Narrow Results

We review the general properties of the intracluster medium (ICM) in clusters that host a cooling flow, and in particular the effects on the ICM of the injection of hot plasma by a powerful active galactic nucleus (AGN). It is observed that, in some cases, the hot plasma produces cavities in the ICM that finally detach and rise, perhaps buoyantly. The gas dynamics induced by the rising bubbles can help in explaining the absence of a cooled gas component in clusters with a cooling flow. This scenario is explored using numerical simulations.

New $0^{+}$ resonance parameters have been extracted and interpreted in terms of the experimental neutron–proton branching ratio of $\mathrm{\text{d+d}}$ reactions in strontium and tantalum metallic lattices. The material-dependent cross-section formula, which includes the new $0^{+}$ resonance parameters, the screening effect, and the effective mass, have been applied to $d{(d,p)}^{3}H$ and ${}^{2}H{(d,n)}^{3}He$ reactions in metallic environments. The contributions of the $0^{+}$ resonance and effective electron mass are crucial to the understanding of the discrepancy between the theoretical and experimental screening values in metallic environments. As a different and connected part of this research, we have proposed a new target material: deuterated metal wire mesh structures. Experimentally confirmed numerical results for Pendry’s effective electron mass for aluminum metal wire have been used to calculate the cross-section and screening energy.

The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear in the literature without firm conclusions. In this paper, we investigate connection between some of the important physical properties like spin-down power, surface magnetic field, equation of state, temperature and gravity associated with the white dwarf in the binary system AR Scorpii and its observational characteristics. We explore the plausible effects of white dwarf surface magnetic field on the absence of substantial accretion in this binary system and also discuss the gravitational wave emission due to magnetic deformation mechanism.

Gravity is so different from other fundamental forces that it is now essentially treated as a nonfundamental force of entropic origin. Several good studies have been carried out in this direction. Quantum gravity is a theory which combines gravity well with quantum physics. However, there are still many impediments to our understanding, especially in the limits of extreme. The effects of quantum gravity start appearing on the scene at Planck length which is the smallest length in nature idealized so far. Although this study incorporates a model which is valid for potential energy corrections at small distances, we have also given a bold try to use it consistently for the corrections at very large distances as well. The model uses two techniques, namely, Boltzmann and Tsallis statistical approaches to explore the thermodynamics within the ambit of the braneworld model giving its further modified version. We have computed the partition function by using both Boltzmann and Tsallis statistical approaches and then used it to study the thermodynamics of the braneworld model. We have analyzed both analytically and graphically, the thermodynamic quantities like Helmholtz free energy and specific heat.

We investigate the matching of continuous gravitational wave (CGW) signals in an all sky search with reference to Earth based laser interferometric detectors. We consider the source location as the parameters of the signal manifold and templates corresponding to different source locations. It has been found that the matching of signals from locations in the sky that differ in their co-latitude and longitude by π radians decreases with source frequency. We have also made an analysis with the other parameters affecting the symmetries. We observe that it may not be relevant to take care of the symmetries in the sky locations for the search of CGW from the output of LIGO-I, GEO600 and TAMA detectors.

We study the Doppler factors for a group blazars at soft X-ray band. In our estimates, we have made the assumptions that (i) blazars can be divided into high-energy-peaked (HEP) objects whose synchrotron peak frequencies ν_p > 10^14.7Hz, and the low-energy-peaked (LEP) objects whose synchrotron peak frequencies ν_p≤10^14.7Hz, and (ii) the intrinsic radiation from a blazar in the energy range from radio to soft X-ray bands is the synchrotron radiation for HEP objects and the soft X-ray emission comes from inverse Compton scattering for LEP objects. Under the above assumptions, we estimate Doppler factors at optical (δ_O) and X-rays (δ_x) for 54 blazars by using the known radio Doppler factors and the observed flux densities in radio, optical and X-ray bands, and Doppler factors at X-ray band in which X-rays are assumed to be produced only by the synchrotron radiation. We get . The Doppler factors are different in various wavebands, and on average, the Doppler factor decreases with frequency from radio to X-ray bands.

