arXiv subjects

Christopher A. Onken

Publications and source records attributed to Christopher A. Onken.

At least 19 recordsLinked to original sources

arXiv2026

OzSSy1: The Australian Southern Seyfert-1 Spectroscopic Atlas and Catalogue at z < 0.1

We present a spectroscopic atlas of 887 broad-line active galactic nuclei (AGNs) in the Southern sky, spanning redshifts $z < 0.1$, declinations $δ< 0$ deg and Galactic latitudes $|b| > 10$ deg. The sample aims at being a largely complete census of nearby broad-line AGN. The atlas is constructed from observations with $R\sim3000$ using the integral-field Wide Field Spectrograph at the Australian National University 2.3 m telescope. Spectra are extracted in a 6.7 arcsec aperture and have a median signal-to-noise ratio of 13 and 23 per Angstrom in the blue and red arms, respectively. Each spectrum is accompanied by a spectral decomposition that models the AGN continuum and host galaxy, along with fits to the emission lines H$γ$, H$β$, H$α$, He II, He I, [O III], [O I], [N II] and [S II], including broad Balmer and Helium components. The data products are publicly available and designed to support studies of population demographics and AGN variability in conjunction with future time-domain surveys.

astro-ph.GA↗

arXiv2026

Optical Variability Structure Function of Low-Luminosity AGN using ATLAS Lightcurves

The origin of the optical flux variability in active galactic nuclei (AGN) is largely unknown. Previous studies have correlated features of the variability structure function (SF) with AGN properties, though they mostly involved high-luminosity AGN to avoid biases from host galaxy flux. In this work, we characterise optical variability in a sample of 246 low-luminosity AGN at $z < 0.1$ from the Six-degree Field Galaxy Survey (6dFGS) through the ensemble variability SF. We use lightcurves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) with a cadence of $\sim$2 days over eight years, and perform host-AGN decomposition on recent spectra to obtain the host fraction. We find that the slope of the SF depends on black hole mass, increasing from $\sim 0.1$ at $\log M_{\mathrm{BH}}/M_\odot \sim 6.5$ to $\sim 0.3$ at $\log M_{\mathrm{BH}}/M_\odot \sim 8$. Contrary to some earlier work, we do not find breaks in the SF, and two-epoch spectra taken 20 years apart suggest that the SF keeps rising into decadal timescales. In addition, we measure an anticorrelation of the amplitude with the luminosity and a positive correlation with the black hole mass. The variability behaviour also suggests that extinction is not the main driver of the variety in Seyfert subtypes.

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arXiv2026

DebrisWatch II: Digging deeper for geosynchronous debris

The geosynchronous (GSO) debris environment is continually evolving. Regular monitoring of the region is consequently of great importance, though the trade-off between coverage and sensitivity makes this challenging for the population of optically faint debris, where collecting area becomes a pivotal factor. Surveys conducted with large-aperture telescopes have provided crucial insights into the nature of this largely uncharacterised population. In this paper, we revisit a survey conducted with the 2.54 m Isaac Newton Telescope (INT), presenting an overhaul of the astrometric calibration and object detection stages of the original analysis pipeline. We apply a blind stacking technique to boost target recovery, unearthing 25 tracklets previously missed by single-frame extraction methods, and pushing the sensitivity limit fainter by 1 magnitude. The same algorithm is applied to a contemporaneous dataset, captured with a 36 cm astrograph, enabling performance benchmarking through the attempted recovery of INT detections from commercial-off-the-shelf observations. We achieve sub-arcsecond astrometric accuracy through a combination of improved star trail centroiding and iterative distortion fitting, allowing short arc initial orbit solutions to be obtained. High-cadence light curves extracted for trailing detections indicate that faint fragments are proportionally more variable than bright derelicts, with many exhibiting photometric signatures of rapid tumbling, often straddling the image noise floor. Lastly, we present preliminary findings from a follow-up multi-national observation campaign, utilising telescopes in Australia, Japan and La Palma. As space traffic management concerns begin to extend beyond GSO altitudes, scientifically-driven surveys of high-altitude orbits have an important role to play in characterising the faint debris environment.

