All posts under tag "Astrophysics"
In this paper we first provide a brief history of the handful of X-ray polarization measurements that have been accomplished to date. We then outline new technological developments that indicate more detailed polarization measurements in this important band of the spectrum can, and should be, accomp
The maximum mass of a neutron star (NS) is poorly defined. Theoretical attempts to define this mass have thus far been unsuccessful. Observational results currently provide the only means of narrowing this mass range down. Eclipsing X-ray binary (XRB) pulsar systems are the only interacting binaries
In fully general relativity, we calculate the images of the radiatively inefficient accretion flow (RIAF) surrounding a Kerr black hole with arbitrary spins, inclination angles, and observational wavelengths. For the same initial conditions, such as the fixed accretion rate, it is found that the int
According to the Carter-Israel conjecture, the end-state of the gravitational collapse of matter is a Kerr-Newman black hole. Nevertheless, neither the theory nor observations can confirm that. In this talk, we discuss the possibility that the collapsing matter can create a super-spinning compact ob
Numerical studies have been performed to interpret the observed 'shock overtaking magnetic cloud (MC)' event by a 2.5 dimensional magnetohydrodynamic (MHD) model in heliospheric meridional plane. Results of an individual MC simulation show that the MC travels with a constant bulk flow speed. The MC
Numerical studies of the interplanetary 'multiple magnetic clouds (Multi-MC)' are performed by a 2.5-dimensional ideal magnetohydrodynamic (MHD) model in the heliospheric meridional plane. Both slow MC1 and fast MC2 are initially emerged along the heliospheric equator, one after another with differe
During its second Antarctic flight, the CREAM (Cosmic Ray Energetics And Mass) balloon experiment collected data for 28 days, measuring the charge and the energy of cosmic rays (CR) with a redundant system of particle identification and an imaging thin ionization calorimeter. Preliminary direct meas
We introduce a new time-domain method for computing the self-force acting on a scalar particle in a Schwarzschild geometry. The principal feature of our method consists in the division of the spatial domain into several subdomains and locating the particle at the interface betweem two them. In this
Tidal disruption by massive black holes is a phenomenon, during which a large part of gravitational energy can be released on a very short time-scale. The time-scales and energies involved during X-ray and IR flares observed in Galactic centre suggest that they may be related to tidal disruption eve
We are leading a comprehensive multi-waveband monitoring program of 34 gamma-ray bright blazars designed to locate the emission regions of blazars from radio to gamma-ray frequencies. The 'maps' are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an an
Assuming that neutrinos of non-zero rest mass dominate the mass density in the universe, and also the mass density on the scale of clusters of galaxies, one obtains the upper limit m < 20 eV/c^2 on their mass, independent of the values of H_0 and q_0, and the lower limit m > 5 eV/c^2 independent of
NASA's Kepler Mission revolutionized exoplanet science in the early part of the 2010's. Looking back from the perspective of the end of that decade, Kepler appears to have burst upon the scene ready for battle, like Athena springing forth, fully formed, from the head of Zeus. The story was not so si
Cygnus X-1 was the first X-ray source widely accepted to be a black hole candidate and remains among the most studied astronomical objects in its class. The detection of non-thermal radio, hard X-rays and gamma rays reveals the fact that this kind of objects are capable of accelerating particles up
We show that the existence of a degenerate halo of sterile neutrinos with rest mass of 17.4 keV near the Galactic Center can account for both the excess 8.7 keV emission observed by the $Suzaku$ mission and the power needed ($10^{40}$ erg s$^{-1}$) to maintain the high temperature of the hot gas (8
The Unruh's thermal state in the vicinity of the event horizon of the black hole provides conditions where impinging particles can radiate other particles. The subsequent decays may eventually lead to observable radiation of photons and neutrinos induced even by massive particles with gravitational
The accretion/ejection coupling in accreting black hole binaries has been described by empirical relations between the X-ray/radio and X-ray/optical-infrared luminosities. These correlations were initially supposed to be universal. However, recently many sources have been found to produce jets that,
