X-Ray Spectral Properties of Four Classical TeV Blazars Using Simultaneous Observations from NICER and NuSTAR

We present a detailed study of X-ray spectral properties observed in four classical TeV (teraelectronvolt) photon-emitting high synchrotron-peaked BL Lacertae (BL Lac) objects using the simultaneous data of the Neutron Star Interior Composition Explorer (NICER) and the Nuclear Spectroscopic Telescope Array Mission (NuSTAR) satellites. We analyzed 13 spectra in total from four BL Lac 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 seven spectra were fitted well using the absorbed LP model, we observed that four spectra of Mrk 421 and two 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 four epochs and Mrk 501 was in a low-flux state during the two epochs. We concluded that there was a contribution from the disk in these six spectra. The moderate- to low-flux state can justify the contribution of disk emission in the X-ray spectra. In the case of four spectra of Mrk 421, we observed a Gaussian feature between 1.42 and 1.70 keV.