YCAA Prize Postdoctoral Fellow
Yale Center for Astronomy & Astrophysics
A short PDF version of my curriculum vitae (as of November 2022), including selected publications, is here.
You can find a complete ADS list of my peer-reviewed publications here. See the collapsable/expandable list below for a breakdown into categoriess according to the level of my direct contribution. Only papers accepted for publication in a journal are shown; those in preparation or review are omitted. Clicking on any paper title will display its abstract and/or relevant external links.
Note on authorship policies: NuSTAR, RoboPol, and BASS collaborations list the people who have significantly contributed to a particular paper as first several authors, with the first author being the lead. This is followed by an alphabetical list of co-authors who contributed to the project leading to the paper in various ways, but whose direct contribution to that particular paper are minor. The EHT collaboration adopted a similar scheme, except for full-collaboration papers, which list all collaboration members alphabetically. For collaboration papers listed below, "et al." marks the start of the alphabetical list.
Paper Abstract:
Obscuration of the innermost parts of active galactic nuclei (AGN) is observed in the majority of the population both in the nearby universe and at high redshift.
However, the nature of the structures causing obscuration, especially in low-luminosity AGN, is poorly understood at present. We present a novel approach to multi-epoch
broadband X-ray spectroscopy, anchored in the long-term average spectrum in the hard X-ray band, applied to the nearby, X-ray bright AGN in the galaxy NGC 1052. From
spectral features due to X-ray reprocessing in the circumnuclear material, based on a simple, uniform-density torus X-ray reprocessing model, we find a covering factor
of 80--100% and a globally averaged column density in the range (1--2)x1023 cm-2. This closely matches the independently measured variable
line-of-sight column density range, leading to a straightforward and self-consistent picture of the obscuring torus in NGC 1052, similar to several other AGN in
recent literature. Comparing this X-ray-constrained torus model with measurements of spatially resolved sub-parsec absorption from radio observations, we find that
it may be possible to account for both X-ray and radio data with a torus model featuring a steep density gradient along the axis of the relativistic jets. This provides
a valuable direction for the development of improved physical models for the circumnuclear environment in NGC 1052 and potentially in a wider class of AGN.
Paper Access:
ADS |
ApJ, 916, 90 |
arXiv:2105.01682
Paper Abstract:
Broadband X-ray spectroscopy of the X-ray emission produced in the coronae of active galactic nuclei (AGNs) can provide important insights into the physical conditions
very close to their central supermassive black holes. The temperature of the Comptonizing plasma that forms the corona is manifested through a high-energy cutoff that
has been difficult to directly constrain even in the brightest AGN because it requires high-quality data at energies above 10 keV. In this paper we present a large
collection of coronal cutoff constraints for obscured AGNs based on a sample of 130 AGNs selected in the hard X-ray band with Swift/BAT and observed nearly simultaneously
with NuSTAR and Swift/XRT. We find that under a reasonable set of assumptions regarding partial constraints the median cutoff is well constrained to 290±20 keV, where
the uncertainty is statistical and given at the 68% confidence level. We investigate the sensitivity of this result to our assumptions and find that consideration of
various known systematic uncertainties robustly places the median cutoff between 240 and 340 keV. The central 68% of the intrinsic cutoff distribution is found to be
between about 140 and 500 keV, with estimated uncertainties of 20 and 100 keV, respectively. In comparison with the literature, we find no clear evidence that the cutoffs
in obscured and unobscured AGNs are substantially different. Our analysis highlights the importance of carefully considering partial and potentially degenerate constraints
on the coronal high-energy cutoff in AGNs.
Paper Access:
ADS |
ApJ, 905, 41 |
arXiv:2011.06583
Paper Abstract:
The basic unified model of active galactic nuclei (AGN) invokes an anisotropic obscuring structure, usually referred to as a torus, to explain AGN obscuration
as an angle-dependent effect. We present a new grid of X-ray spectral templates based on radiative transfer calculations in neutral gas in an approximately
toroidal geometry, appropriate for CCD-resolution X-ray spectra (FWHM > 130 eV). Fitting the templates to broadband X-ray spectra of AGN provides constraints
on two important geometrical parameters of the gas distribution around the supermassive black hole: the average column density and the covering factor. Compared
to the currently available spectral templates, our model is more flexible, and capable of providing constraints on the main torus parameters in a wider range of AGN.
We demonstrate the application of this model using hard X-ray spectra from NuSTAR (3--79 keV) for four AGN covering a variety of classifications: 3C 390.3, NGC 2110,
IC 5063, and NGC 7582. This small set of examples was chosen to illustrate the range of possible torus configurations, from disk-like to sphere-like geometries
with column densities below, as well as above, the Compton-thick threshold. This diversity of torus properties challenges the simple assumption of a standard
geometrically and optically thick toroidal structure commonly invoked in the basic form of the unified model of AGN. Finding broad consistency between the our
constraints and those from infrared modeling, we discuss how the approach from the X-ray band complements similar measurements of AGN structures at other wavelengths.
Paper Access:
ADS |
ApJ, 854, 42 |
arXiv:1801.04938
Paper Abstract:
We present coordinated multiwavelength observations of the bright, nearby BL Lac object Mrk 421 taken in 2013 January-March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT,
MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very-high-energy (VHE) gamma-ray bands. NuSTAR yielded previously unattainable
sensitivity in the 3--79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep power law with
a photon index of approximately 3, with no evidence for an exponential cutoff or additional hard components up to about 80 keV. For the first time, we observed both
the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by
an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure which relates to the two bumps of the broadband SED.
In each bump, the variability increases with energy which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are
more variable. The measured multi-band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack
of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time,
and the significantly longer variability timescale observed in the NuSTAR light curves point toward in-situ electron acceleration, and suggest that there are multiple
compact regions contributing to the broadband emission of Mrk 421 during low-activity states.
Paper Access:
ADS |
ApJ, 819, 156 |
arXiv:1512.02235 |
abstract and figures (VERITAS)
Paper Abstract:
Measurements of the high-energy cut-off in the coronal continuum of active galactic nuclei have long been elusive for all but a small number of the brightest examples.
