Astronomy
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Item Spectral Dependent Degradation of the Solar Diffuser on Suomi-NPP VIIRS Due to Surface Roughness-Induced Rayleigh Scattering(MDPI, 2016-06-17) Shao, Xi; Cao, Changyong; Liu, Tung-ChangThe Visible Infrared Imaging Radiometer Suite (VIIRS) onboard Suomi National Polar Orbiting Partnership (SNPP) uses a solar diffuser (SD) as its radiometric calibrator for the reflective solar band calibration. The SD is made of Spectralon™ (one type of fluoropolymer) and was chosen because of its controlled reflectance in the Visible/Near-Infrared/Shortwave-Infrared region and its near-Lambertian reflectance property. On-orbit changes in VIIRS SD reflectance as monitored by the Solar Diffuser Stability Monitor showed faster degradation of SD reflectance for 0.4 to 0.6 µm channels than the longer wavelength channels. Analysis of VIIRS SD reflectance data show that the spectral dependent degradation of SD reflectance in short wavelength can be explained with a SD Surface Roughness (length scale << wavelength) based Rayleigh Scattering (SRRS) model due to exposure to solar UV radiation and energetic particles. The characteristic length parameter of the SD surface roughness is derived from the long term reflectance data of the VIIRS SD and it changes at approximately the tens of nanometers level over the operational period of VIIRS. This estimated roughness length scale is consistent with the experimental result from radiation exposure of a fluoropolymer sample and validates the applicability of the Rayleigh scattering-based model. The model is also applicable to explaining the spectral dependent degradation of the SDs on other satellites. This novel approach allows us to better understand the physical processes of the SD degradation, and is complementary to previous mathematics based models.Item Unveiling the Origin of the Fermi Bubbles(MDPI, 2018-02-28) Yang, H.-Y. Karen; Ruszkowski, Mateusz; Zweibel, Ellen G.The Fermi bubbles, two giant structures above and below the Galactic center (GC), are among the most important discoveries of the Fermi Gamma-ray Space Telescope. Studying their physical origin has been providing valuable insights into cosmic-ray transport, the Galactic magnetic field, and past activity at the GC in the Milky Way galaxy. Despite their importance, the formation mechanism of the bubbles is still elusive. Over the past few years, there have been numerous efforts, both observational and theoretical, to uncover the nature of the bubbles. In this article, we present an overview of the current status of our understanding of the bubbles’ origin, and discuss possible future directions that will help to distinguish different scenarios of bubble formation.Item Astrophotonic Spectrographs(MDPI, 2019-01-15) Gatkine, Pradip; Veilleux, Sylvain; Dagenais, MarioAstrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs.Item Questions Related to the Equation of State of High-Density Matter(MDPI, 2019-04-30) Miller, M. ColemanAstronomical data about neutron stars can be combined with laboratory nuclear data to give us a strong base from which to infer the equation of state of cold catalyzed matter beyond nuclear density. However, the nuclear and astrophysical communities are largely distinct; each has their own methods, which means that there is often imperfect communication between the communities regarding caveats about claimed measurements and constraints. Here we present a brief summary from one astronomer’s perspective of relevant observations of neutron stars, with warnings as appropriate, followed by a set of questions that are intended to help enhance the dialog between nuclear physicists and astrophysicists.Item ABOVE THE CLOUDS: 1-D MODELING OF OBSERVATIONS OF TIDALLY LOCKED EXTRASOLAR WORLDS(2019) Afrin Badhan, Mahmuda; Deming, L. Drake; Domagal-Goldman, Shawn D.; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Unique and exotic planetary environments give us an opportunity to understand how planetary systems form and evolve over their