Welcome! This page has the list of talks given by the current and past Center for AstroPhysical Surveys (CAPS) fellows (both postdocs and graduate students).
New: CAPS graduate fellowship applications are now open. Please visit this link for more details. Deadline for application: Friday, June 2, 2023.
Nearly two decades of reverberation mapping (RM) studies on nearby AGN have revealed a tight correlation between the size of the Hbeta broad-line region (BLR) and the optical luminosity of the AGN, known the R-L relation. However, recent RM measurements have revealed a systematic offset from the canonical R-L relation. In this talk, I will present my current work on spectral analysis and theoretic photoionization modeling to understand the observed deviations from the R-L relation. This work would provide insight into the AGN accretion activity and photoionization models of the BLR.
All quasars show a common stochastic variability, seen across various observed wavelengths and timescales. The origin of this variability is still uncertain, though variability in the optical is thought to stem from processes in the accretion disk and other structures close to the supermassive black hole (SMBH). Time-series and spectral variability analysis present unique ways to probe a quasar’s geometry and dynamics, including the mass of the SMBH. Much of the analysis used for measuring these structures involves assumed models of radiation and kinematics, which necessitate testing through high-quality, high-cadence data. We present results from modeling quasar optical variability, using high-quality, 20-year-long light curve data, and how it informs our view of the innermost structures around SMBHs. In particular, we find evidence of a characteristic timescale near days long in the optical, and slow, inward-moving propagations in the quasar accretion disk. We mention the future of variability studies, involving high-cadence surveys such as LSST, and multi-epoch spectroscopy campaigns, as well as the finely resolved reverberation mapping data that can be produced.
We perform a rigorous cosmology analysis on simulated type Ia supernovae (SN~Ia) and evaluate the improvement from including photometric host-galaxy redshifts compared to using only the “zspec” subset with spectroscopic redshifts from the host or SN. We use the Deep Drilling Fields (~50 deg^2) from the Photometric LSST Astronomical Time-Series Classification Challenge (PLaSTiCC), in combination with a low-z sample based on Data Challenge2 (DC2). The analysis includes light curve fitting to standardize the SN brightness, a high-statistics simulation to obtain a bias-corrected Hubble diagram, a statistical+systematics covariance matrix including calibration and photo-z uncertainties, and cosmology fitting with a prior from the cosmic microwave background. Compared to using the zspec subset, including events with SN+host photo-z results in i) more precise distances for z>0.5, ii) a Hubble diagram that extends 0.3 further in redshift, and iii) a 50 % increase in the Dark Energy Task Force figure of merit (FoM) based on the w0-wa CDM model. Analyzing 25 simulated data samples, the average bias on w0 and wa is consistent with zero. The host photo-z systematic of 0.01 reduces FoM by only 2 % because i) most z<0.5 events are in the zspec subset, ii) the combined SN+host photo-z has X 2 smaller bias, and iii) the anti-correlation between fitted redshift and color self corrects distance errors. To prepare for analysing real data, the next SNIa-cosmology analysis with photo-z’s should include non SN-Ia contamination and host galaxy mis-associations.
The physics of a magnetically confined, radiation pressure supported column of plasma plays a defining role in understanding the observations of accretion-powered X-ray pulsars, including pulsating ultraluminous X-ray sources (ULXs). Near the neutron star accretor, the accretion flow is constrained by the strong magnetic field to fall along the magnetic field lines. At a sufficiently high accretion rate, the inflow is shocked above the stellar surface and forms a columnar structure below, radiating most of accretion power via sideways emission in a so-called ‘fan-beam’ pattern. The misalignment of the anisotropic radiation emission with respect to the neutron star spin axis results in the observed pulsations. We perform radiative relativistic MHD simulations to study the nonlinear dynamics of the accretion column. The column structure is extremely dynamical and exhibits kHz quasi-periodic oscillations. The existence of the photon bubble instability is identified in simulated accretion columns but proved to be not responsible for triggering the oscillatory behaviors. Instead, the oscillations originate from the inability of the system to resupply heat to locally balance the sideways cooling. The column structure is very sensitive to the shock geometry, which directly determines the cooling efficiency. The time-averaged column structures from the simulations can be approximately reproduced by a 1D stationary model provided one corrects for the actual 2D shape of the time-averaged column. I will also discuss how reduction of scattering opacity by the magnetic field can alter the column structure and variability.
The latest South Pole Telescope survey, SPT-3G, is halfway through a 1500 square degree survey of the southern sky. The result is some of the richest data at millimeter (mm) wavelengths, suitable for large-scale science including cosmic microwave background (CMB) studies as well as small-scale discoveries of galaxies, galaxy clusters, and transient objects. I will present the preliminary point source catalog from three years of winter observing, consisting of over 22,000 significant emissive detections. This catalog already includes 4.5 times as many sources as the previous SPT catalog, and we are able to detect sources 3 times fainter at the central frequency than the previous survey. I will show the first results of cross-matching the 3G catalog to external catalogs, characterizing spectral types, and identifying significant sources with no counterparts as we begin to extract worthy sources for follow-up.
TEMPLATES is a James Webb Space Telescope (JWST) Early Release Science (ERS) program designed to produce high signal-to-noise imaging and Integral Field Unit (IFU) spectroscopic data cubes for four gravitationally lensed galaxies at high redshift (1<z<4.5). The program plan is to spatially resolve the star formation in galaxies across the peak of cosmic star formation. TEMPLATES was awarded ~55 hours to comprehensively study 4 high redshift galaxies out of which ~40 hours have already been completed. At the completion of the program all four target galaxies will have 13 band imaging as well as IFU data covering nebular emission lines such as H-alpha and Paschen-alpha. In this talk, I’ll showcase the new data and discuss how we are tackling some instrumental artifacts. I’ll also give a brief overview of the first science results expected from the program.
Optical variability is a key probe of the AGN population that will be instrumental for studies of AGN demographics with LSST Rubin. We have developed a phenomenological forward model that generates a mock AGN and host galaxy population and simulates light curves given the survey specifications as input. In this talk, I will show how our model can be used to constrain the local black hole mass function (BHMF) using optical variability with LSST Rubin. The ultimate goal is that this relic BHMF, when probed at low masses, will reveal the signature of how supermassive black holes were seeded at high redshifts.
We present the Young Supernova Experiment (YSE) first data release, spanning discoveries from November 24th, 2019 to December 20, 2021. YSE is an active, three year optical time-domain survey on the Pan-STARRS1 and Pan-STARRS2 telescopes, designed to capture young, fast-rising supernovae (SNe) within a few hours to days of explosion. This YSE DR1 includes light curves and metadata for 2008 supernova-like sources, of which 441 transients are spectroscopically-classified. We then uniquely use realistic, multi-survey SNe simulations from YSE and Zwicky Transient Facility (ZTF) data to train the ParSNIP classifier for photometric classification tasks; when validating on spectroscopically-classified YSE SNe, we achieve 82% accuracy across three SN classes (SN Ia, SN II, SN Ibc) and 90% accuracy across two SN classes (SN Ia, CC SNe), with high individual completeness and purity of SN Ia. We then use our classifier to characterize our spectroscopically unclassified sample of 1567 YSE SNe, predicting ~66% SN Ia, ~34% CC SNe. We find that realistic simulations are now sufficient to exclusively train current photometric classification methods without compromising performance on real data. Though, our classifiers have particular difficulty in characterizing transients near the cores of galaxies or exhibit rare photometric or spectral features. In preparation for the forthcoming Rubin Observatory era, griz data sets such as the one presented here will be an important component of building classification and discovery algorithms for transient discovery.
