Chandra Observations of X-Ray-weak Narrow-Line Seyfert 1 Galaxies

We present Chandra observations of 17 optically selected, X-ray-weak narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified by Williams et al. in the Sloan Digital Sky Survey Early Data Release but were not found in the ROSAT All-Sky Survey (RASS) despite having optical properties similar to RASS-detected NLS1s. All objects in this sample were detected by Chandra and exhibit a range of 0.5-2 keV photon indices Γ=1.1-3.4. One object was not detected in the soft band but has a best-fit Γ=0.25 over the full 0.5-8 keV range. These photon indices extend to values far below those normally observed in NLS1s. A composite X-ray spectrum of the hardest objects in this sample does not show any signs of absorption, although the data do not prohibit one or two of the objects from being highly absorbed. We also find a strong correlation between Γ and L_1keV; this may be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such variations are seemingly in conflict with the current paradigm that NLS1s accrete near the Eddington limit. Most importantly, this sample shows that strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in fact, a substantial number may exhibit weak and/or low-Γ X-ray emission.