511 research outputs found
A Complete Sample of Soft X-ray Selected AGN: II. Statistical Analysis
Direct correlations and a Principal Component Analysis (PCA) are presented
for a complete sample of 110 soft X-ray selected AGN of which about half are
Narrow-Line Seyfert 1 galaxies (NLS1s). The direct correlation analyses show
that narrower FWHM(H-beta) correlates with steeper X-ray spectrum, stronger
optical FeII emission, weaker [OIII] emission and stronger short-term X-ray
variability. This direct correlation analysis and the PCA confirm the Boroson &
Green (1992) Eigenvector 1 relationship for AGN: FeII strength anti-correlates
with [OIII] line strength. Eigenvector 1 is well-correlated with the Eddington
luminosity ratio L/L_Edd while Eigenvector 2 shows a very good correlation with
the mass of the central black hole and the mass accretion rate. The Eddington
ratio L/L_Edd correlates with the X-ray spectral index alpha-X and the black
home mass anti-correlates with the X-ray variability chi^2/nu. The Eddington
ration L/L_Edd may be interpreted as the age of an AGN: AGN with steep X-ray
spectra, strong FeII, and weak [OIII] are AGN in an early phase of their
evolution. In this hypothesis NLS1s are young AGN.Comment: Accepted for publication in AJ (April 2004), 21 pages, 13 figure
Radio Loud AGN in the Context of the Eigenvector 1 Parameter Space
We consider the properties of radio-loud (RL) AGN in the context of the
Eigenvector 1 (E1) parameter space. RL sources show a restricted E1 parameter
space occupation relative to the radio-quiet (RQ) majority. The Fanaroff-Riley
II ``parent population'' of relatively un-boosted RL sources (median
radio/optical flux ratio ~490) shows the most restricted occupation. RL sources
have different broad line properties (and inferred black hole masses and
Eddington ratios). FWHM H_beta for the broad line component in RL sources are
at least twice as large as the RQ majority. The average broad FeII emission
line strength is also about half that for RQ sources. Our sample suggests that
the RL cutoff occurs near R_k=70 or logP(6cm)=32.0 ergs/s/Hz. Sources below
this cutoff are RQ although we cannot rule out the existence of a distinct
intermediate population. We show that the Doppler boosted core-dominated RL
sources (median flux ratio ~1000) lie towards smaller FWHM(H_beta_bc) and
stronger FeII in E1 as expected if the lines arise in an accretion disk. Our
subsample of superluminal sources, with orientation inferred from the
synchrotron self Compton model, reinforce this general E1 trend and allow us to
estimate the role of source orientation in driving E1 domain occupation.Comment: 9 pages, 3 figures, accepted for publication in ApJ
An intense soft-excess and evidence for light bending in the luminous narrow-line quasar PHL 1092
The narrow-line quasar PHL 1092 was observed by XMM-Newton at two epochs
separated by nearly thirty months. Timing analyses confirm the extreme
variability observed during previous X-ray missions. A measurement of the
radiative efficiency is in excess of what is expected from a Schwarzschild
black hole. In addition to the rapid X-ray variability, the short UV light
curves (< 4 hours) obtained with the Optical Monitor may also show
fluctuations, albeit at much lower amplitude than the X-rays. In general, the
extreme variability is impressive considering that the broad-band (0.4-10 keV
rest-frame) luminosity of the source is ~10^45 erg/s. During at least one of
the observations, the X-ray and UV light curves show common trends, although
given the short duration of the OM observations, and low significance of the UV
light curves it is difficult to comment on the importance of this possible
correlation. Interestingly, the high-energy photons (> 2 keV) do not appear
highly variable. The X-ray spectrum resembles that of many narrow-line Seyfert
1 type galaxies: an intense soft-excess modelled with a multi-colour disc
blackbody, a power-law component, and an absorption line at ~1.4 keV. The ~1.4
keV feature is curious given that it was not detected in previous observations,
and its presence could be related to the strength of the soft-excess. Of
further interest is curvature in the spectrum above ~2 keV which can be
described by a strong reflection component. The strong reflection component,
lack of high-energy temporal variability, and extreme radiative efficiency
measurements can be understood if we consider gravitational light bending
effects close to a maximally rotating black hole.Comment: To appear in MNRAS. Submitted March 01, 2004. 9 page
A Complete Sample of Soft X-ray Selected AGN: I. The Data
We present the optical spectra and simple statistical analysis for a complete
sample of 110 soft X-ray selected AGN. About half of the sources are
Narrow-Line Seyfert 1 galaxies (NLS1s), which have the steepest X-ray spectra,
strongest FeII emission and slightly weaker [OIII]5007 emission than broad line
Seyfert 1s (BLS1s).
