121 research outputs found
The soft X-ray excess AGN RE J2248-511
We model the spectral energy distribution of the ultrasoft broad-line AGN RE
J2248-511 with Comptonised accretion disc models. These are able to reproduce
the steep optical and ultrasoft X-ray slopes, and the derived black hole mass
is consistent with independent mass estimates. This AGN displays properties of
both broad and narrow line Seyfert 1 galaxies, but we conclude that it is
intrinsically a `normal' Seyfert 1 viewed at high inclination angle.Comment: 4 pages, 1 figure. MG10 Proceeding
Supersoft X-ray sources identified with Be binaries in the Magellanic Clouds
We investigated four luminous supersoft X-ray sources (SSS) in the Magellanic
Clouds suspected to have optical counterparts of Be spectral type. If the
origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in
accreted envelopes of white dwarfs (WDs), like in the majority of SSS, these
objects are close binaries, with very massive WD primaries. Using the South
African Large Telescope (SALT), we obtained the first optical spectra of the
proposed optical counterparts of two candidate Be stars associated with SUZAKU
J0105-72 and XMMU J010147.5-715550, respectively a transient and a recurrent
SSS, and confirmed the proposed Be classification and Small Magellanic Clouds
membership. We also obtained new optical spectra of two other Be stars proposed
as optical counterparts of the transient SSS XMMU J052016.0-692505 and
MAXI-J0158-744. The optical spectra with double peaked emission line profiles,
are typical of Be stars and present characteristics similar to many high mass
X-ray binaries with excretion disks, truncated by the tidal interaction with a
compact object. The presence of a massive WD that sporadically ignites nuclear
burning, accreting only at certain orbital or evolutionary phases, explains the
supersoft X-ray flares. We measured equivalent widths and distances between
lines' peaks, and investigated the variability of the prominent emission lines'
profiles. The excretion disks seem to be small in size, and are likely to be
differentially rotating. We discuss possible future observations and the
relevance of these objects as a new class of type Ia supernovae progenitors.Comment: Accepted for publication in teh Astrophysical Journa
X-ray Spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectral properties of
sources detected in the RIXOS survey, that is nearly complete down to a flux
limit of 3e-14 cgs (0.5-2 keV). We show that for X-ray surveys containing
sources with low count rate spectral slopes estimated using simple hardness
ratios in the ROSAT band can be biased. Instead we analyse three-colour X-ray
data using statistical techniques appropriate to the Poisson regime which
removes the effects of this bias. We have then applied this technique to the
RIXOS survey to study the spectral properties of the sample. For the AGN we
find an average energy index of 1.05+-0.05 with no evidence for spectral
evolution with redshift. Individual AGN are shown to have a range of properties
including soft X-ray excesses and intrinsic absorption. Narrow Emission Line
Galaxies also seem to fit to a power-law spectrum, which may indicate a
non-thermal origin for their X-ray emission. We infer that most of the clusters
in the sample have a bremsstrahlung temperature >3 keV, although some show
evidence for a cooling flow. The stars deviate strongly from a power-law model
but fit to a thermal model. Finally, we have analysed the whole RIXOS sample
containing 1762 sources. We find that the mean spectral slope of the sources
hardens at lower fluxes in agreement with results from other samples. However,
a study of the individual sources demonstrates that the hardening of the mean
is caused by the appearance of a population of very hard sources at the lowest
fluxes. This has implications for the nature of the soft X-ray background.Comment: 31,LaTeX file, 2 PS files with Table 2 and 22 PS figures. MNRAS in
pres
Chandra Multiwavelength Project: Normal Galaxies at Intermediate Redshift
(abridged) We have investigated 136 Chandra extragalactic sources without
