Ginga observations of X-rays from the Seyfert I galaxy Markarian 509

Abstract

We present the X-ray light curves and X-ray spectra of Markarian (Mrk) 509 from two observations with the Ginga satellite. A continuously varying X-ray intensity on hourly time scales was observed on both the occasions. The observed variability is the fastest reported so far from Mrk 509. The X-ray intensity was observed to increase almost linearly before saturating at the maximum on 1988 October 8 and thus showing a quadratic trend. About 5 days later it was observed to decrease linearly. The maximum variation observed is 30%. The X-ray spectra accumulated for different intensity levels were analyzed separately. We find that the 1-30 keV spectra are best explained by (a) a variable power law with the photon index Γ in the range of 1.6-1.7 over the energy bandwidth of 3-30 keY and showing a steepening with increase in the intensity, (b) a variable low-energy excess below 3 ke V with a very steep spectrum (Γ = 4.5 ± 1.0 or a blackbody with kT ⋍ 0.3 keV or an accretion disk with kT ⋍ 0.4 keV), (c) a Gaussian line feature near 6 keV best observed during the peak brightness level and having an equivalent width of 150 ± 49 eV, and (d) good evidence for a flatter (Γ ⋍ 1.47 ± 0.07) spectrum above 10 keV during the maximum brightness. The observations are consistent with the presence of a large amount of cold matter around the dominant nonthermal X-ray source in the active galactic nucleus of Mrk 509 even though no low-energy absorption is observed directly.