Changsu Choi

The Sloan Digital Sky Survey - II:supernova survey: technical summary

The Sloan Digital Sky Survey-II (SDSS-II) has embarked on a multi-year project to identify and measure light curves for intermediate-redshift (0.05 < z < 0.35) Type Ia supernovae (SNe Ia) using repeated five-band (ugriz) imaging over an area of 300 sq. deg. The survey region is a stripe 2.5° wide centered on the celestial equator in the Southern Galactic Cap that has been imaged numerous times in earlier years, enabling construction of a deep reference image for the discovery of new objects. Supernova imaging observations are being acquired between September 1 and November 30 of 2005-7. During the first two seasons, each region was imaged on average every five nights. Spectroscopic follow-up observations to determine supernova type and redshift are carried out on a large number of telescopes. In its first two three-month seasons, the survey has discovered and measured light curves for 327 spectroscopically confirmed SNe Ia, 30 probable SNe Ia, 14 confirmed SNe Ib/c, 32 confirmed SNe II, plus a large number of photometrically identified SNe Ia, 94 of which have host-galaxy spectra taken so far. This paper provides an overview of the project and briefly describes the observations completed during the first two seasons of operation.

The Sloan Digital Sky Survey-II Supernova Survey: Search Algorithm and Follow-up Observations

The Sloan Digital Sky Survey-II Supernova Survey has identified a large number of new transient sources in a 300 deg2 region along the celestial equator during its first two seasons of a three-season campaign. Multi-band (ugriz) light curves were measured for most of the sources, which include solar system objects, galactic variable stars, active galactic nuclei, supernovae (SNe), and other astronomical transients. The imaging survey is augmented by an extensive spectroscopic follow-up program to identify SNe, measure their redshifts, and study the physical conditions of the explosions and their environment through spectroscopic diagnostics. During the survey, light curves are rapidly evaluated to provide an initial photometric type of the SNe, and a selected sample of sources are targeted for spectroscopic observations. In the first two seasons, 476 sources were selected for spectroscopic observations, of which 403 were identified as SNe. For the type Ia SNe, the main driver for the survey, our photometric typing and targeting efficiency is 90%. Only 6% of the photometric SN Ia candidates were spectroscopically classified as non-SN Ia instead, and the remaining 4% resulted in low signal-to-noise, unclassified spectra. This paper describes the search algorithm and the software, and the real-time processing of the SDSS imaging data. We also present the details of the supernova candidate selection procedures and strategies for follow-up spectroscopic and imaging observations of the discovered sources.

A PANCHROMATIC VIEW OF THE RESTLESS SN 2009ip REVEALS THE EXPLOSIVE EJECTION OF A MASSIVE STAR ENVELOPE

The double explosion of SN 2009ip in 2012 raises questions about our understanding of the late stages of massive star evolution. Here we present a comprehensive study of SN 2009ip during its remarkable rebrightenings. High-cadence photometric and spectroscopic observations from the GeV to the radio band obtained from a variety of ground-based and space facilities (including the Very Large Array, Swift, Fermi, Hubble Space Telescope, and XMM) constrain SN 2009ip to be a low energy (E similar to 1050 erg for an ejecta mass similar to 0.5 M-circle dot) and asymmetric explosion in a complex medium shaped by multiple eruptions of the restless progenitor star. Most of the energy is radiated as a result of the shock breaking out through a dense shell of material located at similar to 5 x 10(14) cm with M similar to 0.1 M-circle dot, ejected by the precursor outburst similar to 40 days before the major explosion. We interpret the NIR excess of emission as signature of material located further out, the origin of which has to be connected with documented mass-loss episodes in the previous years. Our modeling predicts bright neutrino emission associated with the shock break-out if the cosmic-ray energy is comparable to the radiated energy. We connect this phenomenology with the explosive ejection of the outer layers of the massive progenitor star, which later interacted with material deposited in the surroundings by previous eruptions. Future observations will reveal if the massive luminous progenitor star survived. Irrespective of whether the explosion was terminal, SN 2009ip brought to light the existence of new channels for sustained episodic mass loss, the physical origin of which has yet to be identified.

