Nathaniel R. Butler

Exclusion of a luminous red giant as a companion star to the progenitor of supernova SN 2011fe

Weidong Li1, Joshua S. Bloom1, Philipp Podsiadlowski2, Adam A. Miller1, S. Bradley Cenko1, Saurabh W. Jha3, Mark Sullivan2, D. Andrew Howell4,5, Peter E. Nugent6,1, Nathaniel R. Butler7, Eran O. Ofek8,9, Mansi M. Kasliwal10, Joseph W. Richards1,11, Alan Stockton12, Hsin-Yi Shih12, Lars Bildsten5,13, Michael M. Shara14, Joanne Bibby14, Alexei V. Filippenko1, Mohan Ganeshalingam1, Jeffrey M. Silverman1, S. R. Kulkarni8, Nicholas M. Law15, Dovi Poznanski16, Robert M. Quimby8, Curtis McCully3, Brandon Patel3, & Kate Maguire2Type Ia supernovae are thought to result from a thermonuclear explosion of an accreting white dwarf in a binary system, but little is known of the precise nature of the companion star and the physical properties of the progenitor system. There are two classes of models: double-degenerate (involving two white dwarfs in a close binary system) and single-degenerate models. In the latter, the primary white dwarf accretes material from a secondary companion until conditions are such that carbon ignites, at a mass of 1.38 times the mass of the Sun. The type Ia supernova SN 2011fe was recently detected in a nearby galaxy. Here we report an analysis of archival images of the location of SN 2011fe. The luminosity of the progenitor system (especially the companion star) is 10–100 times fainter than previous limits on other type Ia supernova progenitor systems, allowing us to rule out luminous red giants and almost all helium stars as the mass-donating companion to the exploding white dwarf.

Optimal time-series selection of quasars

We present a novel method for the optimal selection of quasars using time-series observations in a single photometric bandpass. Utilizing the damped random walk model of Kelly et al., we parameterize the ensemble quasar structure function in Sloan Stripe 82 as a function of observed brightness. The ensemble model fit can then be evaluated rigorously for and calibrated with individual light curves with no parameter fitting. This yields a classification in two statistics—one describing the fit confidence and the other describing the probability of a false alarm—which can be tuned, a priori, to achieve high quasar detection fractions (99% completeness with default cuts), given an acceptable rate of false alarms. We establish the typical rate of false alarms due to known variable stars as 3% (high purity). Applying the classification, we increase the sample of potential quasars relative to those known in Stripe 82 by as much as 29%, and by nearly a factor of two in the redshift range 2.5 99% of quasars exhibiting the expected variability profile. We discuss the utility of the method at high redshift and in the regime of noisy and sparse data. Our time-series selection complements well-independent selection based on quasar colors and has strong potential for identifying high-redshift quasars for Baryon Acoustic Oscillations and other cosmology studies in the LSST era.

The X-Ray Afterglows of GRB 020813 and GRB 021004 with Chandra HETGS: Possible Evidence for a Supernova Prior to GRB 020813

We report on the detection of an emission line near 1.3 keV, which we associate with blueshifted hydrogen-like sulfur (S XVI), in a 76.8 ks Chandra HETGS spectrum of the afterglow of GRB 020813. The line is detected at 3.3 σ significance. We also find marginal evidence for a line possibly due to hydrogen-like silicon (Si XIV) with the same blueshift. A line from Fe is not detected, although a very low significance Ni feature may be present. A thermal model fits the data adequately, but a reflection model may provide a better fit. There is marginal evidence that the equivalent width of the S XVI line decreases as the burst fades. We infer from these results that a supernova likely occurred 2 months prior to the γ-ray burst. We find no discrete or variable spectral features in the Chandra HETGS spectrum of the GRB 021004 afterglow.