We present a direct measurement of the cut-off energy in the nuclear continuum of the nearby Seyfert 1.9 galaxy MCG -05-23-016 with unprecedented precision. The high
sensitivity of NuSTAR up to 79 keV allows us to clearly disentangle the spectral curvature of the primary continuum from that of its reflection component. Using a simple
phenomenological model for the hard X-ray spectrum, we constrain the cut-off energy to 116±6 keV with 90% confidence. Testing for more complex models and nuisance parameters
that could potentially influence the measurement, we find that the cut-off is detected robustly. We further use simple Comptonized plasma models to provide independent
constraints for both the kinetic temperature of the electrons in the corona and its optical depth. At the 90% confidence level, we find kTe=29±2 keV and
τe=1.23±0.08 assuming a slab (disk-like) geometry, and kTe=25±2 keV and τe=3.5±0.2 assuming a spherical geometry. Both geometries
are found to fit the data equally well and their two principal physical parameters are correlated in both cases. With the optical depth in the τe>1 regime,
the data are pushing the currently available theoretical models of the Comptonized plasma to the limits of their validity. Since the spectral features and variability
arising from the inner accretion disk have been observed previously in MCG -05-23-016, the inferred high optical depth implies that a spherical or disk-like corona cannot be homogeneous.
Paper Access:
ADS |
ApJ, 800, 62 |
arXiv:1412.5978
Paper Abstract:
We present X-ray spectral analyses for three Seyfert 2 active galactic nuclei (AGN), NGC 424, NGC 1320, and IC 2560, observed by NuSTAR in the 3--79 keV band.
The high quality hard X-ray spectra allow detailed modeling of the Compton reflection component for the first time in these sources. Using quasi-simultaneous
NuSTAR and Swift/XRT data, as well as archival XMM-Newton data, we find that all three nuclei are obscured by Compton-thick material with column densities
in excess of ~5x1024 cm-2, and that their X-ray spectra above 3 keV are dominated by reflection of the intrinsic continuum on Compton-thick material.
Due to the very high obscuration, absorbed intrinsic continuum components are not formally required by the data in any of the sources. We constrain the intrinsic
photon indices and the column density of the reflecting medium through the shape of the reflection spectra. Using archival multi-wavelength data we recover the
intrinsic X-ray luminosities consistent with the broadband spectral energy distributions. Our results are consistent with the reflecting medium being an edge-on
clumpy torus with a relatively large global covering factor and overall reflection efficiency of the order of 1%. Given the unambiguous confirmation of the
Compton-thick nature of the sources, we investigate whether similar sources are likely to be missed by commonly used selection criteria for Compton-thick AGN,
and explore the possibility of finding their high-redshift counterparts.
Paper Access:
ADS |
ApJ, 794, 111 |
arXiv:1408.5414
Paper Abstract:
We investigate the dichotomy in the radio loudness distribution of quasars by modeling their radio emission and various selection effects using a Monte Carlo approach.
The existence of two physically distinct quasar populations, the radio-loud and radio-quiet quasars, is controversial and over the last decade a bimodal distribution
of radio loudness of quasars has been both affirmed and disputed. We model the quasar radio luminosity distribution with simple unimodal and bimodal distribution functions.
The resulting simulated samples are compared to a fiducial sample of 8300 quasars drawn from the SDSS DR7 Quasar Catalog and combined with radio observations from the
FIRST survey. Our results indicate that the SDSS-FIRST sample is best described by a radio loudness distribution which consists of two components, with (12±1)% of sources
in the radio-loud component. On the other hand, the evidence for a local minimum in the loudness distribution (bimodality) is not strong and we find that previous claims
for its existence were probably affected by the incompleteness of the FIRST survey close to its faint limit. We also investigate the redshift and luminosity dependence
of the radio loudness distribution and find tentative evidence that at high redshift radio-loud quasars were rarer, on average louder, and exhibited a smaller range in
radio loudness. In agreement with other recent work, we conclude that the SDSS-FIRST sample strongly suggests that the radio loudness distribution of quasars is not
a universal function, and that more complex models than presented here are needed to fully explain available observations.
Paper Access:
ADS |
ApJ, 759, 30 |
arXiv:1209.1099
Paper Abstract:
We report the results of joint Chandra/ACIS and NuSTAR deep observations of NGC 1167, the host galaxy of the young radio jet B2 0258+35. In the ACIS data, we detect X-ray emission, extended
both along and orthogonal to the jet. At the end of the southeast radio jet, we find lower-energy X-ray emission that coincides with a region of CO turbulence and fast outflow motions.
This suggests that the hot interstellar medium (ISM) may be compressed by the jet and molecular outflow, resulting in more efficient cooling. Hydrodynamic simulations of jet–ISM interaction
tailored to NGC 1167 are in agreement with this conclusion and with the overall morphology and spectra of the X-ray emission. The faint hard nuclear source detected with Chandra and the
stringent NuSTAR upper limits on the harder X-ray emission show that the active galactic nucleus (AGN) in NGC 1167 is in a very low-accretion state. However, the characteristics of the
extended X-ray emission are more consonant to those of luminous Compton-thick (CT) AGNs, suggesting that we may be observing the remnants of a past high accretion rate episode, with
sustained strong activity lasting ~2x103 years. We conclude that NGC 1167 is presently a Low-Ionization Nuclear Emission-line Region (LINER), but was an AGN in the past,
given the properties of the extended X-ray emission and their similarity with those of CT AGN extended emission.
