code for helpful discussions of the interstellar reddening and E. A. Boldt for valuable comments on the X-ray results. P.A.F. wishes to thank C.R. Purton, P. C. Gregory, and W. M. Turner for assistance with the observations at A.R.O.

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Published - 1981ApJ___243__690K.pdf


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