lifetime, by placing our planetary system in the context of vastly different extrasolar systems. With orbital separations a fraction of the Mercury-Sun distance, these close-in planets provide us with valuable insights regarding interactions between the stellar and planetary atmospheres. Further, observational biases actually allow such planets to be the first to be observed via transit spectroscopy. Observed spectrophotometric signatures from transit measurements can reveal spectrally active species in a planet’s atmosphere. Present observational technologies can also shed light on the atmosphere’s structure and dynamics. Future missions will allow us to constrain these properties with unprecedented accuracy, and are also being designed to observe increasingly smaller, cooler and less extreme planets. The eventual goal, after all, is to identify a world like our own. To interpret the observations with any certainty, however, we must build robust atmospheric models that sufficiently factor both physical and chemical processes expected in those atmospheres. 3-D climate modeling has shown that tidally-locked Earth-like planets, at the inner edge of M dwarf habitable zones, may retain water-vapor-rich stratospheres. However, flaring M dwarfs have strong UV activity, which may photodisassociate H2O. Using synthetical stellar UV within a 1-D photochemical model, I assess whether water vapor loss driven by high stellar UV would affect its detectability in JWST/MIRI transmission spectroscopy. I pseudo-couple a 3-D climate model to our 1-D model to achieve this. In a follow-up study, I also compute 125 additional atmospheric states by varying the Earth-like planet’s orbital distance (thus moistness) and methane production rates. I check for and quantify the simultaneous presence of detectable ozone and methane in an otherwise abiotic anoxic atmosphere. I have also implemented techniques to robustly quantify atmospheric properties of hot Jupiters from data-driven retrievals and built a versatile template for hot Jupiter atmospheres within our 1-D photochemical modeling tool, which was previously only valid for cool rocky worlds. I sketch out a plan for using this work towards mapping non-equilibrated (non-LTE) emissions from methane in the upper atmospheres of observable giants.Item Terrestrial Scanning LiDAR of Kilbourne Hole Maar, Potrillo Volcanic Field, New Mexico(2019-10-23) Whelley, Patrick; Enriquez, Frankie; Richardson, Jacob; Hurtado, José; Young, Kelsey; Bleacher, JacobArchived are point clouds collected using the Goddard Instrument Filed Team's Riegl VZ-400, a Terrestrial Scanning LiDAR.Item Shocks and Cold Fronts in Galaxy Clusters --- Probing the Microphysics of the Intracluster Medium(2018) Wang, Qian; Mushotzky, Richard; Markevitch, Maxim; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Shocks and cold fronts in galaxy clusters, the largest gravitationally bound systems in the universe, are astrophysical laboratories where we can study the microphysics of the intracluster medium (ICM), a very hot ($T\sim10^7$--$10^8$~K) plasma. Being the main baryon content of galaxy clusters, the ICM plays an important role in mediating the energy cascade from gravitational collapse during cosmological structure formation. It is also intricately linked to the evolution of the galaxies within. The scientific enquiries concerning the ICM range from fundamental physics questions to cosmological measurements. In this dissertation, I demonstrate probing ICM microphysics by studying deep X-ray observations of two galaxy clusters, A520 and A2142. For A520, tests for thermal conduction, electron--ion equilibration timescale, and particle acceleration at the shock were carried out. For A2142, a test for the effective viscosity was performed using two apparent Kelvin-Helmholtz eddies along its southern cold front. Other interesting features were discovered and analyzed, such as a low gas fraction subcluster in the A520 outskirts, and X-ray deficient channels that could be plasma depletion sheets in both clusters.Item