Riya Bhowmick, Alok C. Gupta

We present a detailed study of X-ray spectral properties observed in 4 classical TeV (tera-electron volt) photon-emitting high synchrotron-peaked BL Lacertae objects using the simultaneous data of NICER and NuSTAR satellites. We analyzed 13 spectra in total from four BL Lacertae objects: Mrk 421, Mrk 501, PG 1553+113, and PKS 2155-304. We fitted all the spectra using the absorbed Log-Parabolic (LP) model first. While 7 spectra were fitted well using the absorbed LP model, we observed that 4 spectra of Mrk 421 and 2 spectra of Mrk 501 were not fitted satisfactorily using the absorbed LP model. The investigation of the flux states of the sources revealed that Mrk 421 was in a moderate to low-flux state during the 4 epochs and Mrk 501 was in a low-flux state during the 2 epochs. We concluded that there was a contribution from the disk in these 6 spectra. The moderate to low-flux state can justify the contribution of disk emission in the X-ray spectra. In the case of 4 spectra of Mrk 421, we observed a Gaussian feature between 1.42 and 1.70 keV.