Person:

Stubbs, Christopher

We have measured spatial and temporal variability in the y-band sky brightness over the course of four nights above Cerro Tololo near Cerro Pachon, Chile, the planned site for the Large Synoptic Survey Telescope (LSST). Our wide-angle camera lens provided a 41° field of view and a 145" pixel scale. We minimized potential system throughput differences by deploying a deep-depletion CCD and a filter that matches the proposed LSST (y_3) band (970 –1030 nm). Images of the sky exhibited coherent wave structure, attributable to atmospheric gravity waves at 90 km altitude, creating 3%–4% rms spatial sky flux variability on scales of about 2° and larger. Over the course of a full night, the (y_3) band additionally showed highly coherent temporal variability of up to a factor of 2 in flux. We estimate the mean absolute sky level to be approximately (y_3 = 17.8 mag(Vega)), or (y_3 = 18.3 mag(AB)). While our observations were made through a (y_3) filter, the relative sky brightness variability should hold for all proposed y bands, whereas the absolute levels should more strongly depend on spectral response. The spatial variability presents a challenge to wide-field cameras that require illumination correction strategies that make use of stacked sky flats. The temporal variability may warrant an adaptive y band imaging strategy for LSST, to take advantage of times when the sky is darkest.

We have used a precision calibrated photodiode as the fundamental metrology reference in order to determine the relative throughput of the PanSTARRS telescope and the Gigapixel imager, from 400 nm to 1050 nm. Our technique uses a tunable laser as a source of illumination on a transmissive flat-field screen. We determine the full-aperture system throughput as a function of wavelength, including (in a single integral measurement) the mirror reflectivity, the transmission functions of the filters and the corrector optics, and the detector quantum efficiency, by comparing the light seen by each pixel in the CCD array to that measured by a precision-calibrated silicon photodiode. This method allows us to determine the relative throughput of the entire system as a function of wavelength, for each pixel in the instrument, without observations of celestial standards. We present promising initial results from this characterization of the PanSTARRS system, and we use synthetic photometry to assess the photometric perturbations due to throughput variation across the field of view.

Theories that attempt to explain the observed cosmic acceleration by modifying general relativity all introduce a new scalar degree of freedom that is active on large scales, but is screened on small scales to match experiments. We show that if such screening occurrs via the chameleon mechanism such as in (f(R)), it is possible to have order one violation of the equivalence principle, despite the absence of explicit violation in the microscopic action. Namely, extended objects such as galaxies or constituents thereof do not all fall at the same rate. The chameleon mechanism can screen the scalar charge for large objects but not for small ones (large/small is defined by the gravitational potential and controlled by the scalar coupling). This leads to order one fluctuations in the inertial to gravitational mass ratio. In Jordan frame, it is no longer true that all objects move on geodesics. In contrast, if the scalar screening occurrs via strong coupling, such as in the DGP braneworld model, equivalence principle violation occurrs at a much reduced level. We propose several observational tests of the chameleon mechanism: 1. small galaxies should fall faster than large galaxies, even when dynamical friction is negligible; 2. voids defined by small galaxies would be larger compared to standard expectations; 3. stars and diffuse gas in small galaxies should have different velocities, even on the same orbits; 4. lensing and dynamical mass estimates should agree for large galaxies but disagree for small ones. We discuss possible pitfalls in some of these tests. The cleanest is the third one where mass estimate from HI rotational velocity could exceed that from stars by 30 % or more. To avoid blanket screening of all objects, the most promising place to look is in voids.

A long-range fifth force coupled to dark matter can induce a coupling to ordinary matter if the dark matter interacts with Standard Model fields. We consider constraints on such a scenario from both astrophysical observations and laboratory experiments. We also examine the case where the dark matter is a weakly interacting massive particle, and derive relations between the coupling to dark matter and the coupling to ordinary matter for different models. Currently, this scenario is most tightly constrained by galactic dynamics, but improvements in Eotvos experiments can probe unconstrained regions of parameter space.

Optical and infrared observations have thus far detected more celestial cataclysms than have been seen in gravity waves (GW). This argues that we should search for gravity wave signatures that correspond to flux variability seen at optical wavelengths, at precisely known positions. There is an unknown time delay between the optical and gravitational transient, but knowing the source location precisely specifies the corresponding time delays across the gravitational antenna network as a function of the GW-to-optical arrival time difference. Optical searches should detect virtually all supernovae that are plausible gravitational radiation sources. The transient optical signature expected from merging compact objects is not as well understood, but there are good reasons to expect detectable transient optical/IR emission from most of these sources as well. The next generation of deep wide-field surveys (for example PanSTARRS and LSST) will be sensitive to subtle optical variability, but we need to fill the "blind spots" that exist in the Galactic plane, and for optically bright transient sources. In particular, a Galactic plane variability survey at (\lambda \sim 2\mu)m seems worthwhile. Science would benefit from closer coordination between the various optical survey projects and the gravity wave community.

A number of proposals have been put forward to account for the observed accelerating expansion of the Universe through modifications of gravity. One specific scenario, Dvali-Gabadadze-Porrati (DGP) gravity, gives rise to a potentially observable anomaly in the solar system: all planets would exhibit a common anomalous precession, (dw/dt), in excess of the prediction of General Relativity. We have used the Planetary Ephemeris Program (PEP) along with planetary radar and radio tracking data to set a constraint of (|dw/dt|) < 0.02 arcseconds per century on the presence of any such common precession. This sensitivity falls short of that needed to detect the estimated universal precession of (|dw/dt| = 5 \times10^4) arcseconds per century expected in the DGP scenario. We discuss the fact that ranging data between objects that orbit in a common plane cannot constrain the DGP scenario. It is only through the relative inclinations of the planetary orbital planes that solar system ranging data have sensitivity to the DGP-like effect of universal precession. In addition, we illustrate the importance of performing a numerical evaluation of the sensitivity of the data set and model to any perturbative precession.

