X-Ray Observations of the Seyfert Galaxy LB 1727 (1H 0419-577)

Abstract

We discuss the properties of the Seyfert 1.5 galaxy LB 1727, also known as 1H 0419-577, from X-ray observations obtained by ASCA and ROSAT, along with optical observations from earlier epochs. The source flux was F_2-10~=10^-11 ergs cm^-2 s^-1 during the ASCA observations that were carried out during 1996 July-August, and we find only modest (<~20%) variations in the flux in this band within or between these observations. In contrast, a daily monitoring campaign over 1996 June-September by the ROSAT HRI instrument reveals the soft X-ray (0.1-2 keV) flux to have increased by a factor of ~=3. Significant variations were also observed down to timescales of ~40 ks. We find that the 2-10 keV continuum can be parameterized as a power law with a photon index Γ~1.45-1.68 across ~0.7-11 keV in the rest frame. We also report the first detection of iron Kα line emission in this source. Simultaneous ASCA and ROSAT data show the X-ray spectrum to steepen sharply at a rest energy ~0.75 keV, so the spectrum below this energy can be parameterized as a power law of slope Γ~3.6. The X-ray emission appears to be unattenuated, and we find that ionized gas alone cannot produce such a sharp spectral break. Even allowing the presence of such gas, the simultaneous ASCA and HRI data demonstrate that the underlying continuum is required to steepen below ~0.75 keV. Thus LB 1727 is one of the few Seyferts for which we can rule out the possibility that the presence of a warm absorber is solely responsible for the spectral steepening in the soft X-ray regime. Consideration of the overall spectral energy distribution for this source indicates the presence of a pronounced XUV bump visible in optical, ultraviolet, and soft X-ray data. The source appears relatively weak in infrared emission, and so if dust exists in the source, it is not excited by the nuclear radiation.