We compile all the available optical B band data for the quasar 3C 273 from 1887 to 2001 from the literature, and obtain 1,890 data points. Using these data, we analyze the light curve properties by means of the Jurkevich method and the discrete correlation function (DCF) method. The analysis results of the two methods are self consistent; the cross-checked variability period is 13.51 years. The 13.51-year period variation in the optical band is in good agreement with the previous results in the optical and X-ray bands. However, the other claimed periods of the quasar 3C 273 are not confirmed in our work.

It has been suggested that there could be objects even more compact than neutron stars, like the so-called strange stars, P-stars, and magnetars. Strange stars are collapsed stars consisting of u, d, and s quarks. P-stars are a new class of compact stars made of u and d quarks in β-equilibrium with electrons in an Abelian chromomagnetic condensate. It has also been shown that a particle in a circular orbit around a stationary black hole is subject to a centrifugal force that turns out to be directed inwards if the particle orbit radius is between the Schwarzschild radius r_s and 3r_s/2. Here it is proposed that rotation of a sufficiently compact collapsed object may lead to a centrifugal force induced collapse to a black hole that could emit short gamma-ray bursts.

The Tolman test for surface brightness (SB) dimming was originally proposed as a test for the expansion of the universe. The test, which is independent of the details of the assumed cosmology, is based on comparisons of the SB of identical objects at different cosmological distances. Claims have been made that the Tolman test provides compelling evidence against a static model for the universe. In this paper we reconsider this subject by adopting a static Euclidean universe (SEU) with a linear Hubble relation at all z (which is not the standard Einstein–de Sitter model), resulting in a relation between flux and luminosity that is virtually indistinguishable from the one used for ΛCDM models. Based on the analysis of the UV SB of luminous disk galaxies from HUDF and GALEX datasets, reaching from the local universe to z ~ 5, we show that the SB remains constant as expected in a static universe.

A re-analysis of previously published data used for the Tolman test at lower redshift, when treated within the same framework, confirms the results of the present analysis by extending our claim to elliptical galaxies. We conclude that available observations of galactic SB are consistent with a SEU model.

We do not claim that the consistency of the adopted model with SB data is sufficient by itself to confirm what would be a radical transformation in our understanding of the cosmos. However, we believe this result is more than sufficient reason to examine this combination of hypotheses further.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to $\gamma$-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

Extremely wide binary stars represent ideal systems to probe Newtonian dynamics in the low acceleration regimes ($<10^{-10}$ms${}^{-2}$) typical of the external regions of galaxies. Here, we present a study of 60 alleged wide binary stars with projected separation ranging from 0.004pc to 1pc, probing gravitational accelerations well below the limit where dark matter or modified dynamics theories set in. Radial velocities with accuracy $\sim 100$m/s were obtained for each star, in order to constrain their orbital velocity, that, together with proper motion data, can distinguish bound from unbound systems. It was found that about half of the observed pairs do have velocity in the expected range for bound systems, out of the largest separations probed here. In particular, we identified five pairs with projected separation $>0.15$pc that are useful for the proposed test. While it would be premature to draw any conclusion about the validity of Newtonian dynamics at these low accelerations, our main result is that very wide binary stars seem to exist in the harsh environment of the solar neighborhood. This could provide a tool to test Newtonian dynamics versus modified dynamics theories in the low acceleration conditions typical of galaxies. In the near future the GAIA satellite will provide data to increase significantly the number of wide pairs that, with the appropriate follow-up spectroscopic observations, will allow the implementation of this experiment with unprecedented accuracy.

Since Dirac stated his Large Number Hypothesis the space-time variation of fundamental constants has been an active subject of research. Here we analyze the possible spatial variation of two fundamental constants: the fine structure constant $\alpha$ and the speed of light $c$. We study the effects of such variations on the luminosity distance and on the peak luminosity of Type Ia supernovae (SNe Ia). For this, we consider the change of each fundamental constant separately and discuss a dipole model for its variation. Elaborating upon our previous work, we take into account the variation of the peak luminosity of Type Ia supernovae resulting from the variation of each of these fundamental constants. Furthermore, we also include the change of the energy release during the explosion, which was not studied before in the literature. We perform a statistical analysis to compare the predictions of the dipole model for $\alpha$ and $c$ variation with the Union 2.1 and JLA compilations of SNe Ia. Allowing the nuisance parameters of the distance estimator ${\mu }_0$ and the cosmological density matter ${\mathrm{\Omega }}_{\mathrm{\text{m}}}$ to vary. As a result of our analysis, we obtain a first estimate of the possible spatial variation of the speed of light $c$. On the other hand, we find that there is no significant difference between the several phenomenological models studied here and the standard cosmological model, in which fundamental constants do not vary at all. Thus, we conclude that the actual set of data of Type Ia supernovae does not allow to verify the hypothetical spatial variation of fundamental constants.