Paper Access:
ADS |
ApJ, 938, 105 |
arXiv:2209.02549
Paper Abstract:
We present the first high signal-to-noise broad-band X-ray spectrum of the radio-quiet type-2 Seyfert ESO 033-G002, combining data from XMM-Newton and NuSTAR. The nuclear X-ray spectrum
is complex, showing evidence for both neutral and ionized absorption, as well as reflection from both the accretion disc and more distant material, but our broad-band coverage allows us
to disentangle all of these different components. The total neutral column during this epoch is NH= (5--6)x1022 cm-2, consistent with the optical classification
of ESO 033-G002 as a type-2 Seyfert but not so large as to prevent us from robustly determining the properties of the innermost accretion flow. The ionized absorption -- dominated by lines
from Fe XXV and Fe XXVI -- reveals a moderately rapid outflow (vout≈ 5400 km/s) which has a column comparable to the neutral absorption. We find the disc reflection from
the innermost regions to be extreme, with a reflection fraction of Rfrac≈5. This requires strong gravitational lightbending and, in turn, both an extremely compact corona
(within ≈2 RG of the black hole) and a rapidly rotating black hole (a*>0.96). Despite this tight size constraint, with a temperature of kTe= 40--70 keV
the X-ray corona in ESO 033-G002 appears similar to other active galactic nucle in terms of its placement in the compactness-temperature plane, consistent with sitting close to the limit
determined by runaway pair production. Finally, combining X-ray spectroscopy, timing, and updated optical spectroscopy, we also estimate the mass of the black hole to be
log[MBH/MSun]≈ 7.0-7.5.
Paper Access:
ADS |
MNRAS, 506, 1557 |
arXiv:2107.10278
Paper Abstract:
We present torus modelling for the X-ray spectra of a nearby narrow-line Seyfert 1 galaxy Mrk 1239 (z=0.0199), based on archival Suzaku, NuSTAR, and Swift observations. Our model suggests
very soft intrinsic power-law continuum emission of Γ≈2.57 in 2019 and Γ≈2.98 in 2007. By applying a correction factor to the unabsorbed X-ray luminosity, we find
that Mrk 1239 is accreting near or around the Eddington limit. Our best-fitting spectral model also suggests a torus with a column density of log(NH,ave/cm-2)=25.0±0.2
and a high covering factor of 0.90 in Mrk 1239, indicating that this source is most likely to be viewed almost face-on with inclination of about 26 degrees. Our line of sight might cross
the edge of the torus with NH,los= (2--5)x1023 cm-2. The high Eddington ratio and the high line-of-sight column density makes Mrk 1239 one of the active
galactic nuclei that are close to the limit where wind may form near the edge of the torus due to high radiation pressure.
Paper Access:
ADS |
MNRAS, 505, 702 |
arXiv:2105.02078
Paper Abstract:
In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are
consistent with theoretical expectations for a weakly accreting supermassive black hole of mass approximately 6.5 x 109 MSun. The EHTC also partnered with several
international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign,
as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible
to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active
nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone
models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous gamma-ray
emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the gamma-rays can only be produced in the inner jets
(inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.
Paper Access:
ADS |
ApJL, 911, L11 |
arXiv:2104.06855
Data Access:
EHT Data Products |
CyVerse Data Commons
Media Coverage:
link |
link |
link |
link
Paper Abstract:
We present results from modeling the broadband X-ray spectrum of the Type 1 AGN 2MASX J193013.80+341049.5 using NuSTAR, Swift and archival XMM-Newton observations. We find this source
to be highly X-ray obscured, with column densities exceeding 1023 cm-2 across all epochs of X-ray observations, spanning an 8-year period. However, the source
exhibits prominent broad optical emission lines, consistent with an unobscured Type 1 AGN classification. We fit the X-ray spectra with both phenomenological reflection models and
physically motivated torus models to model the X-ray absorption. We examine the spectral energy distribution of this source and investigate some possible scenarios to explain the mismatch
between X-ray and optical classifications. We compare the ratio of reddening to X-ray absorbing column density (EB-V/NH) and find that 2MASX J193013.80+341049.5
likely has a much lower dust-to-gas ratio relative to the Galactic ISM, suggesting that the Broad Line Region (BLR) itself could provide the source of extra X-ray obscuration, being
composed of low-ionization, dust-free gas.
Paper Access:
ADS |
ApJ, 887, 255 |
arXiv:1911.05820
Paper Abstract:
An unprecedented coordinated campaign for the promotion and dissemination of the first black hole image obtained by the Event Horizon Telescope (EHT) collaboration was prepared in a period
spanning more than six months prior to the publication of this result on 10 April 2019. This article describes this unusual campaign and its outcomes.
Paper Access:
ADS |
CAPJ, 26, 11
Paper Abstract:
We present an analysis of a sample of 69 local obscured Swift/BAT active galactic nuclei (AGN) with X-ray spectra from NuSTAR and infrared (IR) spectral energy distributions from Herschel
and WISE. We combine this X-ray and IR phenomenological modeling and find a significant correlation between reflected hard X-ray emission and IR AGN emission, with suggestive indications
that this correlation may be stronger than the one between intrinsic hard X-ray and IR emissions. This relation between the IR and reflected X-ray emission suggests that both are the result
of the processing of intrinsic emission from the corona and accretion disk by the same structure. We explored the resulting implications on the underlying distribution of covering fraction
for all AGN, by generating mock observables for the reflection parameter and IR luminosity ratio using empirical relations found for the covering fraction with each quantity. We find that
the observed distributions of the reflection parameter and IR-to-X-ray ratio are reproduced with broad distributions centered around covering fractions of at least 40--50 %, while narrower
distributions only match our observations when centered around covering fractions of 70--80 %. Our results are consistent with both independent estimates of the covering fractions of
individual objects and the typical covering fraction obtained based on obscured fractions for samples of AGN. These results suggest that the level of reprocessing in AGN, including X-ray
reflection, is related in a relatively straightforward way to the geometry of the obscuring material.
Paper Access:
ADS |
ApJ, 870, 26 |
arXiv:1811.02570
Paper Abstract:
The NuSTAR observatory, with its high sensitivity in hard X-rays, has enabled detailed broadband modeling of the X-ray spectra of Active Galactic Nuclei (AGN), thereby allowing constraints
to be placed on the high-energy cutoff of the X-ray coronal continuum. We investigate the spectral properties of a sample of 46 NuSTAR-observed Seyfert 1 AGN selected from the Swift/BAT
70-month hard X-ray survey. Our measurements of the high-energy cutoff of the continuum from modeling the NuSTAR X-ray spectra are used to map out the temperature--compactness plane for
AGN coronae. We find that most of the coronae lie clustered near the boundary for runaway pair production, suggesting that annihilation and pair production act to regulate the temperature
of the corona. We discuss the implications of coronae whose high-energy cutoff may indicate a low coronal temperature on the heating and thermalization mechanisms in the corona.
