1ES 1927+654 was first detected as a X-ray source during the Einstein
Slew Survey observations with a cumulative 38 s exposure and a count rate
of 0.93
s-1 (Elvis et al. 1992).
Perlman et al. (1996) have measured a redshift of 0.017 for 1ES 1927+654. They have also
demonstrated that the object was not a BL-Lac as previously classified, but a narrow-line radio galaxy. Bauer et al. (2000)
have confirmed this and further classified 1ES 1927+654 as a Seyfert 2.
The 6-cm radio flux density of 1ES 1927+654 is 16.03 mJy (Perlman et al. 1996) and the
optical B magnitude is 15.37.
Although the radio to optical flux density ratio is R = 3.3 (cf. Kellermann et al. 1989),
the low radio power of
W Hz-1 at 5 GHz confirms its radio-quiet nature.
The probability of
a relativistic jet causing the extreme X-ray flux variations is
very unlikely.
In this paper we report on the X-ray spectra of 1ES 1927+654 obtained with ROSAT and recently with Chandra, as well as on the results of an optical follow-up observation in October 2001. 1ES 1927+654 was detected as a X-ray bright source with extremely rapid and strong X-ray variability in the ROSAT All-Sky Survey (Voges et al. 1999) by a systematic analysis of the X-ray timing properties of All-Sky Survey sources. Additionally, a PSPC pointed observation, during the final days of the ROSAT satellite in December 1998, revealed a strong X-ray flare.
The detection of giant X-ray flares on short time scale of less than about
one day is of great importance for the study of the innermost regions of
active galactic nuclei. Models for explaining the presence of giant X-ray
flares include Doppler boosting effects of X-ray hot spots on the accretion
disk (Sunyaev 1973; Guilbert et al. 1983), X-ray flares above
the accretion disk (e.g Reynolds et al. 1999) or partial obscuration
of Compton thick matter close to the central black hole (Brandt et al. 1999;
Boller et al. 1997; Boller et al. 2002). Giant X-ray flares are therefore
thought to give independent evidence for the presence of relativistic motions close
to the central black hole, similar to the conclusions drawn from the
asymmetric Fe
line (Tanaka et al. 1995).
Persistent, strong and rapid variability seems to be a rare phenomenon among radio-quiet AGN and has previously been detected only in the narrow-line Seyfert 1 galaxy IRAS 13224-3809 (Boller et al. 1997). The object shows amplitude X-ray variability of a factor greater than 10, on time scales less than about 1 day. The physical origin of giant and rapid X-ray flares is most probably different from the X-ray variability occuring on much longer time scales of years, or longer, in active and non-active galaxies.
A value of the Hubble constant of
and a cosmological deceleration parameter of
have been adopted throughout.
Copyright ESO 2003