72 research outputs found
Weak Emission Line Quasars in the Context of a Modified Baldwin Effect
We investigate the relationship between the rest-frame equivalent width (EW)
of the C IV \lambda1549 broad-emission line, monochromatic luminosity at
rest-frame 5100 A, and the Hbeta-based Eddington ratio in a sample of 99
ordinary quasars across the widest possible ranges of redshift (0 < z < 3.5)
and bolometric luminosity (10^{44} <~ L <~ 10^{48} erg s^{-1}). We find that
EW(C IV) is primarily anti-correlated with the Eddington ratio, a relation we
refer to as a modified Baldwin effect (MBE), an extension of the result
previously obtained for quasars at z < 0.5. Based on the MBE, weak emission
line quasars (WLQs), typically showing EW(C IV) <~ 10 A, are expected to have
extremely high Eddington ratios. By selecting all WLQs with archival Hbeta and
C IV spectroscopic data, nine sources in total, we find that their Hbeta-based
Eddington ratios are typical of ordinary quasars with similar redshifts and
luminosities. Four of these WLQs can be accommodated by the MBE, but the other
five deviate significantly from this relation, at the >~3 \sigma\ level, by
exhibiting C IV lines much weaker than predicted from their Hbeta-based
Eddington ratios. Assuming the supermassive black-hole masses in all quasars
can be determined reliably using the single-epoch Hbeta-method, our results
indicate that EW(C IV) cannot depend solely on the Eddington ratio. We briefly
discuss a strategy for further investigation into the roles that basic physical
properties play in controlling the relative strengths of broad-emission lines
in quasars.Comment: 7 pages (emulateapj), 2 figures. Accepted for publication in Ap
Supernova 2002ap - The First Month
Supernova (SN) 2002ap in M74 was discovered on January 29, 2002. Being one of
the nearest (10 Mpc) SN events in the last decades, and spectroscopically
similar to the so-called ``hypernovae'' 1997ef and 1998bw, both possibly
associated with gamma-ray bursts (GRBs), it is of great interest. Shortly after
its discovery, we launched an intensive photometric and spectroscopic
monitoring campaign of this event, and here we report the results of the first
month of observations. We use our UBVRI photometry to estimate the magnitudes
at, and dates of, peak brightness. Our data suggest that this object reached
its peak B-band luminosity on Feb. 7.1(-1.3)(+2) UT. Based on its similarity to
SN 1998bw, we estimate the range of possible dates for a GRB that may have been
associated with SN 2002ap. We find that it may include dates outside the time
frame for which all available gamma-ray data have been intensively scanned,
according to recent reports. The absolute magnitude at peak brightness of SN
2002ap (M_B = -16.9) shows that it was significantly fainter than SN 1998bw, or
normal type-Ia SNe, but similar to SN 1997ef. Our spectroscopic observations
confirm that SN 2002ap is strikingly similar to SNe 1998bw and 1997ef. We
briefly describe the spectral evolution of this object. To assist other
observers and to stimulate theoretical models, we make our entire data set
publicly available in digital form.Comment: 5 pages, including 4 figures. MNRAS (pink pages) in press. Data
available electronically from http://wise-obs.tau.ac.il/~avishay/local.htm
Is There a Metallicity--Luminosity Relationship in AGN? The Case of Narrow-Line Seyfert 1 Galaxies
The well known relationship between metallicity and luminosity in AGN is
addressed by introducing new metallicity measurements (based on the method of
Hamann & Ferland 1993; HF93) for a sample of narrow-line Seyfert 1 (NLS1)
galaxies. Our new results, based on a sample of 162 AGN, including 9 NLS1s,
indicate that, while broad-line AGN trace a metallicity--luminosity power law
with an index of ~0.2, NLS1s deviate significantly from this relationship at
low luminosities. Adopting the HF93 method based on the N V/C IV line ratio, we
find that NLS1 metallicities are similar to those of some high-redshift
high-luminosity quasars. We also examined the N IV]/C IV line ratio and
compared it with N V/C IV in a sample of 30 sources including several NLS1s. We
find that the two do not give a consistent answer regarding the N/C abundance
ratio. This result is marginal because of the quality of the data. We suggest
two alternative explanations to these results: 1) The HF93
metallicity--luminosity dependence is not a simple two-parameter dependence and
there is an additional hidden variable in this relationship that has not yet
been discovered. The additional parameter may be the accretion rate, the age of
the central stellar cluster or, perhaps, something else. 2) The strong line
ratios involving N V 1240A suggested by HF93 are not adequate metallicity
indicators for NLS1s and perhaps also other AGN for reasons that are not yet
fully understood.Comment: 4 pages, 2 figures; Figure 2 and Tables 1 and 2 were revised. This
did not affect any of the main results of the Letter (see also the Erratum:
2002, ApJ, 569, L59
Reverberation Mapping of High-Luminosity Quasars: First Results