Gamma‐ray Bursts, Classified Physically

From Galactic binary sources, to extragalactic magnetized neutron stars, to long‐duration GRBs without associated supernovae, the types of sources we now believe capable of producing bursts of gamma‐rays continues to grow apace. With this emergent diversity comes the recognition that the traditional and newly formulated high‐energy observables used for identifying sub‐classes does not provide an adequate one‐to‐one mapping to progenitors. The popular classification of some >100u2009sec duration GRBs as “short bursts” is not only an unpalatable retronym and syntactically oxymoronic but highlights the difficultly of using what was once a purely phenomenological classification to encode our understanding of the physics that gives rise to the events. Here we propose a physically based classification scheme designed to coexist with the phenomenological system already in place and argue for its utility and necessity.

MID-INFRARED PERIOD-LUMINOSITY RELATIONS OF RR LYRAE STARS DERIVED FROM THE WISE PRELIMINARY DATA RELEASE

Interstellar dust presents a significant challenge to extending parallax-determined distances of optically observed pulsational variables to larger volumes. Distance ladder work at mid-infrared wavebands, where dust effects are negligible and metallicity correlations are minimized, has been largely focused on few-epoch Cepheid studies. Here we present the first determination of mid-infrared period-luminosity (PL) relations of RR Lyrae stars from phase-resolved imaging using the preliminary data release of the Wide-field Infrared Survey Explorer (WISE). We present a novel statistical framework to predict posterior distances of 76 well observed RR Lyrae that uses the optically constructed prior distance moduli while simultaneously imposing a power-law PL relation to WISE-determined mean magnitudes. We find that the absolute magnitude in the bluest WISE filter is M{sub W1} = (- 0.421 {+-} 0.014) - (1.681 {+-} 0.147)log{sub 10}(P/0.50118 day), with no evidence for a correlation with metallicity. Combining the results from the three bluest WISE filters, we find that a typical star in our sample has a distance measurement uncertainty of 0.97% (statistical) plus 1.17% (systematic). We do not fundamentalize the periods of RRc stars to improve their fit to the relations. Taking the Hipparcos-derived mean V-band magnitudes, we use the distance posteriors to determine a newmorexa0» optical metallicity-luminosity relation. The results of this analysis will soon be tested by Hubble Space Telescope parallax measurements and, eventually, with the GAIA astrometric mission.«xa0less

An experimental study of charge diffusion in the undepleted silicon of X-ray CCDs

We have experimentally studied the diffused electron clouds formed by X-ray photons interacting within the undepleted bulk of silicon beneath the depleted region of an X-ray CCD. Usually, such events are spread over multiple pixels and are often rejected in data analysis because of incomplete charge collection in the undepleted bulk. An unusual CCD clocking mode where every 100 sequential rows were summed in the serial register allowed us to reduce the charge distribution to a one-dimensional (1-D) representation without losing any information, resulting in a dramatically simplified analysis and improved signal/noise ratio. We have shown that events from the undepleted bulk can be used for retrieving information regarding the energy of the corresponding X-ray photon. In our study, the Mn K/sub /spl alpha// and K/sub /spl beta// lines emitted by the radioactive Fe/sup 55/ source could be clearly separated. Such an analysis can markedly improve the sensitivity of partially depleted CCDs at higher energies. By averaging many events that originate at the same distance from the surface of the device, we were able to extract the shape of the charge cloud after the completion of the diffusion process. The results were found to be in good agreement with theoretical predictions.

IR and Optical Observations of GRB 030115

We present an upper limit on the brightness of the afterglow of the long GRB 030115 measured from Infrared (IR) images taken with the Magellan Classic Cam instrument of Ks > 22 at 6.2 days after the burst. We also present measurments of the host galaxy of GRB 030115 from archival optical and IR HST images taken with the Advanced Camera for Surveys and the Near Infrared Camera and Multi Object Spectrometer 25+ days after the burst. GRB 030115 is classified as an Optically Dark Burst, as its afterglow was found in the J, H, and K IR bands after a null result was reported in the optical. It is the first HETE GRB to have its afterglow found initially in the IR.