We analyzed the longest phase-connected photometric dataset available for NGC 1313 X-2, looking for the ~6 day modulation reported by Liu et al. (2009). The folded B band light curve shows a 6 days periodicity with a significance slightly larger than 3 sigma. The low statistical significance of this
We have recently developed a new method for adjusting weights to minimize sidelobes in the spectral window. Here we show the results of applying this method to published two-site velocity observations of three stars (alpha Cen A, alpha Cen B and beta Hyi). Compared to our previous method of minimizi
An important approach to studying high-energy cosmic rays is the investigation of the properties of extensive air showers; however, the lateral distribution of particles in simulations of such showers strongly depends on the applied model of low-energy hadronic interactions. It has been shown that m
POLAR is a novel compact Compton X-ray polarimeter designed to measure the linear polarization of the prompt emission of Gamma Ray Bursts (GRB) and other strong transient sources such as soft gamma repeaters and solar flares in the energy range 50-500 keV. A detailed measurement of the polarization
The measurement of an excess in the cosmic-ray electron spectrum between 300 and 800 GeV by the ATIC experiment has - together with the PAMELA detection of a rise in the positron fraction up to 100 GeV - motivated many interpretations in terms of dark matter scenarios; alternative explanations assum
We present a wide field study of the Globular Clusters/Low Mass X-ray Binaries connection in the cD elliptical NGC1399, combining HST/ACS and Chandra high resolution data. We find evidence that LMXB formation likelihood is influenced by GCs structural parameters, in addition to the well known effect
In this paper, we present the re-analysis of a set of RXTE-PCA observations of OAO 1657-415 during 1997 August 20-27. We found a revised timing solution which was found to fit better to the data and updated pulse frequency values accordingly. We also verified that there is a marginal correlation bet
We have developed 3D, compressible, non-linear radiative MHD simulations to study the influence of the magnetic field of various strength and geometry on the turbulent convective cells and on the excitation mechanisms of the acoustic oscillations. The results reveal substantial changes of the granul
The geo and solar magnetic fields have long been thought to be very different objects both in terms of spatial structure and temporal behavior. The recently discovered field structure of a fully convective star is more reminiscent of planetary magnetic fields than the Sun's magnetic field (Donati J.
The Telescope Array (TA) is the largest experiment in the northern hemisphere studying ultrahigh energy cosmic rays. TA is a hybrid experiment, which means it has two detector systems: a surface detector and a fluorescence detector. In this paper we report on results from TA on the spectrum, composi
We model the light-curves from radiation-driven clouds near an accreting black hole. Taking into account the multiple images due to strong gravitational lensing, we find that sharp spikes can significantly enhance the observed flux. Following our previous work (Horak & Karas 2006a,b) we assume that
The Geminga gamma ray source was first detected by the SAS-2 gamma-ray satellite observatory and the COS-B x-ray satellite observatory, and has been identified as a radio-quiet pulsar associated with a 300,000 year old supernova remnant. Geminga is one of the brightest GeV sources as seen by the Lar
We present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation
Through detection by low gravitational wave space interferometers, the capture of stars by supermassive black holes will constitute a giant step forward in the understanding of gravitation in strong field. The impact of the perturbations on the motion of the star is computed via the tail, the back-s
Upward through-going muons in the Lake Baikal Neutrino Experiment arriving from the ecliptic plane have been analyzed using NT200 data samples of the years 1998-2002 (1007 live days). We derive upper limits on muon fluxes from annihilation processes of hypothetical WIMP dark matter particles in the
We combine Chandra and XMM-Newton X-ray data from our previous papers with new X-ray observations and with Spitzer mid-infrared data in order to study the nature of the nuclei of radio galaxies and radio-loud quasars with z<1.0 from the 3CRR sample. The significant increase in sample size over our p
We report on the first long (~32 ks) pointed XMM-Newton observation of the supergiant fast X-ray transient IGRJ16479-4514. Results from the timing, spectral and spatial analysis of this observation show that the X-ray source IGRJ16479-4514 underwent an episode of sudden obscuration, possibly an X-
Seven years ago, with the encouragement of the NSF and AURA, NOAO requested proposals from the community to partner with the national observatory to improve instrumentation and/or telescope capabilities at KPNO and CTIO. Of the proposals that were selected, one came from the University of Maryland w
