disk-blackbody spectra in that temperature range are indeed approximated in the [*Chandra*]{} band (at least for the purpose of converting from count rates to fluxes) by power-laws of photon index between $\sim$1.5 (for $T_\mathrm{in} \sim$ 1 keV) and $\sim$2 (for $T_\mathrm{in} \sim$ 0.6 keV), with the most likely value $\Gamma$ $\sim$ 1.8. The source spectrum of N4088–X1 was extracted using the *specextract* script and selecting a circular region of 2$\arcsec$ around the target source and of 15$\arcsec$ in a source-free area of the same chip for the background. The extracted spectrum was grouped to 15 counts per energy bin to allow for $\chi^{2}$ fitting using the tool *grppha*.

*Swift* X-ray Observations
--------------------------

NGC4088 was observed on 23 occasions in X-rays with the *Swift* X-ray Telescope (XRT) between 2009 April 15 and 2009 September 21 as part of a monitoring campaign targeting the supernova (SN) SN2009dd (see Table \[swiftlog\] for a log of the *Swift* observations). We generated images, light curves and spectra, including the background and ancillary response files, with the online XRT data product generator [^7] (@2009MNRAS.397.1177E). We downloaded suitable spectral response files for single and double events in photon-counting mode from the latest calibration database. N4088–X1 was clearly 