41 research outputs found
Reliable Identifications of AGN from the WISE, 2MASS and Rosat all-sky surveys
We have developed the "S_IX" statistic to identify bright, highly-likely
Active Galactic Nucleus (AGN) candidates solely on the basis of WISE, 2MASS and
Rosat all-sky survey data. This statistic was optimized with data from the
preliminary WISE survey and the SDSS, and tested with Lick 3-m Kast
spectroscopy. We find that sources with S_IX < 0 have a <~95% likelihood of
being an AGN (defined in this paper as a Seyfert 1, quasar or blazar). This
statistic was then applied to the full WISE/2MASS/RASS dataset, including the
final WISE data release, to yield the "W2R" sample of 4,316 sources with S_IX <
0. Only 2,209 of these sources are currently in the Veron-Cetty and Veron (VCV)
catalog of spectroscopically confirmed AGN, indicating that the W2R sample
contains nearly 2,000 new, relatively bright (J <~ 16) AGN.
We utilize the W2R sample to quantify biases and incompleteness in the VCV
catalog. We find it is highly complete for bright (J < 14), northern AGN, but
the completeness drops below 50% for fainter, southern samples and for sources
near the Galactic plane. This approach also led to the spectroscopic
identification of 10 new AGN in the Kepler field, more than doubling the number
of AGN being monitored by Kepler. This has identified ~1 bright AGN every 10
square degrees, permitting construction of AGN samples in any sufficiently
large region of sky.Comment: 25 pages, 6 figures, 5 tables. Accepted for publication in ApJ;
tentatively scheduled for the 2012 May 10 issue. This version contains data
from the full WISE release of 13 March 2012, resulting in a W2R sample of
4,316 sources. Tables 3 and 5 can be downloaded from, e.g.
http://www.astro.ucla.edu/~malkan/w2r/table3.xl
High Temporal Resolution XMM Monitoring of PKS 2155-304
The bright, strongly variable BL Lac object PKS 2155-304 was observed by XMM
for two essentially uninterrupted periods of ~11 and 16 hr on 30-31 May 2000.
The strongest variations occurred in the highest energy bands. After scaling
for this effect, the three softest bands (0.1-1.7 keV) showed strong
correlation with no measurable lag to reliable limits of \tau \ls 0.3 hr.
However, the hardest band (~3 keV) was less well-correlated with the other
three, especially on short time scales, showing deviations of ~10-20% in ~1 hr
although, again, no significant interband lag was detected. This result and
examination of previous ASCA and BeppoSAX cross-correlation functions suggest
that previous claims of soft lags on time scales of 0.3-4 hr could well be an
artifact of periodic interruptions due to Earth-occultation every 1.6 hr.
Previous determinations of the magnetic field/bulk Lorentz factor were
therefore premature, as these data provide only a lower limit of B
\gamma^{1/3} \gs 2.5 G. The hardest band encompasses the spectral region above
the high-energy break; its enhanced variability could be indicating that the
break energy of the synchrotron spectrum, and therefore of the underlying
electron energy distribution, changes independently of the lower energies.Comment: 13 pages, 3 figures, accepted by Ap
The Remarkably Featureless High Resolution X-ray Spectrum of Mrk 478
An observation of Mrk 478 using the Chandra Low Energy Transmission Grating
Spectrometer is presented. The source exhibited 30-40% flux variations on
timescales of order 10000 s together with a slow decline in the spectral
softness over the full 80 ks observation. The 0.15--3.0 keV spectrum is well
fitted by a single power law with photon index of Gamma = 2.91 +/- 0.03.
