381 research outputs found
ROSAT all-sky survey on the Einstein EMSS sample
The cosmological evolution and the luminosity function (XLF) of X ray selected Active Galactic Nuclei (AGN's) are discussed. The sample used is extracted from the Einstein Observatory Extended Medium Sensitivity Surveys (EMSS) and consists of more than 420 objects. Preliminary results from the ROSAT All-Sky Survey data confirm the correctness of the optical identification of the EMSS sources, thus giving confidence to the results obtained from the analysis of the AGN's sample. The XLF observed at different redshifts (up to z approx. 2) gives direct evidence of cosmological evolution. Data have been analyzed within the framework of luminosity evolution models and the two most common evolutionary forms, L sub x(Z) = L sub x(0) x e(sup Cr) and L sub x(Z) = L sub x(0) x (1 + z)(exp C), have been considered. Luminosity dependent evolution is required if the evolution function has the exponential form, whereas the simpler pure luminosity evolution model is still acceptable if the evolution function has the power law form. Using the whole sample of objects the number-counts and the de-evolved (z = 0) XLF have been derived. A comparison of the EMSS data with preliminary ROSAT results presented at this meeting indicates an overall agreement
Optical Spectroscopy of the unusual galaxy J2310-43
We present and discuss new spectroscopic observations of the unusual galaxy
J2310-43. The observations cover a wide wavelength range, from 3700 A to 9800 A
allowing the study of both the regions where H alpha and the Ca II ``contrast''
are expected. No evidence for H alpha in emission is found and we thus confirm
the absence of emission lines in the spectrum of J2310-43, ruling out the
possibility that it may host a Seyfert nucleus. The CaII break is clearly
detected and the value of the contrast (38 +/-4 %) is intermediate between that
of a typical elliptical galaxy (about 50 %) and that of a BL Lac object (<25
%). This result imposes limits on the intensity of a possible non-stellar
continuum and, in the light of the radio and X-ray loudness of the source,
draws further attention to the problem of the recognition of a BL Lac object.
Objects like J2310-43 may be more common than previously recognized, and begin
to emerge in surveys of radio-emitting X-ray sources.Comment: 7 pages, 1 figure; to be published in The Astronomical Journa
The X-ray spectral properties of X-ray selected AGN : ROSAT spectra of EMSS AGN
Using a sample of 63 AGNs extracted from the Extended Medium
Sensitivity Survey (EMSS), we study the X-ray spectral properties of X-ray
selected AGN in the 0.12.4 keV ROSAT band. These objects are all the EMSS
AGN detected with more than 300 net counts in ROSAT PSPC images available from
the public archive (as of May 31, 1995). A Maximum-Likelihood analysis is used
to find the mean power-law spectral index and the intrinsic
dispersion $\sigma_p$. We find =1.42 with =0.44. This
value is significantly steeper (0.4) than the mean
/IPC spectral index obtained applying the ML analysis on the whole
sample of EMSS AGN. This result shows that the soft excess already noted in
optically selected AGN is present also in X-ray selected AGN. The relatively
high value obtained for the intrinsic dispersion confirms that in the soft band
AGN are characterized by a variety of spectral indices and the increase with
respect to results obtained from the analysis of Einstein data (0.16) suggests a further broadening of the spectral index
distribution as one moves to softer energies. A comparison between the mean
spectral index of Radio-quiet and Radio-loud subsamples shows that the mean
index of the RL sample is flatter than that of RQ, both in the IPC (0.3) and in the PSPC (0.4) data. This suggests
that the additional X-ray component in RL AGN dominates the X-ray emission of
RL AGN over almost two decades of energy (0.110 keV).Comment: 8 pages LaTex file; mn.sty macro (enclosed), 5 LaTex Tables, 12
Postscript figures; accepted for publication in MNRA
The REX survey: a search for Radio Emitting X-ray sources
We present the scientific goals, the strategy and the first results of the
REX project, an effort aimed at creating a sizable and statistically complete
sample of Radio Emitting X-ray sources (REX) using the available data from a
VLA survey and the ROSAT PSPC archive. Through a positional cross-correlation
of the two data sets we have derived a sample of about 1600 REX. Among the 393
REX identified so far a high fraction is represented by AGNs, typically radio
loud QSOs and BL Lacs. The remaining sources are galaxies, typically radio
galaxies isolated or in cluster. Thanks to the low flux limits in the radio and
in the X-ray band and the large area of sky covered by the survey, we intend to
derive a new complete and unbiased sample of BL Lacs which will contain both
``RBL'' and ``XBL'' type objects. In this way, the apparent dichotomy resulting
from the current samples of BL Lacs will be directly analyzed in a unique
sample. Moreover, the high number of BL Lacs expected in the REX sample (about
200) will allow an accurate estimate of their statistical properties. To date,
we have discovered 15 new BL Lacs and 11 BL Lac candidates with optical
properties intermediate between those of a typical elliptical galaxy and those
of a typical BL Lac object. These objects could harbour weak sources of
non-thermal continuum in their nuclei and, if confirmed, they could represent
the faint tail of the BL Lac population. The existence of such ``weak'' BL Lacs
is matter of discussion in recent literature and could lead to a re-assessment
of the defining criteria of a BL Lac and, consequently, to a revision of their
cosmological and statistical properties.Comment: 30 pages, 13 figures, accepted for publication to Ap
Emission Line AGNs from the REX survey: Results from optical spectroscopy
We present 71 Emission Line objects selected from the REX survey. Except for
3 of them, for which the presence of an active nucleus is dubious, all these
sources are Active Galactic Nuclei (QSOs, Seyfert galaxies, emission line
radiogalaxies). In addition, we present the spectra of other 19 AGNs included
in a preliminary version of the REX catalog but not in the final one. The
majority (80) of the 90 sources presented in this paper is newly discovered.
Finally, we present the general properties in the radio and in the X-ray band
of all the AGNs discovered so far in the REX survey.Comment: 27 pages. To be published in Astronomy and Astrophysics, Supplement
Series. Better quality figures can be asked to the autho
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