Under Newtonian dynamics, the relative motion of the components of a binary star should follow a Keplerian scaling with separation. Once orientation effects and a distribution of ellipticities are accounted for, dynamical evolution can be modeled to include the effects of Galactic tides and stellar mass perturbers, over the lifetime of the solar neighborhood. This furnishes a prediction for the relative velocity between the components of a binary and their projected separation. Taking a carefully selected small sample of 81 solar neighborhood wide binaries from the Hipparcos catalog, we identify these same stars in the recent Gaia DR2, to test the prediction mentioned using the latest and most accurate astrometry available. The results are consistent with the Newtonian prediction for projected separations below 7000 AU, but inconsistent with it at larger separations, where accelerations are expected to be lower than the critical $a_0=1.2\times 10^{-10}$m.s${}^{-2}$ value of MONDian gravity. This result challenges Newtonian gravity at low accelerations and shows clearly the appearance of gravitational anomalies of the type usually attributed to dark matter at galactic scales, now at much smaller stellar scales.

The baryon acoustic oscillation (BAO) peak, seen in the cosmic matter distribution at redshifts up to $\sim$3.5, reflects the continued expansion of the sonic horizon first identified in temperature anisotropies of the cosmic microwave background. The BAO peak position can now be measured to better than $\sim$1% accuracy using galaxies and $\sim$1.4–1.6% precision with Ly-$\alpha$ forests and the clustering of quasars. In conjunction with the Alcock–Paczyński (AP) effect, which arises from the changing ratio of angular to spatial/redshift size of (presumed) spherically-symmetric source distributions with distance, the BAO measurement is viewed as one of the most powerful tools to use in assessing the geometry of the Universe. In this paper, we employ five BAO peak measurements from the final release of the Sloan Digital Sky Survey IV, at average redshifts $〈z〉=0.38$, $0.51$, $0.70$, $1.48$ and $2.33$, to carry out a direct head-to-head comparison of the standard model, $\mathrm{\Lambda }$CDM, and one of its principal competitors, known as the $R_{\mathrm{\text{h}}}=ct$ universe. For completeness, we complement the AP diagnostic with a volume-averaged distance probe that assumes a constant comoving distance scale $r_{\mathrm{\text{d}}}$. Both probes are free of uncertain parameters, such as the Hubble constant, and are therefore ideally suited for this kind of model selection. We find that $R_{\mathrm{\text{h}}}=ct$ is favored by these measurements over the standard model based solely on the AP effect, with a likelihood $\sim$75% versus $\sim$25%, while Planck-$\mathrm{\Lambda }$CDM is favored over $R_{\mathrm{\text{h}}}=ct$ based solely on the volume-averaged distance probe, with a likelihood $\sim 80\%$ versus $\sim$20%. A joint analysis using both probes produces an inconclusive outcome, yielding comparable likelihoods to both models. We are therefore not able to confirm with this work that the BAO data, on their own, support an accelerating Universe.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to γ-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

We present cluster counts and cosmological constraints corresponding to the full Planck mission data set. Our catalogue consists of 439 clusters detected through their Sunyaev-Zel’dovich (SZ) effect and selected with a signal-to-noise cut of 6. Under some modeling assumptions that will be described, we constrain cosmological parameters with a two-dimensional likelihood from the distribution of counts in redshift and signal-to-noise. Cluster model relies on the mass measurement, represented by a mass bias parameter 1 − b. We use priors on 1 − b with mass estimates obtained from gravitational lensing of background galaxies by Planck clusters, and by CMB temperature lensing. We find varying degrees of tension on the present-day amplitude of matter fluctuations with respect to Planck analysis of CMB temperature fluctuations. We also combine CMB and SZ likelihoods to examine constraints on extensions to the base flat ∧CDM model.