Kolmogorov Smirnov tests show that NLS1s and BLS1s have clearly different
distributions of the X-ray spectral slope, X-ray short-term variability, and
FeII equivalent widths and luminosity and FeII/H-beta ratios. The differences
in the [OIII]/H-beta and [OIII] equivalent widths are only marginal. We found
no significant differences between NLS1s and BLS1s in their rest frame 0.2-2.0
X-ray luminosities, rest frame 5100A monochromatic luminosities, bolometric
luminosities, redshifts, and their H equivalent widths.Comment: Accepted for publication in AJ (scheduled for January 2004), 21
pages, 10 figures, The paper including the optical spectra and FeII
subtracted spectra can be retrieved from
http://www.astronomy.ohio-state.edu/~dgrupe/research/sample_paper1.htm
Optical Properties of Radio-selected Narrow Line Seyfert 1 Galaxies
We present results from the analysis of the optical spectra of 47
radio-selected narrow-line Seyfert 1 galaxies (NLS1s). These objects are a
subset of the First Bright Quasar Survey (FBQS) and were initially detected at
20 cm (flux density limit ~1 mJy) in the VLA FIRST Survey. We run Spearman rank
correlation tests on several sets of parameters and conclude that, except for
their radio properties, radio-selected NLS1 galaxies do not exhibit significant
differences from traditional NLS1 galaxies. Our results are also in agreement
with previous studies suggesting that NLS1 galaxies have small black hole
masses that are accreting very close to the Eddington rate. We have found 16
new radio-loud NLS1 galaxies, which increases the number of known radio-loud
NLS1 galaxies by a factor of ~5.Comment: 18 pages, 20 figures, Accepted for publication in Ap
Probing the Ionizing Continuum of Narrow-Line Seyfert 1 Galaxies. I.Observational Results
We present optical spectra and emission-line ratios of 12 Narrow-Line Seyfert
1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A
common feature in the EUV continuum of active galactic nuclei is the big blue
bump (BBB), generally associated with thermal accretion disk emission. While
Galactic absorption prevents direct access to the EUV range, it can be mapped
by measuring the strength of a variety of forbidden optical emission lines that
respond to different EUV continuum regions. We find that narrow emission-line
ratios involving [OII]3727, Hbeta, [OIII]5007, [OI]6300, Halpha,[NII]6583, and
[SII]6716,6731 indicate no significant difference between NLS1s and Broad-Line
Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy
distributions of their ionizing EUV - soft X-ray continua are similar. The
relative strength of important forbidden high ionization lines like [NeV]3426
compared to HeII4686 and the relative strength of [FeX]6374 appear to show the
same range as in BLS1 galaxies. However, a trend of weaker
F([OI]6300)/F(Halpha) emission-line ratios is indicated for NLS1s compared to
BLS1s. To recover the broad emission-line profiles we used Gaussian components.
This approach indicates that the broad Hbeta profile can be well described with
a broad component (FWHM = 3275 +- 800 km/s) and an intermediate broad component
(FWHM = 1200 +- 300 km/s). The width of the broad component is in the typical
range of normal BLS1s. The emission-line flux that is associated with the broad
component in these NLS1s amounts to at least 60% of the total flux. Thus it
dominates the total line flux, similar to BLS1 galaxies.Comment: 34 pages, 9 figures. accepted for publication in the
Astrophys.Journa
Tidal Disruption of a Star By a Black Hole : Observational Signature
We have modeled the time-variable profiles of the Halpha emission line from
the non-axisymmetric disk and debris tail created in the tidal disruption of a
solar-type star by a million solar mass black hole. Two tidal disruption event
simulations were carried out using a three dimensional relativistic
smooth-particle hydrodynamic code, to describe the early evolution of the
debris during the first fifty to ninety days. We have calculated the physical
conditions and radiative processes in the debris using the photoionization code
CLOUDY. We model the emission line profiles in the period immediately after the
accretion rate onto the black hole became significant. We find that the line
profiles at these very early stages of the evolution of the post-disruption
debris do not resemble the double peaked profiles expected from a rotating disk
since the debris has not yet settled into such a stable structure. As a result
of the uneven distribution of the debris and the existence of a ``tidal tail''
(the stream of returning debris), the line profiles depend sensitively on the
orientation of the tail relative to the line of sight. Moreover, the predicted
line profiles vary on fairly short time scales (of order hours to days). Given
the accretion rate onto the black hole we also model the Halpha light curve
from the debris and the evolution of the Halpha line profiles in time.Comment: 20 pages, 9 figures, to appear in ApJ, 1 August 2004 issue; mpeg
simulations of tidal disruption available at
http://www.astro.psu.edu/users/tamarab/tdmovies.htm
Follow-Up Chandra Observations of Three Candidate Tidal Disruption Events
Large-amplitude, high-luminosity soft X-ray flares were detected by the ROSAT
All-Sky Survey in several galaxies with no evidence of Seyfert activity in
their ground-based optical spectra. These flares had the properties predicted
for a tidal disruption of a star by a central supermassive black hole. We
report Chandra observations of three of these galaxies taken a decade after
their flares that reveal weak nuclear X-ray sources that are from 240 to 6000
times fainter than their luminosities at peak, supporting the theory that these
were special events and not ongoing active galactic nucleus (AGN) variability.
