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 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 9, 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 December 20 and 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 9, 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 and 21 observations. The soft X-ray spectral slope α _X,soft = 2.58 _-0.12 ^+0.15 during the high state in 2005 May agrees well with that measured by ROSAT (α _X,soft = 2.54 ± 0.82). This soft X-ray spectrum is superposed on a hard X-ray component with α _X,hard = 0.96 _-0.14 ^+0.16, which is consistent with the hard X-ray spectral slope α _X,hard = 1-11 _-0.19 ^+0.16 found by ASCA. The soft X-ray slope α _X,soft = 1.93 _-0.42 ^+0.58 measured during the December 9 observation agrees well with α _X,soft = 2.03 _-0.20 ^+0.23 measured from the ASCA observation when RX J0148.3-2758 was also in a low state. In contrast to the strong X-ray variability, the analysis of the Swift UV-Optical Telescope (UVOT) photometry from 2005 December of RX J0148.3-2758 shows no significant variability in any of the six UVOT filters. From the simultaneous X-ray and UV observations in 2005 December we measured the X-ray loudness α _ox and found it to vary between α _ox = 1.5 and 1.8. Our Swift observations of RX J0148.3-2758 demonstrate the great potential that the multiwavelength observatory Swift has for active galactic nucleus science.