34 research outputs found
Finding the rarest objects in the universe: A new, efficient method for discovering BL Lacertae objects
We present a new, efficient method for discovering new BL Lac Objects based upon the results of the Einstein Extended Medium Sensitivity Survey (EMSS). We have found that all x-ray selected BL Lacs are radio emitters, and further, that in a 'color-color' diagram (radio/optical and optical/x-ray) the BL Lac Objects occupy an area distinct from both radio loud quasars and the radio quiet QSOs and Seyferts which dominate x-ray selected samples. After obtaining radio counterparts via VLA 'snapshot' observations of a large sample of unidentified x-ray sources, the list of candidates is reduced. These candidates then can be confirmed with optical spectroscopy and/or polarimetry. Since greater than 70 percent of these sources are expected to be BL Lacs, the optical observations are very efficient. We have tested this method using unidentified sources found in the Einstein Slew Survey. The 162 Slew Survey x-ray source positions were observed with the VLA in a mixed B/C configuration at 6 cm resulting in 60 detections within 1.5 position error circle radii. These x-ray/optical/radio sources were then plotted, and 40 BL Lac candidates were identified. To date, 10 candidates have been spectroscopically observed resulting in 10 new BL Lac objects! Radio flux, optical magnitude, and polarization statistics (obtained in white light with the Steward Observatory 2.3 m CCD polarimeter) for each are given
Magnetic Field Amplification and Flat Spectrum Radio Quasars
We perform time-dependent, spatially-resolved simulations of blazar emission
to evaluate several flaring scenarios related to magnetic-field amplification
and enhanced particle acceleration. The code explicitly accounts for
light-travel-time effects and is applied to flares observed in the flat
spectrum radio quasar (FSRQ) PKS 0208-512, which show optical/{\gamma}-ray
correlation at some times, but orphan optical flares at other times. Changes in
both the magnetic field and the particle acceleration efficiency are explored
as causes of flares. Generally, external Compton emission appears to describe
the available data better than a synchrotron self-Compton scenario, and in
particular orphan optical flares are difficult to produce in the SSC framework.
X-ray soft-excesses, {\gamma}-ray spectral hardening, and the detections at
very high energies of certain FSRQs during flares find natural explanations in
the EC scenario with particle acceleration change. Likewise, optical flares
with/without {\gamma}-ray counterparts can be explained by different
allocations of energy between the magnetization and particle acceleration,
which may be related to the orientation of the magnetic field relative to the
jet flow. We also calculate the degree of linear polarization and polarization
angle as a function of time for a jet with helical magnetic field. Tightening
of the magnetic helix immediately downstream of the jet perturbations, where
flares occur, can be sufficient to explain the increases in the degree of
polarization and a rotation by >= 180 degree of the observed polarization
angle, if light-travel-time effects are properly considered.Comment: 12 pages, 9 figures. Accepted for publication in MNRA
SDSS-IV eBOSS Spectroscopy of X-ray and WISE AGN in Stripe 82X: Overview of the Demographics of X-ray and Mid-Infrared Selected Active Galactic Nuclei
We report the results of a Sloan Digital Sky Survey-IV eBOSS program to
target X-ray sources and mid-infrared-selected WISE AGN candidates in a 36.8
deg region of Stripe 82. About half this survey (15.6 deg) covers the
largest contiguous portion of the Stripe 82 X-ray survey. This program
represents the largest spectroscopic survey of AGN candidates selected solely
by their WISE colors. We combine this sample with X-ray and WISE AGN in the
field identified via other sources of spectroscopy, producing a catalog of 4847
sources that is 82% complete to . Based on X-ray luminosities or WISE
colors, 4730 of these sources are AGN, with a median sample redshift of
. About 30% of the AGN are optically obscured (i.e., lack broad lines
in their optical spectra). BPT analysis, however, indicates that 50% of the
WISE AGN at have emission line ratios consistent with star-forming
galaxies, so whether they are buried AGN or star-forming galaxy contaminants is
currently unclear. We find that 61% of X-ray AGN are not selected as MIR AGN,
with 22% of X-ray AGN undetected by WISE. Most of these latter AGN have high
X-ray luminosities ( erg s), indicating that MIR
selection misses a sizable fraction of the highest luminosity AGN, as well as
lower luminosity sources where AGN heated dust is not dominating the MIR
emission. Conversely, 58% of WISE AGN are undetected by X-rays, though we
do not find that they are preferentially redder than the X-ray detected WISE
AGN.Comment: resubmitted to AAS Journals after addressing referee's comments. 21
pages, 17 figures, 5 Tables. Tables 4 & 5 summarize the demographics of the
sample. Figures 15 & 16 compare AGN populations from X-ray and MIR selection.
