Open AccessX‐ray and soft γ‐ray spectra of broad‐line radio galaxies
Author(s) -
Woźniak Przemysław R.,
Zdziarski Andrzej A.,
Smith David,
Madejski Greg M.,
Johnson W. Neil
Publication year - 1998
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1998.01831.x
Subject(s) - physics , astrophysics , spectral line , galaxy , equivalent width , line (geometry) , astronomy , emission spectrum , active galactic nucleus , reflection (computer programming) , k line , x ray , optics , geometry , mathematics , computer science , programming language
We study X‐ray and soft γ‐ray spectral properties of nearby broad‐line radio galaxies (BLRGs) using data from Ginga ASCA , OSSE and EXOSAT . The X‐ray spectra are well fitted by an intrinsic power‐law continuum with an energy index of α ∼ 0.7, moderately absorbed by a cold medium. In addition, the Ginga spectra show fluorescent Fe Kα lines with an average equivalent width of ∼ 100 eV, and, in some cases, Compton reflection humps. However, the latter are significantly weaker than both those seen in radio‐quiet Seyfert 1s and those expected if the Fe Kα lines were due to reflection. We find that this weakness of reflection cannot be explained by dilution by another continuum component, e.g. from a jet. Some ASCA and EXOSAT spectra show soft X‐ray excesses below ∼ 3 keV. When that component is taken into account, the Fe Kα lines in the ASCA data are found to be unresolved in most cases, and to have equivalent widths ≲ 200 eV, consistent with the Ginga data. Multiple observations of 3C 382 and 390.3 show the Fe Kα line approximately constant in flux but accompanied by strong continuum variations. This indicates the bulk of the line is formed by matter at a distance much larger than an accretion‐disc scale, consistent with the ASCA line width measurements. The column density of the matter required to account for the observed line fluxes is N H ≳ 10 23 cm −2 . Such a medium is in the line of sight of 3C 445, but it has to be out of it in other objects in which the observed N H are substantially lower. Thus a cold medium with that N H and covering a large solid angle is common in BLRGs, but in most objects it is out of the line of sight, consistent with the unified AGN model. The spectra of BLRGs break and become softer above ∼ 100 keV, as shown by a simultaneous ASCA /OSSE observation of 3C 120 and by the OSSE spectra being on average much softer than the X‐ray spectra. Finally, we find the X‐ray and γ‐ray spectral properties of Cen A, a bright narrow‐line radio galaxy —α ≃ 0.8, no or weak Compton reflection, N H ≳ 10 23 cm −2 (which is consistent with its Fe Kα line flux), and a high‐energy break at ∼ 100 keV — consistent with Cen A being intrinsically very similar to BLRGs studied here, again in agreement with the unified model.