Combined with high energy data from BeppoSAX, the spectrum from 0.15 to 10 keV
is well fit as the sum of two power laws with Gamma = 3.03 +/- 0.04, which
dominates below 2 keV and 1.4 +/- 0.2, which dominates above 2 keV (quoting 90%
confidence uncertainties). No significant emission or absorption features are
detected in the high resolution spectrum, supporting our previous findings
using the Extreme Ultraviolet Explorer but contradicting the claims of emission
lines by Hwang & Bowyer (1997). There is no evidence of a warm absorber, as
found in the high resolution spectra of many Sy 1 galaxies including others
classified as narrow line Sy 1 galaxies such as Mrk 478. We suggest that the
X-ray continuum may result from Comptonization of disk thermal emission in a
hot corona through a range of optical depths.Comment: 21 pages, 7 figures; accepted for publication in the Astronomical
Journa
A Cutoff in the X-ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516
During 1997 March-July, XTE observed the bright, strongly variable Seyfert 1
galaxy NGC 3516 once every ~12.8 hr for 4.5 months and nearly continuously
(with interruptions due to SAA passage but not Earth occultation) for a 4.2 day
period in the middle. These were followed by ongoing monitoring once every ~4.3
days. These data are used to construct the first well-determined X-ray
fluctuation power density spectrum (PDS) of an active galaxy to span more than
4 decades of usable temporal frequency. The PDS shows no signs of any strict or
quasi-periodicity, but does show a progressive flattening of the power-law
slope from -1.74 at short time scales to -0.73 at longer time scales. This is
the clearest observation to date of the long-predicted cutoff in the PDS. The
characteristic variability time scale corresponding to this cutoff temporal
frequency is 1 month. Although it is unclear how this time scale may be
interpreted in terms of a physical size or process, there are several promising
candidate models. The PDS appears similar to those seen for Galactic black hole
candidates such as Cyg X-1, suggesting that these two classes of objects with
very different luminosities and putative black hole masses (differing by more
than a factor of 10^5) may have similar X-ray generation processes and
structures.Comment: 21 pages, incl. 5 figures, AASTe
Evidence for Rapid Iron K_alpha Line Flux Variability in MCG--6-30-15
This paper employs direct spectral fitting of individual orbital data in
order to measure rapid X-ray iron K_alpha line and continuum spectral slope
variations in Seyfert 1 galaxies with unprecedented temporal resolution.
Application of this technique to a long RXTE observation of MCG--6-30-15
indicates that the line flux does vary on short (~1d) timescales, but that
these variations are not correlated with changes in the continuum flux or
slope. These rapid variations indicate that the line does indeed originate
close to the black hole, confirming predictions based on its very broad
profile. However, the lack of a correlation with the continuum presents
problems for models in which the line variations are driven by those in the
continuum, modified only by light-travel time effects. Instead, it may be that
the line responds according to a physical process with a different time scale,
such as ionization instabilities in the disk, or perhaps that the geometry and
physical picture is more complex than implied by the simplest disk-corona
models.
These data also indicate that the slope of the underlying power-law continuum
(Gamma) shows strong variability and is tightly correlated with the continuum
flux in the sense that the spectrum steepens as the source brightens. All of
these results have been checked with extensive simulations, which also
indicated that a spurious correlation between Gamma and Compton reflection
fraction (R) will result if these quantities are measured from the same
spectra. This casts serious doubts on previous claims of such a Gamma-R
correlation.Comment: Accepted for publication in Ap
X-ray Spectral Variability and Rapid Variability of the Soft X-ray Spectrum Seyfert 1 Galaxies Ark 564 and Ton S180
The bright, soft X-ray spectrum Seyfert 1 galaxies Ark 564 and Ton S180 were
monitored for 35 days and 12 days with ASCA and RXTE (and EUVE for Ton S180).
The short time scale (hours-days) variability patterns were very similar across
energy bands, with no evidence of lags between any of the energy bands studied.
The fractional variability amplitude was almost independent of energy band. It
is difficult to simultaneously explain soft Seyferts stronger variability,
softer spectra, and weaker energy-dependence of the variability relative to
hard Seyferts. The soft and hard band light curves diverged on the longest time
scales probed, consistent with the fluctuation power density spectra that
showed relatively greater power on long time scales in the softest bands. The
simplest explanation is that a relatively hard, rapidly-variable component
dominates the total X-ray spectrum and a slowly-variable soft excess is present
in the lowest energy channels of ASCA. Although it would be natural to identify
the latter with an accretion disk and the former with a corona surrounding it,
a standard thin disk could not get hot enough to radiate significantly in the
ASCA band, and the observed variability time scales are much too short. The
hard component may have a more complex shape than a pure power-law. The most
rapid factor of 2 flares and dips occurred within ~1000 sec in Ark 564 and a
bit more slowly in Ton S180. The speed of the luminosity changes rules out
viscous or thermal processes and limits the size of the individual emission
regions to <~15 Schwarzschild radii (and probably much less), that is, to
either the inner disk or small regions in a corona
X-ray Power Density Spectrum of the Narrow Line Seyfert 1 Galaxy Akn 564
Beginning in 1999 January, the bright, strongly variable Narrow-Line Seyfert
1 (NLS1) galaxy Akn 564 has been observed by RXTE once every ~4.3 days. It was
also monitored every ~3.2 hr throughout 2000 July. These evenly-sampled
observations have allowed the first quantitative comparison of long and short
time-scale X-ray variability in an NLS1 and the derivation of an X-ray Power
Density Spectrum (PDS). The variability amplitude in the short time-scale light
curve is very similar to that in the long time-scale light curve, in marked
contrast to the stronger variability on longer time-scales which is
characteristic of "normal" broad-line Seyfert 1s (BLS1s). Furthermore, the Akn
564 PDS power law cuts off at a frequency of 8.7x10^-7 Hz corresponding to a
timescale of ~13 d, significantly shorter than that seen in the PDS of NGC
3516, a BLS1 of comparable luminosity.
