Mark Edward Huber

Testing LMC Microlensing Scenarios: The Discrimination Power of the SuperMACHO Microlensing Survey

Characterizing the nature and spatial distribution of the lensing objects that produce the previously measured microlensing optical depth toward the Large Magellanic Cloud (LMC) remains an open problem. We present an appraisal of the ability of the SuperMACHO Project, a next-generation microlensing survey directed toward the LMC, to discriminate between various proposed lensing populations. We consider two scenarios: lensing by a uniform foreground screen of objects and self-lensing by LMC stars. The optical depth for screen lensing is essentially constant across the face of the LMC, whereas the optical depth for self-lensing shows a strong spatial dependence. We have carried out extensive simulations, based on data obtained during the first year of the project, to assess the SuperMACHO surveys ability to discriminate between these two scenarios. In our simulations we predict the expected number of observed microlensing events for various LMC models for each of our fields by adding artificial stars to the images and estimating the spatial and temporal efficiency of detecting microlensing events using Monte Carlo methods. We find that the event rate itself shows significant sensitivity to the choice of the LMC luminosity function, limiting the conclusions that can be drawn from the absolute rate. If instead we determine the differential event rate across the LMC, we will decrease the impact of these systematic biases and render our conclusions more robust. With this approach the SuperMACHO Project should be able to distinguish between the two categories of lens populations. This will provide important constraints on the nature of the lensing objects and their contributions to the Galactic dark matter halo.

Spectral Identification of an Ancient Supernova Using Light Echoes in the Large Magellanic Cloud

We report the successful identification of the type of the supernova responsible for the supernova remnant SNR 0509-675 in the Large Magellanic Cloud (LMC) using Gemini spectra of surrounding light echoes. The ability to classify outbursts associated with centuries-old remnants provides a new window into several aspects of supernova research and is likely to be successful in providing new constraints on additional LMC supernovae as well as their historical counterparts in the Milky Way Galaxy (MWG). The combined spectrum of echo light from SNR 0509-675 shows broad emission and absorption lines consistent with a supernova (SN) spectrum. We create a spectral library consisting of 26 SNe Ia and 6 SN Ib/c that are time-integrated, dust-scattered by LMC dust, and reddened by the LMC and MWG. We fit these SN templates to the observed light echo spectrum using � 2 minimization as well as correlation techniques, and we find that overluminous 91T-like SNe Ia with �m15 < 0.9 match the observed spectrum best. Subject headings: ISM: individual(SNR 0509-67.5) — supernova:general — supernova remnants — Magellanic Clouds

Mass Determination and Detection of the Onset of Chromospheric Activity for the Substellar Object in EF Eridani

EF Eri is a magnetic cataclysmic variable that has been in a low accretion state for the past 9 yr. Low-state optical spectra reveal the underlying Zeeman-split white dwarf absorption lines. These features are used to determine a value of 13-14 MG as the white dwarf field strength. Recently, 5-7 yr into the low state, Balmer and other emission lines have appeared in the optical. An analysis of the Hα emission line yields the first radial velocity solution for EF Eri, leading to a spectroscopic ephemeris for the binary and, using the best available white dwarf mass of 0.6 M☉, a mass estimate for the secondary of 0.055 M☉. For a white dwarf mass of 0.95 M☉, the average for magnetic white dwarfs, the secondary mass increases to 0.087 M☉. At EF Eris orbital period of 81 minutes, this higher mass secondary could not be a normal star and still fit within the Roche lobe. The source of the Balmer and other emission lines is confirmed to be from the substellar secondary, and we argue that it is due to stellar activity. We compare EF Eris emission-line spectrum and activity behavior to that recently observed in AM Her and VV Pup and attributed to stellar activity. We explore observations and models originally developed for V471 Tau, for the RS CVn binaries, and for extrasolar planets. We conclude that irradiation of the secondary in EF Eri and similar systems is unlikely and, in polars, the magnetic field interaction between the two stars (with a possible tidal component) is a probable mechanism that would concentrate chromospheric activity on the secondary near the substellar point of the white dwarf.

