103 research outputs found
The Reddest DR3 SDSS/XMM Quasars
We have cross-correlated the SDSS DR3 Schneider et al. (2005) quasar catalog
with the XMM-Newton archive. Color and redshift selections (g - r > 0.5 and 0.9
z < 2.1) result in a sample of 17 red, moderate redshift quasars. The redshift
selection minimizes possible contamination due to host galaxy emission and
Lyalpha forest absorption. Both optical and X-ray information are required to
distinguish between the two likely remaining causes of the red colors: 1)
dust-reddening and 2) an intrinsically red continuum. We find that 7 of 17
quasars can be classified as probable `intrinsically red' objects. These 7
quasars have unusually broad MgII emission lines (=10,500 km s^{-1}),
moderately flat, but unabsorbed X-ray spectra =1.66+/-0.08), and low
accretion rates (mdot/mdot_{Edd}} ~ 0.01). We suggest low accretion rates as a
possible physical explanation for quasars with intrinsically red optical
continua. We find that 8 of 17 quasars can be classified as dust-reddened.
Three of these have upper-limits on the absorption column from X-ray spectral
fits of N_H = 3-13 x 10^{22} cm^2, while the other five quasars must be
absorbed by at least N_H = 10^{23} cm^2 in order to be consistent with a
comparably selected alpha_{ox}-l_{uv} distribution. Two objects in the sample
are unclassified.Comment: 20 pages, 12 figures, Accepted for publication in Ap
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
X-ray selected active galactic nuclei with red optical continua
We discuss the properties of X-ray selected `red' AGN from the RIXOS sample.
These are Seyfert 1 galaxies and quasars whose optical continua are relatively
soft, ie. with an energy index, alpha_opt>2. There are 14 objects in the RIXOS
sample which satisfy this criterion and they cover a range in redshift from
z=0.08 to 1.27. Of these, two have characteristics which suggest that the
continuum is intrinsically red, ie. an optical continuum which does not appear
to have been significantly reddened by dust or to have contaminating light from
the host galaxy. A further three objects show evidence of being absorbed by
cold gas and dust with columns of up to 1e22 cm-2. The data are inconclusive on
the remaining AGN.Comment: 9 pages, 3 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Society. Also available from
http://www.mssl.ucl.ac.uk/www_astro/preprints/preprints.htm
Constraining the black hole mass and accretion rate in the narrow-line Seyfert 1 RE J1034+396
We present a comprehensive study of the spectrum of the narrow-line Seyfert 1
galaxy RE J1034+396, summarizing the information obtained from the optical to
X-rays with observations from the William Herschel 4.2m Telescope (WHT), the
Hubble Space Telescope, the Extreme UltraViolet Explorer, ROSAT, ASCA and
BeppoSAX. The BeppoSAX spectra reveal a soft component which is
well-represented by two blackbodies with kT of about 60 eV and 160 eV,
mimicking that expected from a hot, optically-thick accretion disc around a
low-mass black hole. This is borne out by our modeling of the optical to X-ray
nuclear continuum, which constrains the physical parameters of a NLS1 for the
first time. The models demonstrate that RE J1034+396 is likely to be a system
with a nearly edge-on accretion disk (60 to 75 degrees from the disk axis),
accreting at nearly Eddington rates (0.3 to 0.7 L_edd) onto a low mass (about 2
million solar masses) black hole (abridged).Comment: ApJ accepte
RE J2248-511 - Not all variable, ultrasoft, X-ray AGN have narrow Balmer lines
We present ASCA data on RE J2248-511, extending existing optical and soft
X-ray coverage to 10 keV, and monitoring the soft component. These data show
that, despite a very strong ultrasoft X-ray excess below 0.3 keV and a soft
0.3--2 keV spectral index in earlier ROSAT data, the hard X-ray spectrum
(alpha~ -0.8; 0.6-10 keV) is typical of type 1 AGN, and the soft component has
since disappeared. Optical data taken at two different epochs show that the big
blue bump is also highly variable. The strength of the ultrasoft X-ray
component and the extreme variability in RE J2248-511 are reminiscent of the
behaviour observed in many narrow line Seyfert 1s (NLS1s). However, the high
energy end of the ROSAT spectrum, the ASCA spectrum and the Balmer line full
widths at half maximum of ~3000 km/s in RE J2248-511, are typical of normal
Seyfert 1 AGN.
The change in the soft X-ray spectrum as observed in the ROSAT and ASCA data
is consistent with the behaviour of Galactic Black Hole Candidates (GBHCs) as
they move from a high to a low state, ie. a fall in the ultrasoft component and
a hardening of the X-ray continuum. This GBHC analogy has also been proposed
for NLS1s. Alternatively, the variability may be caused by opacity changes in a
hot, optically-thin corona which surrounds a cold, dense accretion disc; this
was first suggested by Guainazzi et al. for 1H0419-577, an object which shows
remarkably similar properties to RE J2248-511.Comment: 10 pages, 4 figures. Accepted for Monthly Notices of RA
Double-peaked Balmer line emission in the radio-quiet AGN RX J1042+1212
We present optical and X-ray spectra of a radio-quiet X-ray selected AGN, RX
J1042+1212 (=0.271). The Halpha and Hbeta emission lines are very broad
(with full widths at half maximum of ~10000 km s-1) and have double-peaked
profiles. Such features are rarely observed in AGN in general but are even more
unusual in radio-quiet objects. The analysis of the ROSAT PSPC data reveals a
non-varying, unabsorbed spectrum with an energy spectral index, alphax=1.2 and
little or no emission from a soft X-ray excess. The slope of the optical
spectrum is similar, alphaopt=1.0, and is consistent with an extrapolation of
the X-ray spectrum, suggesting that the same power-law continuum may dominate
throughout and that the big blue bump component is relatively weak. We look for
a link between these various properties and investigate models of double-peaked
Balmer line emission in AGN. An accretion disc origin is unlikely in RX
J1042+1212 as this model predicts that lines emitted by a disc should have a
net gravitational redshift (both Halpha and Hbeta have a net blueshift).
Emission from two broad line regions, each gravitationally bound to one
component of a supermassive black hole binary, is a possibility if the two
components are similar in size and nature. Alternatively, the lines (or at
least the narrow peaks of the lines) may be produced by a double-sided jet or
bipolar flow.Comment: TeX file and eps figures, .sty files included. Accepted in MNRA
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