The X-ray counterpart to the gravitational-wave event GW170817

A long-standing paradigm in astrophysics is that collisions—or mergers—of two neutron stars form highly relativistic and collimated outflows (jets) that power γ-ray bursts of short (less than two seconds) duration. The observational support for this model, however, is only indirect. A hitherto outstanding prediction is that gravitational-wave events from such mergers should be associated with γ-ray bursts, and that a majority of these bursts should be seen off-axis, that is, they should point away from Earth. Here we report the discovery observations of the X-ray counterpart associated with the gravitational-wave event GW170817. Although the electromagnetic counterpart at optical and infrared frequencies is dominated by the radioactive glow (known as a ‘kilonova’) from freshly synthesized rapid neutron capture (r-process) material in the merger ejecta, observations at X-ray and, later, radio frequencies are consistent with a short γ-ray burst viewed off-axis. Our detection of X-ray emission at a location coincident with the kilonova transient provides the missing observational link between short γ-ray bursts and gravitational waves from neutron-star mergers, and gives independent confirmation of the collimated nature of the γ-ray-burst emission.

A Measurement of the Rate of Type Ia Supernovae at Redshift z ≈ 0.1 from the First Season of the SDSS-II Supernova Survey

We present a measurement of the rate of Type Ia supernovae (SNe Ia) from the first of three seasons of data from the SDSS-II Supernova Survey. For this measurement, we include 17 SNe Ia at redshift z ≤ 0.12. Assuming a flat cosmology with Ωm = 0.3 = 1 − ΩΛ, we find a volumetric SN Ia rate of [ 2.93+ 0.17−0.04(systematic)+ 0.90−0.71(statistical) ] × 10−5 SNe Mpc −3 h370 yr −1, at a volume-weighted mean redshift of 0.09. This result is consistent with previous measurements of the SN Ia rate in a similar redshift range. The systematic errors are well controlled, resulting in the most precise measurement of the SN Ia rate in this redshift range. We use a maximum likelihood method to fit SN rate models to the SDSS-II Supernova Survey data in combination with other rate measurements, thereby constraining models for the redshift evolution of the SN Ia rate. Fitting the combined data to a simple power-law evolution of the volumetric SN Ia rate, rV ∝ (1 + z)β, we obtain a value of β = 1.5 ± 0.6, i.e., the SN Ia rate is determined to be an increasing function of redshift at the ~2.5 σ level. Fitting the results to a model in which the volumetric SN rate is rV = Aρ(t) + B(t), where ρ (t) is the stellar mass density and (t) is the star formation rate, we find A = (2.8 ± 1.2) × 10−14 SNe M−1☉ yr −1, B = (9.3+ 3.4−3.1) × 10−4 SNe M−1☉.

First-year spectroscopy for the sloan digital sky survey - II. Supernova survey

This paper presents spectroscopy of supernovae (SNe) discovered in the first season of the Sloan Digital Sky Survey-II SN Survey. This program searches for and measures multi-band light curves of SNe in the redshift range z = 0.05-0.4, complementing existing surveys at lower and higher redshifts. Our goal is to better characterize the SN population, with a particular focus on SNe Ia, improving their utility as cosmological distance indicators and as probes of dark energy. Our SN spectroscopy program features rapid-response observations using telescopes of a range of apertures, and provides confirmation of the SN and host-galaxy types as well as precise redshifts. We describe here the target identification and prioritization, data reduction, redshift measurement, and classification of 129 SNe Ia, 16 spectroscopically probable SNe Ia, 7 SNe Ib/c, and 11 SNe II from the first season. We also describe our efforts to measure and remove the substantial host-galaxy contamination existing in the majority of our SN spectra.