astro-ph.IM↗

arXiv2026

The unusually red delay spectrum of the low-mass black hole AGN NGC\,4051 as revealed by intensive continuum reverberation mapping with the Las Cumbres Observatory

We present a two-year optical reverberation mapping campaign of NGC 4051, an active galactic nucleus (AGN) hosting a low-mass black hole ($8\times10^5 M_\odot$), using daily observations in seven photometric bands from Las Cumbres Observatory augmented by archival data from Swift XRT and UVOT. The light curves show correlated variability with wavelength-dependent lags broadly consistent with the standard accretion disc scaling, $τ\propto λ^{4/3}$, and a pronounced u-band excess. However, the $i$ and $z_s$ lags are significantly larger than expected and cannot be explained by a combination of disc emission and diffuse continuum (DC) from the broad-line region (BLR), making NGC 4051 a notable lag-luminosity outlier. The spectral energy distribution (SED) of the variable AGN component is markedly redder than the canonical accretion disc prediction, $F_ν\propto ν^{1/3}$, typically observed in more massive systems. We explore two scenarios to account for the red UV-optical SED and the anomalously large $i$ and $z_s$ lags: (a) SMC-like dust reddening ($E(B-V)\sim0.18$) combined with optically thick emission from the inner edge of the dusty torus; and (b) a dominant diffuse continuum contribution. We discuss the implications of each scenario within a comprehensive multi-wavelength framework.

astro-ph.HE↗

arXiv2026

Pre-perihelion observations of comet P/2023 JN16 (Lemmon) at the 2024 apparition

We present an analysis of the photometric data of new main belt comet P/2023 JN16 (Lemmon) observed with the 1.35-m SkyMapper telescope at Siding Spring Observatory in 2024 during July - September. The faint object showed dusty activity during a long period. Some photometric parameters, such as SkyMapper r band magnitudes, Afrho parameters, and absolute magnitude were estimated. We estimate the nuclear radius for comet P/2023 JN16 to be less than 0.5 km. The activity of the object is possible driven by the release of large, slow-moving particles.

astro-ph.EP↗

arXiv2026

Cepheid-based Distances to NGC 4303 and NGC 1068

We present Cepheid-based distances to two canonical AGN: NGC 4303 (M 61) and NGC 1068. Data were obtained using the Hubble Space Telescope with nonredundant time spacing over 12 visits for each target, and observations were made with the F555W and F814W filters. We found 32,694 point sources in NGC 4303, and 130 of these were determined to be strong Cepheid candidates with periods ranging ~$13-93$ days. In NGC 1068, we found 20,207 point sources, where 51 of these were strong Cepheid candidates with periods ~$14-92$ days. We fit the period$-$luminosity relationship, calibrated based on a geometric distance to the LMC by Riess et al. (2019), to our Cepheid candidates in each galaxy and correct for potential effects of metallicity. Using a distance constraint for the LMC given by Pietrzyński et al. (2019), this yields a distance modulus of $μ= 31.083 \pm 0.035$ mag for NGC 4303 and $μ= 30.150 \pm 0.106$ mag for NGC 1068. Thus, we measure distances of $D = 16.47 \pm 0.27$ Mpc to NGC 4303 and $D = 10.72 \pm 0.52$ Mpc to NGC 1068.

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arXiv2025

The Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) Survey Overview and Initial Data Release