Paper Access:
ADS |
ApJ, 866, 124 |
arXiv:1809.01757
Paper Abstract:
We present the first study of an Iwasawa-Taniguchi ("X-ray Baldwin") effect for Compton-thick active galactic nuclei (AGN). We report a statistically significant anti-correlation between
the rest-frame equivalent width (EW) of the narrow core of the neutral Fe Kα fluorescence emission line, ubiquitously observed in the reflection spectra of obscured AGN, and the
mid-infrared 12 μm continuum luminosity (taken as a proxy for the bolometric AGN luminosity). Our sample consists of 72 Compton-thick AGN selected from pointed and deep-field observations
covering a redshift range of z=0.0014--3.7. We employ a Monte Carlo-based fitting method, which returns a Spearman's Rank correlation coefficient of -0.28±0.12, significant at 98.7% confidence.
The best fit found is log[EW(Fe Kα)] proportional to -0.08±0.04 log(L12 μm), which is consistent with multiple studies of the X-ray Baldwin effect for unobscured and
mildly obscured AGN. This is an unexpected result, as the Fe Kα line is conventionally thought to originate from the same region as the underlying reflection continuum, which together
constitute the reflection spectrum. We discuss the implications this could have if confirmed on larger samples, including a systematic underestimation of the line of sight X-ray obscuring
column density and hence the intrinsic luminosities and growth rates for the most luminous AGN.
Paper Access:
ADS |
MNRAS, 477, 3775 |
arXiv:1804.01100
Paper Abstract:
We present a spectral analysis of the NuSTAR and XMM-Newton observations of the Seyfert 1h galaxy/ULIRG IRAS 05189-2524 taken in 2013. We find evidence for relativistic disk reflection
in the broadband X-ray spectrum: a highly asymmetric broad Fe Kα emission line extending down to 3 keV and a Compton scattering component above 10 keV. Physical modeling with
a self-consistent disk reflection model suggests the accretion disk is viewed at an intermediate angle with a super-solar iron abundance, and a mild constraint can be put on the high-energy
cutoff of the power-law continuum. We test the disk reflection modeling under different absorption scenarios. A rapid black hole spin is favored, however, we cannot place a model-independent
tight constraint on the value. The high reflection fraction (R=2.0--3.2) suggests the coronal illuminating source is compact and close to the black hole (lying within 8.7 gravitational radii above
the central black hole), where light-bending effects are important.
Paper Access:
ADS |
ApJ, 837, 21 |
arXiv:1702.00073
Paper Abstract:
We use new X-ray data obtained with the Nuclear Spectroscopic Telescope Array (NuSTAR), near-infrared (NIR) fluxes and mid-infrared (MIR) spectra of a sample of 24 unobscured type 1
active galactic nuclei (AGN) to study the correlation between various hard X-ray bands between 3 and 80 keV and the infrared (IR) emission. The IR to X-ray correlation spectrum (IRXCS)
shows a maximum at 15--20 μm, coincident with the peak of the AGN contribution to the MIR spectra of the majority of the sample. There is also an NIR correlation peak around 2 μm,
which we associate with the NIR bump observed in some type 1 AGN at 1--5 μm and is likely produced by nuclear hot dust emission. The IRXCS shows practically the same behaviour in
all the X-ray bands considered, indicating a common origin for all of them. We finally evaluated correlations between the X-ray luminosities and various MIR emission lines. All the lines
show a good correlation with the hard X-rays (with Pearson correlation coefficients in excess of 0.7), but we do not find the expected correlation between their ionization potentials and
the strength of the IRXCS.
Paper Access:
ADS |
MNRAS, 469, 110 |
arXiv:1703.09660
Paper Abstract:
Aims. We study the long-term variability of the well-known Seyfert 2 galaxy Mrk 1210 (a.k.a. UGC 4203, or the Phoenix galaxy). Methods. The source was observed by many X-ray
facilities in the last 20 years. Here we present a NuSTAR observation and put the results in context of previously published observations. Results. NuSTAR observed Mrk 1210
in 2012 for 15.4 ks. The source showed Compton-thin obscuration similar to that observed by Chandra, Suzaku, BeppoSAX and XMM-Newton over the past two decades, but different
from the first observation by ASCA in 1995, in which the active nucleus was caught in a low flux state or obscured by Compton-thick matter, with a reflection-dominated spectrum.
Thanks to the high-quality hard X-ray spectrum obtained with NuSTAR and exploiting the long-term spectral coverage spanning 16.9 years, we can precisely disentangle the transmission
and reflection components and put constraints on both the intrinsic long-term variability and hidden nucleus scenarios. In the former case, the distance between the reflector and
the source must be at least ~2 pc, while in the latter one the eclipsing cloud may be identified with a water maser-emitting clump.
Paper Access:
ADS |
A&A, 597, 100 |
arXiv:1609.00374
Paper Abstract:
Narrow-line Seyfert 1 galaxies have been identified by the Fermi Gamma-Ray Space Telescope as a rare class of gamma-ray emitting active galactic nuclei (AGN). The lowest-redshift
candidate among them is the source 1H 0323+342. Here we present quasi-simultaneous Gemini near-infrared and Keck optical spectroscopy for it, from which we derive a black hole mass
based on both the broad Balmer and Paschen emission lines. We supplement these observations with a NuSTAR X-ray spectrum taken about two years earlier, from which we constrain the
black hole mass based on the short timescale spectral variability. Our multiwavelength observations suggest a black hole mass of ~2 x 107 MSun, which agrees well
with previous estimates. We build the spectral energy distribution and show that it is dominated by the thermal and reprocessed emission from the accretion disc rather than the
non-thermal jet component. A detailed spectral fitting with the energy-conserving accretion disc model of Done et al. constrains the Eddington ratio to L/LEdd ~ 0.5
for a (non-rotating) Schwarzschild black hole and to L/LEdd ~ 1 for a Kerr black hole with dimensionless spin of a*=0.8. Higher spin values and so higher Eddington ratios
are excluded, since they would strongly overpredict the observed soft X-ray flux.
