High-resolution X-ray imaging observations of two low-luminosity Seyfert galaxies

Abstract

Results from observations of two nearby, low X-ray luminosity Seyfert-type galaxies, namely NGC 1365 and NGC 4051, using the ROSAT High Resolution Imager (HRI), are presented. The observations, carried out with the aim of detecting and resolving the extranuclear (beyond the central 5–10 arcsec) X-ray emission, show evidence for an extended component of ≃2 kpc size in NGC 1365 and NGC 4051. The extended component contains 56±8 per cent of the total observed flux in the case of NGC 1365, and 21±6 per cent in the case of NGC 4051. In NGC 1365, the extended X-ray emission shows a component aligned with the inner disc structure, another as wings or ears along the east and west direction aligned with the inner spiral arms, and an elongated edge-brightened structure (‘chimney’) breaking out of the disc in the north-west direction. The extended soft X-ray component around the nucleus of NGC 4051 is cospatial with the disc of the galaxy. It also shows an elongation coincident with a ‘banana’-like feature in the north-east seen in the 6-cm radio band. Extensions are also seen towards the south and south-east of the nucleus. Starburst activity driving strong winds through the disc of NGC 1365 can account for most of the extended soft X-ray emission in it. In the case of NGC 4051, extended X-ray emission probably owes its origin to both nuclear activity and starburst-induced activity. The nuclear component of NGC 4051 shows strong soft X-ray variability, with X-ray intensity changing by a factor of 2–3 on a time-scale of a few 100 s. The power spectrum of the variability has been extended to higher frequencies compared with previous observations, and has now reached the true Poissonian noise level of the source. At lower frequencies the power spectrum is best characterized by a power law and a Gaussian. The power-law slope of ≃−1.8 is consistent with the previous low-energy observations with EXOSAT, and the presence of a Gaussian feature signifies the persistence of a quasi-periodic oscillation that was also seen earlier with EXOSAT. During its low intensity and low variability state, however, the extended component of soft X-ray emission dominates the flux in NGC 4051. A list of X-ray sources, some of which are new, detected in the field of view of the HRI during the observations is also presented.