Observation of X‐Ray Variability in the BL Lacertae Object 1ES 1959+65

This paper reports X-ray spectral observations of a relatively nearby z =0.048 BL Lacertae (BL Lac) object 1ES1959+650, which is a potential TeVemitter. The observations include 31 short pointings made by the UnconventionalStellar Aspect (USA) Experiment on board the Advanced Research and GlobalObservation Satellite (ARGOS), and 17 pointings by the PCA on board the RossiX-ray Timing Explorer (RXTE). Most of these observations were spaced by lessthan 1 day. \es was detected by the ARGOS USA detector in the range 1-16 keV,and by the PCA in the 2-16 keV range but at different times. During the closelyspaced RXTE observations beginning on 2000 July 28, an ending of one flare anda start of another are visible, associated with spectral changes, where thephoton index Gamma ranges between ~ 1.4 and 1.7, and the spectrum is harderwhen the source is brighter. This implies that 1ES1959 is an XBL-type blazar,with the X-ray emission likely to originate via the synchrotron process. TheUSA observations reveal another flare that peaked on 2000 November 14 anddoubled the flux within a few days, again associated with spectral changes ofthe same form. The spectral variability correlated with the flux and timingcharacteristics of this object that are similar to those of other nearby BLLacs, and suggest relativistic beaming with a Doppler factor delta > 1.6 andmagnetic fields of the order of a few mG. We also suggest that the steadycomponent of the X-ray emission -- present in this object as well as in otherXBLs -- may be due to the large-scale relativistic jet (such as measured byChandra in many radio-loud AGN), but pointing very closely to our line ofsight.Comment: To appear in ApJ June 1, 2002 issu