Theory predicts that a supermassive black hole binary (SMBHB) could be observed as a luminous active galactic nucleus (AGN) that periodically varies on the order of its orbital timescale. In X-rays, ...periodic variations could be caused by mechanisms including relativistic Doppler boosting and shocks. Here we present the first systematic search for periodic AGNs using 941 hard X-ray light curves (14-195 keV) from the first 105 months of the Swift Burst Alert Telescope (BAT) survey (2004-2013). We do not find evidence for periodic AGNs in Swift-BAT, including the previously reported SMBHB candidate MCG+11−11−032. We find that the null detection is consistent with the combination of the upper-limit binary population in AGNs in our adopted model, their expected periodic variability amplitudes, and the BAT survey characteristics. We have also investigated the detectability of SMBHBs against normal AGN X-ray variability in the context of the extended ROentgen Survey with an Imaging Telescope Array (eROSITA) survey. Under our assumptions of a binary population and the periodic signals they produce, which have long periods of hundreds of days, up to 13% true periodic binaries can be robustly distinguished from normal variable AGNs with the ideal uniform sampling. However, we demonstrate that realistic eROSITA sampling is likely to be insensitive to long-period binaries because longer observing gaps reduce their detectability. In contrast, large observing gaps do not diminish the prospect of detecting binaries of short, few-day periods, as 19% can be successfully recovered, the vast majority of which can be identified by the first half of the survey.
Abstract
We present two independent measurements of stellar velocity dispersions (
σ
⋆
) from the Ca
ii
H+K
λ
3969, 3934 and Mg
i
b
λ
5183, 5172, 5167 region (3880–5550 Å) and the calcium triplet ...region (8350–8750 Å) for 173 hard X-ray-selected Type 1 active galactic nuclei (AGNs;
z
≤ 0.08) from the 105 month Swift-BAT catalog. We construct one of the largest samples of local Type 1 AGNs that have both single-epoch
virial
black hole mass (
M
BH
) estimates and
σ
⋆
measurements obtained from high spectral resolution data, allowing us to test the usage of such methods for supermassive black hole studies. We find that the two independent
σ
⋆
measurements are highly consistent with each other, with an average offset of only 0.002 ± 0.001 dex. Comparing
M
BH
estimates based on broad emission lines and stellar velocity dispersion measurements, we find that the former is systematically lower by ≈0.12 dex. Consequently, Eddington ratios estimated through broad-line
M
BH
determinations are similarly biased (but in the opposite way). We argue that the discrepancy is driven by extinction in the broad-line region. We also find an anticorrelation between the offset from the
M
BH
–
σ
⋆
relation and the Eddington ratio. Our sample of Type 1 AGNs shows a shallower
M
BH
–
σ
⋆
relation (with a power-law exponent of ≈3.5) compared with that of inactive galaxies (with a power-law exponent of ≈4.5), confirming earlier results obtained from smaller samples.
Abstract
We constrain the intrinsic Eddington ratio (
λ
Edd
) distribution function for local active galactic nuclei (AGN) in bins of low and high obscuration
log
(
N
H
/
cm
−
2
)
≤
22
and
22
<
log
...(
N
H
/
cm
−
2
)
<
25
, using the Swift Burst Alert Telescope 70 month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circumnuclear geometry and temporal evolution. Specifically, at low Eddington ratios (
log
λ
Edd
<
−2), obscured AGN outnumber unobscured ones by a factor of ∼4, reflecting the covering factor of the circumnuclear material (0.8, or a torus opening angle of ∼34°). At high Eddington ratios (
log
λ
Edd
>
−1), the trend is reversed, with <30% of AGN having
log
(
N
H
/
cm
−
2
)
>
22
, which we suggest is mainly due to the small fraction of time spent in a highly obscured state. Considering the Eddington ratio distribution function of narrow-line and broad-line AGN from our prior work, we see a qualitatively similar picture. To disentangle temporal and geometric effects at high
λ
Edd
, we explore plausible clearing scenarios such that the time-weighted covering factors agree with the observed population ratio. We find that the low fraction of obscured AGN at high
λ
Edd
is primarily due to the fact that the covering factor drops very rapidly, with more than half the time spent with <10% covering factor. We also find that nearly all obscured AGN at high-
λ
Edd
exhibit some broad lines. We suggest that this is because the height of the depleted torus falls below the height of the broad-line region, making the latter visible from all lines of sight.