broad optical emission lines, including 93 galaxies with narrow emission lines
(NELG) and 43 with only absorption lines (ALG). Based on fx/fo, Lx, X-ray
spectral hardness and optical emission line diagnostics, we have conservatively
classified 36 normal galaxies (20 spirals and 16 ellipticals) and 71 AGNs. We
found no statistically significant evolution in Lx/LB, within the limited z
range. We have built log(N)-log(S), after correcting for completeness based on
a series of simulations. The best-fit slope is -1.5 for both S and B energy
bands, which is considerably steeper than that of the AGN-dominated cosmic
background sources, but slightly flatter than the previous estimate, indicating
normal galaxies will not exceed the AGN population until fx ~ 2 x 10-18 erg s-1
cm-2 (a factor of ~5 lower than the previous estimate). A group of NELGs appear
to be heavily obscured in X-rays, i.e., a typical type 2 AGN. After correcting
for intrinsic absorption, their X-ray luminosities could be Lx > 10^44 erg s-1,
making them type 2 quasar candidates. While most X-ray luminous ALGs (XBONG -
X-ray bright, optically normal galaxy candidates) do not appear to be
significantly absorbed, we found two heavily obscured objects, which could be
as luminous as an unobscured broad-line quasar. Among 43 ALGs, we found two E+A
galaxy candidates with strong Balmer absorption lines, but no [OII] line. The
X-ray spectra of both galaxies are soft and one of them has a nearby close
companion galaxy, supporting the merger/interaction scenario rather than the
dusty starburst hypothesis.Comment: 31 pages, 9 figures, accepted for publication in ApJ (20 June 2006,
v644), replaced with minor correction
ROSAT PSPC spectra of X-ray selected Narrow Emission Line Galaxies
We analyse the ROSAT PSPC spectrum of 19 X-ray selected Narrow Emission Line
Galaxies (NELGs) discovered during the optical identification of sources in the
ROSAT UK Deep Survey. Their properties are compared to those of broad line
Active Galactic Nuclei (AGN) in the same sample. Counts in three spectral bands
have been extracted for all the sources, and have been fitted with a power-law
model assuming the Galactic value for NH. The average slope of NELGs is
\alpha=0.45+-0.09, whilst for the AGN it is \alpha=0.96+-0.03. The power-law
model is a good fit for ~90% of NELGs and ~75% of AGN. Recent work shows that
the fractional surface density of NELGs increases with respect to AGN at faint
fluxes. Thus they are expected to be an important component of the residual
soft (<2 keV) X--ray background. The slope of the X--ray background
(\alpha~0.4, 1-10 keV) is harder than that of AGN (\alpha~1) but our results
show that it is consistent with the summed spectrum of the NELGs in the deep
survey (\alpha~0.4). This may finally reconcile the spectrum of the background
with the properties of the sources that constitute it.Comment: Uuencoded compressed LaTex file and three PS figures plus mn.sty.
Accepted in MNRA
A Medium Survey of the Hard X-Ray Sky with ASCA. II.: The Source's Broad Band X-Ray Spectral Properties
A complete sample of 60 serendipitous hard X-ray sources with flux in the
range \ecs to \ecs (2 - 10
keV), detected in 87 ASCA GIS2 images, was recently presented in literature.
Using this sample it was possible to extend the description of the 2-10 keV
LogN(>S)-LogS down to a flux limit of \ecs (the
faintest detectable flux), resolving about a quarter of the Cosmic X-ray
Background. In this paper we have combined the ASCA GIS2 and GIS3 data of these
sources to investigate their X-ray spectral properties using the "hardness"
ratios and the "stacked" spectra method. Because of the sample statistical
representativeness, the results presented here, that refer to the faintest hard
X-ray sources that can be studied with the current instrumentation, are
relevant to the understanding of the CXB and of the AGN unification scheme.Comment: 28 pages plus 6 figures, LaTex manuscript, Accepted for publication
in the Astrophysical Journal, Figure 5 can retrieved via anonymous ftp at
ftp://ftp.brera.mi.astro.it/pub/ASCA/paper2/fig5.ps.g
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