Paper Access:
ADS |
MNRAS, 464, 2565 |
arXiv:1609.08002
Paper Abstract:
We study the geometry of the AGN obscurer in IRAS 09104+4109, an IR-luminous, radio-intermediate FR-I source at z = 0.442, using infrared data from Spitzer and Herschel, X-ray data
from NuSTAR, Swift, Suzaku, and Chandra, and an optical spectrum from Palomar. The infrared data imply a total rest-frame 1--1000 μm luminosity of 5.5 x 1046 erg s-1
and require both an AGN torus and starburst model. The AGN torus has an anisotropy-corrected IR luminosity of 4.9 x 1046 erg s-1, and a viewing angle and
half opening angle both of approximately 36 degrees from pole-on. The starburst has a star formation rate of 110±34 Mo yr-1 and an age of <50 Myr.
These results are consistent with two epochs of luminous activity in IRAS 09104+4109: one approximately 150 Myr ago, and one ongoing. The X-ray data suggest a photon index of
Γ≈1.8 and a line-of-sight column of NH≈5 x 1023 cm-2. This argues against a reflection-dominated hard X-ray spectrum, which
would have implied a much higher NH and luminosity. The X-ray and infrared data are consistent with a bolometric AGN luminosity of Lbol~(0.5-2.5) x
1047 erg s-1. The X-ray and infrared data are further consistent with coaligned AGN obscurers in which the line of sight "skims" the torus. This is also
consistent with the optical spectra, which show both coronal iron lines and broad lines in polarized but not direct light. Combining constraints from the X-ray, optical, and
infrared data suggests that the AGN obscurer is within a vertical height of 20 pc, and a radius of 125 pc, of the nucleus.
Paper Access:
ADS |
ApJ, 831, 76 |
arXiv:1606.05649
Paper Abstract:
We present a new metric that uses the spectral curvature (SC) above 10 keV to identify Compton-thick AGN in low-quality Swift BAT X-ray data. Using NuSTAR, we observe nine
high-SC-selected AGN. We find that high-sensitivity spectra show the majority are Compton-thick (78% or 7/9) and the remaining two are nearly Compton-thick (NH~5-8
x 1023 cm-2). We find the SCBAT and SCNuSTAR measurements are consistent, suggesting this technique can be applied to future telescopes.
We tested the SC method on well-known Compton-thick AGN and find it is much more effective than broad band ratios (e.g. 100% using SC vs. 20% using 8--24/3--8 keV band ratio).
Our results suggest that using the >10 keV emission may be the only way to identify this population since only two sources show Compton-thick levels of excess in the OIII to X-ray
emission ratio (FOIII/F2--10 keV>1) and WISE colors do not identify most of them as AGN. Based on this small sample, we find that a higher fraction of
these AGN are in the final merger stage than typical BAT AGN. Additionally, these nine obscured AGN have, on average, four times higher accretion rates than other BAT-detected AGN
(Eddington ratio 0.068, compared to 0.016). The robustness of SC at identifying Compton-thick AGN implies a higher fraction of nearby AGN may be Compton-thick (~22%) and the sum
of black hole growth in Compton-thick AGN (Eddington ratio times population percentage), is nearly as large as mildly obscured and unobscured AGN.
Paper Access:
ADS |
ApJ, 825, 85 |
arXiv:1604.07825
Paper Abstract:
We have discovered heavy obscuration in the dual active galactic nucleus (AGN) in the Swift/Burst Alert Telescope (BAT) source SWIFT J2028.5+2543 using Nuclear Spectroscopic
Telescope Array (NuSTAR). While an early XMM-Newton study suggested the emission was mainly from NGC 6921, the superior spatial resolution of NuSTAR above 10 keV resolves the
Swift/BAT emission into two sources associated with the nearby galaxies MCG +04-48-002 and NGC 6921 (z = 0.014) with a projected separation of 25.3 kpc (91"). NuSTAR's sensitivity
above 10 keV finds both are heavily obscured to Compton-thick levels (NH~1.5 x 1024 cm-2) and contribute equally to the BAT detection
(Lint~6 x 1042 erg s-1 in the 10-50 keV band). The observed luminosity of both sources is severely diminished in the 2-10 keV band
(Lobs<0.1 x Lint), illustrating the importance of >10 keV surveys like those with NuSTAR and Swift/BAT. Compared to archival X-ray data, MCG +04-48-002
shows significant variability (by a factor of >3) between observations. Despite being bright X-ray AGNs, they are difficult to detect using optical emission-line diagnostics
because MCG +04-48-002 is identified as a starburst/composite because of the high rates of star formation from a luminous infrared galaxy while NGC 6921 is only classified as
a LINER using line detection limits. SWIFT J2028.5+2543 is the first dual AGN resolved above 10 keV and is the second most heavily obscured dual AGN discovered to date in the
X-ray band other than NGC 6240.
Paper Access:
ADS |
ApJ, 824, L4 |
arXiv:1708.06762
Paper Abstract:
Aims. Study the connection between the masing disk and obscuring torus in Seyfert 2 galaxies. Methods. We present a uniform X-ray spectral analysis of the high energy
properties of 14 nearby megamaser AGN observed by NuSTAR. We use a simple analytical model to localize the maser disk and understand its connection with the torus
by combining NuSTAR spectral parameters with available physical quantities from VLBI mapping. Results. Most of the sources analyzed are heavily obscured, showing a column
density in excess of 1023 cm-2. In particular, 79% are Compton-thick (NH>1.5 x 1024 cm-2). Using column densities
measured by NuSTAR, with the assumption that the torus is the extension of the maser disk, and further assuming a reasonable density profile, the torus dimensions can be
predicted. They are found to be consistent with mid-IR interferometry parsec-scale observations of Circinus and NGC 1068. In this picture, the maser disk is intimately
connected to the inner part of the torus. It is probably made of a large number of molecular clouds connecting the torus and the outer part of the accretion disk, giving
rise to a thin disk rotating in most cases in Keplerian or sub-Keplerian motion. This toy model explains the established close connection between water megamaser
emission and nuclear obscuration as a geometric effect.