Reverberation mapping of nearby active galactic nuclei has led to estimates
of broad-line-region (BLR) sizes and central-object masses for some 37 objects
to date. However, successful reverberation mapping has yet to be performed for
quasars of either high luminosity (above L_opt~10^{46} erg/s) or high redshift
(z>0.3). Over the past six years, we have carried out, at the Hobby-Eberly
Telescope, rest-frame-ultraviolet spectrophotometric monitoring of a sample of
six quasars at redshifts z=2.2--3.2, with luminosities of
L_opt~10^{46.4}--10^{47.6} erg/s, an order of magnitude greater than those of
previously mapped quasars. The six quasars, together with an additional five
having similar redshift and luminosity properties, were monitored
photometrically at the Wise Observatory during the past decade. All 11 quasars
monitored show significant continuum variations of order 10%--70%. This is
about a factor of two smaller variability than for lower luminosity quasars
monitored over the same rest-frame period. In the six objects which have been
spectrophotometrically monitored, significant variability is detected in the
CIV1550 broad emission line. In several cases the variations track the
continuum variations in the same quasar, with amplitudes comparable to, or even
greater than, those of the corresponding continua. In contrast, no significant
Ly\alpha variability is detected in any of the four objects in which it was
observed. Thus, UV lines may have different variability trends in
high-luminosity and low-luminosity AGNs. For one quasar, S5~0836+71 at z=2.172,
we measure a tentative delay of 595 days between CIV and UV-continuum
variations, corresponding to a rest-frame delay of 188 days and a central
black-hole mass of 2.6\times10^9 M_\odot.Comment: 11 pages, 6 figures, emulateapj, accepted for publication in The
Astrophysical Journa
Black-Hole Mass and Growth Rate at High Redshift
We present new H and K bands spectroscopy of 15 high luminosity active
galactic nuclei (AGNs) at redshifts 2.3-3.4 obtained on Gemini South. We
combined the data with spectra of additional 29 high-luminosity sources to
obtain a sample with 10^{45.2}<\lambda L_{\lambda}(5100A)<10^{47.3} ergs/sec
and black hole (BH) mass range, using reverberation mapping relationships based
on the H_beta method, of 10^{8.8}-10^{10.7} M_sun. We do not find a correlation
of L/L_Edd with M_BH but find a correlation with \lambda L_{\lambda}(5100A)
which might be due to selection effects. The L/L_Edd distribution is broad and
covers the range ~0.07-1.6, similar to what is observed in lower redshift,
lower luminosity AGNs. We suggest that this consistently measured and
calibrated sample gives the best representation of L/L_Edd at those redshifts
and note potential discrepancies with recent theoretical and observational
studies. The lower accretion rates are not in accord with growth scenarios for
BHs at such redshifts and the growth times of many of the sources are longer
than the age of the universe at the corresponding epochs. This suggests earlier
episodes of faster growth at z>~3 for those sources. The use of the C IV method
gives considerably different results and a larger scatter; this method seems to
be a poor M_BH and L/L_Edd estimator at very high luminosity.Comment: 8 pages (emulateapj), 4 figures. Accepted for publication in Ap
Exploratory X-ray Monitoring of Luminous Radio-Quiet Quasars at High Redshift: No Evidence for Evolution in X-ray Variability
We report on the second installment of an X-ray monitoring project of seven
luminous radio-quiet quasars (RQQs). New {\sl Chandra} observations of four of
these, at , yield a total of six X-ray epochs, per source,
with temporal baselines of days in the rest frame. These data
provide the best X-ray light curves for RQQs at , to date, enabling
qualitative investigations of the X-ray variability behavior of such sources
for the first time. On average, these sources follow the trend of decreasing
variability amplitude with increasing luminosity, and there is no evidence for
X-ray variability increasing toward higher redshifts, in contrast with earlier
predictions of potential evolutionary scenarios. An ensemble variability
structure function reveals that their variability level remains relatively flat
across days in the rest frame and it is generally lower than
that of three similarly luminous RQQs at over the same
temporal range. We discuss possible explanations for the increased variability
of the lower-redshift subsample and, in particular, whether higher accretion
rates play a leading role. Near-simultaneous optical monitoring of the sources
at indicates that none is variable on -day
timescales, although flux variations of up to \% are observed on
-day timescales, typical of RQQs at similar redshifts. Significant
optical-X-ray spectral slope variations observed in two of these sources are
consistent with the levels observed in luminous RQQs and are dominated by X-ray
variations.Comment: 11 pages (emulateapj), 4 figures. Accepted for publication in Ap
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