Recent observational data on the type Ia supernova rates are in excellent agreement with the old prediction of the population synthesis of binary stars and confirm that the overwhelming majority of type Ia supernovas (~99%) in elliptical galaxies form via mergers of binary white dwarfs with the tota
The NOAO NEWFIRM Pipeline produces instrumentally calibrated data products and data quality measurements from all exposures taken with the NOAO Extremely Wide-Field Infrared Imager (NEWFIRM) at the KPNO Mayall 4-meter telescope. We describe the distributed nature of the NEWFIRM Pipeline, the calibra
I use cosmology examples to illustrate that the second law of thermodynamics is not old and tired, but alive and kicking, continuing to stimulate interesting research on really big puzzles. The question 'Why is the entropy so low?' (despite the second law) suggests that our observable universe is me
Using molecular dynamics simulations, we determine the structure of neutron star crust made of rapid proton capture nucleosynthesis material. We find a regular body centered cubic lattice, even with the large number of impurities that are present. Low charge $Z$ impurities tend to occupy interstitia
Here the results of calculations of pulses in Cherenkov light detectors for the Yakutsk array are presented. As long as the Vavilov-Cherenkov light is used to calibrate signals in scintillation detectors at the Yakutsk array it is vital for these measurements to be precise. The validity of measureme
We discuss the origin of continuum and line X-ray emission observed in the direction the Galactic Center. We predict a significant flux of de-excitation gamma-ray lines in this direction, which can be produced by subrelativistic protons generated by accretion processes.
We analyzed the Subaru/Suprime-Cam images of 73P/Schwassmann-Wachmann 3B and detected no fewer than 154 mini-comets. We applied synchrone-syndyne analysis, modified for rocket effect analysis, to the mini-fragment spatial distribution. We found that most of these mini-comets were ejected from fragme
Adaptive optics will almost completely remove the effects of atmospheric turbulence at 10 microns on the Extremely Large Telescope (ELT) generation of telescopes. In this paper, we observationally confirm that the next most important limitation to image quality is atmospheric dispersion, rather than
The Galactic hard X-ray transient SWIFT J195509+261406 was first observed as a gamma-ray burst GRB 070610. Within 3 days after the burst, more than forty optical flares had been observed. Here we propose that this peculiar event should be associated with a white dwarf. The hard X-ray burst itself ma
PKS 0558-504 is the brightest radio-loud Narrow-Line Seyfert 1 galaxy at X-ray energies. Here we present results from the radio, optical, UV, and X-ray bands obtained with Swift, XMM, and ATCA during a 10-day monitoring campaign in September 2008. The simultaneous coverage at several wavelengths mak
Today, the motion of spacecraft is still described by the classical Newtonian equations of motion plus some relativistic corrections. This approach might become cumbersome due to the increasing precision required. We use the Relativistic Motion Integrator (RMI) approach to numerically integrate the
Broad absorption line quasars (BALQSOs) are key objects for studying the structure and emission/absorption properties of AGN. However, despite their fundamental importance, the properties of BALQSOs are still not well understood. In order to investigate the X-ray nature of these sources, as well as
The reason for the horizontal turn of the Tunguska-1908 bolide trajectory remains difficult to understand. It finds explanation, however, in the New Explosive Cosmogony of minor bodies as having been caused by an explosion of a part (M up to 10^12 g) of the comet nucleus whose ices contained product
Multi-wavelength spectroscopy can be used to constrain the dust and gas properties in debris disks. Circumstellar dust absorbs and scatters incident stellar light. The scattered light is sometimes resolved spatially at visual and near-infrared wavelengths using high contrast imaging techniques that
Politano and Pouquet's law, a generalization of Kolmogorov's four-fifths law to incompressible MHD, makes it possible to measure the energy cascade rate in incompressible MHD turbulence by means of third-order moments. In hydrodynamics, accurate measurement of third-order moments requires large amou
Current projects for large telescopes demand a proper knowledge of atmospheric turbulence to design efficient adaptive optics systems in order to reach large Strehl ratios. However, the proper characterization of the turbulence above a particular site requires long-term monitoring. Due to the lack o
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