Paper Access:
ADS |
A&A, 589, 59 |
arXiv:1602.03185
Paper Abstract:
NGC 7582 is a well-studied X-ray bright Seyfert 2 with moderately heavy (NH~1023-1024 cm-2), highly variable absorption
and strong reflection spectral features. The spectral shape changed around the year 2000, dropping in observed flux and becoming much more highly absorbed.
Two scenarios have been put forth to explain this spectral change: 1) the central X-ray source partially "shut off" around this time, decreasing in intrinsic luminosity,
with a delayed decrease in reflection features due to the light-crossing time of the Compton-thick material or 2) the source became more heavily obscured, with only
a portion of the power law continuum leaking through. NuSTAR observed NGC 7582 twice in 2012, two weeks apart, in order to quantify the reflection using high-quality data
above 10 keV. We find that the most plausible scenario is that NGC 7582 has recently become more heavily absorbed by a patchy torus with a covering fraction of ~80-90%
and an equatorial column density of ~3x1024 cm-2. We find the need for an additional highly variable full-covering absorber with
NH=4-6x1023 cm-2 in the line of sight, possibly associated with a hidden broad line region.
Paper Access:
ADS |
ApJ, 815, 55 |
arXiv:1511.01951
Paper Abstract:
We present the results from a joint Suzaku/NuSTAR broad-band spectral analysis of 3C 390.3. The high quality data enables us to clearly separate the primary continuum
from the reprocessed components allowing us to detect a high energy spectral cut-off (Ecut=117-14+18 keV), and to place constraints on
the Comptonization parameters of the primary continuum for the first time. The hard over soft compactness is 69-24+124 and the optical depth is
4.1-3.6+0.5, this leads to an electron temperature of 30-8+32 keV. Expanding our study of the Comptonization spectrum to
the optical/UV by studying the simultaneous Swift-UVOT data, we find indications that the compactness of the corona allows only a small fraction of the total UV/optical flux
to be Comptonized. Our analysis of the reprocessed emission show that 3C 390.3 only has a small amount of reflection (R~0.3), and of that the vast majority is from distant
neutral matter. However we also discover a soft X-ray excess in the source, which can be described by a weak ionized reflection component from the inner parts of the
accretion disk. In addition to the backscattered emission, we also detect the highly ionized iron emission lines Fe XXV and Fe XXVI.
Paper Access:
ADS |
ApJ, 814, 24 |
arXiv:1510.01333
Paper Abstract:
We present new Nuclear Spectroscopic Telescope Array (NuSTAR) and Chandra observations of NGC 3393, a galaxy reported to host the smallest separation dual active galactic nuclei (AGN)
resolved in the X-rays. While past results suggested a 150 pc separation dual AGN, three times deeper Chandra imaging, combined with adaptive optics and radio imaging suggest a single,
heavily obscured, radio-bright AGN. Using Very Large Array and Very Long Baseline Array data, we find an AGN with a two-sided jet rather than a dual AGN and that the hard X-ray, UV,
optical, near-infrared, and radio emission are all from a single point source with a radius less than 0.2 arcsec. We find that the previously reported dual AGN is most likely a spurious
detection resulting from the low number of X-ray counts at 6-7 keV and Gaussian smoothing of the data on scales much smaller than the point-spread function (PSF) (0.25 versus 0.80 arcsec
FWHM). We show that statistical noise in a single Chandra PSF generates spurious dual peaks of the same separation and flux ratio (39±9% versus 32% counts) as the purported dual AGN.
With NuSTAR, we measure a Compton-thick source (column density 2.2x±0.4 x1024 cm-2) with a large torus half-opening angle of 79-19+1 deg.
which we postulate results from feedback from strong radio jets. This AGN shows a 2-10 keV intrinsic-to-observed flux ratio of 150. Using simulations, we find that even the deepest
Chandra observations would severely underestimate the intrinsic luminosity of NGC 3393 above redshift og 0.2, but would detect an unobscured AGN of this luminosity out to high redshift.
Paper Access:
ADS |
ApJ, 807, 149 |
arXiv:1505.03524
Paper Abstract:
The covering factor of Compton-thick (CT) obscuring material associated with the torus in active galactic nuclei (AGN) is at present best understood through the fraction
of sources exhibiting CT absorption along the line of sight (NH1.5x1024 cm-2) in the X-ray band, which reveals the average covering factor.
Determining this CT fraction is difficult, however, due to the extreme obscuration. With its spectral coverage at hard X-rays (>10 keV), Nuclear Spectroscopic Telescope Array (NuSTAR)
is sensitive to the AGN covering factor since Compton scattering of X-rays off optically thick material dominates at these energies. We present a spectral analysis of 10 AGN
observed with NuSTAR where the obscuring medium is optically thick to Compton scattering, so-called CT AGNs. We use the torus models of Brightman & Nandra (2011) that predict
the X-ray spectrum from reprocessing in a torus and include the torus opening angle as a free parameter and aim to determine the covering factor of the CT gas in these sources
individually. Across the sample we find mild to heavy CT columns, with NH measured from 1024 to 1026 cm-2, and a wide range of
covering factors, where individual measurements range from 0.2 to 0.9. We find that the covering factor, fc, is a strongly decreasing function of the intrinsic
2-10 keV luminosity, LX, where fc=(-0.41±0.13) log10(LX/erg s-1)+18.31±5.33, across more than two orders of magnitude in
LX (1041.5-1044 erg s-1). The covering factors measured here agree well with the obscured fraction as a function of LX
as determined by studies of local AGNs with LX > 1042.5 erg s-1.