We report on the results of two successful, simultaneous observations of Sagittarius A* at the center of the Milky Way. The observations were carried out in 2004 and 2008 using telescopes operating from the mm-radio domain to the X-ray domain, and detected strong flux density variations in all wavelength bands. Modeling suggests that a combination of a synchrotron self Compton process and an adiabatic expansion of source components are at work. The luminous flare emission of Sagittarius A* also supports the presence of an accreting super massive black hole at that position. We also discuss the potential of NIR interferometry for further detailed investigations of the accretion process in SgrA*.

Pulsars are very stable clocks in space which have many applications to problems in physics and astrophysics. Observations of double-neutron-star binary systems have given the first observational evidence for the existence of gravitational waves (GWs) and shown that Einstein’s general theory of relativity is an accurate description of gravitational interactions in the regime of strong gravity. Observations of a large sample of pulsars spread across the celestial sphere forming a “Pulsar Timing Array” (PTA), can in principle enable a positive detection of the GW background in the Galaxy. The Parkes Pulsar Timing Array (PPTA) is making precise timing measurements of 20 millisecond pulsars at three radio frequencies and is approaching the level of timing precision and data spans which are needed for GW detection. These observations will also allow us to establish a “Pulsar Timescale” and to detect or limit errors in the Solar System ephemerides used in pulsar timing analyses. Combination of PPTA data with that of other groups to form an International Pulsar Timing Array (IPTA) will enhance the sensitivity to GWs and facilitate reaching other PTA goals. The principal source of GWs at the nanoHertz frequencies to which PTAs are sensitive is believed to be super-massive binary black holes in the cores of distant galaxies. Current results do not signficantly limit models for formation of such black-hole binary systems, but in a few years we expect that PTAs will either detect GWs or seriously constrain current ideas about black-hole formation and galaxy mergers. Future instruments such as the Square Kilometre Array (SKA) should not only detect GWs from astrophysical sources but also enable detailed studies of the sources and the gravitational theories used to account for the GW emission.

The dark matter paradigm and cold dark matter (CDM) cosmology are widely accepted theories on which much of our understanding of the properties and evolution of the Universe is based. Yet these paradigms face a number of serious problems. A heuristic alternative to dark matter is the theory of modified Newtonian dynamics (MOND) and its relativistic formulation TeVeS. It has been shown that MOND fits the rotation curves of galaxies remarkably well, so additional tests are desirable in order to understand if the dynamics of the Universe on large scales is dominated by dark matter or MOND/TeVeS. Here, we report first results from a new test of MOND by measuring internal velocity dispersions and stellar mass functions of globular clusters in the outer halo of the Milky Way.

The Chandrasekhar limit is of key importance for the evolution of white dwarfs in binary systems and for the formation of neutron stars and black holes in binaries. Mass transfer can drive a white dwarf in a binary over the Chandrasekhar limit, which may lead to a Type Ia supernova (in case of a CO white dwarf) or an Accretion-Induced Collapse (AIC, in the case of an O-Ne-Mg white dwarf; and possibly also in some CO white dwarfs) which produces a neutron star. The direct formation of neutron stars or black holes out of degenerate stellar cores that exceed the Chandrasekhar limit, occurs in binaries with components that started out with masses ≥ 8 M_⊙.

This paper first discusses possible models for Type Ia supernovae, and then focusses on the formation of neutron stars in binary systems, by direct core collapse and by the AIC of O-Ne-Mg white dwarfs in binaries. Observational evidence is reviewed for the existence of two different direct neutron-star formation mechanisms in binaries: (i) by electron-capture collapse of the degenerate O-Ne-Mg core in stars with initial masses in the range of 8 to about 12 M_⊙, and (ii) by iron-core collapse in stars with inital masses above this range. Observations of neutron stars in binaries are consistent with a picture in which neutron stars produced by e-capture collapse have relatively low masses, ˜1.25M_⊙, and received hardly any velocity kick at birth, whereas neutron stars produced by iron-core collapses are more massive and received large velocity kicks at birth. Many of the globular cluster neutron stars and also some of the neutron stars in low-mass binaries in the Galactic disk are likely to have been produced by AIC of O-Ne-Mg white dwarfs in binaries. AIC is expected to produce normal strongly magnetized neutron stars, which in binaries can evolve into millisecond pulsars through the usual recycling scenario.