This paper reports X–ray spectral observations of a relatively nearby ( z = 0.048 ) BL Lacertae ( BL Lac ) object 1ES1959 + 65 , which is a potential TeV emitter .
The observations include 31 short pointings made by the Unconventional Stellar Aspect ( USA ) Experiment on board the Advanced Research and Global Observation Satellite ( ARGOS ) , and 17 pointings by the PCA on board the Rossi X–ray Timing Explorer ( RXTE ) .
Most of these observations were spaced by less than 1 day .
1ES1959 + 65 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 closely spaced RXTE observations beginning on 2000 July 28 , an ending of one flare and a start of another are visible , associated with spectral changes , where the photon index \Gamma ranges between \sim 1.4 and 1.7 , and the spectrum is harder when the source is brighter .
This implies that 1ES1959 + 65 is an XBL-type blazar , with the X–ray emission likely to originate via the synchrotron process .
The USA observations reveal another flare that peaked on 2000 November 14 and doubled the flux within a few days , again associated with spectral changes of the same form .
The spectral variability correlated with the flux and timing characteristics of this object that are similar to those of other nearby BL Lacs , and suggest relativistic beaming with a Doppler factor \delta \geq 1.6 and magnetic fields of the order of a few mG. We also suggest that the steady component of the X–ray emission – present in this object as well as in other XBLs – may be due to the large-scale relativistic jet ( such as measured by Chandra in many radio-loud AGN ) , but pointing very closely to our line of sight .