Abstract
Spectral energy distributions (SEDs) from X-ray to far-infrared (FIR) wavelengths are presented for a sample of 1246 X-ray-luminous active galactic nuclei (AGNs;
L
0.5–10 keV
> 10
43
erg s
...−1
), with
z
spec
< 1.2, selected from Stripe 82X, COSMOS, and GOODS-N/S. The rest-frame SEDs show a wide spread (∼2.5 dex) in the relative strengths of broad continuum features at X-ray, ultraviolet (UV), mid-infrared (MIR), and FIR wavelengths. A linear correlation (log–log slope of 0.7 ± 0.04) is found between
L
MIR
and
L
X
. There is significant scatter in the relation between the
L
UV
and
L
X
owing to heavy obscuration; however, the most luminous and unobscured AGNs show a linear correlation (log–log slope of 0.8 ± 0.06) in the relation above this scatter. The relation between
L
FIR
and
L
X
is predominantly flat, but with decreasing dispersion at
L
X
> 10
44
erg s
−1
. The ratio between the “galaxy-subtracted” bolometric luminosity and the intrinsic
L
X
increases from a factor of ∼10 to 70 from log
L
bol
/(erg s
−1
) = 44.5 to 46.5. Characteristic SED shapes have been determined by grouping AGNs based on relative strengths of the UV and MIR emission. The average
L
1
μ
m
is constant for the majority of these SED shapes, while AGNs with the strongest UV and MIR emission have elevated
L
1
μ
m
, consistent with the AGN emission dominating their SEDs at optical and near-infrared wavelengths. A strong correlation is found between the SED shape and both the
L
X
and
L
bol
, such that
L
bol
/
L
X
= 20.4 ± 1.8, independent of the SED shape. This is consistent with an evolutionary scenario of increasing
L
bol
with decreasing obscuration as the AGN blows away circumnuclear gas.
Abstract
The optical-ultraviolet transient AT 2021loi is located at the center of its host galaxy. Its spectral features identify it as a member of the Bowen fluorescence flare (BFF) class. The first ...member of this class was considered to be related to a tidal disruption event, but enhanced accretion onto an already active supermassive black hole was suggested as an alternative explanation. Having occurred in a previously known unobscured active galactic nucleus, AT 2021loi strengthens the latter interpretation. Its light curve is similar to those of previous BFFs, showing a rebrightening approximately 1 yr after the main peak (which was not explicitly identified but might be the case in all previous BFFs). An emission feature around 4680 Å, seen in the preflare spectrum, strengthens by a factor of ∼2 around the optical peak of the flare and is clearly seen as a double-peaked feature then, suggesting a blend of N
iii
λ
4640 with He
ii
λ
4686 as its origin. The appearance of O
iii
λ
3133 and possible N
iii
λλ
4097, 4103 (blended with H
δ
) during the flare further support a Bowen fluorescence classification. Here we present ZTF, ATLAS, Keck, Las Cumbres Observatory, NEOWISE-R, Swift AMI, and Very Large Array observations of AT 2021loi, making it one of the best-observed BFFs to date. It thus provides some clarity on the nature of BFFs but also further demonstrates the diversity of nuclear transients.
We investigate the observed relationship between black hole mass (M sub( BH)), bolometric luminosity (L sub( bol)) and Eddington ratio ( lambda sub( Edd)) with optical emission-line ratios (NII ...lambda 6583/H alpha , SII lambda lambda 6716, 6731/H alpha , OI lambda 6300/H alpha , OIII lambda 5007/H beta , NeIII lambda 3869/H beta and He II lambda 4686/H beta ) of hard X-ray-selected active galactic nuclei (AGN) from the BAT AGN Spectroscopic Survey. We show that the NII lambda 6583/H alpha ratio exhibits a significant correlation with lambda sub( Edd) (R sub( Pear) = -0.44, p-value = 3 x 10 super( -13), sigma = 0.28 dex), and the correlation is not solely driven by M sub( BH) or L sub( bol). The observed correlation between NII lambda 6583/H alpha ratio and M sub( BH) is stronger than the correlation with L sub( bol), but both are weaker than the lambda sub( Edd) correlation. This implies that the large-scale narrow lines of AGN host galaxies carry information about the accretion state of the AGN central engine. We propose that NII lambda 6583/H alpha is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure lambda sub( Edd) and thus M sub( BH) from the measured L sub( bol), even for high-redshift obscured AGN. We briefly discuss possible physical mechanisms behind this correlation, such as the mass-metallicity relation, X-ray heating, and radiatively driven outflows.
Abstract
This work studies the relationship between accretion-disk size and quasar properties, using a sample of 95 quasars from the Sloan Digital Sky Survey Reverberation Mapping Project with ...measured lags between the
g
and
i
photometric bands. Our sample includes disk lags that are both longer and shorter than predicted by the Shakura and Sunyaev model, requiring explanations that satisfy both cases. Although our quasars each have one lag measurement, we explore the wavelength-dependent effects of diffuse broad-line region (BLR) contamination through our sample’s broad redshift range, 0.1 <
z
< 1.2. We do not find significant evidence of variable diffuse Fe
ii
and Balmer nebular emission in the rms spectra, nor from Anderson–Darling tests of quasars in redshift ranges with and without diffuse nebular emission falling in the observed-frame filters. Contrary to previous work, we do not detect a significant correlation between the measured continuum and BLR lags in our luminous quasar sample, similarly suggesting that our continuum lags are not dominated by diffuse nebular emission. Similar to other studies, we find that quasars with larger-than-expected continuum lags have lower 3000 Å luminosities, and we additionally find longer continuum lags with lower X-ray luminosities and black hole masses. Our lack of evidence for diffuse BLR contribution to the lags indicates that the anticorrelation between continuum lag and luminosity is not likely to be due to the Baldwin effect. Instead, these anticorrelations favor models in which the continuum lag increases in lower-luminosity active galactic nuclei, including scenarios featuring magnetic coupling between the accretion disk and X-ray corona, and/or ripples or rims in the disk.
