Level Of Systematic Uncertainty Research Articles

Starlight-polarization-based tomography of the magnetized ISM: PASIPHAE’s line-of-sight inversion method

We present the first Bayesian method for tomographic decomposition of the plane-of-sky orientation of the magnetic field with the use of stellar polarimetry and distance. This standalone tomographic inversion method presents an important step forward in reconstructing the magnetized interstellar medium (ISM) in three dimensions within dusty regions. We develop a model in which the polarization signal from the magnetized and dusty ISM is described by thin layers at various distances, a working assumption which should be satisfied in small-angular circular apertures. Our modeling makes it possible to infer the mean polarization (amplitude and orientation) induced by individual dusty clouds and to account for the turbulence-induced scatter in a generic way. We present a likelihood function that explicitly accounts for uncertainties in polarization and parallax. We develop a framework for reconstructing the magnetized ISM through the maximization of the log-likelihood using a nested sampling method. We test our Bayesian inversion method on mock data, representative of the high Galactic latitude sky, taking into account realistic uncertainties from Gaia and as expected for the optical polarization survey PASIPHAE according to the currently planned observing strategy. We demonstrate that our method is effective at recovering the cloud properties as soon as the polarization induced by a cloud to its background stars is higher than ~0.1% for the adopted survey exposure time and level of systematic uncertainty. The larger the induced polarization is, the better the method’s performance, and the lower the number of required stars. Our method makes it possible to recover not only the mean polarization properties but also to characterize the intrinsic scatter, thus creating new ways to characterize ISM turbulence and the magnetic field strength. Finally, we apply our method to an existing data set of starlight polarization with known line-of-sight decomposition, demonstrating agreement with previous results and an improved quantification of uncertainties in cloud properties.

EXTP perspectives for the νMSM sterile neutrino dark matter model

We discuss the potential of the eXTP X-ray telescope, in particular its Spectroscopic Focusing Array (SFA), Large Area Detector (LAD) and Wide Field Monitor (WFM) for the detection of a signal from keV-scale decaying dark matter. We show that the sensitivity of the eXTP is sufficient to improve existing constraints on the mixing angle of the neutrino Minimal extension of the Standard Model ($\nu$MSM) by a factor of 5-10 within the dark matter mass range 2-50 keV, assuming 1% level of systematic uncertainty. We assert that the eXTP will be able to probe previously inaccessible range of $\nu$MSM parameters and serve as a precursor for the Athena mission in decaying dark matter searches.

Quantifying the accuracy of the Alcock-Paczyński scaling of baryon acoustic oscillation measurements

We investigate—in a generic setting—the regime of applicability of the Alcock-Paczyński (AP) scaling conventionally applied to test different cosmological models, given a fiducial measurement of the baryon acoustic oscillation (BAO) characteristic scale in the galaxy 2-point correlation function. We quantify the error in conventional AP scaling methods, for which our ignorance about the true cosmology is parameterised in terms of two constant AP scaling parameters. We propose a new, and as it turns out, improved version of the constant AP scaling, also consisting of two scaling parameters.The two constant AP scaling methods are almost indistinguishable when the fiducial model used in data reduction and the “true” underlying cosmology are not differing substantially in terms of metric gradients, but are otherwise expected to differ. Our new methods can be applied to existing analyses through a reinterpretation of the results of the conventional AP scaling. This reinterpretation might be important in model universes where curvature gradients above the scale of galaxies are significant.We test our theoretical findings on ΛCDM mock catalogues. The conventional constant AP scaling methods are surprisingly successful for pairs of large-scale metrics, but eventually break down when toy models allowing for large metric gradients are tested. The new constant AP scaling methods proposed in this paper are efficient for all test models examined. We find systematic errors of ∼1% in the recovery of the BAO scale when the true model is distant from the fiducial, which cannot be attributed to any constant AP approximation. The level of systematic uncertainty is robust to the exact fitting method employed. This indicates that caution must be taken with the error budget when extrapolating the BAO acoustic scale measurements obtained in the standard literature.

CHEERS: The chemical evolution RGS sample

The chemical yields of supernovae and the metal enrichment of the hot intra-cluster medium (ICM) are not well understood. This paper introduces the CHEmical Enrichment RGS Sample (CHEERS), which is a sample of 44 bright local giant ellipticals, groups and clusters of galaxies observed with XMM-Newton. This paper focuses on the abundance measurements of O and Fe using the reflection grating spectrometer (RGS). The deep exposures and the size of the sample allow us to quantify the intrinsic scatter and the systematic uncertainties in the abundances using spectral modeling techniques. We report the oxygen and iron abundances as measured with RGS in the core regions of all objects in the sample. We do not find a significant trend of O/Fe as a function of cluster temperature, but we do find an intrinsic scatter in the O and Fe abundances from cluster to cluster. The level of systematic uncertainties in the O/Fe ratio is estimated to be around 20-30%, while the systematic uncertainties in the absolute O and Fe abundances can be as high as 50% in extreme cases. We were able to identify and correct a systematic bias in the oxygen abundance determination, which was due to an inaccuracy in the spectral model. The lack of dependence of O/Fe on temperature suggests that the enrichment of the ICM does not depend on cluster mass and that most of the enrichment likely took place before the ICM was formed. We find that the observed scatter in the O/Fe ratio is due to a combination of intrinsic scatter in the source and systematic uncertainties in the spectral fitting, which we are unable to disentangle. The astrophysical source of intrinsic scatter could be due to differences in AGN activity and ongoing star formation in the BCG. The systematic scatter is due to uncertainties in the spatial line broadening, absorption column, multi-temperature structure and the thermal plasma models. (Abbreviated).