The decline of RX J1624.9+7554 by a factor of 6000 is consistent with the
(t-t_D)^(-5/3) decay predicted for the fall-back phase of a tidal disruption
event, but only if ROSAT was lucky enough to catch the event exactly at its
peak in 1990 October. RX J1242.6-1119A has declined by a factor of 240, also
consistent with (t-t_D)^(-5/3). In the H II galaxy NGC 5905 we find only
resolved, soft X-ray emission that is undoubtedly associated with starburst
activity. When accounting for the starburst component, the ROSAT observations
of NGC 5905, as well as the Chandra upper limit on its nuclear flux, are
consistent with a (t-t_D)^(-5/3) decay by at least a factor of 1000. Although
we found weak Seyfert~2 emission lines in Hubble Space Telescope spectra of NGC
5905, indicating that a low-luminosity AGN was present prior to the X-ray
flare, we favor a tidal disruption explanation for the flare itself.Comment: 17 pages, 4 figures, to appear in ApJ April 1 200
Chandra Observations of X-ray Weak Narrow-Line Seyfert 1 Galaxies
We present Chandra observations of 17 optically-selected, X-ray weak
narrow-line Seyfert 1 (NLS1) galaxies. These objects were optically identified
by Williams et al. (2002) in the Sloan Digital Sky Survey Early Data Release,
but were not found in the ROSAT All-Sky Survey (RASS) despite having optical
properties similar to RASS-detected NLS1s. All objects in this sample were
detected by Chandra and exhibit a range of 0.5-2 keV photon indices
Gamma=1.1-3.4. One object was not detected in the soft band, but has a best-fit
Gamma=0.25 over the full 0.5-8 keV range. These photon indices extend to values
far below what are normally observed in NLS1s. A composite X-ray spectrum of
the hardest objects in this sample does not show any signs of absorption,
although the data do not prohibit one or two of the objects from being highly
absorbed. We also find a strong correlation between Gamma and L_1keV; this may
be due to differences in L_bol/L_Edd among the NLS1s in this sample. Such
variations are seemingly in conflict with the current paradigm that NLS1s
accrete near the Eddington limit. Most importantly, this sample shows that
strong, ultrasoft X-ray emission is not a universal characteristic of NLS1s; in
fact, a substantial number may exhibit weak and/or low-Gamma X-ray emission.Comment: Minor changes, added section on X-ray weakness. 25 pages incl. 6
figures, 3 tables, accepted to Ap
SDSS J143030.22-001115.1: A misclassified narrow-line Seyfert 1 galaxy with flat X-ray spectrum
We used multi-component profiles to model H and [O III]4959,5007 lines for SDSS J143030.22-001115.1, a narrow-line Seyfert 1
galaxy (NLS1) in a sample of 150 NLS1s candidates selected from the Sloan
Digital Sky Survey (SDSS) Early Data Release (EDR). After subtracting the
H contribution from narrow line regions (NLRs), we found that its full
width half maximum (FWHM) of broad H line is nearly 2900 \kms,
significantly larger than the customarily adopted criterion of 2000 \kms. With
its weak Fe II multiples, we think that SDSS J143030.22-001115.1 can't be
classified as a genuine NLS1. When we calculate the virial black hole masses of
NLS1s, we should use the H linewidth after subtracting the H
contribution from NLRs.Comment: 7 pages, 1 table, accepted by ChJA
- …