The catalog (in fits format) can be downloaded at
http://doi.org/10.5281/zenodo.257735
The Discovery of the First "Changing Look" Quasar: New Insights into the Physics & Phenomenology of AGN
SDSS J015957.64+003310.5 is an X-ray selected, AGN from the Stripe
82X survey that transitioned from a Type 1 quasar to a Type 1.9 AGN between
2000 and 2010. This is the most distant AGN, and first quasar, yet observed to
have undergone such a dramatic change. We re-observed the source with the
double spectrograph on the Palomar 5m telescope in July 2014 and found that the
spectrum is unchanged since 2010. From fitting the optical spectra, we find
that the AGN flux dropped by a factor of 6 between 2000 and 2010 while the
broad H emission faded and broadened. Serendipitous X-ray observations
caught the source in both the bright and dim state, showing a similar 2-10 keV
flux diminution as the optical while lacking signatures of obscuration. The
optical and X-ray changes coincide with -band magnitude variations over
multiple epochs of Stripe 82 observations. We demonstrate that variable
absorption, as might be expected from the simplest AGN unification paradigm,
does not explain the observed photometric or spectral properties. We interpret
the changing state of J0159+0033 to be caused by dimming of the AGN continuum,
reducing the supply of ionizing photons available to excite gas in the
immediate vicinity around the black hole. J0159+0033 provides insight into the
intermittency of black hole growth in quasars, as well as an unprecedented
opportunity to study quasar physics (in the bright state) and the host galaxy
(in the dim state), which has been impossible to do in a single sources until
now.Comment: accepted for publication in Ap
BAT AGN spectroscopic survey - XV: the high frequency radio cores of ultra-hard X-ray selected AGN
We have conducted 22 GHz radio imaging at 1 arcsec resolution of 100 low-redshift AGN selected at 14–195 keV by the Swift-BAT. We find a radio core detection fraction of 96 per cent, much higher than lower frequency radio surveys. Of the 96 radio-detected AGN, 55 have compact morphologies, 30 have morphologies consistent with nuclear star formation, and 11 have sub-kpc to kpc-scale jets. We find that the total radio power does not distinguish between nuclear star formation and jets as the origin of the radio emission. For 87 objects, we use optical spectroscopy to test whether AGN physical parameters are distinct between radio morphological types. We find that X-ray luminosities tend to be higher if the 22 GHz morphology is jet-like, but find no significant difference in other physical parameters. We find that the relationship between the X-ray and core radio luminosities is consistent with the L_R/L_X ∼ 10⁻⁵ of coronally active stars. We further find that the canonical fundamental planes of black hole activity systematically overpredict our radio luminosities, particularly for objects with star formation morphologies
On the cosmic evolution of AGN obscuration and the X-ray luminosity function: XMM-Newton and Chandra spectral analysis of the 31.3 deg Stripe 82X
We present X-ray spectral analysis of XMM and Chandra observations in the
31.3 deg Stripe-82X (S82X) field. Of the 6181 X-ray sources in this field,
we analyze a sample of 2937 active galactic nuclei (AGN) with solid redshifts
and sufficient counts determined by simulations. Our results show a population
with median values of spectral index , column
density log and
intrinsic, de-absorbed, 2-10 keV luminosity
log, in the redshift
range 0-4. We derive the intrinsic fraction of AGN that are obscured
(), finding a
significant increase in the obscured AGN fraction with redshift and a decline
with increasing luminosity. The average obscured AGN fraction is for
log. This work constrains the AGN
obscuration and spectral shape of the still uncertain high-luminosity and
high-redshift regimes (log, ),
where the obscured AGN fraction rises to . We report a luminosity
and density evolution of the X-ray luminosity function, with obscured AGN
dominating at all luminosities at and unobscured sources prevailing at
log at lower redshifts. Our results
agree with evolutionary models in which the bulk of AGN activity is triggered
by gas-rich environments and in a downsizing scenario. Also, the black hole
accretion density (BHAD) is found to evolve similarly to the star formation
rate density, confirming the co-evolution between AGN and host-galaxy, but
suggesting different time scales in their growing history. The derived BHAD
evolution shows that Compton-thick AGN contribute to the accretion history of
AGN as much as all other AGN populations combined.Comment: 31 pages, 35 figures, Accepted for publication in Ap
Dust Reddened Quasars in FIRST and UKIDSS: Beyond the Tip of the Iceberg
We present the results of a pilot survey to find dust-reddened quasars by matching the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) radio catalog to the UKIDSS near-infrared survey and using optical data from Sloan Digital Sky Survey to select objects with very red colors. The deep K-band limit provided by UKIDSS allows for finding more heavily reddened quasars at higher redshifts as compared with previous work using FIRST and Two Micron All Sky Survey (2MASS). We selected 87 candidates with K ≤ 17.0 from the UKIDSS Large Area Survey (LAS) First Data Release (DR1), which covers 190 deg^2. These candidates reach up to ~1.5 mag below the 2MASS limit and obey the color criteria developed to identify dust-reddened quasars. We have obtained 61 spectroscopic observations in the optical and/or near-infrared, as well as classifications in the literature, and have identified 14 reddened quasars with E(B – V) > 0.1, including 3 at z > 2. We study the infrared properties of the sample using photometry from the Wide-Field Infrared Survey Explorer and find that infrared colors improve the efficiency of red quasar selection, removing many contaminants in an infrared-to-optical color-selected sample alone. The highest-redshift quasars (z ≳ 2) are only moderately reddened, with E(B – V) ~ 0.2-0.3. We find that the surface density of red quasars rises sharply with faintness, comprising up to 17% of blue quasars at the same apparent K-band flux limit. We estimate that to reach more heavily reddened quasars (i.e., E(B – V) ≳ 0.5) at z > 2 and a depth of K = 17, we would need to survey at least ~2.5 times more area