We present an overview of the Weighing Halos Accurately, Locally, and Efficiently with Supernovae (WHALES) survey, the first to discover and measure Type Ia supernovae (SNe Ia) in and around galaxy superclusters. By building a sample of SNe~Ia around these massive environments, we aim to provide new constraints on bulk-flow models while laying the groundwork for improved estimates of supercluster masses. Here, we present data from the first two seasons targeting the Shapley Supercluster (0.02<z<0.06), which is responsible for a large but unknown fraction of our local group's motion. Until now, no supernovae had been analyzed in the direction of Shapley. Through the WHALES survey, we have identified 12 likely SNe Ia in this region using SkyMapper, including 8 with spectroscopic confirmation. We present the first light curves of these SNe and combine our observations with data from ATLAS. We demonstrate that the low number of discovered SNe Ia per season is consistent with various rate calculations, highlighting the need for future surveys to monitor superclusters over a multi-year timespan. Finally, we present simulations of SN Ia observations in the environments of massive galaxy clusters, demonstrating how the inferred peculiar velocities can constrain cluster masses, and highlighting the added complexity within superclusters. We find that a sample of 100 SNe Ia would enable a 25% precision measurement of the total mass of the Shapley Supercluster.

astro-ph.CO↗

arXiv2025

AllBRICQS: The Discovery of Luminous Quasars in the Northern Hemisphere

We present the second catalog of bright quasars from the All-sky BRIght, Complete Quasar Survey (AllBRICQS), focusing on spectroscopically observed quasars in the Northern Hemisphere with Galactic latitude $|b| > 10^\circ$. This catalog includes their spectral data, redshifts, and luminosities. AllBRICQS aims to identify the last remaining optically bright quasars using data from the Wide-field Infrared Survey Explorer (WISE) and Gaia all-sky survey Data Release 3 (DR3). AllBRICQS searches for quasars that are brighter than $B_P = 16.5$ or $R_P = 16$ mag in Gaia DR3, based on simple selection criteria. Here, we report 62 new AllBRICQS quasars spanning various types, which include typical broad emission line quasars and the most luminous iron low-ionization broad absorption line quasars discovered to date. Spectroscopic observations were conducted using the Long-Slit Spectrograph on the 1.8-meter telescope at Bohyunsan Optical Astronomy Observatory, YFOSC on the 2.4-meter telescope at Lijiang Observatory, and BFOSC on the 2.16-meter telescope at Xinglong Observatory. We applied flux calibration using ZTF broadband photometry to correct for attenuation due to intermittent thin clouds during the observations. Redshifts were determined using inverse-variance weighted cross-correlation methods. Our targets span the bolometric luminosity range of $44.9<\log \left( L_{\rm bol} / {\rm erg~s^{-1}} \right)<48.0$ at redshifts between 0.09 and 2.48. These confirmed AllBRICQS quasars provide a valuable resource for future research into quasar evolution, black holes, their environments, and their host galaxies across multiple wavelengths.

astro-ph.GA↗

arXiv2025

An Extremely-High Velocity Outflow in SMSS J2157-3602, the most luminous quasar in the first 1.3 Gyr

We report the discovery of an extremely high-velocity outflow (EHVO) in the most luminous ($L\rm_{Bol}$ $\sim$ 2.29 $\times$ 10$^{48}$ erg/s) QSO, SMSS J2157-3602, at z=4.692. Combined XSHOOTER and NIRES observations reveal that the EHVO reaches a maximum velocity of v$_\mathrm{max} \sim 0.13c$ and persists over rest-frame timescales of a few months to one year. SMSS J2157-3602 also exhibits one of the highest balnicity index discovered in an EHVO so far. In addition, the blueshifted CIV emission traces a high-velocity (v$\rm_{CIV}^{50}\sim$ 4660 km/s) outflow from the broad-line region. Thanks to an XMM-Newton observation, we also discover the X-ray weak nature of this QSO, which likely prevents the overionization of the innermost disk atmosphere and facilitates the efficient launch of the detected EHVO and BLR winds. The extraordinary luminosity of SMSS J2157-3602 and the extreme velocity of the EHVO make it a unique laboratory for testing AGN driven feedback under extreme conditions. Current uncertainties on the outflow's location and column density strengthen the case for dedicated follow-up, which will be essential to assess the full feedback potential of this remarkable quasar.

