Rosanne Di Stefano

Ten Simple Rules for the Care and Feeding of Scientific Data

In the early 1600s, Galileo Galilei turned a telescope toward Jupiter. In his log book each night, he drew to-scale schematic diagrams of Jupiter and some oddly moving points of light near it. Galileo labeled each drawing with the date. Eventually he used his observations to conclude that the Earth orbits the Sun, just as the four Galilean moons orbit Jupiter. History shows Galileo to be much more than an astronomical hero, though. His clear and careful record keeping and publication style not only let Galileo understand the solar system, they continue to let anyone understand how Galileo did it. Galileos notes directly integrated his data (drawings of Jupiter and its moons), key metadata (timing of each observation, weather, and telescope properties), and text (descriptions of methods, analysis, and conclusions). Critically, when Galileo included the information from those notes in Sidereus Nuncius, this integration of text, data, and metadata was preserved, as shown in Figure 1. Galileos work advanced the “Scientific Revolution,” and his approach to observation and analysis contributed significantly to the shaping of todays modern “scientific method”.

A SYNOPTIC X-RAY STUDY OF M31 WITH THE CHANDRA HIGH RESOLUTION CAMERA

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of 2.5 years and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term lightcurves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well-fit by a power-law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X > 10^{37} erg/s . We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up HST WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 +/- 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 solar mass black hole X-ray nova with an orbital period of ~20 days. In the other case, the optical counterpart varies between F336W = 20.82 +/- 0.06 mag and F336W = 21.11 +/- 0.02 mag. Ground-based and HST observations show this object is bright (V = 18.8 +/- 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.

THE ULTRALUMINOUS X-RAY SOURCE NGC 1313 X-2 (MS 0317.7 6647) AND ITS ENVIRONMENT

We present new optical and Chandra observations of the field containing the ultraluminous X-ray source NGC 1313 X-2. On an ESO 3.6 mi mage, theChandra error box embraces an R ¼ 21:6 pointlike object and excludes a previously proposed optical counterpart. The resulting X-ray/optical flux ratio of NGC 1313 X-2 is � 500. The value of fX=fopt, the X-ray variability history, and the spectral distribution derived from a reanalysis of the ROSAT, ASCA ,a ndXMM-Newton data indicate a luminous X-ray binary in NGC 1313 as a likely explanation for NGC 1313 X-2. If the X-ray soft component observed in the XMM-Newton EPIC spectrum originates from an accretion disk, the inferred mass of the compact remnant is � 100 M� , making it an intermediate-mass black hole.

A New Channel for the Detection of Planetary Systems through Microlensing. I. Isolated Events due to Planet Lenses

We propose and evaluate the feasibility of a new strategy to search for planets via microlensing observations. This new strategy is designed to detect planets in wide orbits, i.e., with orbital separation, a, greater than ~1.5RE. Planets in wide orbits may provide the dominant channel for the discovery of planets via microlensing, particularly low-mass (e.g., Earth-mass) planets. This paper concentrates on events in which a single planet serves as a lens, leading to an isolated event of short duration. We point out that a distribution of events due to lensing by stars with wide-orbit planets is necessarily accompanied by a distribution of shorter duration events. The fraction of events in the latter distribution is proportional to the average value of q1/2, where q is the ratio between planet and stellar masses. The position of the peak or peaks also provides a measure of the mass ratios typical of planetary systems. We study detection strategies that can optimize our ability to discover isolated short-duration events due to lensing by planets and find that monitoring employing sensitive photometry is particularly useful. If planetary systems similar to our own are common, even modest changes in detection strategy should lead to the discovery a few isolated events of short duration every year. We therefore also address the issue of the contamination due to stellar populations of any microlensing signal due to low-mass MACHOs. We describe how, even for isolated events of short duration, it will be possible to test the hypothesis that the lens was a planet instead of a low-mass MACHO, if the central star of the planetary system contributes a measurable fraction of the baseline flux.

MESOLENSING EXPLORATIONS OF NEARBY MASSES : FROM PLANETS TO BLACK HOLES

Nearby masses can have a high probability of lensing stars in a distant background field. This high-probability lensing, or mesolensing, can be used to dramatically increase our knowledge of dark and dim objects in the solar neighborhood, where it can discover and facilitate the study of members of the local dark matter population (free-floating planets, low-mass dwarfs, white dwarfs, neutron stars, and stellar-mass black holes). We can measure the mass and transverse velocity of those masses discovered (or already known) and determine whether or not they are in binaries with dim companions. Here we explore these and other applications of mesolensing, including the study of forms of matter that have been hypothesized but not discovered, such as intermediate-mass black holes, dark matter objects free-streaming through the Galactic disk, and planets in the outermost regions of the solar system. In each case we discuss the feasibility of deriving results based on present-day monitoring systems, and we also consider the vistas that will open with the advent of all-sky monitoring in the era of Pan-STARRS and LSST.

