X-RAY OBSERVATIONS OF THE SEYFERT-1 GALAXY MARKARIAN-464

In this paper we present results of EXOSAT (0.05-10 keV) and hard X-ray (25-45 keV) balloon observations (XG-81) of the Seyfert 1 galaxy Mrk 464. The spectrum observed with EXOSAT is well fitted by a power law with a photon spectral index 1.70 +/- 0.15, in agreement with a previous measurement by HEAO 1 A-2. The absorption column density is found consistent with the galactic column density. Both the EXOSAT and the HEAO 1 upper limit (N(H) < 10(22) cm-2) are in disagreement with the very high column density of 2.5 x 10(23) cm-2 observed by Ariel 5. The XG-81 experiment detected a bright hard X-ray source in a field of view centered on Mrk 464. This measurement is most likely explained by an outburst of this Seyfert galaxy, with a peak luminosity one order of magnitude greater than the average one. Similar variations have been observed already in two other objects of this class, namely Centaurus A and NGC 4151