PIC simulation data for "Magnetic reconnection in a quasi-parallel shock: two-dimensional local particle-in-cell simulation"(2019) Bessho, Naoki; Chen, Li-Jen; Wang, Shan; Hesse, Michael; Wilson, Lynn, III2-dimensional particle-in-cell simulation data for a paper "Fully kinetic simulations of magnetic reconnection in the Earth’s quasi-parallel bow shock" Data for fig.1(a): by1250-fig1a.txt, by1563-fig1a.txt, by1875-fig1a.txt, format: x/d_i, B_y/B_0. For fig.1(b)(c)(d): az1250.txt, az1563.txt, az1875.txt, format: x/d_i, y/d_i, A_z/(B_0d_i). For fig1(e)(f)(g)(h)(i)(j): ex1875.txt, cuz1875.txt, bz1875.txt, ne1875.txt, vxe1875.txt, vye1875.txt, format: x/d_i, y/d_i, Q, where Q is each normalized field quantity (E_x, J_z, B_z, N_e, V_ex, V_ey). Normalization is given in the figure. Data for fig.2(a) for J_z, fig.2(b) for B_z, fig.2(d) for V_ex, fig.2(e) for V_ey are the same as given in fig1. For fig.2(c): cde1875.txt. For fig.2(g)(h): vxi1875.txt, vyi1875.txt. For contour of A_z for each plot, use az1875.txt. For vector plot of fluid velocities, use vxe1875.txt, vye1875.txt, vxi1875.txt, vyi1875.txt. For fig.2(f)(i): az1888.txt, az1900.txt. Format is the same as other 2-D plots. Data for fig.3(a), use ne1875.txt. For fig.3(b): ez1875.txt. For vector plot of fluid velocities, use vxe1875.txt, vye1875.txt. For vector plot of E-field: exav1875.txt, eyav1875.txt. Format is the same as other 2-D plots. For contour of A_z for each plot, use az1875.txt. For fig.3(c)(d)(e): bjc#.txt, ejc#.txt, cujc#.txt, vjec#.txt, vjic#.txt, nec#.txt, cdec#.txt, where the letter j represents a component of the field (x, y, z), and # represents a number (1, 2, or 3), format: y/d_i, Q. For fig.3(f)(g)(h)(i): fj-fig3-?#.txt, where the letter j represents e (electron) or i (ion), ? represents a figure alphabet (f, g, h, or i), and # represents a number 1 (left panel) or 2 (right panel), format: v_k/v_A, v_l/v_A, count, where v_k is the x axis of the velocity and v_l is the y axis of the velocity. Data for fig.4(a): vyi1797.txt, az1797.txt, the same format as other 2-D plots. For fig.4(b), use vyi1875.txt and az1875.txt. For fig.4(c)(d), use vyi1875.txt, vye1875.txt, for A_z contour az1875.txt, and for velocity vectors, also use vxi1875.txt and vxe1875.txt. For fig.4(e), viin-x1875.txt, for velocity vector: vxi-x1875.txt, vyi-x1875.txt, and use az1875.txt for A_z contour. For fig.4(f), vein-x1875.txt, for velocity vector: vxe-x1875.txt, vye-x1875.txt, and use az1875.txt for A_z contour. For fig.4(g)(h): bjc#.txt, ejc#.txt, cujc#.txt, vjec#.txt, vjic#.txt, nec#.txt, cdec#.txt, where the letter j represents a component of the field (x, y, z), and # represents a number (4 or 5), format: x/d_i, Q. For fig.4(i), viinc4.txt, viinc5.txt, format x/d_i, |v_i-xy|/v_A. For fig.4(j), veinc4.txt, veinc5.txt, format x/d_i, |v_e-xy|/v_A. For fig4.(k): fj-fig4-k#.txt, where the letter j represents e or i, and # represents a number 1 (top panel), 2 (middle panel), or 3 (bottom panel), format: v_k/v_A, v_l/v_A, count, where v_k is the x axis of the velocity and v_l is the y axis of the velocity.Item The Search for Supermassive Black Hole Binaries in the Time Domain(2018) Liu, Tingting; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Supermassive black hole binaries (SMBHBs) are expected due to galaxy mergers and the ubiquity of central supermassive black holes (SMBHs) in galaxies, but direct evidence for close-separation SMBHBs has been elusive. This thesis presents my search for SMBHBs in the optical time domain, {\it i.e.