\begin{table}%t4
\par
\caption {\label{tab:pexrav}Best-fit results for the simultaneous spectral fit of the
three epochs (see text for details). We report the values obtained
for the {\it nominal} Crab cross-calibration factor of ISGRI
($C_{\rm ISGRI}=0.99$), and for $C_{\rm ISGRI}=0.89$ and $C_{\rm ISGRI}=1.09$.
$N_{\rm Pex}$ and $N_{\rm Bkp}$ are the normalization factors for the PEXRAV and
broken power law models respectively (reported as photon flux at 1~keV in units of $10^{-3} ~\rm ph/cm^2/s/keV$).
 $\ \ \ ^*$ error obtained fixing the PEXRAV normalization.}
 %\centering
\par
\begin{tabular}{lccc}
\cline{2-4}\\[-3.5mm]
\cline{2-4}
                     & $C_{\rm ISGRI}=0.99$    & $C_{\rm ISGRI}=0.89$  & $C_{\rm ISGRI}=1.09$ \\
                     & (nominal from Crab)&                  &  \\ \hline
$N_{\rm Pex}$ ($10^{-3} ~\rm  ph/cm^2/s/keV$) & $13.3 \pm 5.0$ ($\pm $0)$^*$ & $19.4 \pm 5.3$ & $7.9 \pm 4.7$\\
$N_{\rm Bkp}$ for rev. 633 ($10^{-3} \rm \ ph/cm^2/s/keV$)  & $27.1 \pm 9.3$ ($\pm 5.1$)$^*$& $20.3 \pm 9.5$ & $33.8 \pm 9.3$\\
$N_{\rm Bkp}$ for rev. 635 ($10^{-3} \rm \ ph/cm^2/s/keV$)  & $19.5 \pm 7.9$ ($\pm 3.8$)$^*$& $13.5 \pm 7.8$ & $25.7 \pm 8.0$\\
$N_{\rm Bkp}$ for rev. 637 ($10^{-3} \rm \ ph/cm^2/s/keV$)  & $25.4 \pm 8.6$ ($\pm 4.7$)$^*$& $18.7 \pm 8.7$ & $32.0 \pm 8.6$\\
photon index 1       & $1.46 \pm 0.12$ ($\pm $$0.10$)$^*$& $1.38 \pm 0.15$& $1.51 \pm 0.10$ \\
photon index 2       & $1.71 \pm 0.05$ ($\pm$$ 0.04$)$^*$& $1.66 \pm 0.05$& $1.75 \pm 0.04$\\
break Energy (keV)   & $4.   \pm 2.$ ($\pm$$ 2.$)$^*$& $4.   \pm 2.$ & $4.   \pm 2.$\\
XRT cross-calib      & $1.01 \pm 0.11$ ($\pm $$0.11$)$^*$& $1.02 \pm 0.11$ & $0.98 \pm 0.11$  \\
$\chi^2/\rm d.o.f.$      & 38.0/45 & 40.7/45& 36.4/45\\
null hypothesis probability    & 0.76    & 0.65  & 0.82 \\
\hline
\end{tabular}
\end{table}