Swift Observations of the highly X-ray variable Narrow Line Seyfert 1 galaxy RX J0148.3-2758

We report on Swift observations of the Narrow-Line Seyfert 1 galaxy (NLS1) RX J0148.3--2758. It was observed for 41.6 ks in 2005 May and for 15.8 ks in 2005 December. On short as well as on long timescales RX J0148.3--2758 is a highly variable source. It doubles its X-ray flux within 18-25 ks. The observation of 2005 December 09, which had a flux 4 times lower than during the 2005 May observations, shows a significant hardening of the X-ray hardness ratio compared with the 2005-May and 2005-December 20/21 observations. A detailed analysis of the X-ray spectra shows that we actually observe two spectral changes in RX J0148.3-2758: First a decrease of the soft X-ray component between 2005 May and December 09, which is most likely due to an increase of the intrinsic absorber column, and second a decrease of the hard X-ray flux in the December 20/21 observations. The soft X-ray spectral slope $\alpha_{\rm X, soft}$=2.58$^{+0.15}_{-0.12}$ during the high state in 2005 May agrees well with that measured by ROSAT (\axs=2.54\plm0.82). In contrast to the strong X-ray variability, the analysis of the Swift UVOT photometry from December 2005 of RX J0148.3--2758 shows no significant variability in any of the 6 UVOT filters. From the simultaneous X-ray and UV observations in 2005 December we measured the X-ray loudness alpha-ox varies between alpha-ox=1.5 and 1.8. Our Swift observations of RX J0148.3-2758 demonstrate the great potential the multi-wavelength observatory Swift has for AGN science. (shortened)