We present an analysis of the spectral properties observed in X-rays from active galactic nucleus BL Lacertae using RXTE , Suzaku , ASCA , Beppo SAX , and Swift observations .
The total time covered by these observations is approximately 20 years .
We show strong observational evidence that this source undergoes X-ray spectral transitions from the low hard state ( LHS ) through the intermediate state ( IS ) to the high soft state ( HSS ) during these observations .
During the RXTE observations ( 1997 – 2001 , 180 ks , for a total 145 datasets ) , the source was approximately \sim 75 % , \sim 20 % and only \sim 5 % of the time in the IS , LHS , and HSS , respectively .
We also used Swift observations ( 470 datasets , for a total 800 ks ) , which occurred during 12 years ( 2005 – 2016 ) , the broadband ( 0.3 – 200 keV ) data of Beppo SAX ( 1997 – 2000 , 160 ks ) , and the low X-ray energy ( 0.3 – 10 keV ) data of ASCA ( 1995 – 1999 , 160 ks ) .
Two observations of Suzaku ( 2006 , 2013 ; 50 ks ) in combinations with long-term RXTE and Swift data-sets fortunately allow us to describe all spectral states of BL Lac .
The spectra of BL Lac are well fitted by the so-called bulk motion Comptonization ( BMC ) model for all spectral states .
We have established the photon index saturation level , \Gamma _ { sat } =2.2 \pm 0.1 , in the \Gamma versus mass accretion rate ( \dot { M } ) correlation .
This \Gamma - \dot { M } correlation allows us to estimate the black-hole ( BH ) mass in BL Lac to be M _ { BH } \sim 3 \times 10 ^ { 7 } M _ { \odot } for a distance of 300 Mpc .
For the BH mass estimate , we use the scaling method taking stellar-mass Galactic BHs 4U 1543–47 and GX 339–4 as reference sources .
The \Gamma - \dot { M } correlation revealed in BL Lac is similar to those in a number of stellar-mass Galactic BHs and two recently studied intermediate-mass extragalactic BHs .
It clearly shows the correlation along with the very extended \Gamma saturation at \sim 2.2 .
This is robust observational evidence for the presence of a BH in BL Lac .
We also reveal that the seed ( disk ) photon temperatures are relatively low , of order of 100 eV , which are consistent with a high BH mass in BL Lac .
It is worthwhile to emphasize that we found particular events when X-ray emission anti-correlates with radio emission .
This effect indicates that mass accretion rate ( and thus X-ray radiation ) is higher when the mass outflow is lower .