1806.02841
Calibrating long period variables as standard candles with machine learning
Rau, Koposov, Trac, Mandelbaum
Variable stars with well-calibrated period-luminosity relationships provide accurate distance measurements to nearby galaxies and are therefore a vital tool for cosmology and astrophysics. While these measurements typically rely on samples of Cepheid and RR-Lyrae stars, abundant populations of luminous variable stars with longer periods of 10-1000 days remain largely unused. Apply machine learning to derive a mapping between light curve features of these variable stars and their magnitude to extend the traditional period-luminosity (PL) relation commonly used for Cepheid samples. Using photometric data for long period variable stars in the LMC, demonstrate that the predictions produce residual errors comparable to those obtained on the corresponding Cepheid population. Show that the model generalizes well to other samples by performing a blind test on photometric data from the SMC. The predictions on the SMC again show small residual errors and biases, comparable to results that employ PL relations fitted on Cepheid samples. The residual biases are complementary between the long period variable and Cepheid fits, which provides exciting prospects to better control sources of systematic error in cosmo distance measurements. Finally show that the proposed methodology can be used to optimize samples of variable stars as standard candles independent of any prior variable star classification.
1806.03356
A family-based method of quantifying NEOWISE diameter errors
Masiero, Mainzer, Wright
Quantifying the accuracy with which physical properties of asteroids can be determined from thermal modeling is critical to measuring the impact of IR data on the understanding of asteroids. Previous work (Mainzer+ 2011) has used independently-derived diameters (from asteroid radar, occultation, and spacecraft visits) to test the accuracy of the NEOWISE diameter determinations. Here, present a new and different method for bounding the actual NEOWISE diameter errors in the Main Belt based on the knowledge of the albedos of asteroid families. Show the 1 sigma relative diameter error for the Main Belt population must be less than 17.5% for the vast majority of objects. For a typical uncertainty on H magnitude of 0.2 mag, the relative error on diameter for the population would be ~10%.
1806.03681
CAMELOT: Cubesats Appiled for MEasuring and LOcalising Transients - mission overview
Werner, et al
Propose a fleet of nano satellites to perform an all-sky monitoring and timing based localization of gamma-ray transients. The fleet of at least nine 3U cubesats shall be equipped with large and thin CsI (Tl) scintillator based soft gamma-ray detectors read out by multi-pixel photon counters. For bright short gamma-ray bursts (GRBs), by cross-correlating their light curves, the fleet shall be able to determine the time difference of the arriving GRB signal between the satellites and thus determine the source position with an accuracy of ~10'. This requirement demands precise time synchronization and accurate time stamping of the detected gamma-ray photons, which will be achieved by using on-board GPS receivers. Rapid follow up observations at other wavelengths require the capability for fast, nearly simultaneous downlink of data using a global inter satellite communication network. In terms of all-sky coverage, the proposed fleet will outperform all GRB monitoring missions.
1806.04150
The distance to the galaxy coma P
Anand, et al
If the extremely low surface brightness galaxy Coma P lies at 5.5±0.3 Mpc as recently proposed, then it would have an extraordinarily deviant peculiar velocity of ~900 km/s at a location where differential velocities between galaxies are low. Access images from the HST archives used to derive the literature distance from the magnitude of the tip of the red giant branch. This paper's analysis gives the distance to be 10.9±1.0 Mpc. At this location the galaxy lies within the infall region of the Virgo Cluster, such that its still considerable peculiar velocity of ~500 km/s is consistent with an established model. Coma P has an unusually pronounced asymptotic giant branch relative to its red giant branch. The dominant stellar population is just a few Gyr old.
1806.04292
A new measure of tension between experiments
Adhikari, Huterer
Tensions between cosmo measurements by different surveys or probes have always been important --- and are presently much discussed --- as they may lead to evidence of new physics. Several tests have been devised to probe the consistency of datasets given a cosmological model, but they often have undesired features such as dependence on the Bayesian priors given to parameters, or burdensome requirements such as that of near-Gaussian posterior distributions. Propose a new quantity, defined in a similar way as the Bayesian evidence ratio and therefore calibrated on the familiar Jeffreys scale, in which these undesired properties are absent. Test the quantity on simple models with Gaussian and non-Gaussian likelihoods. Then apply it to data from the Planck satellite: Investigate the consistency of the TT and EE angular power spectrum measurements, as well as the mutual consistency of small- and large-angle portion of each measurement, finding mild (~2-3 sigma) discrepancies in agreement with previous work.
1806.04649
Concordance and Discordance in Cosmology
Raveri, Hu
The success of present and future cosmological studies is tied to the ability to detect discrepancies in complex data sets within the framework of a cosmological model. Tensions caused by the presence of unknown systematic effects need to be isolated and corrected to increase the overall accuracy of parameter constraints, while discrepancies due to new physical phenomena need to be promptly identified. Develop a full set of estimators of internal and mutual agreement and disagreement, whose strengths complement each other. These allow to take into account the effect of prior information and compute the statistical significance of both tensions and confirmatory biases. Apply them to a wide range of state of the art cosmo probes and show that these estimators can be easily used, regardless of model and data complexity. Derive a series of results that show that discrepancies indeed arise within the standard LCDM model. Several of them exceed the probability threshold of 95% and deserve a dedicated effort to understand their origin.
1806.04668
The Universe at extreme magnification
Diego
Extreme magnifications of distant objects by factors of several thousand have recently become a reality. Small very luminous compact objects, such as SNe, giant stars at z=1-2, Pop III stars at z>7 and even gravitational waves from merging binary black holes near caustics of gravitational lenses can be magnified to many thousands or even tens of thousands thanks to their small size. Explore the probability of such extreme magnifications in a cosmological context including also the effect of micro lenses near critical curves. Show how a natural limit to the maximum magnification appears due to the presence of micro lenses near critical curves. Use a combination of state of the art halo mass functions, high-resolution analytical models for the density profiles and inverse ray tracing to estimate the probability of magnification near caustics. Estimate the rate of highly-magnified events in the case of SNe, GW and very luminous stars including Pop III stars. The findings reveal that future observations will increase the number of events at extreme magnifications opening the door not only to study individual sources at cosmic distances but also to constrain compact dark matter candidates.
1806.05189
A dynamical origin for planets in triple star systems
Fragile, Loeb, Ginsburg
Recent radial velocity and transit data discovered ~100 planets in binary or triple stellar systems out of the entire population of a few thousand known planets. Stellar companions are expected to strongly influence both the formation and the dynamical evolution of planets in multiple star systems. Explore the possibility that planets in triples are formed as a consequence of the dynamical interactions of binaries in star clusters. Simulations show that the probability of forming triple star systems with a planet is in the range 0.5-3%, potentially accounting for most of the observed population. The recently launched TESS satellite is expected to find a larger sample of planets in triple systems.
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