Multiwavelength observations of the quasar Q 1821+643 during the ROSAT All Sky Survey.

Abstract

Temporal variations of the flux in the optical, UV, soft X-ray and medium X-ray ranges occur in most AGN studied, but no general correlation has emerged between the variations in these different energy ranges. Because of its high ecliptic latitude the radio-quiet quasar Q 1821 + 643 was surveyed for 37 days by ROSAT during the ROSAT All Sky Survey, thereby providing the rare opportunity for repeated simultaneous observations of a bright quasar in the X-ray, ultraviolet and optical ranges. The first result is that no flux variation by more than 10% has been detected during the 37 d period of visibility in the range (0.1-2.4) keV, and no variation by more than 5% has been detected in the UV and optical ranges. This indicates a steady state of the emission mechanisms during this period. The most interesting result is that the fit of the soft X-ray spectrum of Q 1821 + 643 and of 3C273 (a flat radio spectrum quasar of similar optical luminosity) obtained during the ROSAT Survey with a thermal bremsstrahlung indicates a definitely lower temperature for Q1821 + 643 than for 3C 273 (kT= 73 and 173eV respectively).