Engineering topics

Linder, Eric V.

Publications and source records attributed to Linder, Eric V..

10 records

DOE OSTI2024

Model independent approach for calculating galaxy rotation curves for low S/N MaNGA galaxies

Internal kinematics of galaxies, traced through the stellar rotation curve or two dimensional velocity map, carry important information on galactic structure and dark matter. With upcoming surveys, the velocity map may play a key role in the development of kinematic lensing as an astrophysical probe. Here, we improve techniques for extracting velocity information from integral field spectroscopy at low signal-to-noise (S/N), without a template, and demonstrate substantial advantages over the standard Penalized PiXel-Fitting method (pPXF) approach. Robust rotation curves can be derived down to S/N ≈ 2 using our method.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Testing α -attractor quintessential inflation against CMB and low-redshift data

Due to universality and attractor properties, α-attractor quintessential inflation establishes direct relations between inflationary observables such as the scalar tilt n s and the tensor-to-scalar ratio r, and late-time dark energy equation of state parameters w 0 and w a . In this work, we examine three different physically motivated regimes, considering complete freedom in the parameter α, models inspired by supergravity where α takes on values up to α = 7/3, and Starobinsky inflation (α = 1). We investigate the consistency and constraints imposed by Cosmic Microwave Background measurements from the Planck satellite, B-mode polarization data from the BICEP/Keck collaboration, and low-redshift observations. Additionally, we consider small-scale CMB measurements released by the Atacama Cosmology Telescope, which give results approaching the Harrison– Zel’dovich spectrum (n s ≈ 1). Here α-attractors lead to an improved fit over $\Lambda$CDM. For the large-scale CMB measurements, α ≳ 2 models can provide equally good fits as $\Lambda$CDM

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1994

Do gamma-ray bursts originate from an extended Galactic Halo of high-velocity neutron stars?

The gamma-ray burst brightness distribution is inhomogeneous and the distribution on the sky is nearly isotropic. These features argue against an association of gamma-ray bursts with those Galactic objects that are known to exhibit a strong concentration toward the Galactic center or plane. The observed statistical properties indicate a cosmological origin. Circumstantial evidence suggests that neutron stars are involved in the burst phenomenon. Here we consider Population II neutron stars in an extended Galactic Halo (EGH) as an alternative to cosmological scenarios. The Burst and Transient Source Experiment (BATSE) data indicate a small deviation from isotropy near the 2 sigma level of statistical significance. If confirmed for an increasing number of bursts, these anisotropies could rule out cosmological scenarios. On the other hand, EGH models require small anisotropies like those observed by BATSE. We consider simple distribution models to determine the generic properties such halos must have to be consistent with the observations and discuss the implications of the corresponding distance scale on burst models.

Hartmann, Dieter H.↗

NASA NTRS1992

Cosmic gamma-ray bursts from BATSE - Another great debate

The BATSE detectors aboard Compton Observatory record about one cosmic gamma-ray burst (GRB) per day. Preliminary data analysis shows a highly isotropic sky map and a nonuniform brightness distribution. Anisotropies expected from a Galactic neutron star population, the most frequently considered source model, did not emerge from the data. Taken at face value, the data seem to suggest a heliocentric solution of the GRB puzzle. The observed isotropy can be achieved if sources are either very near or extragalactic. Pop I neutron stars in the disk do not simultaneously fit sky and brightness distributions. A possibility are sources in an extended Galactic halo with scale length large enough to avoid strong anisotropies due to the solar offset from the Galactic center. If GRBs are located in an extended halo we ask whether the neutron star paradigm can survive. We show that the recently discovered high velocity radio pulsars may provide a natural source population for GRBs. If these pulsars formed in the halo, as suggested by the radio data, the possibility arises that GRBs and high velocity pulsars are two related phenomena that provide observational evidence of the dark Galactic corona. We also discuss cosmological redshift constraints that follow from the observed brightness distribution.