Abstract
Weak emission-line quasars (WLQs) are a subset of type 1 quasars that exhibit extremely weak Ly
α
+ N
v
λ
1240 and/or C
iv
λ
1549 emission lines. We investigate the relationship between ...emission-line properties and accretion rate for a sample of 230 “ordinary” type 1 quasars and 18 WLQs at
z
< 0.5 and 1.5 <
z
< 3.5 that have rest-frame ultraviolet and optical spectral measurements. We apply a correction to the H
β
-based black hole mass (
M
BH
) estimates of these quasars using the strength of the optical Fe
ii
emission. We confirm previous findings that WLQs’
M
BH
values are overestimated by up to an order of magnitude using the traditional broad-emission-line region size–luminosity relation. With this
M
BH
correction, we find a significant correlation between H
β
-based Eddington luminosity ratios and a combination of the rest-frame C
iv
equivalent width and C
iv
blueshift with respect to the systemic redshift. This correlation holds for both ordinary quasars and WLQs, which suggests that the two-dimensional C
iv
parameter space can serve as an indicator of accretion rate in all type 1 quasars across a wide range of spectral properties.
Abstract
We present a catalog of the millimeter-wave (mm-wave) continuum properties of 98 nearby (
z
< 0.05) active galactic nuclei (AGNs) selected from the 70 month Swift/BAT hard-X-ray catalog that ...have precisely determined X-ray spectral properties and subarcsecond-resolution Atacama Large Millimeter/submillimeter Array Band 6 (211–275 GHz) observations as of 2021 April. Due to the hard-X-ray (>10 keV) selection, the sample is nearly unbiased for obscured systems at least up to Compton-thick-level obscuration, and provides the largest number of AGNs with high-physical-resolution mm-wave data (≲100–200 pc). Our catalog reports emission peak coordinates, spectral indices, and peak fluxes and luminosities at 1.3 mm (230 GHz). Additionally, high-resolution mm-wave images are provided. Using the images and creating radial surface brightness profiles of mm-wave emission, we identify emission extending from the central sources and isolated blob-like emission. Flags indicating the presence of these emission features are tabulated. Among 90 AGNs with significant detections of nuclear emission, 37 AGNs (≈41%) appear to have both or one of extended or blob-like components. We, in particular, investigate AGNs that show well-resolved mm-wave components and find that these seem to have a variety of origins (i.e., a jet, radio lobes, a secondary AGN, stellar clusters, a narrow-line region, galaxy disk, active star formation regions, or AGN-driven outflows), and some components have currently unclear origins.
Abstract “Changing-look” active galactic nuclei (CL-AGNs) challenge our basic ideas about the physics of accretion flows and circumnuclear gas around supermassive black holes. Using first-year Sloan ...Digital Sky Survey V (SDSS-V) repeated spectroscopy of nearly 29,000 previously known active galactic nuclei (AGNs), combined with dedicated follow-up spectroscopy, and publicly available optical light curves, we have identified 116 CL-AGNs where (at least) one broad emission line has essentially (dis-)appeared, as well as 88 other extremely variable systems. Our CL-AGN sample, with 107 newly identified cases, is the largest reported to date, and includes ∼0.4% of the AGNs reobserved in first-year SDSS-V operations. Among our CL-AGNs, 67% exhibit dimming while 33% exhibit brightening. Our sample probes extreme AGN spectral variability on months to decades timescales, including some cases of recurring transitions on surprisingly short timescales (≲2 months in the rest frame). We find that CL events are preferentially found in lower-Eddington-ratio ( f Edd ) systems: Our CL-AGNs have a f Edd distribution that significantly differs from that of a carefully constructed, redshift- and luminosity-matched control sample (Anderson–Darling test yielding p AD ≈ 6 × 10 −5 ; median f Edd ≈ 0.025 versus 0.043). This preference for low f Edd strengthens previous findings of higher CL-AGN incidence at lower f Edd , found in smaller samples. Finally, we show that the broad Mg ii emission line in our CL-AGN sample tends to vary significantly less than the broad H β emission line. Our large CL-AGN sample demonstrates the advantages and challenges in using multi-epoch spectroscopy from large surveys to study extreme AGN variability and physics.
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