Paper Access:
ADS |
ApJ, 805, 41 |
arXiv:1502.07353
Paper Abstract:
NuSTAR observed the bright Compton-thin, narrow-line Seyfert 1 galaxy, NGC 5506, for about 56 ks. In agreement with past observations, the spectrum is well fitted
by a power law with Γ~1.9, a distant reflection component and narrow ionized iron lines. A relativistically blurred reflection component is not required by
the data. When an exponential high-energy cutoff is added to the power law, a value of 720-190+130 keV (90 per cent confidence level) is found.
Even allowing for systematic uncertainties, we find a 3σ lower limit to the high-energy cutoff of 350 keV, the highest lower limit to the cutoff energy found
so far in an AGN by NuSTAR.
Paper Access:
ADS |
MNRAS, 447, 3029 |
arXiv:1412.4541
Paper Abstract:
B2 1023+25 is an extremely radio-loud quasar at z=5.3 that was first identified as a likely high-redshift blazar candidate in the SDSS+FIRST quasar catalog.
Here, we use the Nuclear Spectroscopic Telescope Array (NuSTAR) to investigate its non-thermal jet emission, whose high-energy component we detected in the
hard X-ray energy band. The X-ray flux is ~5.5x10-14 erg cm-2 s-1 (5--10 keV) and the photon spectral index is ΓX=1.3--1.6.
Modeling the full spectral energy distribution, we find that the jet is oriented close to the line of sight, with a viewing angle of ~3o, and has
significant Doppler boosting, with a large bulk Lorentz factor ~13, which confirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the first object
at redshift larger than 5 detected by NuSTAR, demonstrating the ability of NuSTAR to investigate the early X-ray universe and to study extremely active supermassive
black holes located at very high redshift.
Paper Access:
ADS |
ApJ, 777, 147 |
arXiv:1309.3280
Papers listed here are organized by topic or collaboration. Overall chronological ordering is available at the ADS link above.
----- Non-blazar AGN Studies, Informal Collaborations:
Paper Access:
ADS |
arXiv:2302.04425
Paper Access:
ADS |
ApJ, 943, 162 |
arXiv:2210.08030
Paper Access:
ADS |
A&A, 669, 114 |
arXiv:2210.15376
Paper Access:
ADS |
ApJ, 939, L13 |
arXiv:2210.08401
Paper Access:
ADS |
ApJ, 927, 42 |
arXiv:2202.00895
Paper Access:
ADS |
ApJ, 922, 259 |
arXiv:2109.00572
Paper Access:
ADS |
ApJ, 922, 252 |
arXiv:2109.00599
Paper Access:
ADS |
A&A, 651, 588 |
arXiv:2106.08331
Paper Access:
ADS |
MNRAS, 497, 229 |
arXiv:2006.13583
Paper Access:
ADS |
ApJ, 900, 164 |
arXiv:2008.02175
Paper Access:
ADS |
ApJ, 894, 71 |
arXiv:2003.12190
Paper Access:
ADS |
ApJ, 872, 8 |
arXiv:1812.09217
Paper Access:
ADS |
ApJ, 871, 88 |
arXiv:1812.03194
Paper Access:
ADS |
Nat. Astro., 3, 242 |
arXiv:1901.03731
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 867, 110 |
arXiv:1805.12140
Media Coverage:
link |
link
Paper Access:
ADS |
ApJ, 864, 27 |
arXiv:1805.06920
Paper Access:
ADS |
MNRAS, 474, 2444 |
arXiv:1711.06270
Paper Access:
ADS |
ApJ, 843, 89 |
arXiv:1705.09260
Paper Access:
ADS |
MNRAS, 456, L94 |
arXiv:1511.03503
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 789, 56 |
arXiv:1405.3674
----- BAT AGN Spectroscopic Survey (BASS):
Paper Access:
ADS |
ApJ, 940, 77 |
arXiv:2209.09929
Paper Access:
ADS |
ApJ, 938, 87 |
arXiv:2208.03880
Paper Access:
ADS |
ApJS, 261, 8 |
arXiv:2111.13720
Paper Access:
ADS |
ApJS, 261, 2 |
arXiv:2207.12432
Summary and Popular Article:
link |
link
Paper Access:
ADS |
ApJS, 261, 1 |
arXiv:2207.12428
Summary and Popular Article:
link |
link
Paper Access:
ADS |
ApJS, 233, 17 |
arXiv:1709.03989
Data Access:
link
Paper Access:
ADS |
ApJ, 850, 74 |
arXiv:1707.08123
Data Access:
link
Paper Access:
ADS |
MNRAS, 470, 800 |
arXiv:1705.01550
Paper Access:
ADS |
MNRAS, 454, 3622 |
arXiv:1509.05425
----- NuSTAR AGN Science Groups, Excluding Blazars:
Paper Access:
ADS |
ApJ, 846, 20 |
arXiv:1707.06651
Paper Access:
ADS |
ApJ, 844, 10 |
arXiv:1705.10804
Paper Access:
ADS |
ApJ, 836, 99 |
arXiv:1612.06389
Data Access:
link
Paper Access:
ADS |
ApJ, 831, 185 |
arXiv:1511.04183
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 826, 93 |
arXiv:1606.09265
Paper Access:
ADS |
ApJ, 819, 150 |
arXiv:1511.01915
Paper Access:
ADS |
A&A, 585, 157 |
arXiv:1510.04477
Paper Access:
ADS |
ApJ, 815, 66 |
arXiv:1511.04184
Paper Access:
ADS |
ApJ, 815, 36 |
arXiv:1509.03322
Paper Access:
ADS |
ApJ, 812, 116 |
arXiv:1411.0670
Paper Access:
ADS |
ApJ, 809, 115 |
arXiv:1506.05120
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 808, 184 |
