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Astron. Astrophys. 342, 124-130 (1999)

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1. Introduction

Mkn 421 was the first BL Lac object to be detected in X-rays (Ricketts et al. 1976). Subsequent observations have detected the source up to 100 keV (e.g.: Ubertini et al. 1984). The X-ray emission of Mkn 421 is highly variable, with distinct behavioral differences between soft and hard X-rays. In the 0.5-10 keV band the source is variable on time scales ranging from several hours to days, occasionally exhibiting large X-ray outbursts characterized by a marked hardening of the spectrum. EXOSAT observations showed that during the quiescent state the source hardens as it brightens (George et al. 1988). Giommi et al. (1990) showed that flux variations in the hard X-ray band (0.7-8 keV) were more pronounced than those in the soft band (0.06-0.3 keV).

Observations with EXOSAT (Giommi et al. 1990), ROSAT (Fink et al. 1991) and ASCA (Kubo et al. 1998) reveal that variability of Mkn 421 in the soft X-ray band can be represented by small-amplitude variations about a quiescent level that remains temporally relatively constant. The timescale of these variations is of the order of days to weeks (George et al. 1988).

Mkn 421 has also been detected in [FORMULA]-rays by Compton GRO/EGRET (Lin et al. 1996), and it was the first extragalactic source to be detected at TeV energies (Punch et al. 1992). Multiwavelength observations have shown that variability in the X-ray band is accompanied by significant variability in the TeV region, with little or no change in other wave bands (Macomb et al. 1995).

In this paper we report on three observations of Mkn 421 in 1997 April and May with the Narrow Field Instruments (NFI) on board the BeppoSAX X-ray observatory (see Table 1 for a log of the observations).


Table 1. Mkn 421 observations log. Note that the LECS is only operated during spacecraft nighttime and collimated detectors spend about [FORMULA] of the time observing the background

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© European Southern Observatory (ESO) 1999

Online publication: December 22, 1998