Soft X-ray observations of two BL Lacertae objects: Markarian 421 and 501

Abstract

In this paper we report on the soft X-ray (0.15-2.8 keY) observations of two BL Lacertae-type objects, viz., Mrk 421 and Mrk 501. The observations were made with the low-energy detectors on the HEAO 1 satellite during the period 1977 August-1978 December. We find steep, soft X-ray power-law spectra with photon index Γ ≈ 3 for both Mrk 421 and Mrk 501. The power-law models are found to give a significantly better fit than the thermal.models to the observed pulse-height spectra of Mrk 421 and Mrk 501. Day-to-day soft X-ray (0.25 keV band) itensity variations are observed only in Mrk 501. No significant change is found in Γ from both the BL Lac objects during our period of observations. However, the sum of all the X-ray observations from 1976 until 1980 can be understood in terms of two spectral components of variable intensity to account for the X-ray emission observed between 0.15 and 20 keY from Mrk 421 and Mrk 501. No correlation is observed between intensity variations and the spectral form.