We present Stellar Locus Regression (SLR), a method of directly adjusting the instrumental broadband optical colors of stars to bring them into accord with a universal stellar color-color locus, producing accurately calibrated colors for both stars and galaxies. This is achieved without first establishing individual zeropoints for each passband, and can be performed in real-time at the telescope. We demonstrate how SLR naturally makes one wholesale correction for differences in instrumental response, for atmospheric transparency, for atmospheric extinction, and for Galactic extinction. We perform an example SLR treatment of SDSS data over a wide range of Galactic dust values and independently recover the direction and magnitude of the canonical Galactic reddening vector with 14--18 mmag RMS uncertainties. We then isolate the effect of atmospheric extinction, showing that SLR accounts for this and returns precise colors over a wide of airmass, with 5--14 mmag RMS residuals. We demonstrate that SLR-corrected colors are sufficiently accurate to allow photometric redshift estimates for galaxy clusters (using red sequence galaxies) with an uncertainty (\sigma(z)/(1+z)) = 0.6% per cluster for redshifts 0.09< (z) <0.25. Finally, we identify our objects in the 2MASS all-sky catalog, and produce i-band zeropoints typically accurate to 18 mmag using only SLR. We offer open-source access to our IDL routines, validated and verified for the implementation of this technique, at http://stellar-locus-regression.googlecode.com

We present results of X-ray observations of a sample of 15 clusters selected via their imprint on the cosmic microwave background from the thermal Sunyaev-Zel'dovich (SZ) effect. These clusters are a subset of the first SZ-selected cluster catalog, obtained from observations of 178 deg2 of sky surveyed by the South Pole Telescope (SPT). Using X-ray observations with Chandra and XMM-Newton, we estimate the temperature, TX , and mass, Mg , of the intracluster medium within r 500 for each cluster. From these, we calculate YX = MgTX and estimate the total cluster mass using an M 500-YX scaling relation measured from previous X-ray studies. The integrated Comptonization, Y SZ, is derived from the SZ measurements, using additional information from the X-ray-measured gas density profiles and a universal temperature profile. We calculate scaling relations between the X-ray and SZ observables and find results generally consistent with other measurements and the expectations from simple self-similar behavior. Specifically, we fit a Y SZ-YX relation and find a normalization of 0.82 ± 0.07, marginally consistent with the predicted ratio of Y SZ/YX = 0.91 ± 0.01 that would be expected from the density and temperature models used in this work. Using the YX -derived mass estimates, we fit a Y SZ-M 500 relation and find a slope consistent with the self-similar expectation of Y SZvpropM 5/3 with a normalization consistent with predictions from other X-ray studies. We find that the SZ mass estimates, derived from cosmological simulations of the SPT survey, are lower by a factor of 0.78 ± 0.06 relative to the X-ray mass estimates. This offset is at a level of 1.3σ when considering the ~15% systematic uncertainty for the simulation-based SZ masses. Overall, the X-ray measurements confirm that the scaling relations of the SZ-selected clusters are consistent with the properties of other X-ray-selected samples of massive clusters, even allowing for the broad redshift range (0.29 < z < 1.08) of the sample.

The flare of radiation from the tidal disruption and accretion of a star can be used as a marker for supermassive black holes that otherwise lie dormant and undetected in the centres of distant galaxies1. Previous candidate flares2, 3, 4, 5, 6 have had declining light curves in good agreement with expectations, but with poor constraints on the time of disruption and the type of star disrupted, because the rising emission was not observed. Recently, two ‘relativistic’ candidate tidal disruption events were discovered, each of whose extreme X-ray luminosity and synchrotron radio emission were interpreted as the onset of emission from a relativistic jet7, 8, 9, 10. Here we report a luminous ultraviolet–optical flare from the nuclear region of an inactive galaxy at a redshift of 0.1696. The observed continuum is cooler than expected for a simple accreting debris disk, but the well-sampled rise and decay of the light curve follow the predicted mass accretion rate and can be modelled to determine the time of disruption to an accuracy of two days. The black hole has a mass of about two million solar masses, modulo a factor dependent on the mass and radius of the star disrupted. On the basis of the spectroscopic signature of ionized helium from the unbound debris, we determine that the disrupted star was a helium-rich stellar core.

We present a novel quantitative scheme of cluster classification based on the morphological properties that are manifested in X-ray images. We use a conventional radial surface brightness concentration parameter (c_SB) as defined previously by others and a new asymmetry parameter, which we define in this paper. Our asymmetry parameter, which we refer to as photon asymmetry (A_phot), was developed as a robust substructure statistic for cluster observations with only a few thousand counts. To demonstrate that photon asymmetry exhibits better stability than currently popular power ratios and centroid shifts, we artificially degrade the X-ray image quality by (1) adding extra background counts, (2) eliminating a fraction of the counts, (3) increasing the width of the smoothing kernel, and (4) simulating cluster observations at higher redshift. The asymmetry statistic presented here has a smaller statistical uncertainty than competing substructure parameters, allowing for low levels of substructure to be measured with confidence. A phot is less sensitive to the total number of counts than competing substructure statistics, making it an ideal candidate for quantifying substructure in samples of distant clusters covering a wide range of observational signal-to-noise ratios. Additionally, we show that the asymmetry-concentration classification separates relaxed, cool-core clusters from morphologically disturbed mergers, in agreement with by-eye classifications. Our algorithms, freely available as Python scripts (https://github.com/ndaniyar/aphot), are completely automatic and can be used to rapidly classify galaxy cluster morphology for large numbers of clusters without human intervention.