Light Curves of Type Ia Supernovae from Near the Time of Explosion

We present a set of 11 Type Ia supernova (SN Ia) light curves with dense, premaximum sampling. These supernovae (SNe), in galaxies behind the Large Magellanic Cloud (LMC), were discovered by the SuperMACHO survey. The SNe span a redshift range of z = 0.11-0.35. Our light curves contain some of the earliest premaximum observations of SNe Ia to date. We also give a functional model that describes the SN Ia light-curve shape (in our VR band). Our function uses the expanding fireball model of Goldhaber et al. to describe the rising light curve immediately after explosion but constrains it to smoothly join the remainder of the light curve. We fit this model to a composite observed VR-band light curve of three SNe between redshifts of 0.135 and 0.165. These SNe have not been K-corrected or adjusted to account for reddening. In this redshift range, the observed VR band most closely matches the rest-frame V band. Using the best fit to our functional description of the light curve, we find the time between explosion and observed VR-band maximum to be 17.6 ± 1.3(stat) ± 0.07(sys) rest-frame days for a SN Ia with a VR-band Δm-10 of 0.52 mag. For the redshifts sampled, the observed VR-band time of maximum brightness should be the same as the rest-frame V-band maximum to within 1.1 rest-frame days.

Color and Variability Characteristics of Point Sources in the Faint Sky Variability Survey

We present an analysis of the color and variability characteristics for point sources in the Faint Sky Variability Survey (FSVS). The FSVS cataloged ~23 deg2 in BVI filters from ~16 to 24 mag to investigate variability in faint sources at moderate-to-high Galactic latitudes. Point-source completeness is found to be >83% for a selected representative sample (V = 17.5-22.0 mag, B - V = 0.0-1.5) containing both photometric B, V detections and 80% of the time-sampled V data available compared to a basic internal source completeness of 99%. Multiepoch (10-30) observations in V spanning minutes to years modeled by light-curve simulations reveal amplitude sensitivities to ~0.015-0.075 mag over a representative V = 18-22 mag range. Periodicity determinations appear viable to timescales of order 1 day or less using the most sampled fields (~30 epochs). The fraction of point sources is found to be generally variable at 5%-8% over V = 17.5-22.0 mag. For V brighter than 19 mag, the variable population is dominated by low-amplitude (<0.05 mag) and blue (B - V < 0.35) sources, possibly representing a population of γ Doradus stars. Overall, the dominant population of variable sources are bluer than B - V = 0.65 and have main-sequence colors, likely reflecting larger populations of RR Lyrae, SX Phe, γ Doradus, and W UMa variables.

OPTICAL AND INFRARED OBSERVATIONS OF TWO MAGNETIC INTERACTING BINARIES: TAU 4 (RXJ0502.8+1624) & SDSS J121209.31+013627.7

We present new optical photometric and spectroscopic observations and K-band spectroscopy of two magnetic interacting binaries: Tau 4 (RXJ0502.8+1624) and SDSS J121209.31+013627.7. Tau 4 shows short-term, highly modulated optical light but no orbital period is firmly detected. K-band spectroscopy shows H and He I emission and reveals a remarkable phase-resolved dataset. We detect clear evidence for cyclotron humps in the K-band implying a magnetic white dwarf with B ~ 7-11 MG and confirming Tau 4 as a magnetic binary. A short self eclipse by the gas stream is evident as the spectrum changes from emission to absorption and back within a few minutes. None of our K-band spectra of Tau 4 appear similar to the previously reported observation. Optical spectroscopy of SDSS J121209.31+013627.7 in 2007 May and June shows Zeeman-split Balmer absorption lines as previously noted in the literature but only weak, sporadic H? emission compared to past data. However, stronger H? emission returned in 2008 February, looking similar to that previously detected in 2006. The variable nature of this emission line makes irradiation from the white dwarf an unlikely cause of the H? emission in J1212. The nature of J1212 is reminiscent of that observed in its near twin, EF Eri.

Light echoes of SNe in the LMC

The SuperMACHO project has discovered light echoes from 3 ancient SNe in the LMC. These SNRs are three of the six youngest in the LMC, and are classified as likely SN Ia based on X-ray data.