Hartmann, Dieter H.↗

NASA NTRS1991

Fuzzy correlations of gamma-ray bursts

The origin of gamma-ray bursts is not known, both in the sense of the nature of the source emitting the radiation and literally, the position of the burst on the sky. Lacking unambiguously identified counterparts in any wavelength band studied to date, statistical approaches are required to determine the burster distance scale. Angular correlation analysis is one of the most powerful tools in this regard. However, poor detector resolution gives large localization errors, effectively beam smearing the positions. The resulting fuzzy angular correlation function is investigated and the generic isotropization that smearing induces on any intrinsic clustering is discussed. In particular, the extent to which gamma-ray burst observations by the BATSE detector aboard the Gamma-Ray Observatory might recover an intrinsic source correlation is investigated.

Hartmann, Dieter H.↗

NASA NTRS1988

Clustering correlations and limits on cosmological gravitational waves

A cosmological background of gravitational waves induces angular deviations in the propagation of light traversing it. All observed astrophysical sources might therefore have varying apparent positions, with the time dependence set by the wave period. Wavelengths greater than a kiloparsec are examined, so the positions are frozen but in general correlated. Comparison with observed galaxy-galaxy n-point correlation functions provide a spectrum-independent limit on the energy density of gravitational waves with wavelengths between a few tens of kiloparsecs and a few hundred megaparsecs of Omega(GW) less than 0.001.

Linder, Eric V.↗

NASA NTRS1988

Microwave background anisotropy induced by gravitational waves

A cosmological background of gravitational waves induces redshift perturbations in light transversing it. Calculations of this Sachs-Wolfe effect on the microwave background are presented in an Omega = 1 Friedmann universe as a function of angular scale and gravitational wave spectrum. Blurriness of the last-scattering surface can cause nonnegligible dilution of the anisotropy for wavelengths less than about 100 Mpc. The limit implied for the energy density of the gravitational waves is given. A difficulty in associating a linear scale with an angular anisotropy, due to the clumpiness of the universe, is also pointed out.

Linder, Eric V.↗

NASA NTRS1988

Gravitational lensing statistics of amplified supernovae

Amplification statistics of gravitationally lensed supernovae can provide a valuable probe of the lensing matter in the universe. A general probability distribution for amplification by compact objects is derived which allows calculation of the lensed fraction of supernovae at or greater than an amplification A and at or less than an apparent magnitude. Comparison of the computed fractions with future results from ongoing supernova searches can lead to determination of the mass density of compact dark matter components with masses greater than about 0.001 solar mass, while the time-dependent amplification (and polarization) of the expanding supernovae constrain the individual masses. Type II supernovae are found to give the largest fraction for deep surveys, and the optimum flux-limited search is found to be at approximately 23d magnitude, if evolution of the supernova rate is neglected.

Linder, Eric V.↗

NASA NTRS1988

Cosmological gravitational waves

A cosmological background of gravitational waves would alter the propagation of radiation, inducing redshift fluctuations, apparent source position deflections, and luminosity variations. By comparing these astrophysical effects with observations, it is possible to deduce upper limits on the energy density present in gravitational waves. Emphasis is placed on microwave background anisotropy from the redshift deviations and galaxy clustering correlation functions from the angular deviations. Many of the gravitational wave effects are shown to be generalizations of the gravitational lensing formalism.

Linder, Eric V.↗

NASA NTRS1987

Gravitational lensing of supernovae by dark matter candidates of mass M greater than about 0.001 solar masses

A review is presented concerning the gravitational lensing of supernovae by intervening condensed objects, including dark matter candidates such as dim stars and black holes. the expansion of the supernova beam within the lens produces characteristic time-dependent amplification and polarization which depend upon the mass of the lens. The effects of the shearing of the beam due to surrounding masses are considered, although the study of these effects is confined to isolated masses whose size is much less than that of the supernova (about 10 to the 15th cm). Equations for the effects of lensing and graphs comparing these effects in different classes of supernovae are compared. It is found that candidates for lensing would be those supernovae at least as bright as their parent galaxy, or above the range of luminosities expected for their spectral class.

Wagoner, Robert V.↗