What's wrong with AGN models for the X-ray background ?

The origin of the hard X-ray background (XRB) as a superposition of unabsorbed and absorbed Active Galactic Nuclei is now widely accepted as the standard model. The identification of faint X-ray sources in ROSAT, ASCA, and BeppoSAX medium-deep surveys and their average spectral properties are in broad agreement with the model predictions. However, AGN models, at least in their simplified version, seem to be at odds with some of the most recent findings calling for substantial revisions. I will review the recent XRB "best fit" models and discuss how the foreseen XMM and Chandra surveys will be able to constrain the allowed parameter space.Comment: 10 pages, 5 figures. Invited talk to appear in the Proceedings of the Conference X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray Background. (September 6-10 - 1999

On the low detection efficiency of disk water megamasers in Seyfert 2 AGN

Disk megamasers are a unique tool to study active galactic nuclei (AGN) sub-pc environment, and precisely measure some of their fundamental parameters. While the majority of disk megamasers are hosted in heavily obscured (i.e., Seyfert 2, Sy2) AGN, the converse is not true, and disk megamasers are very rarely found even in obscured AGN. The very low detection rate of such systems in Sy2 AGN could be due to the geometry of the maser beaming, which requires a strict edge-on condition. We explore some other fundamental factors which could play a role in a volume-limited survey of disk megamasers in Sy2 galaxies, most importantly the radio luminosity.Comment: 2 pages, 2 figures. To appear in the Proceedings IAU Symposium No. 336, 2017 "Astrophysical Masers: Unlocking the Mysteries of the Universe

The Iron Line Background

We investigate the presence of iron line emission among faint X-ray sources identified in the 1Ms Chandra Deep Field South and in the 2Ms Chandra Deep Field North. Individual source spectra are stacked in seven redshift bins over the range z=0.5-4. We find that iron line emission is an ubiquitous property of X-ray sources up to z~3. The measured line strengths are in good agreement with those expected by simple pre-Chandra estimates based on X-ray background synthesis models. The average rest frame equivalent width of the iron line does not show significant changes with redshift.Comment: 5 pages, 2 figures, ApJ Letters in press (include emulateapj.sty

ROSAT observations of radio-selected BL Lac objects

ROSAT observations of a homogeneous sample of radio--selected BL Lac objects are presented. Results of a detailed spectral analysis in the soft 0.1-2.0 keV energy range are discussed and compared with similar previously published results. The X--ray spectral shape is discussed in relation to the overall energy distribution with particular emphasys on the high energy gamma--ray emission recently detected by CGRO--EGRET for about half of the objects in the sample. Dividing the objects in our sample on the basis of the radio to X--ray flux ratio (alpha_rx) we show that objects with alpha_rx > 0.75 have flatter X--ray spectra and are likely to be stronger gamma--ray emitters than objects with alpha_rx < 0.75. Moreover we note that the value of the peak energy of the synchrotron component, in a E * F(E) plot, correlates with alpha_rx and alpha_x.Comment: to appear on MNRAS, 18 pages (latex file plus 5 tables and 3 figures) uuencoded compressed tar postscript file

The Steep Spectrum Quasar PG1404+226 with ASCA, HST and ROSAT

(abridged) We present and discuss our observations of the NL quasar PG1404+226 with ASCA and HST, and a re-analysis of our earlier observations with ROSAT. The soft X-ray spectrum is very steep and displays an absorption feature (edge or line at ~1.1 keV). We have applied a variety of models to the ASCA and ROSAT spectra without finding a completely satisfactory fit, and the identification of the edge remains uncertain. A satisfactory fit of the ASCA spectrum assuming that the edge is produced by highly ionized iron (using the code absori in XSPEC) is obtained with an overabundance of iron by a factor > 25 compared to solar, a suggestion supported by the extremely high equivalent width of the Fe K_alpha line at 6.4 keV. A warm absorber model fitting the absorption feature with NeVII-NeX edges and assuming a peculiar oxygen/neon abundance ratio is consistent with the ROSAT data but not the ASCA data. Finally, it is also possible that the observed edge is caused by a OVIII or OVII edge or line, blueshifted by z_abs=0.2 to 0.5 depending on the specific identification, as has been suggested previously for 2 other NL quasars, but there are no other features in the UV and X-ray spectra in support of this suggestion. Two systems of UV absorption lines, one nearly at rest in the source frame, the other blueshifted by ~1900 km/s are identified in the HST/FOS spectra. Photoionization models indicate that the UV absorption and the ~1 keV absorption are probably caused by absorbers with different physical conditions. PG1404+226 is one more case of AGN where both UV and X-ray absorption features are detected, thereby increasing further the significance of the previously noted statistical association of the two types of absorbers.Comment: 11 pages, A&A in pres