SEOUL NATIONAL UNIVERSITY 4K × 4K CAMERA (SNUCAM) FOR MAIDANAK OBSERVATORY

We present the characteristics of the Seoul National University 4k Camera (SNUCAM) and report its performance on the 1.5m telescope at the Maidanak observatory in Uzbekistan. SNUCAM is a CCD camera with a pixel scale of 0.266” in 4096 × 4096 format, covering 18.1’×18.1’ field of view on the 1.5m. The camera is currently equipped with Bessell UBVRI, Hα, SDSS ugriz, and Y-band filters, allowing us to carry out a variety of scientific programs ranging from exoplanet studies to survey of quasars at high redshift. We examine properties of SNUCAM such as the bias level and its temporal variation, the dark current, the readout noise, the gain, the linearity, the fringe patterns, the amplifier bias, and the bad pixels. From our observations, we also constructed the master fringe frames in I-, z-, and Y-band. We outline some of the current scientific programs being carried out with SNUCAM, and demonstrate that SNUCAM on the 1.5m can deliver excellent images that reach to the 5-σ detection limits of R ~ 25.5 mag and z ~ 22.7 mag in 1 hour total integration.

The effects of transplacental porcine circovirus type 2 infection on porcine epidemic diarrhoea virus-induced enteritis in preweaning piglets

Abstract The effects of transplacental porcine circovirus type 2 (PCV2) infection on porcine epidemic diarrhoea virus (PEDV)-induced enteritis were examined in neonatal piglets. Six pregnant sows were randomly allocated to an infected (n =3) or control group (n =3). Three pregnant sows were inoculated intranasally with 6mL of tissue culture fluid containing 1.2×105 tissue culture infective doses 50% (TCID50)/mL of PCV2 strain SNUVR000470 three weeks before the expected farrowing date. Three control pregnant sows were similarly exposed to uninfected cell culture supernatants. Thirty piglets from PCV2-infected sows were randomly assigned to two groups (A and B) of 15 piglets each. Another 30 piglets from noninfected sows were randomly assigned to two groups (C and D) of 15 piglets each. The piglets in groups A and C were dosed orally at three days of age with 2mL of virus stock (1×106.5 TCID50/mL) of the PEDV strain, SNUVR971496, at the third passage. The mean villous height and crypt depth (VH:CD) ratio in PEDV-infected piglets from PCV2-infected sows (group A) were significantly different from those of the PEDV-infected piglets from PCV2 negative sows (group C) at 36, 48, and 72h post-inoculation (hpi) (P <0.05). In PEDV-infected piglets from PCV2-infected sows (group A), significantly more PEDV nucleic acid was detected in the jejunal tissues (P <0.05) at 24hpi than in the same tissues of the PEDV-infected piglets from PCV2 negative sows (group C). Thereafter, at 36, 48, 60, and 70hpi significantly more PEDV nucleic acid (P <0.05) was detected in the jejunal tissues of the PEDV-infected piglets from PCV2 negative sows (group C) than those of the PEDV-infected piglets from the PCV2-infected sows (group A). It is concluded that the clinical course of PEDV disease was markedly affected by transplacental infection of PCV2.

Seoul National University Bright Quasar Survey in Optical (SNUQSO). II. Discovery of 40 Bright Quasars Near the Galactic Plane

We report the discovery of 40 bright quasars and active galactic nuclei (AGNs) at low Galactic latitude (b < ). The low Galactic latitude region has been considered a place to avoid when searching for extragalactic sources, because of the high Galactic extinction, as well as a large number of stars contaminating the sample selection. Bright quasars (R 17) suffer more from such difficulties because they look like bright stars, which are numerous at low b, yet their surface number density is very low. In order to find quasars in this region of the sky less explored for extragalactic sources, we have started a survey of low Galactic latitude bright quasars as a part of the Seoul National University Quasar Survey in Optical (SNUQSO). Quasar candidates have been selected from radio and near-infrared (NIR) data. Out of 88 targets, we identify 29 bright quasars/AGNs around the antigalactic center, and 11 bright quasars/AGNs in the outskirts of the Galactic center, from two observing runs in 2006 at the Bohyunsan Optical Astronomical Observatory (BOAO) in Korea. Our finding demonstrates that quasars/AGNs can be discovered effectively, even at low Galactic latitude, using multiwavelength data.

DISCOVERY OF A FAINT QUASAR AT z ∼ 6 AND IMPLICATIONS FOR COSMIC REIONIZATION

Recent studies suggest that faint active galactic nuclei may be responsible for the reionization of the universe. Confirmation of this scenario requires spectroscopic identification of faint quasars (