astro-ph.GA↗

arXiv2025

A Stellar Dynamical Mass for the Central Black Hole in MCG$-$06-30-15

We present the stellar dynamical mass of the central black hole in the nearby Seyfert galaxy MCG$-$06-30-15 using the Schwarzschild orbit-superposition method implemented in the open-source code FORSTAND. We obtained spatially resolved $K$-band nuclear stellar spectra for this galaxy with SINFONI on the VLT. We extracted the bulk stellar kinematics using Gauss$-$Hermite (GH) parameterization of the line-of-sight velocity distributions. A multicomponent surface brightness profile of the galaxy was determined from an $HST$ medium-band $V$ image. Our best-fit models indicate a black hole mass of $M_{BH}=(4.4\pm1.4) \times 10^7 M_{\odot}$ and a stellar mass-to-light ratio of $M/L$=($3.0\pm0.3$) $M_{\odot}$/$L_{\odot}$, within 1$σ$ confidence intervals. Our constraint on $M_{BH}$ agrees with an upper limit on the mass from stellar dynamics based on the Jeans Anisotropic Method, but is $\sim$10 times larger than the reported mass from reverberation mapping. However, our best-fit $M_{BH}$ may be systematically biased high due to the counter-rotating disk in the nucleus of MCG$-$06-30-15 and the inability of the GH parameterization to fully describe such a complicated set of stellar kinematics. In addition, a dynamical $M_{BH}$ value depends heavily on the assumed source distance, which is not yet accurately constrained for this galaxy. MCG$-$06-30-15 is only the fourth galaxy in which we can compare $M_{BH}$ from stellar dynamical modeling with that from reverberation mapping. A direct comparison of $M_{BH}$ allows us to identify and investigate the possible sources of bias associated with different mass measurement techniques, which may influence our understanding of black hole and galaxy coevolution across cosmological timescales.

astro-ph.GA↗

arXiv2024

Discovering Changing-Look AGN in the 6dF Galaxy Survey using ATLAS Light curves

Changing-Look Active Galactic Nuclei (CLAGN) are characterised by extreme variations in line emission over short timescales, mostly affecting broad H$β$ lines. While a few hundred CLAGN are known, a complete sample of turn-on CLAGN is still elusive. Here, we present a search for turn-on CLAGN in a complete sample of galaxies, using archival spectra and recent light curves. We obtained light curves from the Asteroid Terrestrial Impact Last Alert System (ATLAS) for 16,232 emission line galaxies, including both star-forming and active galaxies, at $z<0.1$ with spectra from the Six-degree Field Galaxy Survey (6dFGS). We first establish typical variability behaviour for different AGN types, as recorded between 2001 and 2009, and then select outliers from the bulk behaviour as CLAGN candidates. We obtain new spectra for the candidates and identify 12 new turn-on CLAGN (appearing broad H$β$ line) and 19 new turn-off CLAGN (disappearing broad H$β$ line). We may have missed AGN that changed and reverted their state over the 15$-$20 years since 6dFGS spectra were taken, and thus our CLAGN rates of 1.7$\%$ for turn-on and 9.6$\%$ for turn-off are lower limits. The turn-on rate is naturally much lower as the type 1.9/2 sample is dominated by obscured AGN due to orientation, which are not expected to change. However, the number of turn-on (27) and turn-off (24) CLAGN we find are similar, suggesting that our parent AGN sample is reasonably complete in our search volume at $z<0.1$.