} by searching for their optical variability signatures. It is a novel approach that can potentially yield SMBHBs in close, sub-pc orbits, a population of SMBH pairs or binaries that can not be directly imaged or resolved by current telescopes or techniques. Further, searches in the optical time domain are sensitive to SMBHBs in the low-frequency gravitational wave-emitting regime, opening up the possibility of studying them in the era of multi-messenger astronomy. I developed a custom pipeline to systematically search in the Pan-STARRS1 Medium Deep Survey (PS1 MDS) for periodically varying quasars, which have been predicted as the manifestations of SMBHBs at close separations. I constructed a spectroscopically-complete sample of SMBHB candidates using observations with the Gemini Telescope or the Discovery Channel Telescope and measured their black hole masses and redshifts. I also followed up the candidates with a dedicated monitoring program on the Las Cumbres Observatory telescopes, in order to put their periodicity to the test and identify false positives that are due to the stochastic variability of regular quasars that do not host SMBHBs. I set up the expectations for a true periodic signal by modeling normal quasar variability and showed that evidence for a true signal should strengthen over a longer temporal baseline. I then used the expectations as a guide and applied a range of statistical criteria to select more robust candidates from PS1 MDS. From this down-selected sample, I was able to determine an upper limit on the SMBHB rate. I also discussed the search for SMBHBs in the era of the Large Synoptic Survey Telescope and SMBHB candidates as possible gravitational wave sources for the pulsar timing arrays.Item SIFTING FOR SAPPHIRES IN THE TRANSIENT SKY: THE SEARCH FOR TIDAL DISRUPTION EVENTS IN THE OPTICAL TIME DOMAIN(2018) HUNG, TZU-YU; Gezari, Suvi; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tidal disruption events (TDEs) refer to the scenario where a star passes within the tidal disruption sphere of a supermassive black hole (SMBH) and becomes torn apart by tidal stresses. In the classical picture, a thermal flare is expected once the bound stellar debris circularize to form an accretion disk that feeds onto the black hole. This flare of radiation provides a unique window to study the demographics of black holes within distant and quiescent galaxies that cannot be probed by other means. In addition, TDEs serve as a powerful probe of the accretion process, where the mass fallback rate can be super-Eddington for \Mbh{} $<$ a few $\times$ 10$^7$ \Msun{}. In recent years, ground-based wide-field optical surveys have successfully detected about a dozen of TDEs. Yet our knowledge of these events is still limited due to their low occurrence rate ($\approx$ 10$^{-4}$--10$^{-5}$ gal$^{-1}$ yr$^{-1}$). In the first part of this thesis, we present results from a systematic selection of TDEs in the Intermediate Palomar Transient Factory (iPTF). Our selection targets typical optically-selected TDEs: blue transients ($g-r$ $<$ 0 mag) residing in the center of resolved red galaxies that are absent of previous nuclear activity. Our photometric selection has led to discoveries of two TDEs in $\sim$4 months, iPTF16axa and iPTF16fnl, in 2016. With the most stringent criteria, we significantly reduced the contamination rate from SN Ia and AGN from 200:1 to 4.5:1. We derived a TDE rate of 1.7$^{+2.9}_{-1.3}$ $\times$ 10$^{-4}$ gal$^{-1}$ yr$^{-1}$ and forecast a discovery rate of 32$^{+41}_{-25}$ TDEs per year for ZTF. The second part of this thesis features a detailed analysis of the photometric and spectroscopic observations on iPTF16axa. We compared iPTF16axa with 11 other TDEs in the literature with well-sampled optical light curves. We concluded that most of these TDE candidates have peak luminosities confined between log(L [erg s$^{-1}$]) = 43.4--44.4, with constant temperatures of a few $\times$ 10$^4$ K during their power-law declines, implying blackbody radii on the order of ten times the tidal disruption radius, that decrease monotonically with time. For TDE candidates with hydrogen and helium emission, the high helium-to-hydrogen ratios suggest that the emission arises from high-density gas, where nebular arguments break down. In the last part of this thesis, I present statistical analyses on the Zwicky Transient Facility (ZTF) data and comments on the TDE rate from the first few months of the survey. Finally, I close this chapter with an analysis on the optical spectra of the first ZTF TDE -- AT2018zr.