Bounding the Higgs width at the LHC: Complementary results fromH→WW

We investigate the potential of the process $gg \to H \to WW$ to provide bounds on the Higgs width. Recent studies using off-shell $H\rightarrow ZZ$ events have shown that Run 1 LHC data can constrain the Higgs width, $\Gamma_H < (25-45) \Gamma_{H}^{\rm SM}$. Using 20 fb-1 of 8 TeV ATLAS data, we estimate a bound on the Higgs boson width from the WW channel between $\Gamma_H < (100-500) \Gamma_H^{SM}$. The large spread in limits is due to the range of cuts applied in the existing experimental analysis. The stricter cuts designed to search for the on-shell Higgs boson limit the potential number of off-shell events, weakening the constraints. As some of the cuts are lifted the bounds improve. We show that there is potential in the high transverse mass region to produce upper bounds of the order of $(25-50) \Gamma_H^{SM}$, depending strongly on the level of systematic uncertainty that can be obtained. Thus, if these systematics can be controlled, a constraint on the Higgs boson width from the $H \to WW$ decay mode can complement a corresponding limit from $H \to ZZ$.

SYSTEMATIC PROBLEMS WITH USING DARK MATTER SIMULATIONS TO MODEL STELLAR HALOS

The limits of available computing power have forced models for the structure of stellar halos to adopt one or both of the following simplifying assumptions: (1) stellar mass can be "painted" onto dark matter particles in progenitor satellites; (2) pure dark matter simulations that do not form a luminous galaxy can be used. We estimate the magnitude of the systematic errors introduced by these assumptions using a controlled set of stellar halo models where we independently vary whether we look at star particles or painted dark matter particles, and whether we use a simulation in which a baryonic disk galaxy forms or a matching pure dark matter simulation that does not form a baryonic disk. We find that the "painting" simplification reduces the halo concentration and internal structure, predominantly because painted dark matter particles have different kinematics than star particles even when both are buried deep in the potential well of the satellite. The simplification of using pure dark matter simulations reduces the concentration further, but increases the internal structure, and results in a more prolate stellar halo. These differences can be a factor of 1.5-7 in concentration (as measured by the half-mass radius) and 2-7 in internal density structure. Given this level of systematic uncertainty, one should be wary of overinterpreting differences between observations and the current generation of stellar halo models based on dark matter only simulations when such differences are less than an order of magnitude.

WAVELENGTH ACCURACY OF THE KECK HIRES SPECTROGRAPH AND MEASURING CHANGES IN THE FINE STRUCTURE CONSTANT

We report on an attempt to accurately wavelength calibrate four nights of data taken with the Keck HIRES spectrograph on QSO PHL957, for the purpose of determining whether the fine structure constant was different in the past. Using new software and techniques, we measured the redshifts of various Ni II, Fe II, Si II, etc. lines in a damped Lyalpha system at z = 2.309. Roughly half the data were taken through the Keck iodine cell which contains thousands of well calibrated iodine lines. Using these iodine exposures to calibrate the normal Th-Ar Keck data pipeline output, we found absolute wavelength offsets of 500 m s{sup -1} to 1000 m s{sup -1} with drifts of more than 500 m s{sup -1} over a single night, and drifts of nearly 2000 m s{sup -1} over several nights. These offsets correspond to an absolute redshift of uncertainty of about DELTAz approx 10{sup -5}(DELTAlambda approx 0.02 A), with daily drifts of around DELTAz approx 5 x 10{sup -6} (DELTAlambda approx 0.01 A), and multiday drifts of nearly DELTAz approx 2 x 10{sup -5}(approx0.04 A). The causes of the wavelength offsets are not known, but since claimed shifts in the fine structure constant wouldmore » result in velocity shifts of less than 100 m s{sup -1}, this level of systematic uncertainty may make it difficult to use Keck HIRES data to constrain the change in the fine structure constant. Using our calibrated data, we applied both our own fitting software and standard fitting software to measure DELTAalpha/alpha, but discovered that we could obtain results ranging from significant detection of either sign, to strong null limits, depending upon which sets of lines and which fitting method were used. We thus speculate that the discrepant results on DELTAalpha/alpha reported in the literature may be due to random fluctuations coming from underestimated systematic errors in wavelength calibration and fitting procedure.« less