Identifying Microlensing by Binaries

The microlensing monitoring programs have studied large numbers of standard light curves that seem to be due to lensing by a dark point mass. Theory predicts that many microlensing events should display significant deviations from the standard form. Lens binarity, in particular, is expected to be common. So far, however, only a handful of light curves exhibit evidence that the lens is a binary; all of these display dramatic deviations from the standard light curve, exhibiting pronounced multiple peaks, caustic crossings, or both. Binary lens events in which the light curve is less dramatically perturbed should also exist in the data set. Why, then, have we not detected them? The answer may lie in the fact that the perturbations, though often significant, tend to be less distinctive than those associated with caustic crossings. It is therefore possible that some of these more gently perturbed events have been misclassified, and that others have simply been missed. Reliable estimates of the overall detection efficiency, and hence derivation of the fraction of the Galactic halo mass that may be in MACHOs, rely on resolving this issue. Microlensing can also be used to determine the form of the initial mass function (IMF) beyond the solar neighborhood; accurate determination of the IMF also relies on the ability to correctly identify binary light curves. We present a method to determine whether a light curve is due to lensing by a binary. The method works for both gently and dramatically perturbed binary-lens light curves. Our method identifies all degenerate solutions—i.e., all possible lensing events that might have given rise to the observed light curve. It also enables us to eliminate from consideration large ranges of possible false positive identifications associated with light curves that might mimic microlensing by a binary. This method, or a generalization of it, can also be applied to the analysis of light curves that deviate from the standard point-mass lens form because of astronomical effects other than lens binarity.

An Ultraluminous Supersoft X-Ray Source in M81: An Intermediate-Mass Black Hole?

Ultraluminous supersoft X-ray sources (ULSSSs) exhibit supersoft spectra with blackbody temperatures of 50-100 eV and bolometric luminosities above 1039 ergs s−1 and are possibly intermediate-mass black holes (IMBHs) of ≥103 M☉ or massive white dwarfs that are progenitors of Type Ia supernovae. In this Letter we report our optical studies of such a source in M81, M81-ULS1, with HST archive observations. M81-ULS1 is identified with a pointlike object, the spectral energy distribution of which reveals a blue component in addition to the companion of an asymptotic giant branch star. The blue component is consistent with the power law as expected from the geometrically thin accretion disk around an IMBH accretor but inconsistent with the power law as expected from the X-ray-irradiated flared accretion disk around a white dwarf accretor. This result is strong evidence that M81-ULS1 is an IMBH instead of a white dwarf.

ON THE NATURE OF THE PROGENITOR OF THE Type Ia SN2011fe IN M101

The explosion of a Type Ia supernova, SN2011fe, in the nearby Pinwheel galaxy (M101 at 6.4 Mpc) provides an opportunity to study pre-explosion images and search for the progenitor, which should consist of a white dwarf (WD), possibly surrounded by an accretion disk, in orbit with another star. We report on our use of deep Chandra observations and Hubble Space Telescope observations to limit the luminosity and temperature of the pre-explosion WD. It is found that if the spectrum was a blackbody, then pre-SN WDs with steady nuclear burning of the highest possible temperatures and luminosities are excluded assuming moderate n{sub H} values, but values of kT between roughly 10 eV and 60 eV are permitted even if the WD was emitting at the Eddington luminosity. This allows the progenitor to be an accreting nuclear-burning WD with an expanded photosphere 4-100 times the WD itself, or a super-critically accreting WD blowing off an optically thick strong wind, or possibly a recurrent nova with luminosities an order of magnitude lower than Eddington. The observations are also consistent with a double degenerate scenario, or a spinning down WD that has been spun up by accretion from the donor.

A SEARCH FOR ASTEROIDS, MOONS, AND RINGS ORBITING WHITE DWARFS

Do white dwarfs host asteroid systems? Although several lines of argument suggest that white dwarfs may be orbited by large populations of asteroids, transits would provide the most direct evidence. We demonstrate that the Kepler mission has the capability to detect transits of white dwarfs by asteroids. Because white-dwarf asteroid systems, if they exist, are likely to contain many asteroids orbiting in a spatially extended distribution, discoveries of asteroid transits can be made by monitoring only a small number of white dwarfs, compatible with Keplers primary mission, which is to monitor stars with potentially habitable planets. Possible future missions that survey 10 times as many stars with similar sensitivity and minute-cadence monitoring can establish the characteristics of asteroid systems around white dwarfs, such as the distribution of asteroid sizes and semimajor axes. Transits by planets would be more dramatic, but the probability that they will occur is lower. Ensembles of planetary moons and/or the presence of rings around planets can also produce transits detectable by Kepler. The presence of moons and rings can significantly increase the probability that Kepler will discover planets orbiting white dwarfs, even while monitoring only a small number of them.

SHORT-DURATION LENSING EVENTS. I. WIDE-ORBIT PLANETS? FREE-FLOATING LOW-MASS OBJECTS? OR HIGH-VELOCITY STARS?

Short-duration lensing events tend to be generated by low-mass lenses or by lenses with high transverse velocities. Furthermore, for any given lens mass and speed, events of short duration are preferentially caused by nearby lenses (mesolenses) that can be studied in detail, or else by lenses so close to the source star that finite-source-size effects may be detected, yielding information about both the Einstein ring radius and the surface of the lensed star. Planets causing short-duration events may be in orbits with any orientation, and may have semimajor axes smaller than 1?AU, or they may reach the outer limits of their planetary systems, in the region corresponding to the solar systems Oort Cloud. They can have masses larger than Jupiters or smaller than Plutos. Lensing therefore has a unique potential to expand our understanding of planetary systems. A particular advantage of lensing is that it can provide precision measurements of system parameters, including the masses of and projected separation between star and planet. We demonstrate how the parameters can be extracted and show that a great deal can be learned. For example, it is remarkable that the gravitational mass of nearby free-floating planet-mass lenses can be measured by complementing observations of a photometric event with deep images that detect the planet itself. A fraction of short events may be caused by high-velocity stars located within a?kiloparsec. Many high-velocity lenses are likely to be neutron stars that received large natal kicks. Other high-speed stars may be members of the halo population. Still others may be hypervelocity stars that have been ejected from the Galactic center, or runaway stars escaped from close binaries, possibly including the progenitor binaries of Type Ia supernovae.