515 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
Complex Optical-X-ray Correlations in the Narrow-Line Seyfert 1 Galaxy NGC 4051
This paper presents the results of a dense and intensive X-ray and optical
monitoring of the narrow-line Seyfert 1 galaxy NGC 4051 carried out in 2000.
Results of the optical analysis are consistent with previous measurements. The
amplitude of optical emission line variability is a factor of two larger than
that of the underlying optical continuum, but part or all of the difference can
be due to host-galaxy starlight contamination or due to the lines being driven
by the unseen UV continuum, which is more variable than the optical continuum.
We measured the lag between optical lines and continuum and found a lower, more
accurate broad line region size of 3.0+-1.5 light days in this object. The
implied black hole mass is M_BH=5(+6,-3)x10^5 M_sun; this is the lowest mass
found, so far, for an active nucleus. We find significant evidence for an
X-ray-optical (XO) correlation with a peak lag of about <1 day, although the
centroid of the asymmetric correlation function reveals that part of the
optical flux varies in advance of the X-ray flux by 2.4+-1.0 days. This complex
XO correlation is explained as a possible combination of X-ray reprocessing and
perturbations propagating from the outer (optically emitting) parts of the
accretion disc into its inner (X-ray emitting) region.Comment: 8 pages, 4 figures included, LaTeX mn.sty, accepted for publication
in MNRA
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
Hard X-ray photon index as an indicator of bolometric correction in active galactic nuclei
We propose the rest-frame 2-10 keV photon index, \ga, acting as an indicator
of the bolometric correction, \lb/ (where \lb~ is the
bolometric luminosity and is the rest-frame 2-10 keV
luminosity), in radio-quiet active galactic nuclei (AGNs). Correlations between
\ga~ and both bolometric correction and Eddington ratio are presented, based on
simultaneous X-ray, UV, and optical observations of reverberation -mapped AGNs.
These correlations can be compared with those for high-redshift AGNs to check
for any evolutionary effect. Assuming no evolutionary effect in AGNs' spectral
properties, together with the independent estimates of , the
bolometric correction, Eddington ratio, and black hole (BH) mass can all be
estimated from these correlations for high-redshift AGNs, with the mean
uncertainty of a factor of 2-3. If there are independent estimates of BH
masses, \ga~ for high-redshift AGNs can be used to determine their true \lb~
and , and in conjunction with the redshift, can be potentially
used to place constraints on cosmology by comparison with the rest-frame 2-10
keV flux. We find that the true estimated from \ga~ for the
brightest Type I AGNs with in the Lockman Hole is generally in agreement
with the observed .
However, there are still many uncertainties, such as the accurate
determination of the intrinsic \ga~ for distant AGNs and the large uncertainty
in the luminosities obtained, which call for significant further study before
``AGN cosmology'' can be considered a viable technique.Comment: 2 figures, accepted for publication in the ApJ Lette
The Discovery of a Nearby M Dwarf
We report the discovery of a nearby M dwarf star, found accidentally while observing the old nova DN Gem at the Wise Observatory. The star is designated 1200-05296925 in the PMM USNO-A1.0 catalogue and its coordinates, calculated for 1997 November 27 are: RA=6 55 05.13 Dec = +32 09 54.1 (Equinox J2000, Epoch J1997.90). Astrometric measurements for the star yielded a yearly proper motion rate of 0.1550.002 arcseconds in right ascension and negligible yearly proper motion rate in declination. The apparent V magnitude of the star was measured as m_V=13.870.16 mag and spectral identification yielded a spectral type of M3.5Ve0.5 subclasses. Using relations between spectral type and absolute V magnitude in M dwarfs, we arrive at an absolute magnitude of M_V=12.3^{+1.2}_{-1.1} mag, which corresponds to a distance of 21^{+15}_{-10} pc
The Ultraviolet-to-Mid-Infrared Spectral Energy Distribution of Weak Emission Line Quasars
We present Spitzer Space Telescope photometry of 18 Sloan Digital Sky Survey
(SDSS) quasars at 2.7 <= z <= 5.9 which have weak or undetectable
high-ionization emission lines in their rest-frame ultraviolet (UV) spectra
(hereafter weak-lined quasars, or WLQs). The Spitzer data are combined with
SDSS spectra and ground-based, near-infrared (IR) photometry of these sources
to produce a large inventory of spectral energy distributions (SEDs) of WLQs
across the rest-frame ~0.1-5 mum spectral band. The SEDs of our sources are
inconsistent with those of BL Lacertae objects which are dominated by
synchrotron emission due to a jet aligned close to our line-of-sight, but are
consistent with the SED of ordinary quasars with similar luminosities and
redshifts that exhibit a near-to-mid-IR 'bump', characteristic of hot dust
emission. This indicates that broad emission lines in WLQs are intrinsically
weak, rather than suffering continuum dilution from a jet, and that such
sources cannot be selected efficiently from traditional photometric surveys.Comment: 10 pages (emulateapj), 4 figures. Accepted for publication in Ap
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