arXiv:1511.04186
Paper Access:
ADS |
MNRAS, 447, 160 |
arXiv:1411.5151
Paper Access:
ADS |
ApJ, 794, 102 |
arXiv:1403.3078
Paper Access:
ADS |
ApJ, 794, 62 |
arXiv:1408.5281
Paper Access:
ADS |
ApJ, 792, 117 |
erratum: ApJ, 794, 176 |
arXiv:1407.1844
Paper Access:
ADS |
MNRAS, 440, 2347 |
arXiv:1412.4541
Paper Access:
ADS |
ApJ, 786, 16 |
arXiv:1403.2491
Paper Access:
ADS |
ApJ, 785, 17 |
arXiv:1402.2666
Paper Access:
ADS |
ApJ, 770, 103 |
arXiv:1301.7307
----- Blazars and Jets, Excluding EHT and RoboPol Collaborations:
Paper Access:
ADS |
arXiv:2210.02547
Paper Access:
ADS |
A&A, 655, 89 |
arXiv:2106.05516
Data Access:
link
Paper Access:
ADS |
A&A, 639, 42 |
arXiv:1912.07273
Paper Access:
ADS |
ApJS, 248, 29 |
arXiv:2001.08678
Paper Access:
ADS |
MNRAS, 487, 181 |
arXiv:1904.12863
Media Coverage:
link
Paper Access:
ADS |
ApJ, 839, 96 |
arXiv:1703.10657
Paper Access:
ADS |
ApJ, 834, 2 |
arXiv:1611.04626
Paper Access:
ADS |
ApJ, 831, 142 |
arXiv:1609.02203
Paper Access:
ADS |
ApJ, 826, 76 |
arXiv:1602.06446
Paper Access:
ADS |
ApJ, 816, 53 | (not on arXiv)
Paper Access:
ADS |
MNRAS, 456, 171 |
arXiv:1511.02654
Paper Access:
ADS |
ApJ, 815, L23 |
arXiv:1512.04435
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 807, 167 |
arXiv:1503.04848
Paper Access:
ADS |
ApJ, 807, 79 |
arXiv:1502.04699
Paper Access:
ADS |
MNRAS, 448, 3121 |
arXiv:1501.07407
----- Event Horizon Telescope (EHT) Collaboration:
Paper Access:
ADS |
ApJ, 943, 170 |
arXiv:2302.04622
Paper Access:
ADS |
ApJ, 934, 145 |
arXiv:2208.01662
Paper Access:
ADS |
ApJ, 930, L19 |
arXiv:2207.06829
Summary, Press Release, Media:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 925, 13 |
arXiv:2111.01317
Paper Access:
ADS |
Nat. Astro., 5, 1017 |
arXiv:2111.03356
Data Access:
link
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
Phys. Rev. D, 103, 104047 |
arXiv:2105.09343
Paper Access:
ADS |
ApJ, 912, 35 |
arXiv:2105.01804
Paper Access:
ADS |
ApJ, 910, L14 |
arXiv:2105.02272
Paper Access:
ADS |
ApJ, 910, L13 |
arXiv:2105.01173
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 910, L12 |
arXiv:2105.01169
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
Phys. Rev. Lett., 125, 141104 |
arXiv:2010.01055
Paper Access:
ADS |
ApJ, 901, 67 |
arXiv:2009.11842
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 897, 148
Paper Access:
ADS |
ApJ, 897, 139
Paper Access:
ADS |
A&A, 636, 5 |
arXiv:2004.01161
Paper Access:
ADS |
ApJS, 243, 26 |
arXiv:1904.04923
Paper Access:
ADS |
ApJ, 875, L6 |
arXiv:1906.11243
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 875, L5 |
arXiv:1906.11242
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 875, L4 |
arXiv:1906.11241
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 875, L3 |
arXiv:1906.11240
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 875, L2 |
arXiv:1906.11239
Media Coverage:
link |
link |
link |
link
Paper Access:
ADS |
ApJ, 875, L1 |
arXiv:1906.11238
Media Coverage:
link |
link |
link |
link
----- RoboPol Collaboration:
Paper Access:
ADS |
MNRAS, 474, 1296 |
arXiv:1710.08922
Paper Access:
ADS |
MNRAS, 463, 3365 |
arXiv:1609.00640
Paper Access:
ADS |
MNRAS, 462, 1775 |
arXiv:1607.04292
Paper Access:
ADS |
MNRAS, 457, 2252 |
arXiv:1601.03392
Paper Access:
ADS |
MNRAS, 452, 1669 |
arXiv:1505.07467
Paper Access:
ADS |
MNRAS, 452, 715 |
erratum: MNRAS, 462, 2011 |
arXiv:1503.03054
Paper Access:
ADS |
MNRAS, 445, L114 |
arXiv:1409.2417
Paper Access:
ADS |
MNRAS, 442, 1706 |
arXiv:1310.7555
Paper Access:
ADS |
MNRAS, 442, 1693 |
arXiv:1311.3304
----- Other Surveys and Catalogs:
Paper Access:
ADS |
A&A, 602, 1 |
arXiv:1703.09713
Paper Access:
ADS |
ApJS, 227, 11 |
arXiv:1608.03279
Data Access:
link
Paper Access:
ADS |
ApJ, 783, 122 |
arXiv:1402.0661
Paper Access:
ADS |
Proc. of 37th ICRC, id. 834
Paper Access:
ADS |
RNAAS, 3, 173 |
PDF (local)
Paper Access:
ADS |
Proc. of 36th ICRC, id. 624
Paper Access:
ADS |
BAAS, 51g, 468 |
arXiv:1905.11439
Paper Access:
ADS |
BAAS, 51c, 468 |
arXiv:1905.11439
Paper Access:
ADS |
BAAS, 51c, 126 |
arXiv:1903.05241
Paper Access:
ADS |
arXiv:1903.03035
Paper Access:
ADS |
Caltech Library
Paper Access:
ADS |
EPJWC, Vol. 61, #04013 |
arXiv:1309.4494
Paper Access:
ADS |
ATel #5107
Paper Access:
ADS |
ATel #4947
Paper Access:
ADS |
ATel #4861
Paper Access:
CARMA Memo. #55 |
PDF (local)
Paper Access:
ADS |
15th YSC Proc. T.o.C. |
PDF (local)
Mislav Baloković -- last updated 2023 Mar 08