astro-ph.GA↗

arXiv2024

Supermassive black holes are growing slowly by $z\sim5$

We investigate the black hole mass function at $z\sim5$ using XQz5, our recent sample of the most luminous quasars between the redshifts $4.5 < z < 5.3$. We include 72 quasars with black hole masses estimated from velocity-broadened emission-line measurements and single-epoch virial prescriptions in the footprint of a highly complete parent survey. The sample mean Eddington ratio and standard deviation is $\logλ\approx -0.20\pm0.24$. The completeness-corrected mass function is modelled as a double power-law, and we constrain its evolution across redshift assuming accretion-dominated mass growth. We estimate the evolution of the mass function from $z=5-4$, presenting joint constraints on accretion properties through a measured dimensionless e-folding parameter, $k_{\rm{ef}} \equiv \langleλ\rangle U (1-ε)/ε= 1.79\pm0.06$, where $\langleλ\rangle$ is the mean Eddington ratio, $U$ is the duty cycle, and $ε$ is the radiative efficiency. If these supermassive black holes were to form from seeds smaller than $10^8\,M_{\odot}$, the growth rate must have been considerably faster at $z\gg5$ than observed from $z=5-4$. A growth rate exceeding $3\times$ the observed rate would reduce the initial heavy seed mass to $10^{5-6}\,M_{\odot}$, aligning with supermassive star and/or direct collapse seed masses. Stellar mass ($10^2\,M_{\odot}$) black hole seeds would require $\gtrsim4.5\times$ the observed growth rate at $z\gg5$ to reproduce the measured active black hole mass function. A possible pathway to produce the most extreme quasars is radiatively inefficient accretion flow, suggesting black holes with low angular momentum or photon trapping in supercritically accreting thick discs.

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arXiv2024

Discovery of A New Blue Large-Amplitude Pulsator in the SkyMapper DR2: SMSS J184506.82-300804.7

We report the discovery of a new Blue Large-Amplitude Pulsator, SMSS J184506-300804 (SMSS-BLAP-1) in Data Release 2 of the SkyMapper Southern Sky Survey. We conduct high-cadence photometric observations in the $u$ band to confirm a periodic modulation of the lightcurve. SMSS-BLAP-1 has a ~19-min pulsation period with an amplitude of 0.2 mag in $u$ band, and is similar to the classical BLAPs found by OGLE. From spectroscopic observations with the Wide-Field Spectrograph on the ANU 2.3m telescope, we confirm it as a low-gravity BLAP: best-fit parameters from the non-LTE Tlusty model are estimated as $T_\mathrm{eff}$ = 29,020$^{+193}_{-34}$ K, $\log g$ = 4.661$^{+0.008}_{-0.143}$ (cm s$^{-2}$), and $\log$ n(He)/n(H) = -2.722$^{+0.057}_{-0.074}$ dex. However, our BLAP exhibits a very He-deficient atmosphere compared to both low- and high-gravity BLAPs, which have $\log$ n(He)/n(H) in the range -0.41~-2.4.

astro-ph.SR↗

arXiv2024

The accretion of a solar mass per day by a 17-billion solar mass black hole

Around a million quasars have been catalogued in the Universe by probing deeper and using new methods for discovery. However, the hardest ones to find seem to be the rarest and brightest specimen. In this work, we study the properties of the most luminous of all quasars found so far. It has been overlooked until recently, which demonstrates that modern all-sky surveys have much to reveal. The black hole in this quasar accretes around one solar mass per day onto an existing mass of $\sim$17 billion solar masses. In this process its accretion disc alone releases a radiative energy of $2\times 10^{41}$ Watts. If the quasar is not strongly gravitationally lensed, then its broad line region (BLR) is expected to have the largest physical and angular diameter occurring in the Universe, and will allow the Very Large Telescope Interferometer to image its rotation and measure its black hole mass directly. This will be an important test for BLR size-luminosity relations, whose extrapolation has underpinned common black-hole mass estimates at high redshift.

astro-ph.CO↗

arXiv2024

SkyMapper Southern Survey: Data Release 4

We present the fourth data release (DR4) of the SkyMapper Southern Survey (SMSS), the last major step in our hemispheric survey with six optical filters: u, v, g, r, i, z. SMSS DR4 covers 26,000 sq.deg from over 400,000 images acquired by the 1.3m SkyMapper telescope between 2014-03 and 2021-09. The 6-band sky coverage extends from the South Celestial Pole to Dec = +16deg, with some images reaching Dec ~ +28deg. In contrast to previous DRs, we include all good-quality images from the facility taken during that time span, not only those explicitly taken for the public Survey. From the image dataset, we produce a catalogue of nearly 13 billion detections made from ~700 million unique astrophysical objects. The typical 10sigma depths for each field range between 18.5 and 20.5 mag, depending on the filter, but certain sky regions include longer exposures that reach as deep as 22 mag in some filters. As with previous SMSS catalogues, we have cross-matched with a host of other imaging and spectroscopic datasets to facilitate additional science outcomes. SMSS DR4 is now available to the worldwide astronomical community.

