First detection of hard X-ray photons in the soft X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007: The X-ray photon distribution observed by Swift

TL;DR

This study reports the first detection of hard X-ray photons in the soft X-ray transient galaxy WPVS 007, revealing changes in its X-ray spectrum and absorption features over time, and suggesting a possible disappearance of the absorbing material.

Contribution

First detection of hard X-ray photons in WPVS 007 and analysis of its evolving X-ray spectrum and absorption properties using Swift data.

Findings

WPVS 007 detected at 2 x 10^{-17} W m^{-2} in 0.3-10 keV band

X-ray spectrum consistent with partial covering absorber (~10^{23} cm^{-2})

Absorber may be disappearing, indicated by spectral hardness change

Abstract

We report on the first detection of hard X-ray photons (E>2.5 keV) in the X-ray transient Narrow-Line Seyfert 1 galaxy WPVS 007 which was the AGN with the softest X-ray spectrum during the ROSAT All-Sky Survey. The AGN is clearly detected at a level of about 2 x 10^{-17} W m^{-2} in the observed 0.3-10.0 keV band by Swift in a 50 ks observation in 2007 September. For the first time since the ROSAT All-Sky Survey observation in 1990 it was possible to derive an X-ray photon distribution by adding together all Swift observations that have been performed so far (85.5 ks in total). This photon distribution is consistent with an X-ray spectrum of an AGN with a partial covering absorber with a column density in the order of ~ 1 x 10^{23} cm^{-2} and a covering fraction of about 90%. A comparison with the 2002 Chandra data suggests that WPVS 007 has become brighter by a factor of about 4. The Swift data also suggest that the absorber which is causing the current low-state may have started to disappear. This disappearance is indicated by a significant change in the hardness ratio from a very soft X-ray state during the 2005 October to 2007 January observations to a rather hard X-ray state in the 2007 September observations. In the UV, WPVS 007 seems to become fainter by up to 0.5 mag over the last two years. The optical to X-ray spectral slope derived from the spectral energy distribution is alpha-ox=2.5 which classifies WPVS 007 as an X-ray weak AGN. After correcting for reddening and X-ray absorption, alpha-ox becomes 1.9 and the luminosity in the Big-Blue-Bump is log LBBB=37.7 [W], which translates into an Eddington ratio L/LEdd ~ 1.0.