This result is consistent with NLS1s showing faster (as opposed to larger
amplitude) variations than BLS1s, providing further evidence that NLS1s harbour
lower mass black holes than BLS1s of similar luminosity, accreting at a
correspondingly higher relative rate.Comment: Accepted for publication in Ap
X-ray Observations of the Broad-Line Radio Galaxy 3C 390.3
We present the data and preliminary analysis for a series of 90 ROSAT HRI and
two ASCA observations of the broad-line radio galaxy 3C 390.3. These data were
obtained during the period 1995 January 2 to 1995 October 6 as part of an
intensive multiwavelength monitoring campaign. The soft X-ray flux in the ROSAT
band varied by nearly a factor of four during the campaign, and the
well-resolved light-curve shows several distinct features. Several large
amplitude flares were observed, including one in which the flux increased by a
factor of about 3 in 12 days. Periods of reduced variability were also seen,
including one nearly 30 days long. While the HRI hardness ratio decreased
significantly, it is apparently consistent with that expected due to the
detector during the monitoring period.
The two ASCA observations were made on 1995 January 15 and 1995 May 5. The
0.5-10.0 keV spectra can be adequately described by an absorbed power-law.
There is no evidence for a soft excess in the ASCA spectra, indicating that the
ROSAT HRI is sampling variability of the X-ray power-law. A broad iron line was
observed in a longer 1993 ASCA observation, and while there is statistical
evidence that the line is present in the 1995 spectra, it could not be resolved
clearly. There is evidence, significant at >90% confidence, that the photon
index changed from 1.7 to 1.82 while the flux increased by 63%. The spectral
change can be detected in the spectra below 5 keV, indicating that the origin
cannot be a change in ratio of reflected to power-law flux. A compilation of
results from ASCA and Ginga observations show that on long time scales the
intrinsic photon index is correlated with the flux.Comment: 17 pages using (AASTeX) aaspp4.sty and 4 Postscript figures.
Astrophysical Journal, in pres
Be Stars in the Open Cluster NGC 6830
We report the discovery of 2 new Be stars, and re-identify one known Be star
in the open cluster NGC 6830. Eleven H-alpha emitters were discovered using the
H-alpha imaging photometry of the Palomar Transient Factory Survey. Stellar
membership of the candidates was verified with photometric and kinematic
information using 2MASS data and proper motions. The spectroscopic confirmation
was carried out by using the Shane 3-m telescope at Lick observatory. Based on
their spectral types, three H-alpha emitters were confirmed as Be stars with
H-alpha equivalent widths > -10 Angstrom. Two objects were also observed by the
new spectrograph SED-Machine on the Palomar 60 inch Telescope. The SED-Machine
results show strong H-alpha emission lines, which are consistent with the
results of the Lick observations. The high efficiency of the SED-Machine can
provide rapid observations for Be stars in a comprehensive survey in the
future.Comment: 11 pages, 8 figures, AJ in pres
Reverberation Mapping of the Kepler-Field AGN KA1858+4850
KA1858+4850 is a narrow-line Seyfert 1 galaxy at redshift 0.078 and is among
the brightest active galaxies monitored by the Kepler mission. We have carried
out a reverberation mapping campaign designed to measure the broad-line region
size and estimate the mass of the black hole in this galaxy. We obtained 74
epochs of spectroscopic data using the Kast Spectrograph at the Lick 3-m
telescope from February to November of 2012, and obtained complementary V-band
images from five other ground-based telescopes. We measured the H-beta light
curve lag with respect to the V-band continuum light curve using both
cross-correlation techniques (CCF) and continuum light curve variability
modeling with the JAVELIN method, and found rest-frame lags of lag_CCF = 13.53
(+2.03, -2.32) days and lag_JAVELIN = 13.15 (+1.08, -1.00) days. The H-beta
root-mean-square line profile has a width of sigma_line = 770 +/- 49 km/s.
Combining these two results and assuming a virial scale factor of f = 5.13, we
obtained a virial estimate of M_BH = 8.06 (+1.59, -1.72) x 10^6 M_sun for the
mass of the central black hole and an Eddington ratio of L/L_Edd ~ 0.2. We also
obtained consistent but slightly shorter emission-line lags with respect to the
Kepler light curve. Thanks to the Kepler mission, the light curve of
KA1858+4850 has among the highest cadences and signal-to-noise ratios ever
measured for an active galactic nucleus; thus, our black hole mass measurement
will serve as a reference point for relations between black hole mass and
continuum variability characteristics in active galactic nuclei