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arXiv2023

Velocity-Resolved Reverberation Mapping of NGC 3227

We describe the results of a new reverberation mapping program focused on the nearby Seyfert galaxy NGC 3227. Photometric and spectroscopic monitoring were carried out from 2022 December to 2023 June with the Las Cumbres Observatory network of telescopes. We detected time delays in several optical broad emission lines, with H$β$ having the longest delay at $τ_{\rm cent}=4.0^{+0.9}_{-0.9}$ days and He II having the shortest delay with $τ_{\rm cent}=0.9^{+1.1}_{-0.8}$ days. We also detect velocity-resolved behavior of the H$β$ emission line, with different line-of-sight velocities corresponding to different observed time delays. Combining the integrated H$β$ time delay with the width of the variable component of the emission line and a standard scale factor suggests a black hole mass of $M_{\rm BH}=1.1^{+0.2}_{-0.3} \times 10^7 M_{\odot}$. Modeling of the full velocity-resolved response of the H$β$ emission line with the phenomenological code CARAMEL finds a similar mass of $M_{\rm BH}=1.2^{+1.5}_{-0.7} \times 10^7 M_{\odot}$, and suggests that the H$β$-emitting broad line region (BLR) may be represented by a biconical or flared disk structure that we are viewing at an inclination angle of $θ_i \approx 33^{\circ}$ and with gas motions that are dominated by rotation. The new photoionization-based BLR modeling tool BELMAC finds general agreement with the observations when assuming the best-fit CARAMEL results, however BELMAC prefers a thick disk geometry and kinematics that are equally comprised of rotation and inflow. Both codes infer a radially extended and flattened BLR that is not outflowing.

astro-ph.GA↗

arXiv2023

Virial Black Hole Mass Estimates of Quasars in the XQ-100 Legacy Survey

The black hole (BH) mass and luminosity are key factors in determining how a quasar interacts with its environment. In this study, we utilise data from the European Southern Observatory Large Programme XQ-100, a high-quality sample of 100 X-shooter spectra of the most luminous quasars in the redshift range $3.5 < z < 4.5$, and measure the properties of three prominent optical and ultraviolet broad emission-lines present in the wide wavelength coverage of X-shooter: CIV, MgII, and H$β$. The line properties of all three broad lines are used for virial estimates of the BH mass and their resulting mass estimates for this sample are tightly correlated. The BH mass range is $\log{(\rm{M_{BH}}/\rm{M_\odot})} = 8.6-10.3$ with bolometric luminosities estimated from the 3000A continuum in the range $\log{(\rm{L_{bol}}/\rm{erg\,s^{-1}})} = 46.7-48.0$. Robustly determined properties of these quasars enable a variety of follow-up research in quasar astrophysics, from chemical abundance and evolution in the broad-line region to radiatively driven quasar outflows.

astro-ph.GA↗

arXiv2023

Ultraluminous Quasars At High Redshift Show Evolution In Their Radio-Loudness Fraction In Both Redshift And Ultraviolet Luminosity

We take a sample of 94 ultraluminous, optical quasars from the search of over 14,486 deg^2 by Onken et al. 2022 in the range 4.4<redshift<5.2 and match them against the Rapid ASKAP Continuum Survey (RACS) observed on the Australian Square Kilometre Array Pathfinder (ASKAP). From this most complete sample of the bright end of the redshift ~5 quasar luminosity function, there are 10 radio continuum detections of which 8 are considered radio-loud quasars. The radio-loud fraction for this sample is 8.5 \pm 2.9 per cent. Jiang et al. 2007 found that there is a decrease in the radio-loud fraction of quasars with increasing redshift and an increase with increasing absolute magnitude at rest frame 2500 Angstroms. We show that the radio-loud fraction of our quasar sample is consistent with that predicted by Jiang et al. 2007, extending their result to higher redshifts.

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