X-ray spectral variations of synchrotron peak in BL Lacs
  The spectral energy distribution of blazars around the synchrotron peak can
be well described by the log-parabolic model that has three parameters: peak
energy ($E_\textrm{p}$), peak luminosity ($L_\textrm{p}$) and the curvature
parameter ($b$). It has been suggested that $E_\textrm{p}$ shows relations with
$L_\textrm{p}$ and $b$ in several sources, which can be used to constrain the
physical properties of the emitting region and/or acceleration processes of the
emitting particles. We systematically study the $E_\textrm{p}$-$L_\textrm{p}$
and $E_\textrm{p}$-(1$/b$) relations for 14 BL Lac objects using the 3-25~keV
$RXTE$/PCA and 0.3-10~keV $Swift$/XRT data. Most objects (9/14) exhibit
positive $E_\textrm{p}$-$L_\textrm{p}$ correlations, three sources show no
correlation, and two sources display negative correlations. In addition, most
targets (7/14) present no correlation between $E_\textrm{p}$ and 1$/b$, five
sources pose negative correlations, and two sources demonstrate positive
correlations. 1ES~1959+650 displays two different $E_\textrm{p}$-$L_\textrm{p}$
relations in 2002 and 2016. We also analyze $E_\textrm{p}$-$L_\textrm{p}$ and
$E_\textrm{p}$-(1$/b$) relations during flares lasting for several days. The
$E_\textrm{p}$-$L_\textrm{p}$ relation does not exhibit significant differences
between flares, while the $E_\textrm{p}$-(1$/b$) relation varies from flare to
flare. For the total sample, when $L_\textrm{p}$ < $\textrm{10}^\textrm{45}\
\textrm{erg}\ \textrm{s}^\textrm{-1}$, there seems to be a positive
$E_\textrm{p}$-$L_\textrm{p}$ correlation. $L_\textrm{p}$ and the slope of
$E_\textrm{p}$-$L_\textrm{p}$ relation present an anti-correlation, which
indicates that the causes of spectral variations might be different between
luminous and faint sources. $E_\textrm{p}$ shows a positive correlation with
the black hole mass. We discuss the implications of these results.
